ESSENTIALS

ESSENTIALS

section 22  Haematological disorders 5520 and often mild, as compared with thrombocytopenia associated with inadequate platelet production (mentioned previously). Rarely, an autoantibody to a specific clotting factor, such as factor VIII, produces a severe acquired bleeding disorder. An acquired von Willebrand syndrome may be due to either an autoantibody (often an IgG paraprotein and occasionally IgM) or consumption of VWF in patients with uncontrolled essential thrombocythaemia. Hyperfibrinolysis A bleeding tendency due to heritable defects of fibrinolysis is excep- tionally rare. Pharmacological doses of fibrinolytic activators, such as recombinant tissue factor activator, have an immediate lytic effect, and hence clinical utility as a ‘clot-​busting drugs’, but their adminis- tration is associated with a very high incidence of bleeding. Bleeding in the neonate Bleeding in the neonate may be due to a rare heritable defect or an acquired abnormality occurring in utero or soon after delivery. Thrombocytopenia is present in 30 to 40% of neonates in special care baby units. Thrombocytopenia presenting in the first 72 h is most often due to chronic fetal hypoxia, often with intrauterine growth retardation, but is rarely due to transplacental passage of an alloantibody to a platelet antigen, FNAIT, previously known as NAIT (neonatal alloimmune thrombocytopenia, now fetal and neonatal). Thrombocytopenia presenting after 72 h is most often in association with sepsis or necrotizing enterocolitis and is often associated with DIC. Bleeding from the umbilical stump or intracranial haemor- rhage not explained by DIC or severe thrombocytopenia may be due to severe deficiency of factors VIII, IX, XIII or afibrinogenaemia. Management of bleeding Acute bleeding Effective treatment depends on a critical assessment of the extent and nature of bleeding and the likely cause. When a defined haemo- static abnormality is identified, specific therapy can be given. Drugs that cause bleeding should be stopped and if appropriate (based on an assessment of relative benefit and risk), specific reversal agents administered, such as PCC for patients over-​anticoagulated with VKA. Nonhaematological causes of bleeding should be managed ap- propriately, for example, dialysis and red cell transfusion in patients with renal failure. Vitamin K should be given to critically ill patients and patients with liver disease. Early and sufficient blood product support should be given to patients with massive blood loss and to those with dilutional coagulopathy. Patients with overt haemato- logical disorders such as myelodysplasia, ITP, or factor VIII inhibi- tors will require specialist care. Pharmacological agents can be used to increase haemostatic capacity but should be used by clinicians with appropriate experience. Such drugs include DDAVP, tranex- amic acid, and off-​licence use of drugs such as recombinant factor VIIa. Aprotinin was used extensively in the past but is now used with caution because of thrombotic complications, including death, and renal impairment. Nonacute bleeding It is important to identify the circumstances that contribute to ab- normal bleeding and to determine the likelihood of an underlying persistent bleeding tendency as this will influence future man- agement, for example, at times of surgery or decisions regarding antithrombotic therapy. A comprehensive drug history should iden- tify drugs that may have to be stopped. Some individuals are par- ticularly sensitive to the usually mild anticoagulant effect of aspirin or ADP antagonists or even NSAIDs. A single episode of abnormal surgical bleeding may not be readily explained but this should be taken into consideration at times of future surgery so that mechan- ical rather than pharmacological thromboprophylaxis is used and any antiplatelet or anticoagulant therapy stopped with certainty. FURTHER READING Balduini CL, Savoia A, Seri M (2013). Inherited thrombocytopenias frequently diagnosed in adults. J Thromb Haemost, 11, 1006–​19. Federici AB (2014). Clinical and laboratory diagnosis of VWD. Hematology Am Soc Hematol Educ Program, 2014, 524–​30. Harrison P, et al. (2011). Guidelines for the laboratory investigation of heritable disorders of platelet function. Br J Haematol, 155, 30–​44. Hunt B, et al. (2015). A practical guideline for the haematological man- agement of major haemorrhage. Br J Haematol, 2015, 170, 788–​803. Keeling D, et al. (2011). Guidelines on oral anticoagulation with war- farin –​ fourth edition. Br J Haematol, 154, 311–​24. Makris M, et al. (2012). Guideline on the management of bleeding in patients on antithrombotic agents. Br J Haematol, 160, 35–​46. Monroe DM, Hoffman M (2006). What does it take to make the perfect clot? Arterioscler Thromb Vasc Biol, 26, 41–​8. Toh CH, Alhamdi Y (2013). Current consideration and management of disseminated intravascular coagulation. Hematology Am Soc Hematol Educ Program, 2013, 286–​91. 22.7.3  Thrombocytopenia and disorders of platelet function Nicola Curry and Susie Shapiro ESSENTIALS The platelet is the smallest circulating blood cell. In health, it plays a vital role in haemostasis, and in disease contributes to prob- lems of bleeding and/​or thrombosis. The number of platelets pro- duced is under tight homeostatic control, regulated by the cytokine thrombopoietin. A normal platelet count lies within the range 150 to 450 × 109/​litre. Vessel injury initiates a set of highly regulated and specialized responses from the platelet, which include adhesion, activation, granule release reactions, and platelet aggregation. A stable haemo- static plug is thereby formed at the site of injury, where platelets are bound tightly to fibrinogen via the glycoprotein IIb/​IIIa receptor. Thrombocytopenia Thrombocytopenia is defined as a reduction in the number of cir- culating platelets to fewer than the normal reference range (typically

22.7.3  Thrombocytopenia and disorders of platelet function 5521 <150 × 109/​litre). Spontaneous bleeding is uncommon unless the platelet count falls below 10 to 20 × 109/​litre or unless there is ab- normal platelet function. Thrombocytopenia can be classified according to three main pathologies: (1) increased platelet destruc- tion, (2)  reduced platelet production, and (3)  increased platelet sequestration. Disorders of increased platelet destruction may be immune medi- ated or nonimmune. Primary immune thrombocytopenia (ITP) is an acquired disorder affecting both adults and children, characterized by an isolated thrombocytopenia (platelet count <100 × 109 /​litre) for which no precipitant can be found. Primary ITP is a diagnosis of exclusion. Corticosteroids are the main first-​line therapy for adult ITP, commonly prednisolone. Nonimmune causes of platelet destruction include microangiopathic haemolytic disorders such as thrombotic thrombocytopenic purpura, haemolytic uraemic syndrome, and dis- seminated intravascular coagulation. Decreased platelet production—​most cases are acquired, with common or important causes being toxins (drugs, alcohol), nutri- tional deficiencies (folate or vitamin B12), bone marrow infiltration, and myelodysplastic syndrome. Disorders of platelet distribution and platelet sequestration—​these include splenomegaly and hypersplenism, haemodilution (in pa- tients who have received large volumes of crystalloid solutions or blood products), and extracorporeal circulation. Disorders of platelet function Disorders of platelet function are usually acquired. The most common causes are medications and toxins (aspirin, nonsteroidal anti-​ inflammatory agents, ticlopidine, clopidogrel, glycoprotein IIb/​IIIa inhibitors), systemic disorders (chronic kidney disease), and haemato- logical diseases (chronic myeloproliferative disorders, myelodysplastic syndromes, dysproteinaemias). Congenital disorders, which can affect platelet adhesion and aggregation, secretion, or procoagulant activity, are a rare cause of symptomatic bleeding. Introduction The platelet is the smallest circulating blood cell. It is a 2-​ to 4-​µm diameter, discoid, anucleate cell that circulates in the blood fol- lowing release from the megakaryocyte. In health, it plays a vital role in haemostasis (see Chapter 22.7.1), and in disease contributes to problems of bleeding as well as thrombosis. It plays important roles in other processes such as promotion of vessel constriction as well as vessel wall repair. Arguably, the most important additional function for the platelet is the central role it takes in the cross-​talk between coagulation and inflammation. Platelets are involved in the promotion of leucocyte recruitment, the formation of platelet microparticles, and the formation of neutrophil extracellular traps which are increasingly being recognized as an important host re- sponse to infection as well as playing a central role in thrombotic and inflammatory diseases. The number of platelets produced in the body is under tight homeostatic control, regulated by the cytokine thrombopoietin (TPO) which is produced by the liver. TPO binds to its receptor, c-​ MPL, found on both megakaryocyte and platelet surfaces and is then internalized. TPO acts to stimulate megakaryopoiesis. A  normal platelet count lies within the range 150 × 109/​litre to 450 × 109/​litre. Platelets have a lifespan of approximately 7 to 10 days and they usu- ally circulate in the bloodstream in a quiescent state. Their small shape and size, in comparison to red blood cells and plasma proteins, means that they circulate close to the endothelial surface facilitating their role in clot formation following endothelial injury. Intact, un- damaged vessel walls help to maintain the platelets in an inactive state by releasing nitric oxide which acts both to dilate the vessel wall and inhibit adhesion, activation, and aggregation of the platelet. Vessel injury initiates a set of highly regulated and specialized responses from the platelet which includes adhesion, activation, granule release reactions, and platelet aggregation. In simple terms, a platelet can be viewed as acting principally to form ‘haemostatic bricks’ at the sites of vessel injury which are then ‘glued’ in place by fibrin. The term ‘platelet disorder’ covers a very large and heterogeneous group of diseases with myriad causes. Platelet disorders can be in- herited or acquired and may be classified in the following way: 1. An abnormality of platelet number (quantitative disorder), for example, thrombocytopenia (platelet count <150 × 109/​litre) or thrombocytosis (platelet count >450 × 109/​litre) (thrombocytosis is covered in Chapter 22.3.6). 2. An abnormality of platelet function (qualitative disorder). 3. A combination of quantitative and qualitative abnormalities. Acquired disorders are more common and therefore are more fre- quently encountered in everyday clinical practice. This chapter will focus on thrombocytopenia and disorders of platelet function. Thrombocytopenia and platelet dysfunction Thrombocytopenia is defined as a reduction in the number of circu- lating platelets to fewer than a laboratory’s normal reference range (typically <150 × 109/​litre). Spontaneous bleeding is uncommon un- less the platelet count falls below 10 to 20 × 109/​litre or unless there is abnormal platelet function. Thrombocytopenia can be classified according to three main pathologies: (1) reduced platelet produc- tion, (2) increased platelet destruction, and (3) increased platelet se- questration (Table 22.7.3.1). History and physical examination of the thrombocytopenic patient/​patient with platelet dysfunction A single platelet count should not be evaluated in isolation and a broader assessment of the clinical and laboratory picture must be sought when investigating thrombocytopenia. The history will pro- vide many of the important details and should focus on: bleeding history, family history (if an inherited disorder is considered), drug history and history of other symptoms related to a potentially causa- tive acquired disorder. Bleeding history (See also Chapter 22.7.2.) Typical symptoms of thrombocytopenia or platelet dysfunction relate to increased mucosal surface bleeding: skin bruising/​bleeding (petechiae, ecchymoses), gum bleeding, epistaxis, heavy menstrual bleeding, and gastrointestinal bleeding. However, in some patients there may be no symptoms at all until a patient bleeds

section 22  Haematological disorders 5522 excessively after a haemostatic challenge such as surgery, dental ex- traction, or childbirth. Less commonly, a patient may present with symptoms of anaemia, due to continued occult blood loss and the development of iron deficiency. A bleeding history is a subjective method of assessment and mild platelet disorders can be difficult to distinguish from normality. There are standardized guides or ‘bleeding questionnaires’ that can improve the accuracy of taking a bleeding history and the International Society for Thrombosis and Hemostasis (ISTH) recommend the ISTH Bleeding Assessment Tool (2010). In women with heavy menstrual bleeding, pictorial men- strual blood loss charts are a useful method of quantifying bleeding and can be helpful as a means of monitoring response to treatment. Drug history It is important to also take a full drug history, and specifically to ask about over-​the-​counter medicines that may contain aspirin or other antiplatelet agents. The use of other drugs that may cause an im- mune thrombocytopenia should be specifically sought (i.e. heparin, sulfonamides, and quinine). Family history A patient is more likely to have an inherited disease if they give a positive family history and present at a young age. Some of the in- herited platelet disorders are autosomal recessive—​consider asking if the patient’s parents are in a consanguineous relationship. Previous platelet counts can prove invaluable as they may pro- vide evidence of a longstanding thrombocytopenia suggesting an inherited disease, or may show a rapid recent fall in a platelet count more in keeping with an acquired problem. Von Willebrand disease presents with symptoms that are very similar to a platelet dis- order and should be excluded during the investigation, particularly platelet-​type and type 2B von Willebrand disease where thrombo- cytopenia is a common feature. Connective tissue disorders such as Ehlers–​Danlos syndrome can present with problematic bruising and should also be considered in the differential. Examination of pa- tients with thrombocytopenia or platelet dysfunction should focus on documenting the sites of bruising and bleeding—​particularly noting sites of active bleeding. The examination should then focus on evaluation of the lymph nodes, liver, spleen, and joints (to look for signs of disease that can be associated with thrombocytopenia—​ see Box 22.7.3.1). Table 22.7.3.1  Classification of thrombocytopenia by aetiology Acquired Platelet destruction Immune: Immune thrombocytopenic purpura (ITP) Immune thrombocytopenia associated with other autoimmune disorders: Evan’s syndrome, systemic lupus erythematosus, lymphoproliferative disorders Drug induced: penicillins, bendroflumethiazide, digoxin, quinine, gold, heparin Infection: HIV, hepatitis, malaria Post-​transfusion purpura Fetal and neonatal alloimmune thrombocytopenia Nonimmune: Thrombotic thrombocytopenic purpura Haemolytic uraemic syndrome Disseminated intravascular coagulation Pregnancy related: haemolysis with elevated liver enzymes and low platelets (HELLP), pre-​eclampsia Cardiopulmonary bypass Reduced production Marrow failure: aplastic anaemia Marrow infiltration: metastatic cancer, haematological malignancies (leukaemia, lymphoma, myeloma), myelofibrosis, storage disorders (Gaucher’s disease), granulomatous disorders (sarcoidosis) Nutritional deficiency: vitamin B12 and folate Toxins: alcohol, drugs (co-​trimoxazole, penicillamine), viral infections (HIV, hepatitis) Altered distribution/​sequestration Splenomegaly, massive transfusion, cardiopulmonary bypass Congenital Platelet destruction Thrombotic thrombocytopenic purpura (TTP) Reduced production ± altered platelet function Small platelets: Wiskott–​Aldrich syndrome Normal-​sized platelets: congenital amegakaryocytic thrombocytopenia, thrombocytopenia with absent radius Large platelets: MYH-​9-​related disorders including May–​Hegglin anomaly Box 22.7.3.1  Secondary associations of immune thrombocytopenia • Infections:

— HIV

— Hepatitis C

— Varicella

— Epstein–​Barr virus • Collagen vascular disease:

— Systemic lupus erythematosus

— Rheumatoid arthritis • Lymphoproliferative disorders

— Hodgkin disease

— Non-​Hodgkin lymphoma, including CLL/SLL • Other:

— Antiphospholipid antibody syndrome

— Autoimmune thyroid dysfunction

— Sarcoidosis

— Post bone marrow transplantation

22.7.3  Thrombocytopenia and disorders of platelet function 5523 Laboratory evaluation of the thrombocytopenic patient/​patient with platelet dysfunction Routine laboratory tests First-​line laboratory tests and the reasons for performing these tests are shown in Table 22.7.3.2. The blood film is essential. Pseudothrombocytopenia is a common reason for a low platelet count. This is an artefact that occurs when platelets clump to- gether after blood is collected into ethylenediaminetetraacetic acid (EDTA). This effect occurs in 0.1% of blood samples and is caused by a clinically insignificant autoantibody which agglutinates platelets. Often the artefact can be avoided by using an anticoagulant other than EDTA (e.g. citrate). Fragmented red cells (schistocytes) may be seen in thrombotic thrombocytopenic purpura (TTP), haemolytic uraemic syndrome (HUS), and disseminated intravascular coagu- lation (DIC). Leukoerythroblastic changes, such as teardrop-​shaped red blood cells, nucleated red blood cells, and immature white cells suggest infiltration of the bone marrow. The presence of abnormal circulating cells such as lymphoblasts or myeloblasts suggests a haematological malignancy. Typical changes on the peripheral smear such as megaloblastic red blood cells and hypersegmented neutro- phils suggest vitamin B12 or folate deficiency. Atypical lymphocytes suggest a viral infection. Large platelets suggest the diagnosis of im- mune thrombocytopenia but if there are many giant platelets a diag- nosis of an inherited thrombocytopenia should be entertained. Other laboratory investigations that may be indicated include antinuclear antibody, rheumatoid factor, thyroid-​stimulating hor- mone, antiphospholipid antibodies, and testing for HIV, hepatitis C, and other infectious causes (Epstein–​Barr virus, varicella), vitamin B12 and folate, and lactate dehydrogenase. More specialist tests Examination of the bone marrow should be considered if the aeti- ology of the thrombocytopenia is uncertain. A bone marrow exam- ination is required when abnormalities are seen on the blood film suggestive of haematological malignancy or infiltration of the marrow. Assessing platelet function Many platelet function tests are specialized and can only be per- formed at certain laboratories. Tests should be performed within 2 h of venepuncture, because platelets can undergo in vitro activation or desensitization if left in blood bottles. Methods such as the ‘Ivy’ or the ‘Template’ bleeding time were used historically to assess primary haemostasis. This test is no longer recommended as it is subject to many confounding factors. Another test, the PFA-​100 or PFA-​200 test, goes some way to replacing the bleeding time and provides an automated means of assessing pri- mary haemostasis. However, it cannot be used to exclude a platelet disorder when used alone as it is not sufficiently sensitive to reli- ably detect a mild platelet disorder. Two cartridges are used for each test; an ADP/​collagen and an adrenaline/​collagen cartridge. It is im- portant to note that results are unreliable in the face of a platelet count less than 80–100 × 109 /​litre. Platelet aggregation is generally considered the gold standard test of platelet function. It involves analysing the response of a patient’s platelets to a variety of agonists. The aggregometer monitors changes in light transmission through a sample of platelet-​rich plasma, over time. Light transmission increases as platelet aggregation in- creases. The following are commonly used agonists: ADP, collagen, ristocetin, arachidonic acid, and epinephrine. Each agonist is tested at multiple concentrations. In concert with aggregometry, platelet secretion tests and/or platelet nucleotides are commonly evaluated. Assessment of platelet ADP and ATP levels are useful for the diag- nosis of storage pool disorders. Many of these platelet tests are difficult to perform and patients may need to be tested more than once. Other specialist tests in- clude platelet flow cytometry and electron microscopy. Finally, molecular genetic testing of families with inherited disorders of platelets may be helpful. A United Kingdom-​wide clinical study (BRIDGE-​BPD study; http://​www.bridgestudy.org) is recruiting patients with platelet defects (and other rare bleeding disorders) where the genetic cause is not yet elucidated. This study has used exome sequencing to investigate the associations between geno- type and phenotype. General approach to management Once a patient has been diagnosed with thrombocytopenia, gen- eral measures to reduce their bleeding risk should be implemented. These include avoidance of antiplatelet agents, in particular non­ steroidal medications; avoidance of trauma, and this should in- clude consideration of contact sports such as rugby or martial arts; consideration of hormonal inhibition of the menses; and control of blood pressure. Tranexamic acid, an antifibrinolytic, is a useful drug that patients can self-​administer for minor bleeding such as gum bleeding or epistaxis and is also a helpful adjunctive therapy for menorrhagia. Specific treatment for certain conditions are de- scribed in the relevant sections. If a patient requires a surgical intervention and has normal platelet function, the United Kingdom British Committee for Standards in Haematology (BCSH) guidelines recommend minimum platelet thresholds for invasive procedures (Table 22.7.3.3). Table 22.7.3.2  First-​line laboratory tests for thrombocytopenia Historical full blood count (FBC) Trends in platelet counts provide clues to underlying diagnosis FBC and reticulocyte count A reticulocyte count is helpful in the presence of anaemia, as if high it suggests haemolysis and if low it points towards reduced red cell production. Cytopenias involving other cell lineages are suggestive of disorders involving the bone marrow Blood film Essential (see text). Prothrombin time (PT), activated partial thromboplastin
time (APTT), Clauss fibrinogen ± D-​dimers Exclude disseminated intravascular coagulation or an additional coagulation abnormality Renal and liver function Exclude haemolytic uraemic syndrome, liver dysfunction

section 22  Haematological disorders 5524 Disorders of increased platelet destruction Disorders of increased platelet destruction can be subdivided into two principal categories: immune and nonimmune. Nonimmune causes include the microangiopathic haemolytic disorders such as TTP, HUS, and DIC. Immune-​mediated platelet disorders Immune-​mediated platelet disorders can be classified according to the type of antibody involved. These include autoantibodies (immune thrombocytopenic purpura), alloantibodies (NAIT or post-​transfusion purpura (PTP)), and immune complex formation (heparin-​induced thrombocytopenia). Heparin-​induced thrombo- cytopenia is described in detail in Chapter 22.7.5 and will not be covered here. Autoimmune thrombocytopenia is classified as pri- mary (idiopathic) if there is no underlying cause and secondary if it is associated with a systemic disease. Primary immune thrombocytopenia Primary ITP is an acquired immune-​mediated disorder af- fecting both adults and children. It is characterized by an isolated thrombocytopenia, defined as a platelet count of less than 100 × 109 /​litre, for which no precipitant can be found. Primary ITP is a diagnosis of exclusion and there are no reliable clinical or labora- tory parameters that allow specific diagnosis. Historically, ITP was thought to arise solely due to the effects of an IgG autoantibody that coated the platelet and led to the increased clearance of platelets by Fcɤ receptors in the liver and spleen. Recent research has recognized that more complex mechanisms including direct T-​cell-​mediated cytotoxic effects and impaired platelet production are also important. ITP affects men and women equally, except between the ages of 30 to 60 years where it is more common in women. Adults who pre- sent with ITP often describe an insidious onset of symptoms and are more likely to have chronic disease. Childhood ITP often follows a discrete viral illness (60% of cases), and presents with acute onset of symptoms. Signs and symptoms of ITP vary widely. Many patients have no symptoms and their thrombocytopenia may be picked up by a routine full blood count. Other patients experience significant bleeding. The severity of the thrombocytopenia, age (>60 years), and a previous history of haemorrhage are predictors of increased bleeding risk. Overall, adults with ITP have a four-​ to fivefold in- creased risk of death from bleeding and infection although the ab- solute rate of fatal haemorrhage from ITP is low, 0.016 to 0.039 cases per adult patient-​year at risk. Diagnostic approach A thorough history and examination, as outlined previously, should be completed. Examination should be normal, aside from signs of mucosal bleeding. Mild splenomegaly may be evident in younger patients, but significant splenomegaly, lymphadenopathy, or hepato- megaly would point to another cause. Additional investigations for ITP, over and above those set out previously, should be considered (Table 22.7.3.4). A bone marrow examination is not of value rou- tinely, but is recommended within the international ITP consensus guidelines in the following groups: patients older than 60 years, pa- tients presenting with systemic symptoms or abnormal signs, or in patients where splenectomy is being considered. Treatment of adult ITP An individualized approach should be taken for the treatment of patients. Patients with a platelet count greater than 50 × 109/​litre rarely require therapy unless there is evidence of active bleeding or Table 22.7.3.3  Recommended minimum platelet counts for invasive procedures Clinical indication Treatment value (×109/​litre) Therapeutic Massive transfusion

50 Massive transfusion and multiple trauma or TBI 100 DIC and bleeding 50 Intracerebral bleeding 100 Prophylactic Pre-​invasive procedure, i.e. LP, CVC, epidural 50 Pre-​surgery 50-​75 Pre-​surgery at high-​risk sites: i.e. brain/​eye 100 CVC, central venous catheter; DIC, disseminated intravascular coagulation; LP, lumbar puncture; TBI, traumatic brain injury. Adapted from UK British Committee for Standards in Haematology guidelines and European trauma guidelines. Table 22.7.3.4  Additional tests for investigation of ITP Test Reason for test Reticulocyte count and Direct antiglobulin test Determines presence of haemolysis Important if considering anti-​D therapy Immunoglobulin quantification Looking for: common variable immunodeficiency and IgA deficiency Blood group and Rh status Informative if considering anti-​D therapy Helicobacter pylori serology Urea breath test Eradication of H. pylori has been shown to lead to resolution of thrombocytopenia in some instances Antiphospholipid antibodies and lupus anticoagulant These are positive in 40% of individuals with ITP Pregnancy test in women of childbearing age Management may differ in pregnancy Antinuclear antibodies Can help diagnose systemic lupus erythematosus and is also an indicator of chronicity in childhood ITP Viral PCR for CMV and parvovirus Chronic infection can cause thrombocytopenia CMV, cytomegalovirus; PCR, polymerase chain reaction.

22.7.3  Thrombocytopenia and disorders of platelet function 5525 the patient needs to undergo a surgery. Platelet transfusions should not be used in patients with ITP unless bleeding exists which re- quires immediate therapy. First-​line therapy Corticosteroids  Corticosteroids are the mainstay first-​line therapy for adult ITP. Prednisone is the most commonly used medication, and is effective and easy to administer. An alternative to prednisone is the steroid dexamethasone (Table 22.7.3.5). The duration and dose of steroids should be kept to a minimum. Intravenous anti-​D  Reticuloendothelial blockade results in a more rapid rise in the platelet count than corticosteroids. Anti-​D can be used for Rh D-​positive, nonsplenectomized patients. Blood group, direct antiglobulin test, and reticulocyte count must be known prior to the use of this therapy. Anti-​D is a pooled plasma product and patients should be made aware of this prior to treatment. In 2010, the Food and Drug Administration issued a black box warning against anti-​D highlighting the risk of haemolysis, DIC, and renal failure. The incidence of severe haemolytic reactions is es- timated at 1 in 1115 patients and it tends to occur within 4 h of treat- ment. The risk appears highest in adults over the age of 65 years or in patients with baseline evidence of haemolysis or renal impairment. It is recommended that anti-​D is avoided in these groups of patients. Intravenous immunoglobulin  Intravenous immunoglobulin (IVIg) is often administered when a rapid rise in platelet count is needed, for example, prior to surgery. Side effects are common during IVIg in- fusions and concomitant corticosteroid therapy can both reduce side effects and may enhance platelet response. Second-​line therapies Approximately one-​third of patients with ITP fail to respond to first-​ line therapy or relapse. Optimal second-​line therapy is uncertain and patients should be actively involved in treatment decisions. The aim of second-​line therapy is to achieve a sustained haemostatic platelet response. Rituximab  Rituximab is a chimeric monoclonal antibody directed against CD20, a pan B-​cell antigen. Studies comparing standard-​ dose (375 mg/​m2) versus low-​dose rituximab (100 mg) found no difference in response rate, although the duration of response was shorter with the lower dose. A disadvantage of rituximab is that the average time to a response is long, leaving patients vulnerable to bleeding, as well as a lack of sustained response. If a response is durable (i.e. lasts for ≥12 months), a patient is likely to respond to a second cycle of treatment at relapse. Rituximab is contraindicated in patients with active hepatitis B infection. The optimum time to administer rituximab is not yet clear and informative randomized controlled trials are lacking. Newer-​generation anti-​CD20 treatments are being developed. Only one has been tested so far in ITP—​veltuzumab, a humanized monoclonal antibody. It led to an observable response in 55% of pa- tients with some showing a durable response to 4.3 years. The poten- tial benefit is that it can be administered subcutaneously. Thrombopoietin receptor agonists  TPO receptor agonists bind and activate the TPO receptor and initiate megakaryocyte differen- tiation and platelet production. There are two main types of TPO receptor agonists: TPO peptide mimetics (romiplostim) and TPO nonpeptide mimetics (eltrombopag). These drugs are used as main- tenance therapy and as soon as they are discontinued, platelet counts return to baseline levels. Splenectomy  Historically, splenectomy was the second-​line treat- ment of choice. Now, with greater choice of drug therapies there is an increasing reluctance to offer splenectomy. If used, it should be deferred until at least 6 months, since spontaneous remission of Table 22.7.3.5  Common treatments for immune thrombocytopenia Therapy Dose Initial response rate and speed of response Side effects Long-​term response rate First-​line therapies Corticosteroids:    Prednisone    Dexamethasone 1 mg/​kg (range 0.5–​2 mg/​kg) given until platelets rise above 30–​50 × 109/​litre 40 mg/​kg for 4 days every
2–​4 weeks for 1–​4 cycles 70–​80% respond within
a few weeks 90% respond within a
few weeks Variable and include weight gain, lability of mood, diabetes, hypertension, cataracts, avascular necrosis, increased infection risk 15% at 10 years 50–​60% at 2–​5 years Intravenous anti-​D 50–​75 mcg/​kg 80% respond within 4–​5 days Haemolysis, fever, rigors Fatal intravascular haemolysis, DIC, and renal failure are rare Typical response lasts 3–​4 weeks IVIg 0.4 g/​kg per day for 5 days or 1 g/​kg per day for 1–​2 days 80% respond within 2–​4 days (occasionally by 24 h) Headaches, fever, rigors, fatigue, transient neutropenia, thrombosis, and renal failure Response duration short-​lived. Baseline platelet counts seen at 4 weeks Second-​line therapies Rituximab 375 mg/​m2 intravenously every week for 4 weeks 60% respond with median time to response 5.5 weeks Few side effects, most commonly fevers, rigors, or itching with first infusion 20% at 3–​5 years TPO receptor agonists:    Romiplostim    Eltrombopag 1–​10 μg/​kg subcutaneously weekly 25–​75 mg/​day orally 80–​90% within 1–​4 weeks 70–​80% within 2–​3 weeks Headache, fatigue, arthralgia, increased bone marrow reticulin Headache, increased bone marrow reticulin, thrombosis Response seen at 4 years with continued administration Response seen at 1.5 years with continued administration Splenectomy –​ 80% respond within
3 weeks Lifelong risk of infection 60–​70% response at 5 years

section 22  Haematological disorders 5526 ITP can occur up to 6 to 12 months from diagnosis. Splenectomy can be performed as an open or laparoscopic procedure, with mor- tality rates of 1.0 and 0.2%, respectively. Indium-​labelled platelet scans can be used to look for platelet destruction in the spleen. If splenic destruction is confirmed, approximately 90% of patients respond to splenectomy. Splenectomized patients are at lifelong increased risk of infection, particularly from encapsulated organ- isms, and should be vaccinated at least 4 weeks prior to surgery. Patients should be given a polyvalent pneumococcal, meningo- coccal C conjugate, and Haemophilus influenzae B vaccine. Patients in receipt of rituximab within 6 months of splenectomy may not respond to vaccination and revaccination should be given after B-​ cell recovery. No consensus has been reached about whether long-​ term prophylactic antibiotics are useful, but asplenic patients are often given phenoxymethylpenicillin 250 to 500 mg twice daily or erythromycin 500 mg twice daily. An alternative approach is to offer splenectomized patients a home supply of antibiotics to use in case of need for a febrile illness. Patients need to be well educated about their risk of infection. Third-​line therapies Approximately 20% of patients will not achieve an acceptable platelet count after first-​ and second-​line treatments and splenectomy. Some of these non-​ or poor-responders will maintain a good quality of life with a low platelet count (sometimes as low as 10 × 109/​litre) and will only require therapy for surgical intervention. Other patients have increased bleeding rates and a higher risk of death. Options for them are limited but include combination chemotherapy and Campath-​1H. Emergency treatment of ITP Patients with ITP may require an urgent increase in platelet count, most commonly for surgical procedures, and sometimes for signifi- cant active bleeding. Combination therapy is of particular benefit in these situations and prednisone plus IVIg are recommended for pa- tients with uncontrolled bleeding. High-​dose methylprednisolone may also be useful. Platelet transfusion is appropriate as treatment for a patient with significant active bleeding (and responses may be better if transfusion is given with IVIg), along with other standard measures to treat major haemorrhage. Vinca alkaloid drugs (i.e. vincristine) in combination with other therapies have been re- ported to lead to a better and more rapid rise in platelet count and may be considered for emergency treatment. ITP during pregnancy ITP is estimated to occur in 1 in 1000 to 1 in 10 000 pregnancies. Women who have previously had a diagnosis of ITP may relapse during pregnancy. During the first two trimesters, treatment is initi- ated only if the patient is symptomatic, the platelet count falls below 20–​30 × 109/​litre, or the patient needs to undergo a procedure. High-​quality data are lacking to guide safe platelet thresholds for procedures, but as a general rule caesarean section or spontaneous vaginal delivery are thought safe with a platelet count above 50 × 109/​litre and neuraxial anaesthesia requires a count of at least 80 × 109/​litre. There is no evidence to recommend a caesarean section over vaginal delivery in maternal ITP. Primary treatments for pregnant patients with ITP are similar to nonpregnancy and corticosteroids and IVIg remain the first-​line treatments. Prednisone is often started at a lower dose (10–​20 mg/​ day) and adjusted according to response. IVIg doses and response rates are similar to nonpregnant patients and IVIg is commonly used in situations where the platelet count needs to rise rapidly. First-​ line therapies can fail and combination therapy, such as high-​dose methylprednisolone (1 g) in combination with IVIg or azathioprine has been used especially in the weeks just prior to delivery to estab- lish a safe platelet count. Splenectomy has been performed in preg- nancy, and if required is recommended in the early second trimester using a laparoscopic technique. Neonatal ITP occurs in approximately 10% of cases. Procedures during delivery that may increase intracranial haemorrhage, such as Ventouse extraction, fetal scalp electrodes, or forceps de- livery should be avoided. A cord blood sample should be drawn immediately after delivery. The neonate should be monitored for between 2 and 5 days after delivery. If a platelet count falls below 20 × 109/​litre or the neonate is bleeding, a single dose of IVIg is recommended and if there is significant haemorrhage, a platelet transfusion should be given. Transcranial ultrasound scanning should be routine for all neonates with a platelet count less than 50 × 109/​litre. ITP in childhood The diagnosis of ITP in a child is one of exclusion. Patients com- monly present with bruises and petechiae and only very few (3%) have clinically significant bleeding. The incidence of intracerebral haemorrhage in children is in the order of 0.1 to 3%. Most chil- dren can be managed expectantly. This approach requires the parents to understand the attendant risks of thrombocytopenia for their child. Two-​thirds of children who are managed using a watch and wait policy will spontaneously remit within 6 months of diagnosis. Time to remission is highly variable. Admission to hospital is only required for children with significant bleeding. Treatment for children mirrors adult management but many treatments including corticosteroids, immunosuppression, and splenectomy, are not thought to be curative and may cause more problems than thrombocytopenia. Prednisone is often used first line at a dose of 1 to 2 mg/​kg per day for a maximum of 14 days or 4 mg/​kg per day for 4 days; 75% of patients respond and platelet recovery is seen rapidly by 2 to 7 days. IVIg leads to a response in 80% of patients and recovery is rapid within 1 to 2 days. Children are more commonly treated with a single dose of IVIg at 0.8 to 1.0 g/​kg. Anti-​D is also used in childhood ITP. Treatment of life-​ threatening bleeding mirrors adult therapy with the use of platelet transfusions in combination with intravenous high-​dose cortico- steroids and IVIg. Some children develop chronic ITP. A child should be re-​evaluated 3–​6 months after primary diagnosis. Investigations should include a bone marrow examination to look for haematological malig- nancy, antinuclear antibodies, tests for antiphospholipid syndrome, immunoglobulin quantification, and a review of medications. The aim of therapy for chronic ITP is to maintain a haemostatic platelet count while minimizing corticosteroid exposure. Splenectomy may also be performed in children with ITP but risks of infection are high (3% overwhelming sepsis) and this risk must be weighed against the low overall mortality for childhood ITP (0.5%). If first-​ line therapies prove ineffective, treatments such as rituximab can be used. A  systematic review of uncontrolled studies evaluating

22.7.3  Thrombocytopenia and disorders of platelet function 5527 rituximab therapy in 323 children reported a pooled complete re- sponse of 39% (platelets >100 × 109/​litre) and an overall response rate of 68% (platelets >30 × 109/​litre) with a median response dur- ation of 12.8 months. TPO receptor agonist therapies have also been used in the paediatric setting and a recently published placebo-​ controlled randomized controlled trial, PETIT2 (NCT01520909), evaluated the safety and efficacy of eltrombopag in chronic ITP. Eltrombopag led to a sustained response in 40% of patients and no safety concerns were raised. Secondary immune thrombocytopenia A variety of medical disorders cause secondary immune thrombo- cytopenia (Box 22.7.3.1). The treatment for secondary immune thrombocytopenia is similar to that of ITP. Alloimmune thrombocytopenia Alloimmune thrombocytopenia is caused by alloantibodies directed against platelet glycoproteins (GPs). There are two alloimmune thrombocytopenic disorders: NAIT and PTP. Fetal and neonatal alloimmune thrombocytopenia NAIT is a condition in which maternal alloantibodies cross the placenta and destroy fetal platelets. The mother’s immune system recognizes paternal platelet antigens, expressed on fetal platelets, as foreign. This disorder can cause severe and life-​threatening fetal thrombocytopenia that may lead to intracranial haemorrhage and death in utero. About 80% of NAIT cases are caused by anti-​HPA-​1a and 15% anti-​HPA-​5b; other HPA antibodies are detected occasion- ally. The diagnosis of NAIT requires the demonstration of maternal platelet  alloantibodies that react against platelet-​specific antigens present in the father and infant but not in the mother. There is no laboratory parameter which reliably predicts the severity of fetal/​ neonatal thrombocytopenia. Management of the bleeding neonate is with platelets which are negative for the antigen (usually HPA-​1a negative); use random donor platelets in an emergency. NAIT recurs in 85 to 90% of sub- sequent pregnancies. For subsequent pregnancies, cordocentesis is performed at about 20 weeks for platelet count and phenotype. If the neonate is affected, the mainstay of treatment is IVIg given to the mother every week (1 g/​kg). A beneficial effect of IVIg on the fetal/​neonatal platelet count occurs in about 67% of cases. Weekly intrauterine platelet transfusions may be used if other treatment fails. Post-​transfusion purpura PTP is rare (<1 in 700 000 transfusions) and is manifest by a severe thrombocytopenia (platelet count <10 × 109/​litre) developing 5 to 12 days after a blood transfusion. Bleeding is common and may be fatal. PTP is seen after transfusion of packed red cells, whole blood and platelets. It occurs in patients who have been previously sensi- tized, either by pregnancy or a previous transfusion, to foreign platelet antigens. These patients develop an alloantibody against a platelet antigen that they lack, usually HPA-​1a (or less commonly HPA-​5b). PTP occurs most often in multiparous women. The alloantibodies cause platelet destruction. The diagnosis of PTP is confirmed by the demonstration of IgG alloantibodies in the patient’s serum against one of the HPA antigens. It is unclear why alloantibodies attack the patient’s own, as well as the transfused platelets. IVIg therapy is the primary treatment (1 g/​kg intravenously for 2 days) and 85% of pa- tients respond. Platelet transfusion should be avoided, unless there is significant bleeding. Plasmapheresis is an option for IVIg-​refractory patients. The incidence of PTP has fallen since the introduction of leucodepletion. Drug-​induced thrombocytopenia Many drugs can cause thrombocytopenia. The medications most commonly implicated include heparin, penicillins, sulfonamides, valproic acid, and quinine. However, virtually every medication has been associated with thrombocytopenia. The diagnosis of drug induced thrombocytopenia is mostly empiric. Laboratory confirmation can be sought and involves the demonstration of drug-​dependent antiplatelet antibodies using methods such as enzyme-​linked immunosorbent assay (ELISA). Patients with drug-​induced thrombocytopenia typically have moderate to se- vere thrombocytopenia. Thrombocytopenia is usually seen after 2 to 3 days if the medication has previously been used, or after 1 to 3 weeks for a new medication. The thrombocytopenia usually resolves within 5–​10 days of stopping the causative drug. In cases of severe thrombocytopenia, the drug should be discontinued and the patient may be treated using either IVIg or anti-​D. Treatment with cortico- steroids is less effective. In cases of life-​threatening haemorrhage, platelet transfusions may be required. Patients should not take the drug causing the thrombocytopenia again as it will cause thrombo- cytopenia with subsequent exposure. Nonimmune platelet destruction Microangiopathic haemolytic anaemia and related disorders A microangiopathic haemolytic anaemia (MAHA) with thrombo- cytopenia may be caused by TTP, HUS, DIC, HELLP (haemolysis, elevated liver enzymes, and low platelets), and pre-​eclampsia/​ eclampsia. Thrombotic thrombocytopenic purpura This is a rare disorder characterized by thrombocytopenia and microangiopathic haemolytic anaemia. Its incidence is approxi- mately 6 per million per year. Recognition and early diagnosis is im- portant as untreated the mortality is 90%. Even with optimal current therapy mortality is about 20%. It may be acquired (95% of cases) or congenital. TTP is caused by a deficiency of the von Willebrand factor (VWF) cleaving protease ADAMTS13 (a disintegrin and metalloprotease with thrombospondin type 1 motif). Deficiency of ADAMTS13 results in circulation of ultra-​large VWF multimers. These are haemostatically active and spontaneously bind to platelets, par- ticularly under conditions of high shear. The VWF/​platelet aggre- gates block small blood vessels. These microthromboses cause tissue damage, consume platelets, and result in fragmentation of passing red blood cells. Acquired TTP results from autoantibodies against ADAMTS13. Most cases are idiopathic but TTP can be associated with other diseases and conditions (Box 22.7.3.2). Acquired TTP Acquired TTP has a peak incidence of 40 years and slightly more woman are affected then men. The only clinical criteria required for

section 22  Haematological disorders 5528 its diagnosis are thrombocytopenia and microangiopathic haemo- lytic anaemia. Patients may also present with: • bleeding: bruising, petechiae, haematuria, retinal haemorrhage • neurological signs (in about 70% of patients) which may be tran- sient: confusion, headache, visual problems, aphasia, paresis, coma • renal signs (about 30%):  acute kidney injury, proteinuria, microhaematuria • cardiac signs (about 40%): chest pain, hypotension • gastrointestinal tract signs (about 30%) • nonspecific symptoms: fever, arthralgia, myalgia, pallor, jaundice, abdominal pain Diagnosis of TTP is challenging as there is significant overlap with other disorders such as HUS, pregnancy-​related disorders, DIC, and autoimmune disorders (Box 22.7.3.3). The initial diag- nosis is made on clinical history and examination combined with blood parameters consistent with microangiopathic haemolytic anaemia: anaemia, thrombocytopenia, red cell fragments on blood film, elevated reticulocytes/​bilirubin/​lactate dehydrogenase, and low haptoglobin. A  negative Coombs’ test helps to exclude an autoimmune haemolytic anaemia. A  coagulation screen should be normal, rather than deranged as it is in DIC. The diagnosis is confirmed by an ADAMTS13 activity assay less than 5 to 10% with or without anti-​ADAMTS antibody, but treatment should not be delayed while these results are awaited. Investigations should also be sent to look for an underlying cause such as HIV, pregnancy, pancreatitis, and malignancy (Table 22.7.3.6). Acute acquired TTP should be treated as a medical emergency. The mainstay of treatment is plasma exchange (PEX) with fresh frozen plasma, although the optimum regimen has not yet been determined. PEX significantly improves time to remission and chance of survival compared to fresh frozen plasma infusion. PEX removes the ultra-​large VWF and is a source of ADAMTS13. PEX should be started within hours of the patient presenting. If there is any delay than an infusion of fresh frozen plasma can be given as a temporary holding measure. Patients require daily plasma exchange of 1 to 1.5 blood volumes. This may be intensified in patients who present with cardiac or neurological involvement, or who are refractory to initial therapy. Plasma exchange should be continued daily until the platelet count has normalized for 2 days and then it can be stopped (median time to remission is about 2 weeks). Given that autoantibodies are the most frequent pathogenic mechanism, adjunctive therapies include corticosteroids and rituximab (monoclonal anti-​CD20). High-​dose prednisone at 1 to 2 mg/​kg is commonly given although its efficacy has not been unequivocally demonstrated. In prospective studies, with limited patient numbers, rituximab has been shown to reduce time to re- mission if given within the first 3 days of presentation, and may prolong time between relapses. Other immunosuppressants have also been tried, generally in the pre-​rituximab era and with less evidence of effectiveness than is currently available for rituximab, but they may be considered for refractory cases. These include ciclosporin, cyclophosphamide and vincristine. Splenectomy is associated with high mortality in the acute setting (40%) and has limited proven benefit. Major bleeding is rare and platelet transfusions should be avoided as they can precipitate widespread thrombosis. Patients can be supported with transfusion of packed red blood cells as required. While the clinical efficacy of antiplatelets has not been proven they are relatively safe—​and given the risk of micro- vascular thrombosis, and the risk of venous thrombosis in an un- well medical patient, it is generally recommended that patients should be started on low-​dose aspirin (75 mg) and low molecular weight heparin thromboprophylaxis as platelet count recovers (>50 × 109/​litre) (expert opinion). Relapse is defined as an episode of acute TTP more than 30 days after remission. Relapse occurs in 20 to 50% of patients. It is recom- mended that patients are counselled with regard to the risk of relapse and advised to seek medical help early if they experience symptoms of potential relapse. Patients should be monitored long term using Box 22.7.3.3  Differential diagnosis of microangiopathic haemolytic anaemia and thrombocytopenia • Thrombotic thrombocytopenic purpura (TTP) • Haemolytic uraemic syndrome (HUS) • Disseminated intravascular coagulation (DIC) • Pregnancy related: pre-​eclampsia, HELLP, HUS • Vasculitis • Catastrophic antiphospholipid syndrome • Evans syndrome (autoimmune haemolytic anaemia and thrombo­­ cytopenia) • Malignant hypertension • Infections, typically viral (adenovirus, cytomegalovirus) or severe bac- terial infections • Disseminated malignancy Table 22.7.3.6  Investigations in a patient with suspected thrombotic thrombocytopenic purpura To establish MAHA Full blood count, blood film, reticulocytes, lactate dehydrogenase, bilirubin, coagulation screen (PT, APTT, fibrinogen), Coombs’ test To look for organ involvement Renal function, troponin, ECG and consider echocardiogram and CT/​MRI To look for underlying cause Pregnancy test, HIV, hepatitis A/​B/​C serology, autoantibody screen including antinuclear antibodies, dsDNA, anticardiolipin antibody, β2-​glycoprotein-​1, lupus anticoagulant screen, vitamin B12, folate, thyroid function, amylase, stool culture, CT chest/​abdomen/​pelvis To confirm diagnosis ADAMTS13 activity assay, anti-​ADAMTS13 antibody assay ECG, electrocardiogram; MAHA, microangiopathic haemolytic anaemia; MRI, magnetic resonance imaging. Box 22.7.3.2  Conditions and diseases associated with thrombotic thrombocytopenic purpura • Pregnancy and postpartum • Infections: HIV • Drugs:  ciclosporin, quinine, ticlopidine, clopidogrel, interferon-​α, simvastatin • Connective tissue disorder: lupus erythematosus and scleroderma • Allogenic bone marrow transplantation

22.7.3  Thrombocytopenia and disorders of platelet function 5529 ADAMTS13 activity assays and anti-​ADAMTS13 antibody assays. It has been shown that patients with an ADAMTS13 activity less than 10% or a detectable anti-​ADAMTS13 antibody have a threefold increase risk of relapse at 1 year. In such cases, elective rituximab has been successfully used to normalize ADAMTS13 activity in the majority of patients. Congenital TTP Only about 100 cases of congenital TTP have been reported world- wide, but this is likely to increase given the better understanding of the disease and availability of ADAMTS13 assays. It is caused by a mutation in the ADAMTS13 gene (chromosome 9) which re- sults in a quantitative or qualitative deficiency. There is a wide spec- trum of disease with the severest cases manifesting in the neonatal period; milder cases may manifest in middle age or in pregnancy. Treatment is with infusion of an intermediate-​purity FVIII concen- trate (which contains ADAMTS13) or with infusion of fresh frozen plasma. Haemolytic uraemic syndrome This syndrome includes microangiopathic haemolytic anaemia and thrombocytopenia but with more marked renal failure then is generally seen in TTP. It is discussed in more detail in Chapter 22.7.3. Classical HUS typically affects children and is associated with verotoxin-​positive bloody diarrhoea, Escherichia coli serotype O157:H7, or Shigella dysenteriae serotype I. Treatment is supportive and may include renal dialysis. Atypical HUS (aHUS) is generally not associated with diar- rhoea. It may have multisystem symptoms and can be extremely difficult to differentiate from TTP. The primary feature of HUS is renal impairment/​failure, in association with thrombocyto- penia and MAHA. As opposed to TTP, ADAMTS13 activity levels are greater than 10%. Patients should be treated urgently with plasma exchange, especially if TTP has not been excluded. There is increasing evidence for complement dysfunction in aHUS and many patients benefit from administration of the monoclonal anti- body C5 inhibitor, eculizumab. Pregnancy-​associated thrombocytopenia and microangiopathic haemolytic anaemia Several disorders may present for the first time in pregnancy or post- partum period including congenital and acquired TTP, aHUS, and DIC. Pregnancy-​specific disorders of pre-​eclampsia/​eclampsia and HELLP are also associated with low platelets and MAHA. A multi-​ disciplinary approach with close liaison between obstetricians and haematologists is vital. Disseminated intravascular coagulation DIC is an acquired syndrome characterized by widespread intra- vascular activation of the coagulation cascade. The most frequent clinical presentation of DIC is bleeding, although organ dysfunc- tion can also result from microthrombi. Excessive thrombin over- whelms the physiological inhibitors of coagulation and results in excess fibrin, platelet activation, and fibrin/​platelet thrombosis, and bleeding secondary to thrombocytopenia and coagulation factor consumption. First-​line tests for its diagnosis include platelet count, elevated fibrin degradation products, prolonged prothrombin time, and low fibrinogen. DIC is discussed further in Chapter 22.7.5. Disorders of platelet distribution and platelet sequestration Splenomegaly and hypersplenism Approximately 30% of circulating platelets are normally pooled in the spleen. Splenomegaly results in an increase in the size of the pool of platelets sequestered in the spleen and may result in moderate thrombocytopenia (platelets >40 × 10/​9litre). Haemodilutional disorders Dilutional thrombocytopenia is seen after major surgery or large volume blood transfusion. Incidental or gestational thrombocyto- penia occurs in up to 75% of pregnancies and is usually mild—​ increased blood volume is likely to be a significant component. Extracorporeal circulation Thrombocytopenia associated with cardiopulmonary bypass is multifactorial: haemodilution, blood loss, as well as activation of platelets by the synthetic surface may contribute. The thrombocyto- penia is usually mild but it is often accompanied by platelet dysfunc- tion, secondary to activation on the synthetic surface as well as the use of antiplatelet agents. Disorders of decreased platelet production Decreased platelet production results from abnormalities affecting the megakaryocyte progenitor cells, megakaryocytes, or the bone marrow stroma. An isolated reduction in platelet production is rare—​it is generally associated with abnormal production of other cell lines. Diagnosis is usually made by bone marrow to examine megakaryocyte numbers and morphology. Causes may be acquired or congenital. Acquired disorders of decreased platelet production Toxins Many drugs and toxins can cause thrombocytopenia as a result of bone marrow suppression. Common drugs include chemotherapy agents, ionizing radiation, chloramphenicol, and nonsteroidal anti-​ inflammatory drugs (NSAIDs). Alcohol directly suppresses platelet production, but additionally thrombocytopenia may result from hypersplenism and nutritional deficiency. The thrombocytopenia may be associated with a megalo- blastic anaemia and ringed sideroblasts. Nutritional deficiencies Folate or vitamin B12 deficiency can result in thrombocytopenia which may be severe. The thrombocytopenia may be associated with a megaloblastic anaemia and hypersegmented neutrophils. The platelet count recovers with replacement of the deficient vitamin.

section 22  Haematological disorders 5530 Infection Systemic infections (viral, bacterial, and fungal) may result in thrombocytopenia of multifactorial aetiology. Viral infec- tions may suppress platelet production directly by infection of the megakaryocyte, toxic effects of viral proteins of cyto- kines, haemophagocytosis, or immune destruction of platelets. Thrombocytopenia has been associated with HIV, Epstein–​Barr virus, adenovirus, measles, mumps, varicella, hepatitis, and parvo- virus (erythrovirus) B19. Bacterial and fungal infections may also cause thrombocytopenia by direct toxicity or haemophagocytosis. Thrombocytopenia associated with malaria is thought to be due to direct infection of the platelets, dysregulated cytokines and im- mune function, and increased splenic removal. Treatment would generally be supportive, with platelet transfusions if required, although the thrombocytopenia is usually relatively mild and platelet count increases as the infection resolves. Infiltration of the bone marrow Infiltration of the bone marrow generally results in a degree of pan- cytopenia. Infiltration may be with nonhaematopoietic cells such as metastatic cancer, granulomatous or storage disorders, or by haem- atological malignancies (leukaemia, lymphoma, myeloma) and myelofibrosis. Myelodysplasia may present initially with an isolated thrombocytopenia. Congenital disorders of decreased platelet production Thrombocytopenia in infancy is usually secondary to platelet de- struction. Inherited causes of reduced platelet production are rare but should be considered when there is a family history of bleeding or when thrombocytopenia in infants and children persists and is otherwise unexplained. A few of the better characterized disorders are outlined in the following sections, but these are only a fraction of the inherited thrombocytopenias. Treatment depends on the severity of the bleeding disorder and associated platelet dysfunction. Options include local measures including hormone treatment for menorrhagia, antifibrinolytics such as tranexamic acid, desmopressin (DDAVP) for platelet dys- function disorders, and platelet transfusions. Inherited thrombocytopenia with reduced platelet size Wiskott–​Aldrich syndrome:  an X-​linked disorder characterized by micro-​thrombocytopenia, combined immunodeficiency, ec- zema, and increased risk of developing autoimmune disorders and malignancy. Inherited thrombocytopenia with normal platelet size Congenital amegakaryocytic thrombocytopenia:  an autosomal recessive disorder due to mutations in the MPL gene, character- ized by severe thrombocytopenia, and almost complete absence of megakaryocytes in the bone marrow. Individuals develop progressive bone marrow failure over 5 to 10 years although it can be more rapid. Thrombocytopenia with absent radius:  an autosomal recessive disorder characterized by a severe thrombocytopenia which clas- sically improves throughout childhood. Individuals have bilaterally absent radii and often other associated features including skeletal defects of the lower limb, cow’s milk intolerance, and renal and car- diac abnormalities. Inherited thrombocytopenias with increased platelet size MYH-​9 related disorders including May–​Hegglin anomaly: this is an autosomal dominant macrothrombocytopenia caused by deletion within the MYH9 gene, which encodes nonmuscle my- osin II-​A heavy chain. Individuals usually have a mild bleeding phenotype, although it may be more severe than expected from the platelet count due to associated platelet dysfunction. Associated features include sensorineural hearing loss, glomerulonephritis, and cataracts. Neutrophils may have inclusions on blood film, called Döhle-​like bodies. Bernard–​Soulier syndrome and grey platelet syndrome are dis- cussed under platelet function disorders, as the platelet dysfunction is generally more marked than the thrombocytopenia. Disorders of platelet function Disorders of platelet function are usually acquired. Acquired disorders of platelet function Drugs Numerous drugs have been shown to affect platelet function. Some drugs have been designed for this purpose, while for others it is a side effect. Antiplatelet agents such as aspirin, thienopyridine derivatives, and GPIIb/​IIIa inhibitors are used in cardiovascular disorders. Clinical trial evidence is discussed further in Chapter  22.7.3. Aspirin irreversibly inhibits cyclooxygenase within platelets preventing formation of thromboxane, resulting in reduced platelet aggregation. Thienopyridine derivatives (e.g. clopidogrel, ticlopidine, and prasugrel) inhibit platelet function by inhibiting the P2Y12 ADP receptor and therefore the ADP-​induced pathway of platelet activation. Three commercially available GPIIb/​IIIa inhibitors are abciximab (a monoclonal antibody), eptifibatide (a synthetic cyclic heptapeptide), and tirofiban (a nonpeptide an- tagonist). They block platelet aggregation by directly inhibiting the platelet receptor for fibrinogen. Of note, there is a 5% risk of thrombocytopenia and 1 to 2% risk of severe thrombocytopenia (platelets <50 × 109/​litre). NSAIDs inhibit cyclooxygenase (COX):  COX-​1 is found in most cells, including platelets and the gastrointestinal epithe- lium, and COX-​2 is induced by inflammation. COX-​2 selective inhibitors, such as etoricoxib, have less activity on COX-​1 com- pared to traditional NSAIDs. This reduces the effect on platelet function and gastrointestinal side effects such as ulceration. If an anti-​inflammatory is required in a patient with an inherited bleeding disorder, COX-​2 selective inhibitors are therefore pre- ferred. Other drugs described as potentially reducing platelet aggregation are nitrates, calcium channel blockers, β-​blockers, β-​lactam antibiotics, antiepileptics, tricyclic antidepressants, and phenothiazines. Chronic renal failure Uraemia can result in defects in adhesion and aggregation of plate- lets. The exact pathogenesis is unknown. DDAVP is occasionally used to improve platelet function in a bleeding patient. Platelet func- tion may also improve after dialysis.

22.7.3  Thrombocytopenia and disorders of platelet function 5531 Chronic myeloproliferative disorders and myelodysplastic syndromes Chronic myeloproliferative disorders and myelodysplasia (see Chapters  22.3.5, 22.3.6, and 22.3.2) may be associated with ab- normalities in platelet function as well as platelet count. Platelet function tests may show impaired aggregation to a range of agon- ists and storage pool defects. If the patient is bleeding then treat- ment is supportive—​antifibrinolytics (tranexamic acid) and platelet transfusions if necessary. The platelet function defect may respond to treatment of the underlying disease. Dysproteinaemias Paraproteins, associated with Waldenström macroglobulinaemia, monoclonal gammopathy of uncertain significance, and multiple myeloma, may be associated with an acquired coagulation disorder (acquired haemophilia and acquired VWD) but also can result in abnormalities of platelet function. Nonspecific binding of the para- protein may disrupt the platelet membrane receptors. Treatment options include plasmapheresis to transiently remove the parapro- tein, treatment of the underlying disorder, antifibrinolytics, and po- tentially platelet transfusions. Congenital disorders of platelet function Inherited platelet function disorders are an uncommon cause of symptomatic bleeding. They are heterogeneous in severity, difficult to diagnose, and therefore mild platelet function disorders in par- ticular are likely to be under-​diagnosed. Patients may present with a history of easy bruising, epistaxis, menorrhagia, and prolonged bleeding after surgery or dental procedures and other family mem- bers may be affected. These disorders may be classified functionally into abnormalities of platelet adhesion, aggregation, signalling and secretion, and pro- coagulant activity. A few of the most well-​characterized disorders are outlined in the following sections. As for inherited thrombocyto- penia disorders, treatment depends on the severity of the bleeding disorder. Options include local measures including hormone treat- ment for menorrhagia, antifibrinolytics such as tranexamic acid, desmopressin (DDAVP) (see Chapter  22.7.4 for further detail of practical administration), and platelet transfusions. Disorders of platelet adhesion Bernard–​Soulier syndrome is caused by a deficiency or abnor- mality of platelet GPIb/​IX. This results in defective binding to VWF, and a markedly reduced ability to adhere to sites of vascular injury, where subendothelial VWF is exposed. It is rare, with an approximate frequency of one in a million. It is autosomal re- cessive and consanguinity is common in reported kindreds. It is characterized by a mild–​moderate thrombocytopenia and giant platelets. On platelet function testing, platelets do not agglutinate in response to ristocetin, but show normal aggregation with other agonists including ADP, collagen, and thromboxane. Diagnosis can be confirmed by flow cytometry using analysis of GPIb-​α density. It is a moderate to severe bleeding disorder. If antifibrinolytics and local measures fail to control bleeding then platelet transfusions are usually effective but there is a risk of alloimmunization by HLA antigens or GPIb-​α. Normal infants of women who have antiplatelet antibodies are at risk of alloimmune thrombocyto- penia. Of note, one copy of the gene that encodes GP1b-​α is lost in individuals with a chromosome 22q11 deletion (diGeorge and velocardiofacial syndromes). Some of these individuals develop macrothrombocytopenia although their platelet function is normal. Disorders of platelet aggregation Platelet aggregation occurs after the adhesion of platelets to the damaged vessel wall, and occurs when activated platelets interact with one another. Glanzmann thrombasthenia is caused by quantitative or qualita- tive abnormalities of GPIIb–​IIIa (platelet integrin αIIbβ3) resulting in the absence of platelet aggregation. It is rare, autosomal recessive, and consanguinity is common within reported kindreds. Platelet function tests show absent aggregation with agonists such as ADP, adrenaline, collagen, and arachidonic acid but agglutination with ristocetin is present. Flow cytometry using antibodies to GPIIb (CD41) and GPIIIa (CD61) is used for definitive diagnosis. It is generally a severe bleeding disorder—​the clinical features are those usually expected with platelet dysfunction: easy bruising, epistaxis, and menorrhagia. Haemarthroses are very rarely reported. Similar to Bernard–​Soulier syndrome, platelet transfusions are effective in controlling bleeding, but there is a risk of alloimmunization espe- cially to the missing GPs, resulting in refractoriness to subsequent transfusions. Recombinant factor VIIa is licensed for use in those patients who are refractory to platelet transfusions. Disorders of signalling Defects of platelet ADP receptors have been reported and associ- ated with a bleeding tendency: G protein-​coupled receptors P2Y1 and P2Y12 and a ligand-​gated ion channel P2X1. Storage pool disorders Storage pool deficiency (SPD) syndromes result in secondary dis- orders of aggregation. This is a heterogeneous group of disorders characterized by a reduction in secretable substances stored in platelet granules. It may result from reduced dense granules (δ-​SPD) or reduced α-​granules (α-​SPD) or from both (αδ-​SPD). Patients with δ-​SPD or αδ-​SPD usually have absent secondary aggregation waves to ADP and adrenaline, although primary waves are present. Collagen-​induced aggregation is usually ab- sent or markedly reduced, but ristocetin-​induced agglutination is present. In the majority of cases, SPDs are isolated disorders and inheritance may be autosomal dominant, but this has not been de- termined for many. These SPDs can also be associated with other syndromes including Wiskott–​Aldrich syndrome, thrombocyto- penia with absent radius, Hermansky–​Pudlak syndrome, and Chediak–​Higashi syndrome. Grey platelet syndrome is an α-​SPD. It is extremely rare, with fewer than 100 cases reported worldwide. Both autosomal dom- inant and autosomal recessive inheritance has been described. On the blood film there is a thrombocytopenia, and platelets appear agranular and misshapen. Electron microscopy demonstrates re- duced or absent numbers of α-​granules