11 - 2. Consciousness

2. Consciousness

© SPMM Course 2. Consciousness  Consciousness is a state characterised by an awareness of self and environment and an ability to respond to environmental factors; it is made up of two components – arousal (wakefulness) and awareness (attentional processing).  Arousal depends on intact functioning of ARAS – Ascending Reticular activating System. Thalamocortical connections generate rhythmical bursts of neuronal activity (20 – 40 Hz) which are in desynchrony by default. ARAS acting via the thalamic intralaminar nuclei synchronises these oscillations. Arousal is directly proportional to the degree of such synchrony achieved. The absence of arousal produces stupor and coma.  The maintenance of attention appears to require an intact right frontal lobe  Small lesions of ARAS are enough to produce a stuporous state, but large bilateral lesions are required at the cortical level to cause the same depression in alertness.  Stupor: In this state the individual appears to be asleep and yet, when vigorously stimulated, may become alert as manifest by eye opening and ocular movement (Cartlidge 2001). Most patients in stupor have diffuse organic cerebral dysfunction. Caloric testing in organic stupor will usually reveal tonic deviation whereas in a psychiatric stupor (catatonia/depression) ocular nystagmus will be seen (Cartlidge 2001). This is because the following tonic deviation in a conscious subject, a fast phase of correction appears resulting in nystagmus.  Akinetic mutism: It is seen in patients with diencephalic or bilateral anterior cingulate damage. The syndrome is characterised by immobility and eye closure with little or no vocalisation. Sleep/wake cycles can be seen, as indicated by eye opening. There is little in the way of movement to painful stimuli, and the hallmark is the absence of spasticity and rigidity (Cartlidge 2001). Akinetic mutism can arise as a result of lesions that interfere with reticular/cortical integration but spare the corticospinal pathways. There is some debate about whether or not the syndrome should be clearly differentiated from the vegetative state. CJD can also present with akinetic mutism before death.  Vegetative state: This results from the isolated actions of the ARAS and the thalamus in the absence of higher cortical influence due to extensive cortical damage. A patient in the fully established vegetative state will almost invariably show spasticity and rigidity of the limbs, which are absent in patients with the syndrome of akinetic mutism. In the early stages of the vegetative state, the two clinical syndromes are indistinguishable.  Locked in syndrome: Acording to Cartlidge (2001), the ventral pontine or locked in syndrome describes a condition of total paralysis below the level of the third nerve nuclei. Such patients can open their eyes and elevate and depress their eyes to command. Horizontal eye movements are usually lost, and no other voluntary movement is possible. The diagnosis of this state depends on the recognition that the patient can open his eyes voluntarily rather than spontaneously in the vegetative state. This generally results from infarction of the ventral pons, pontine tumours, pontine haemorrhage, central pontine myelinolysis, head injury or brain stem encephalitis.