# 20 - Disorders of memory

# Disorders of memory

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 Other regions: In most cases of memory loss procedural memory is intact. A deficit in procedural 
memory with preservation of declarative memory may be seen in persons with Parkinson's disease, 
in whom dopaminergic neurons of the nigrostriatal tract degenerate. Though speculative, 
cerebellum, striatum, amygdala and certain parts of the neocortex (including motor area) are 
thought to be involved in non-declarative procedural memory storage. The anterior temporal 
lobe is the key area for semantic memory. 
 Long-term potentiation: Strengthening of the connection between two neurons on repeated 
communication is called long-term potentiation - LTP. This may be the neuronal basis of memory. 
It is mediated by NMDA mediated Ca2+ entry in glutamate neurons. Learning increases branching 
and synapse formation and may also influence neurogenesis. 
 
Disorders of memory 
 Amnesia is a term used either for pure memory deficits (mostly episodic) or cognitive deficits 
where memory loss is predominant and not congruent with the level of loss in other domains. 
 Generally both anterograde and retrograde memory loss occur in parallel, such as in Alzheimer’s 
disease or head injury. 
 Relatively pure anterograde 
amnesia may be seen when 
there is hippocampal damage, 
e.g. herpes simplex 
encephalitis, focal temporal 
lobe tumours, or infarction. 
 Confabulation—for example, in 
Korsakoff’s syndrome—might 
be grandiose or delusional, 
but more often involves the 
misordering and fusion of real 
memories which end up being 
retrieved out of context. 
 A transient amnesic syndrome with pronounced anterograde, and variable retrograde, amnesia is 
seen in transient global amnesia (TGA), while ‘‘memory lacunes’’, and repeated brief episodes of 
memory loss suggest transient epileptic amnesia (TEA). 
 Ribot's Law of retrograde amnesia: ‘The dissolution of memory is inversely related to the recency 
of the event’. Recent memories are more likely to be lost than the more remote memories in organic 
amnesia (not always the case though). 
 Semantic dementia: It is a variant of frontotemporal dementia. Patients with semantic breakdown 
typically complain of loss of words. Vocabulary diminishes, and patients use substitute words 
such as ‘‘thing’’. There is a parallel impairment in appreciating the meaning of individual words, 
which first involves infrequent or unusual words. 
 A word finding difficulty is common in both anxiety and aging, but variable and not associated 
with impaired comprehension. This is in stark contrast to the anomia in semantic dementia which 
Anterograde 
amnesia 
Forgetting newly encountered information from 
the time of a lesion. 
Presents as forgetfulness regarding appointments, 
losing items around the home, inability to 
remember conversation leading to repeated 
questions etc. 
Retrograde 
amnesia 
Loss of memory of past events that happened 
before the lesion was sustained. 
Presents as loss of memory of past events such as 
jobs, holidays, not able to remember the 
topography of a route and getting lost.

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is relentlessly progressive and associated with atrophy of the anterior temporal lobe, usually on 
the left. 
 Working memory deficits can present as lapses in concentration and attention e.g. losing one’s 
train of thought, inability to process a complex task as the components are not retained long 
enough in memory to be processed. Basal ganglia and white matter diseases may present with 
predominantly working memory deficits. 
 Dissociative amnesia is not an organic syndrome, but centred on the loss of memory of important 
recent events that is partial, patchy and selective. It can occur as a part of dissociative fugue. The 
characters of dissociative amnesia are episodic memory loss (retrograde only with no anterograde 
deficits) for events that happened in a discrete period of time from minutes to years. In dissociative 
amnesia, the problem is not inefficient retrieval but the strikingly complete unavailability of 
memories which were formed normally and were previously accessible. The forgotten events are 
generally traumatic or stressful.