A taxonomy of memory disorders

AMNESIA

Neurological Psychogenic

Selective Amnesia

PsychoticConditions

Permanent Transient

Transient Global Amnesia

Post-Traumatic Amnesia

Post ECT or Convulsion

Progressive

Stable

MaterialSpecific

Global

Frontal Amnesia

Amnesic Syndrome

Cerebral hemispheres

Frontal Amnesia

A. Organisational Deficits

Simple registration and recall not affected by frontal lesions

Memory problems may be secondary to an inability to organise material for the purpose of committing it to memory

i.e. failure to impose a meaningful structure on the information, to generate appropriate learning strategies

Frequent concomitant of traumatic brain injury

Manifest on tests such as Rey Complex Figure, Rey Auditory Verbal Learning

Frontal Amnesia

B. Retrieval Problems

Retrieval involves strategic problem-solving. Often disturbed following frontal lesions

Patient with a retrieval deficit will demonstrate a disturbance of free recall

Recognition memory should, however, be intacteg. RAVLT: poor score on recall of list A (trials 1-

6) recognition 15/15

One advantage of WMS-III relative to WMS-R

Frontal Amnesia

C. Temporal Discrimination

Increasing attention being devoted to this aspect of memory

Patients with frontal lesions are markedly impaired in making temporal discriminations. Great difficulty in judging recency and temporal order and in reconstructing sequences.

Note, deficits of temporal ordering may be seen in the absence of fontal lobe pathology

Two processes involved:a) Encoding of information needed for temporal

memoryb) Effective processing of retrieved information

regarding temporal order

In patients with lesions of the frontal lobes deficit lies in b) ie. Is one of faulty processing (c.f. WKS patients where the deficit lies in a)).

General Amnesic Syndrome

Definition

A permanent, stable and global disorder of memory due to organic brain dysfunction which occurs in the absence of any other extensive perceptual or cognitive disturbance.

NB. PermanencyStabilityPervasivenessSpecificity

Clinical Features of the Amnesic Syndrome

1. Profound difficulty or total inability to acquire new material (anterograde amnesia)

2. Preservation of immediate memory as measured by tasks such as digit span

3. Preservation of semantic memory

4. Preservation of procedural learning

5. Some retrograde amnesia (variable across patients)

Neuopathology

Brain structures implicated:1. Bilateral damage to the mesial temporal

lobes of both the right and left hemispheres

Within these areas the hippocampus has been seen to represent the crucial structure

2. Structures within the diencephalon and specifically:Nuclei within the thalamusMamillary bodiesMamillo-thalamic tractFornix

All above structures represent part of the limbic system

Aetiology

GAS typically seen in association with

1. Wernicke-Korsakoff Syndrome2. Herpes Simplex Encephalitis3. Hypoxia4. Anterior Communicating Artery

Aneurysm5. Thalamic Infarction6. Temporal Lobe Resection

Other causes:CVATumour

Wernicke-Korsakoff Syndrome

Typically the result of chronic alcoholism

Principle cause: Thiamine deficiency

Results in damage to the subcortical structures and in particular the diencephalon

Minimal requirement: Lesion of the mamillary bodies and dorsomedial nucleus of the thalamus

Typically additional lesions in the frontal lobes (atrophy) due to alcohol neurotoxicity and often the medial temporal structures including the hippocampus

Treatment. Thiamine. Amnesia often persists

WKS: Characteristics

1. Normal memory span2. Severe anterograde amnesia3. Normal rate of forgetting4. Extensive, temporally graded

retrograde amnesia5. Confabulation present6. Cued recall better than

spontaneous recall7. Recognition relatively intact8. Poor at recency judgements9. Frontal lobe dysfunction

typically present

CASE ST

49 yo male Hx: 20 years of heavy

alcohol consumption Dx: WKS Ax: four weeks after

detoxification and thiamine Tx

S.T.

Verbal IQ = 106 PIQ = 108 Working Memory = 109 Processing Speed = 90 Full Scale IQ = 97 General Ability = 107

S.T.

INDEX Auditory Mem = 65 Visual Mem = 68 Visual Working Mem = 89 Immediate Mem = 67 Delayed Mem = 62

Index Predicted Actual Base-Rate

Aud M 104 65 <1%

Visual M 104 68 <1%

Vis Wrk 105 90 10%

Immed 105 67 <1%

Delayed 104 62 <1%

ST

Rey Auditory Verbal Learning TestTrials I-V = 6, 6, 6, 6, 7Recall B = 5Recall A = 2Recognition = 14/15

Recognition Memory TestWords = Scaled score of 9Faces = Scaled score of 9

15-Item Visual Memory TestRaw Score = 15/15

COWATWords = 16

Wisconsin Card Sorting TestCategories = 2, Perseverative Responses = 83

Rey Complex Figure

Thalamic Infarction

Primary thalamic haemorrhage relatively frequent, accounting for over 10% if all intracranial haemorrhage.

Amnesia a principle and chronic symptom

Occurrence of thalamic amnesia dependent on infarction occurring in a particular region of the thalamus (dorsomedial nucleus)

Thalamic infarction

Amnesia characterised by:

1. Normal span2. Severe anterograde amnesia3. RA in majority of cases – no temporal gradient4. Confabulation particularly in the early stages

post-stroke5. May have visuospatial, frontal and language

impairments

Famous case of NA – Penetrating head injury when fencing foil entered the right nostril, damaging the left thalamus. Demonstrated profound amnesia. RA initially extensive (years) then shrank to weeks.

Frequently cited as an example of thalamic damage. However, MRI scanning revealed additional damage to structures including mamillary bodies, mamillo-thalamic tract and anterior (left) temporal lobe.

Anterior Communicating Artery Aneurysm

Aneurysm: Localised dilatation of the walls of a blood vessel, usually an artery, due to weakening through infection, injury, degenerative illness or a congenital defect. Rupture frequently associated with a lasting impairment of memory.

ACoA particularly prone to the development of saccular aneurysms. Estimated that 30 to 40% of all ruptured aneurysms represent those of the ACoA.

ACoA supplies basal forebrain, anterior cingulate, anterior hypothalamus, anterior commissure and the genu of the corpus callosum.

Accordingly, frontal lobe damage a frequent concomitant of rupture of the ACoA

ACoA Amnesia

Characterised by:1. Normal span2. Impaired learning (less severely disrupted

than in many other conditions)3. Facilitated by the provision of strategies

suggesting an executive basis4. Recognition memory relatively intact5. Confabulation present both momentary and

fantastic6. Impaired temporal discrimination7. Typically demonstrate frontal impairments

*Group variable in terms of the severity of the memory impairment.

Mesial temporal structures implicated in disorders such as

1. Herpes Simplex Encephalitis

Viral infection causing encephalitis

Associated with Type 1 Herpes Simplex (that characterised by the oral lesions)

Neuropathology has consistently been reported to principally involve the lateral and medial temporal cortex including the hippocampus after which the virus may extend to the orbito-frontal cortex, the cingulate gyrus and the parietal lobe.

Illness initially characterised by an acute confusional state. After this resolves the patient will invariably be left with a severe memory disorder.

HSE amnesia characterised by:

1. Normal memory span

2. Profound anterograde amnesia

3. Rapid forgetting

4. RA: Variable though usually extensive

5. Intact implicit memory

6. Semantic memory deficit

Case PD

37 yo male HSE 2 years previously Currently on pension Previously manager of a

Woolworths store

P.D.

Verbal IQ = 112PIQ = 120Working Memory = 119Processing Speed = 115Full Scale IQ = 117General Ability = 116

PD: WMS-IV

Index Index Score

Auditory Memory 47

Visual Memory 54

V. Working Mem 110

Immediate Mem 56

Delayed Memory 40

Discrepancy Analysis: PD

Index Obt.

Score

Pred.

Score

Freq.

Aud.M 47 108 <1%

Vis.M 54 109 <1%

VWM 110 111 n.s.

Imm 56 111 <1%

Delyd 40 109 <1%

PD

Recognition Memory Test

Words = Scaled score of 6

Faces = Scaled score of 5

Rey Complex Figure Test

Copy = 34

Recall = 2.5

15-Item Visual Memory Test

Raw Score = 10/15

Mesial temporal structures (con’t)

2. Bilateral temporal lobe resection

Best known case HM

23 yo man operated on in 1957 by W. Scoville

Bilateral resection to control intractable epilepsy. Produced one of the densest amnesias ever recorded

HM’s amnesia characterised by:1. Memory span intact2. Semantic memory relatively intact3. Implicit memory intact4. Frontal dysfunction not observed5. Profound anterograde amnesia6. Anosmic7. RA of at least 2 years

Other cases of unilateral resection resulting in global amnesia described (presumably pathology on the other side). Typically a unilateral resection will result in a material specific memory disorder

Anoxia

Typically secondary to cardiac arrest or the result of carbon monoxide poisoning.

Certain cerebral structures are more susceptible to damage than are others. Medial temporal structures (and the hippocampus in particular) are susceptible to damage as are the ‘watershed’ areas of the cortex – areas which lie at junctions between distinct cerebrovascular systems and which are dependent for their blood supply on the most distant radiations of the cerebral arteries.

Transient Memory Disorders

Thought to be related to cerebrovascular disturbance (CVD)

1. Transient Global Amnesia

Most commonly reported in association with disturbances of the vertebrobasilar arterial system although has also been reported in association with bilateral MCA disturbance.

Patients typically over 50 years and usually experience only one attack (although there have been reports of recurrent episodes)

Attacks start suddenly and generally involve:i) An inability to form new memories of events

experienced during the episodeii) A temporary amnesia for events in the hours/days

preceding the attackiii) Individual remains alert but perplexed during the

episodeiv) Invariably leaves the patient with memory loss for

events that occurred during the episode.

2. Transient Ischaemic Attacks (TIA’s)

TIA: attack of short-lived focal neurological deficit produced by temporary ischaemia. By definition, recovery should occur within 24 hours.

Result of narrowing (stenosis) of an arterial vessel

Whether memory impairment evident dependent on site and extent of stenosis

Retrograde Amnesia

Loss of memory or difficulty in recalling events that occurred prior to onset of the amnesia.

??Double dissociation of RA and AA