Transient Global Amnesia

Transient global amnesia is a self-limiting syndrome of sudden, isolated, profound anterograde amnesia. An episode typically lasts several hours and resolves completely within twenty four hours.

It occurs predominantly in middle aged and older adults. The underlying pathophysiology remains incompletely understood despite extensive investigation.

Diagnosis rests on the characteristic, highly stereotyped clinical presentation. It also requires exclusion of more serious, sometimes urgent, alternative causes of acute amnesia, most notably posterior circulation stroke and seizure related amnesia. Clinical overlap is substantial and the management implications differ critically.

Pathophysiology

• The precise mechanism remains debated. Proposed theories are transient hippocampal ischaemia (the vascular hypothesis, given the hippocampus's role in memory formation and vulnerability to transient hypoperfusion), venous congestion within the hippocampal region, and a migrainous mechanism.
• The venous hypothesis is supported by some association between episodes and preceding Valsalva-type manoeuvres. The migraine theory reflects a recognised, though incompletely characterised, association with migraine in some individuals.
• The true mechanism may be a heterogeneous group of processes converging on a final common pathway of transient hippocampal dysfunction rather than a single mechanism.
• Recognised precipitants in a proportion of cases include emotional stress, significant physical exertion, sudden immersion in cold or hot water, sexual intercourse, and Valsalva-type manoeuvres (including significant straining or breath-holding).
• A substantial proportion of episodes occur without an identifiable precipitant.
• The condition characteristically does not recur, and only a small minority experience a further episode. This is a reassuring feature for counselling after a confirmed, typical episode.

• The classic presentation is sudden onset of profound anterograde amnesia (inability to form new memories from onset), with repetitive questioning. The person repeatedly asks the same questions, typically about their location, the time or recent events, because they cannot retain the answer.
• The episode is generally witnessed and reported by a family member or bystander, as the affected person is unaware of the memory disturbance.
• Retrograde amnesia is variably present. It typically affects a period of hours to, less commonly, days or weeks before onset. It generally resolves as the episode resolves, whereas anterograde amnesia persists for the duration of the episode.
• A defining feature is that the person retains personal identity throughout. This contrasts with the loss of personal identity in a dissociative fugue state, which is generally psychogenic rather than organic.
• Procedural memory and the ability to perform complex, previously learned tasks are preserved. This includes driving, which has important safety implications requiring careful discussion.
• General cognitive function and language ability are preserved outside the isolated memory domain.
• The absence of any other neurological symptom or sign supports the diagnosis and argues against posterior circulation stroke. This covers headache, focal weakness, sensory disturbance, visual field defect and any alteration in level of consciousness. Any such feature warrants urgent reconsideration of the more serious diagnosis.
• The episode resolves gradually over several hours. Retrograde amnesia resolves first, then anterograde amnesia. The great majority of episodes resolve completely within twenty four hours.
• A permanent, dense amnestic gap remains for the period of the episode, since new memories could not form. Cognitive function otherwise returns entirely to baseline.

• Diagnosis rests on recognition of the characteristic, highly stereotyped presentation, together with exclusion of more serious alternative diagnoses.
• Posterior circulation (vertebrobasilar territory) stroke or transient ischaemic attack can occasionally present with a broadly similar, though generally not identical, pattern of amnesia. It characteristically features additional neurological signs or symptoms not present in typical, uncomplicated transient global amnesia: diplopia, vertigo, ataxia, dysarthria or visual field defect.
• Seizure related amnesia (transient epileptic amnesia) more typically presents with shorter, recurrent episodes, often on waking. Transient global amnesia is a single, prolonged episode lasting several hours.
• Transient epileptic amnesia may be accompanied by other subtle seizure-related features, including brief automatisms or a preceding aura.
• The essential clinical approach is a full neurological examination confirming no focal deficit beyond the isolated memory impairment. It is combined with structured history taking, ideally with collateral history from a witness, characterising the onset, duration and evolution of the amnesia and confirming no additional neurological symptom.
• Magnetic resonance imaging of the brain, including diffusion weighted sequences, may show small, punctate hippocampal lesions in a proportion of confirmed cases. Imaging is typically most sensitive at twenty four to seventy two hours after symptom onset.
• This finding is neither universally present nor required for diagnosis. Imaging is performed mainly to exclude alternative diagnoses, particularly acute stroke, rather than to confirm transient global amnesia.
• Electroencephalography may be considered where the pattern of presentation raises clinical suspicion of a seizure-related mechanism, to support exclusion of transient epileptic amnesia.

• The acute episode needs no specific treatment. It is self-limiting, with spontaneous, complete resolution typically within twenty four hours.
• Management during the episode is reassurance and support for the affected individual and family, with structured clinical assessment to confirm the diagnosis and exclude more serious alternative causes.
• Once the diagnosis is confirmed and alternatives excluded, reassure the patient and family about the generally excellent prognosis and the characteristically low, though not absent, risk of recurrence.
• Explain the nature of the episode and the expectation of a permanent, isolated and circumscribed amnestic gap for the period of the episode.
• There is no specific evidence of an increased risk of subsequent stroke or dementia after a confirmed, typical episode. This is an important, reassuring counselling point and distinguishes the prognosis from that of a genuine transient ischaemic attack.
• The diagnostic distinction from transient ischaemic attack remains essential because the mechanisms differ. Where posterior circulation transient ischaemic attack is confirmed or remains a genuine possibility, urgent secondary stroke prevention is required instead.
• After a confirmed, typical, uncomplicated episode with full resolution and no evidence of an alternative cause, no ongoing treatment or investigation is generally required.
• Recurrence occurs in a small minority and is managed with the same reassurance and diagnostic confirmation as the initial episode.