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Transient Global Amnesia (TGA)

Summary

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  • Acute, temporary loss of anterograde memory with preserved retrograde memory
  • Typically lasts 4-6 hours, resolving within 24 hours
  • Characteristic imaging findings on diffusion-weighted MRI1

Pathophysiology

  • Exact mechanism remains unclear, but leading hypotheses include:
    • Transient ischaemia in the medial temporal lobe
    • Venous congestion leading to hippocampal dysfunction
    • Migraine-related phenomenon
  • Hippocampal CA1 region particularly vulnerable to metabolic stress

Demographics

  • Incidence: 5-10 per 100,000 persons per year
  • Peak age: 50-70 years
  • Slight female predominance (1.2:1 female-to-male ratio)
  • Rare in individuals under 40 years of age

Diagnosis

  • Clinical diagnosis based on:
    • Sudden onset of anterograde amnesia
    • Preserved retrograde memory
    • No focal neurological deficits
    • Resolution within 24 hours
  • Exclusion of other causes (e.g., stroke, seizure, head trauma)
  • Diagnostic criteria proposed by Hodges and Warlow

Imaging

  • CT:
    • Usually normal
    • Useful to exclude other pathologies (e.g., haemorrhage)
  • MRI:
    • Diffusion-weighted imaging (DWI):
    • Characteristic punctate 1-3 mm hyperintense lesions in lateral hippocampus
    • Typically unilateral (60-70%) or bilateral (30-40%)
    • Best visualised 24-72 hours after symptom onset
    • T2-weighted and FLAIR:
    • May show corresponding hyperintensities, but less sensitive than DWI
  • Functional imaging:
    • PET and SPECT may show transient hypoperfusion in medial temporal lobes

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  • 55-year-old male presenting with 24 hours of anterograde amnesia.
  • DWI showed a focus of diffusion restriction in body of the right hippocampus.

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  • A 50-year-old patient presented with sudden onset confusion and anterograde amnesia followed by a headache (that differed from usual migraine).
  • MRI showed a punctate diffusion-weighted hyperintensity, with some degree of restriction, in the tail of the left hippocampus.

Treatment

  • None required; self-limiting with a good prognosis. The punctate DWI dot in the lateral hippocampus (CA1) is best seen at 24–72 hours and may be absent if imaged too early

Differential diagnosis

Differential Diagnosis Distinguishing Feature
Acute hippocampal infarct Persisting restricted diffusion in a PCA/anterior choroidal territory; larger or irregular lesion
Limbic encephalitis More generalised medial temporal T2/FLAIR hyperintensity without the focal punctate DWI spot of TGA
Herpes simplex encephalitis Asymmetric medial temporal swelling with haemorrhage and leptomeningeal enhancement
Status epilepticus (mesial temporal) Hippocampal/cortical swelling with FLAIR hyperintensity and restricted diffusion beyond CA1

  1. A Ropper. Transient Global Amnesia. 2023. The New England journal of medicine - Open in new tab. ↩