Opioid-associated amnestic syndrome

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Opioid-associated amnestic syndrome is a form of opioid neurotoxicity characterized by acute memory loss and bilateral hippocampal diffusion restriction and/or T2-FLAIR hyperintensity on MRI.

Epidemiology

About 40 cases have been reported in the English literature as of 2020 .

Fentanyl is the most commonly associated opioid in this syndrome . Polysubstance overdose is common, particularly with cocaine .

Clinical presentation

The proposed case definition consists of new onset amnesia that lasts longer than 24 hours and the following supporting factors :

  • confirmed case: positive toxicology for opioid and bilateral hippocampal injury on CT or MRI
  • probable case: known history of opioid use and bilateral hippocampal injury on CT or MRI
  • possible case: positive toxicology, history of opioid use, or bilateral hippocampal injury on CT or MRI

    • Radiographic features

    The bilateral hippocampi are symmetrically and diffusely involved . A wide range of extrahippocampal structures may also be abnormal, including the basal ganglia and cerebellum (cerebellar, hippocampal, and basal nuclei transient edema with restricted diffusion (CHANTER) syndrome).

    CT

    Noncontrast head CT shows hypodensity of the hippocampi but this finding may be subtle.

    MRI
    • FLAIR: diffuse hyperintensity of the hippocampi is characteristic
    • DWI: diffusion restriction is a hallmark in the initial days to weeks after injury but resolves thereafter

    Differential diagnosis

    The differential for causes of bilateral hippocampal restricted diffusion includes hypoxic-ischemic injury (eg, cardiac arrest), seizures, cardioembolic infarcts, hypoglycemia, and encephalitis . Transient global amnesia appears as punctate foci of restricted diffusion.