Severe hypoxia

Severe hypoxia can give rise to an amnesic syndrome following carbon monoxide poisoning, cardiac and respiratory arrests, or suicide attempts by hanging or poisoning with the exhaust gases from a car. Drug overdoses may precipitate prolonged unconsciousness and cerebral hypoxia, and this quite commonly occurs in heroin abusers. Zola-Morgan et al.2) described a patient with repeated episodes of hypoxia and/or cardiovascular problems who developed a moderately severe anterograde amnesia. At autopsy 6 years later, this patient was shown to have a severe loss of pyramidal cells in the CA1 region of the hippocampi bilaterally, with the rest of the brain appearing relatively normal. Hippocampal atrophy on MRI has been reported in amnesic patients in whom hypoxia may have been the cause. The author's research group has also produced evidence of medial temporal lobe atrophy in hypoxic patients, but this same group has also found thalamic hypometabolism in hypoxic patients on FDG-PET scanning, and this finding is consistent with other reports. In brief, the memory disorder is likely to result from a combination of hippocampal and thalamic changes, related to the many common neural pathways between these two structures.(28)

There have been claims that selective damage to these circuits produces an impairment in recall memory, but not in recognition memory. (28) However, there are many commonalities in the pattern of episodic memory impairment between patients who have experienced severe hypoxic episodes and those whose amnesic disorder results from the more extensive temporal lobe damage found in herpes encephalitis.

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