Tuesday, 9 February 2010

Accelerated Forgetting and the Neuropsychological Assessment of Memory in Epilepsy

Patients with epilepsy frequently complain of memory difficulties yet often perform normally on standard neuropsychological tests of memory. It has been suggested that this may be due to an impairment of very long-term memory consolidation processes, beyond those normally assessed in the neuropsychological clinic.

Blake et al. (2000) found despite normal learning and retention over 30 min, patients with epileptic foci in the left temporal lobe performed disproportionately poorly on the long-term test compared with both patients with epileptic foci in the right temporal lobe and controls. Findings provide evidence for an extended period of memory consolidation and point to the critical region for this process, at least for verbal material, in the left temporal lobe.

Zeman et al. 1998 studied the concept of transient epileptic amnesia (TEA). TEA usually begins in later life, with a mean age of 65 years in this series. Episodes are typically brief, lasting less than one hour, and recurrent, with a mean frequency of three a year. Attacks on waking are characteristic. Repetitive questioning occurs commonly during attacks. The anterograde amnesia during episodes is, however, often incomplete so that patients may later be able to “remember not being able to remember”. The extent of the retrograde amnesia during attacks varies from days to years. Most patients experience other seizure types compatible with an origin in the temporal lobes, but transient amnesia is the only manifestation of epilepsy in about one third of patients. Epileptiform abnormalities arising from the temporal lobes are most often detected on interictal sleep EEG. Despite normal performance on tests of anterograde memory, many patients complain of persistent interictal disturbance of autobiographical memory, involving a significant but variable loss of recall for salient personal episodes. He hypothesises that post ictal states (5-30 mintues following seizure) may be responsible for disrupting the consolidation of long term memories, thus explaining accelerated forgetting. Direct links between temporal lobe epilepsy and memory difficulties is complicated by a number of confounding variables:

• Anti-convulsent medication side effects
• Age of epilepsy onset
• Seizure frequency
• Structural damage arising from epileptic activity

See Butler and Zeman for a comprehensive and up to date review of the issues

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