Memory and Cognition in Narcolepsy

Abstract

Many patients suffering from the sleep disorder of narcolepsy complain about cognitive dysfunction, most about frequent memory and concentration problems. These self-reports have not consistently been corroborated by standardised assessment of cognitive function, with many investigations not finding significant differences in memory function between narcoleptic patients and healthy control subjects or reporting only mild impairments. In recent years, however, an increasing number of studies have reported significant deficits and the focus of current research is on the mechanisms causing these impairments. For attention and executive control, narcolepsy patients are primarily impaired on tasks with higher demands on attentional resources or in tasks which require concentration across an extended period of time, suggesting an increased need for allocating cognitive resources to the maintenance of alertness, thereby compensating for increased levels of sleepiness. In the memory domain, impairments, if present, mostly affect encoding, probably related to disturbed sleep patterns with reduced proportions of non-REM sleep. Decision-making and other reward-associated behaviours are only beginning to be examined in narcolepsy. Hypotheses are derived from the anatomical distribution of hypocretin neurons, whose number is reduced in narcolepsy and who project to regions involved in reward processing. Decision-making under ambiguity has been found to be specifically affected with a potential role of altered amygdala function in narcolepsy patients. Taken together, narcolepsy impairs cognitive function in many patients suffering from the disease. The mechanisms responsible for the cognitive decline, although not yet fully characterised, involve sleepiness, altered sleep patterns and direct structural and functional consequences of hypocretin deficiency.