The influence of narcolepsy on olfactory function: a review.

INTRODUCTION: Narcolepsy is a sleep disorder associated with loss of hypocretin cells characterized by irrepressible need to sleep, often accompanied by cataplexy, sleep fragmentation, hypnagogical and hypnopompic hallucinations, and sleep paralysis. It is also correlated with alterations in the sleep-wake cycle, dysautonomia, olfactory dysfunction, and eating disorders.

METHODS: This is a review about influence of narcolepsy on human olfaction. Pubmed, Embase, Ovid and Cochrane databases were searched for articles on the evaluation of olfactory function in narcoleptic patients including terms as narcolepsy, olfaction disorder, amongst others.

RESULTS: Seven articles met the inclusion criteria. In five of them, the olfaction of narcoleptic patients was diminished in comparison with healthy control groups. The diagnosis of narcolepsy relates to worse performance in olfactory tests. Experimental researches showed that hypocretin and hypocretin receptors are present in the olfactory system, and this neuropeptide may have a role on olfactory sensitivity and on the olfactory modulation. The cause of hyposmia appears to be multifactorial. Among them, it stands out the hypocretin deficiency, therefore, that seems to be involved in the olfactory impairment in narcoleptic patients.