Sleep disorders with neurological disease: Case

Case discussion

This girl carries the textbook presentation of narcolepsy type 1, and the case is built to test whether the candidate recognises the cataplexy, understands its mechanism, sequences the right investigations, and builds a defensible management plan. The daytime sleepiness over six months in a previously well child already narrows the differential, but it is the laughter-triggered loss of muscle tone with preserved consciousness that makes the diagnosis, because cataplexy is pathognomonic for narcolepsy type 1 and changes the entire investigation pathway. [1]

The mechanism to defend at viva is the loss of the hypothalamic orexin neurons that normally stabilise wakefulness and gate REM sleep to the night. Lose them, and two things follow: the child cannot sustain wakefulness by day, giving the irresistible sleep, and REM sleep breaks through into wakefulness, giving the cataplexy, the sleep paralysis, and the vivid hypnagogic hallucinations. Her weight gain and withdrawal are the recognised psychosocial consequences of the disease and of the disrupted sleep, and her normal examination is exactly what is expected, because narcolepsy has no motor or cognitive signs between the episodes. [1] [12]

The investigation you would arrange

The definitive pathway is an overnight polysomnography followed the next morning by a multiple sleep latency test. The polysomnography excludes other causes of sleepiness such as obstructive sleep apnoea and periodic limb movement disorder, and confirms adequate total sleep. The multiple sleep latency test measures how quickly she falls asleep in a series of daytime nap opportunities, and a mean sleep latency of eight minutes or less with two or more sleep-onset REM periods confirms a central disorder of hypersomnolence. Because her cataplexy is clear and classical, this combination is narcolepsy type 1, and the diagnostic lumbar puncture for cerebrospinal fluid orexin may be avoidable, though orexin below 110 picograms per millilitre remains the definitive biomarker if there is any ambiguity. [1] [11]