This is the third one in the four-post series on Narcolepsy. Let's commence.
Diagnosing a not so common disorder, that too neurological is quite tricky. The greatest difficulty is separating it from the normal daytime and postprandial sleepiness of most people. Admit it, most of us feel like sleeping in the afternoon period after lunch, especially while reclining in the sofa, while watching television or in the theatres watching matinee shows. :p
What distinguishes the typical narcoleptic attack from commonplace postprandial drowsiness and napping is the frequent occurrence of the former(2 to 6 times everyday), their irresistibility, and their occurrence in unusual situations, as while eating, standing, or carrying on a conversation.
Excessive daytime sleepiness can also be present with heart failure, hypothyroidism, use of antihistaminics, alcohol intake, head trauma, certain brain tumours like craniopharyngioma etc.
Overnight polysomnography followed by a standardized multiple sleep latency test can exclude other causes of excessive daytime sleepiness like obstructive sleep apnea. In the test, the patient is given 5 opportunities to nap at 2 hour intervals in a day
If there are more than 2 Sleep-onset REM periods and a mean sleep latency of less than 8 minutes, it strongly suggests narcolepsy.
Actually, in Narcolepsy, there is characteristic reversal in the order of the two states of sleep, with REM (rather than NREM) phase occurring at the onset of sleep attacks. And the sleep latency is nothing but the interval between the point when an individual tries to sleep and the point of onset of sleep with the respective EEG patterns.
Measurement of hypocretin(orexin) levels in the CSF may help establish the diagnosis; a level lower than 110pg/ml is diagnostic of narcolepsy.
Cataplexy must also be distinguished from syncope, drop attacks and atonic seizures. In atonic seizures, there is temporary loss of consciousness, while in narcolepsy consciousness is perfectly preserved.
That's all! Do go through the other posts in this series.
Role of Orexins in Narcolepsy
Clinical features of Narcolepsy
Treatment of Narcolepsy