A 28-year-old man reports sudden bilateral loss of muscle tone triggered by laughter or surprise, causing him to collapse without losing consciousness. He also has excessive daytime sleepiness despite adequate night sleep and vivid dream-like images at sleep onset. PSG shows sleep-onset REM periods. What is the most likely diagnosis?

• A REM sleep behaviour disorder
• B Idiopathic hypersomnia
• C Kleine-Levin syndrome
• D Narcolepsy type 1 (with cataplexy) ✓

Explanation

Narcolepsy type 1 is characterised by the tetrad of excessive daytime sleepiness, cataplexy (sudden loss of muscle tone triggered by strong emotion, typically positive emotions like laughter), hypnagogic or hypnopompic hallucinations, and sleep paralysis. Cataplexy is pathognomonic of type 1 and is caused by loss of hypothalamic orexin (hypocretin)-producing neurons, with CSF hypocretin-1 below 110 pg/mL. PSG and MSLT demonstrate sleep-onset REM periods. REM sleep behaviour disorder involves acting out dreams during REM sleep, not cataplexy.

Reference: Kaplan & Sadock's Synopsis of Psychiatry, 11th ed.

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Written and medically reviewed by the StethoPrep medical team.