Central Disorders of Hypersomnolence
Narcolepsy
Narcolepsy type 1 requires CSF hypocretin-1 ≤110 pg/mL or cataplexy plus objective sleepiness; type 2 lacks cataplexy and has normal hypocretin. The classic tetrad: excessive daytime sleepiness, cataplexy, hypnagogic/hypnopompic hallucinations, and sleep paralysis-REM intrusion phenomena. Cataplexy is sudden loss of muscle tone triggered by emotion, with preserved consciousness.
The MSLT requires a preceding adequate sleep period (typically ≥6 hours on PSG) and no confounding factors. Mean sleep latency ≤8 minutes plus ≥2 sleep-onset REM periods (SOREMPs)-one of which may be the preceding PSG-supports narcolepsy. A single SOREMP on PSG counts. Orexin deficiency is irreversible; treatment includes modafinil, stimulants, sodium oxybate, and pitolisant.
Idiopathic hypersomnia and Kleine-Levin
Idiopathic hypersomnia features long sleep times, unrefreshing naps, and typically fewer than 2 SOREMPs. Kleine-Levin syndrome is recurrent episodic hypersomnia with cognitive and behavioral changes, often with hyperphagia and hypersexuality.
Traps: diagnosing narcolepsy on symptoms alone without MSLT/CSF; ignoring insufficient sleep or circadian delay as mimics (both can produce SOREMPs); and forgetting that antidepressants suppress REM and can invalidate MSLT interpretation. Also, cataplexy is not fainting-consciousness is preserved.
Rule of thumb: Before an MSLT, ensure adequate sleep and screen for OSA, circadian delay, and REM-suppressing drugs.
Worked example: Mean sleep latency 5 minutes with 2 SOREMPs and clear cataplexy supports narcolepsy type 1; if cataplexy is absent and hypocretin normal, type 2 is more likely.