A 28-year-old male from Delhi presents with a 2-year history of irresistible daytime sleep attacks and sudden loss of muscle tone triggered by laughter. Overnight polysomnography confirms 7 hours of sleep with no obstructive apnoea. The Multiple Sleep Latency Test (MSLT) is performed with five 20-minute nap opportunities at 2-hour intervals. The diagram shows the sleep architecture during one of the nap periods. The structure marked **A** in the diagram represents a sleep-onset REM period occurring within 15 minutes of sleep onset. If two such events are documented across the MSLT naps, combined with a mean sleep latency of 6 minutes, what is the most likely diagnosis?
A. Idiopathic Hypersomnia with long sleep time
B. Circadian Rhythm Sleep-Wake Disorder, delayed sleep phase type
C. Obstructive Sleep Apnoea with excessive daytime sleepiness
D. Narcolepsy Type 1 (if cataplexy or CSF hypocretin-1 <110 pg/mL is confirmed)
Explanation
Why Narcolepsy Type 1 is right
The ICSD-3 diagnostic criteria for Narcolepsy Type 1 require a mean sleep latency ≤8 minutes AND ≥2 sleep-onset REM periods (SOREMPs) on the MSLT, PLUS either documented cataplexy or CSF hypocretin-1 (orexin-A) deficiency <110 pg/mL. This patient meets the MSLT quantitative criteria (mean latency 6 minutes, ≥2 SOREMPs marked as A) and has cataplexy (sudden loss of muscle tone triggered by emotion), which is pathognomonic when present. The overnight polysomnogram excluded obstructive sleep apnoea, satisfying the prerequisite for MSLT interpretation. Narcolepsy Type 1 is driven by autoimmune destruction of hypocretin/orexin-producing neurons in the lateral hypothalamus, almost universally associated with HLA-DQB1*06:02 and often triggered by H1N1 influenza or PANDEMRIX vaccination in genetically susceptible individuals. [ICSD-3; Harrison's 21e Ch 31]
Why each distractor is wrong
Idiopathic Hypersomnia with long sleep time: This condition presents with excessive daytime sleepiness but does NOT produce SOREMPs on MSLT; instead, it shows normal or prolonged sleep latency with no REM sleep at sleep onset. The presence of ≥2 SOREMPs is incompatible with this diagnosis.
Obstructive Sleep Apnoea with excessive daytime sleepiness: The overnight polysomnogram explicitly excluded obstructive sleep apnoea. Moreover, OSA does not produce SOREMPs on MSLT; it causes fragmented sleep with arousals and hypoxia, not REM sleep at sleep onset.
Circadian Rhythm Sleep-Wake Disorder, delayed sleep phase type: This disorder presents with difficulty initiating sleep at the desired clock time but normal sleep architecture and no SOREMPs on MSLT. It does not produce the characteristic combination of short sleep latency and multiple SOREMPs.
High-YieldNEET PG
The MSLT is the gold-standard objective test for narcolepsy; ≥2 SOREMPs within 15 minutes of sleep onset (marked A) + mean latency ≤8 min + cataplexy or low CSF hypocretin = Narcolepsy Type 1.
ICSD-3 (AASM); Harrison's 21e Ch 31
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