What is the expected result on the multiple sleep latency test (MSLT) for a patient with excessive daytime sleepiness, sleeping 11 hours per night, and unremarkable overnight polysomnography (PSG) results, without symptoms of cataplexy or hypnagogic/hypnopompic hallucinations?

Expected MSLT Findings in Idiopathic Hypersomnia

The expected result is option B: mean sleep latency of less than eight minutes with fewer than two sleep-onset REM periods. 1, 2

Clinical Reasoning

This 25-year-old woman presents with a classic picture of idiopathic hypersomnia without long sleep time:

• Excessive daytime sleepiness despite 11 hours of nocturnal sleep (total sleep time 6-10 hours defines this subtype) 1, 2
• Unrefreshing naps (90 minutes but wakes unrefreshed—a hallmark feature) 2
• Absence of cataplexy, hypnagogic/hypnopompic hallucinations, and sleep paralysis (ruling out narcolepsy) 1
• Unremarkable overnight polysomnography (excluding sleep-disordered breathing and other nocturnal disorders) 1
• No depression or anxiety 1

MSLT Diagnostic Criteria by Disorder

Narcolepsy (Type 1 or 2)

• Mean sleep latency ≤8 minutes PLUS ≥2 sleep-onset REM periods (SOREMPs) 1, 2
• The presence of multiple SOREMPs is the distinguishing feature of narcolepsy 3, 4
• Narcolepsy Type 1 requires definite cataplexy; Type 2 lacks cataplexy but has the characteristic MSLT findings 2

Idiopathic Hypersomnia

• Mean sleep latency ≤8 minutes with <2 SOREMPs 1, 2
• This patient lacks the REM sleep dysregulation seen in narcolepsy 5
• Research shows idiopathic hypersomnia patients demonstrate longer sustained sleep latency compared to initial sleep onset, reflecting fluctuation between wake and Stage 1 NREM before sustained sleep (unlike the abrupt sleep onset in narcolepsy) 5

Normal/Non-Pathological Sleepiness

• Mean sleep latency >8 minutes would suggest insufficient objective sleepiness to support a central hypersomnia diagnosis 1, 6
• This would be inconsistent with her clinical presentation of severe daytime sleepiness 1

Key Diagnostic Pitfalls

The critical distinction is the number of SOREMPs, not just the mean sleep latency:

• A mean sleep latency <8 minutes confirms objective excessive sleepiness but is non-specific 1
• ≥2 SOREMPs indicate REM sleep dysregulation characteristic of narcolepsy 1, 3, 4
• <2 SOREMPs with mean sleep latency ≤8 minutes points to idiopathic hypersomnia 1, 2

The specificity of ≥2 SOREMPs for narcolepsy is approximately 97-99%, though sensitivity is only 70% 4. When three or more SOREMPs occur with mean sleep latency <5 minutes, specificity reaches 99.2% 4. However, this patient's lack of cataplexy and REM-related phenomena (hallucinations, sleep paralysis) makes narcolepsy unlikely 1.

Clinical Context

The American Geriatrics Society emphasizes that proper MSLT interpretation requires correlation with clinical findings—the test cannot be used in isolation 1, 4. This patient's clinical picture of prolonged, unrefreshing sleep without REM-related symptoms strongly suggests idiopathic hypersomnia, which the MSLT should confirm by demonstrating objective sleepiness (mean sleep latency ≤8 minutes) without the REM dysregulation (≥2 SOREMPs) that defines narcolepsy 1, 2.