Clinical significance of nocturnal sleep onset REM periods in narcolepsy diagnosis and diagnostic stability

Purpose: Narcolepsy diagnosis relies on polysomnography (PSG) and the Multiple Sleep Latency Test (MSLT), though concerns remain regarding their diagnostic accuracy and reliability. Nocturnal sleep-onset rapid eye movement periods (SOREMPs) are a known feature of narcolepsy. This study investigated the clinical significance of nocturnal SOREMP in narcolepsy diagnosis and its association with test–retest reliability. Methods: We retrospectively reviewed 109 patients who underwent at least two MSLTs. Patients were categorized into nocturnal SOREMP (n = 40) and non-nocturnal SOREMP (n = 69) groups based on PSG findings. Only MSLT-identified SOREMPs were used for diagnosis. Group comparisons and follow-up changes were analyzed using t-tests, chi-square tests, or Wilcoxon signed-rank tests. Results: The nocturnal SOREMP group had more SOREMPs (3.50 vs. 2.23), shorter mean sleep latency (1.99 vs. 4.77), and higher initial narcolepsy diagnosis rates (92.5% vs. 63.8%). Diagnostic stability was significantly higher in the nocturnal group (90.0% vs. 50.7%). Among narcolepsy type 2 (NT2) patients, those with nocturnal SOREMP showed complete diagnostic retention (100%), whereas only 27.3% of those without nocturnal SOREMP did. In a subgroup analysis, patients who had a nocturnal SOREMP on at least one of the two PSGs exhibited significantly higher diagnostic stability (93.9%) than those without nocturnal SOREMP on either test (50.0%). Similarly, NT2 retention was 100% in the former versus 16.7% in the latter. Conclusions: Nocturnal SOREMP may serve as a specific marker for narcolepsy, suggesting potential utility in supporting diagnostic consistency. Its presence appears to be associated with improved diagnostic reliability, particularly in NT2.