Sleep time and SOREMPs features support idiopathic hypersomnia and narcolepsy type 2 as a single disease entity

Idiopathic hypersomnia (IH) and narcolepsy type 2 (NT2) are rare hypersomnolence disorders with unknown pathophysiology. Recent evidence suggests they may belong to the same spectrum. This study compared objective sleep duration and REM sleep features, including sleep-onset REM periods (SOREMPs), during a 32-h continuous polysomnography (PSG) in patients with confirmed NT2 or IH.

Consecutive drug-free patients with sleepiness complaints underwent baseline PSG and multiple sleep latency test (MSLT) and were included if their mean sleep latency was ≤8 min, with a clear NT2 or IH diagnosis. They were classified into three groups based on baseline SOREMPs (0, 1, or ≥2). During a second visit, they underwent a 32-h bedrest protocol with continuous PSG under controlled conditions.

Of 84 patients (mean age 28.6 ± 8.9 years, 70.2% female), 22 had ≥2 SOREMPs (NT2), 16 had 1 (IH), and 46 had none (IH) at baseline PSG/MLST. Surprisingly, total sleep duration during the 32-h bedrest and SOREMP count did not differ between the three groups. However, the ≥2 SOREMPs group had longer REM sleep duration, more REM sleep bouts, and more REM/Wake transitions during bedrest. Patients with 1 or ≥2 SOREMPs also showed a higher REM sleep proportion at baseline than those without SOREMPs.

Total sleep time and number of SOREMPs during prolonged PSG did not differ between IH and NT2. NT2 patients, however, exhibited more REM sleep and REM/Wake transitions. Overall, our data on sleep time and SOREMPs frequency support IH and NT2 as a single disease entity. Statement of Significance Idiopathic hypersomnia (IH) and narcolepsy type 2 (NT2) are rare central hypersomnolence disorders with unknown pathophysiology, with recent evidence suggesting they may belong to the same spectrum. We found that patients with objective EDS (MSLT ≤8 min), without cataplexy, with ≥2 SOREMPs on the MSLT (NT2), compared to the others (IH; 0 or 1 SOREMPs), had similar total sleep duration during the 32-h bedrest protocol with continuous PSG. The number of SOREMPs was not higher in patients with NT2 during this continuous recording, although they exhibited longer REM sleep duration, more REM sleep bouts, and more REM/Wake transitions. Overall, these data on sleep time and number of SOREMPs support IH and NT2 as a single disease entity.