1330 Recurrent Loss of Consciousness in a Patient with Narcolepsy and OSA: A Hidden Underlying Etiology of Chronic Hypoventilation

Abstract Introduction Narcolepsy-like features have been reported in patients with myotonic dystrophy, particularly type 1 (DM1), but true narcolepsy is rare. Sodium oxybate, while effective for narcolepsy with cataplexy, is known to cause hypoventilation and central apneas, with risk heightened in patients with neuromuscular weakness or pre-existing ventilatory compromise. We present a case of recurrent loss-of-consciousness episodes in a patient with established narcolepsy and OSA, ultimately revealing significant nocturnal hypoventilation and raising diagnostic uncertainty about whether his symptoms were due to oxybate therapy or underlying neuromuscular disease. Report of case(s) 41-year-old man with a history of previously diagnosed narcolepsy and OSA (AHI 15) presented after five episodes of altered consciousness over one year. Extensive cardiac and neurologic evaluations conducted during hospital admissions were unrevealing, except for consistently elevated serum bicarbonate (31–33 mEq/L) and hypercapnia (PCO2 52–57 mmHg). He had longstanding EDS beginning in young adulthood, with diagnosis of narcolepsy confirmed on PSG/MSLT. He was treated with multiple stimulants before initiation of sodium oxybate (Xyrem), which improved daytime alertness. Following two hospitalizations for loss of consciousness with documented apneas and hypercapnia, inpatient BiPAP (14/10 cm H2O) was initiated. Post-discharge polysomnography on Xyrem showed moderate OSA (AHI 23.5) and persistent nocturnal hypoventilation with CO2 levels 50–56 mmHg. After discontinuing Xyrem, repeat PSG demonstrated marked improvement in OSA (AHI 2.4) but persistent nocturnal hypercapnia (55–58 mmHg). Pulmonary function testing revealed normal spirometry but reduced MIP (55%) and MEP (21%), with normal supine FVC drop (7%), suggesting effort-related limitation rather than clear neuromuscular weakness. He remained off oxybates, started solriamfetol with partial improvement, and experienced no further LOC events. Neurologic exam revealed subtle grip percussion myotonia, prompting genetic testing for myotonic dystrophy type 1 (DM1). Genetic testing for DM1 was positive. Over subsequent months he experienced psychiatric decompensation unrelated to sleep events but later achieved clinical stability with normalization of Epworth Sleepiness Scale (ESS 5). Conclusion Myotonic dystrophy type 1 (DM1) is associated with narcolepsy-like features, including excessive daytime sleepiness (EDS), sleep-onset REM periods, and REM sleep dysregulation, which can mimic narcolepsy. Caution is advised when prescribing oxybate in neuromuscular disease, and polysomnographic monitoring is recommended to detect respiratory complications. Support (if any)