Objective: We present a case of narcolepsy in a patient with obstructive sleep apnea （OSA） who complained of residual sleepiness without improvement in Epworth sleepiness scale （ESS） after oral appliance （OA） therapy, and who underwent the Multiple Sleep Latency Test （MSLT）.
Method: The patient was a 48-year-old male who had no significant past medical history and family history, and had been experiencing excessive daytime sleepiness. The initial interview at the Department of Sleep Medicine did not reveal any cataplexy, sleep paralysis, and hypnagogic hallucinations. He was diagnosed with mild OSA （apnea hypopnea index: AHI 10.5/h） based on polysomnography （PSG）, and was referred to our department for OA therapy. Excessive daytime sleepiness did not improve after the initiation of OA therapy. Thereafter, follow-up sleep study with PSG and the MSLT were performed.
Results: The PSG with OA showed an improvement of OSA in AHI from 10.5 to 3.6/h, and sleep-onset REM sleep period （SOREMP） was not observed. MSLT showed that the mean sleep latency was 4min 6s/five naps, and number of SOREMP was two times; therefore, the patient was diagnosed with narcolepsy type 2. Use of Modafinil 100 mg/day decreased the ESS score from 15 to 4 and improved daytime sleepiness.
Conclusion: In patients with OSA and narcolepsy, it is not possible to improve excessive daytime sleepiness by providing treatment for OSA only. Patients who complain of residual sleepiness even after OA treatment should be assessed further for other sleep disorders including hypersomnia.
