Excessive daytime somnolence is one of the common complaints in patients with myotonic dystrophy. Here we report a 60-year-old female case of myotonic dystrophy type 1 with narcolepsy due to medical condition. The size of the CTG repeat in the 3' untranslated region of the DMPK gene was 1,800-2,400 repeats. Brain MRI was normal. Polysomnography revealed sleep apnea and chronic alveolar hypoventilation. Multiple sleep latency tests revealed normal sleep latencies and sleep onset REM was not observed. Orexin/hypocretin in the cerebrospinal fluid was markedly decreased to an undetectable level. Such sleep-related disorders may worsen the quality of life and possibly cause sudden death in patients with myotonic dystrophy. Narcolepsy associated with myotonic dystrophy should be evaluated appropriately.
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