A case of startle disease with atypical clinical features.

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A 9-year-old girl with a 3-year history of episodes of falling down every morning due to exaggerated startle response was reported. She was diagnosed as having startle disease or hyperekplexia, because of her exaggerated startle response, clear consciousness during falling, no epileptic discharge on ictal EEG and no family history. Besides her exaggerated responses, she had frequent episodes of mild nocturnal myoclonus. Other atypical features were mild mental retardation, frequent pavor nocturnus, interictal EEG abnormalities and circadian rhythm of exaggeration of startle responses. Mental retardation has been reported in a few other reports. These findings indicate that this disorder is related to diffuse cerebral lesion. Sleep disorders such as nocturnal jerks, somnambulism or nightmares reported by Gastaut et al. were observed occasionally, and the pavor nocturnus observed in our case was thought to be one of these sleep disorders. Polysomnography revealed a low rate of slow wave sleep in this case. Sleep mechanisms may be related to this disorder. The exaggerated startle responses occurred only in the morning. This phenomenon indicates that the pathological changes in the center of circadian rhythm is involved in this disorder. Clonazepam was effective in this case. Considering the variability of clinical features, this disorder should be regarded as a syndrome rather than a disease.
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