Myoclonus & Startle

Contributed by Marcelo Merello, MD
Director, Neuroscience Department
Head Movement Disorders Section
Institute for Neurological Research Raul Carrea (FLENI)
Buenos Aires, Argentina

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Startle is a stereotypical response to a sudden and unexpected stimulus. In most instances, the stimulus is acoustic, but other modalities such as tactile, visual, or vestibular are also effective stimuli. The motor component of startle satisfies the criteria for myoclonus.

Exaggerated startle, is a feature of various neurologic and psychiatric conditions. Hyperekplexia is an uncommon clinical syndrome that is characterized by brisk and generalized startle in response to trivial (most often acoustic or tactile) stimulation. Habituation of startle with repeated stimulation or attenuation with pre-warning is poor.

Hyperekplexia may occur with a genetic background or as an acquired disorder. Hereditary hyperekplexia (or startle disease) manifests shortly after birth with violent jerking to noise and touch, and massive and sustained stiffening of the trunk and limbs, clenching fists, and attacks of a high-frequency trembling.

Newborns are at risk for sudden infant death due to laryngospasm and cardiorespiratory failure. Symptomatic hyperekplexia is a clinical sign of brain or brainstem disorders without known specificity for the nature or the precise site of the lesion. 

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