Abstract
The term “startle” describes a sudden involuntary movement of the body in response to an unexpected stimulus. The startle reflex in humans is a normal physiologic symmetric flexor response present starting around 6 weeks of age and remaining for life. Conditions with an abnormal or exaggerated startle are collectively referred to as startle syndromes, and are a rare, heterogeneous group of disorders. The startle syndromes are categorized into three broad groups: (1) hyperekplexia, (2) stimulus-induced disorders, and (3) neuropsychiatric startle disorders. While startle syndromes are often relatively benign, medical emergencies can arise as complications from the abnormal startle. Most concerning is the increased morbidity and mortality from apneic episodes in patients with hyperekplexia, which can unfortunately lead to sudden death. Therefore, prompt recognition and treatment of this disorder is imperative. In this chapter, we review the normal human startle reflex and then provide an overview of the startle syndromes, with a particular focus on hyperekplexia given the movement disorder emergencies seen in this syndrome.
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Two patients with hereditary hyperekplexia due to a glycine receptor mutation appear in this video segment. Lack of habituation to nose tap, exaggerated startle response, and marked axial hypertonia are evident. The next video, taken in the neonatal intensive care unit, is an example of how not to examine these babies: one nose tap is sufficient, and the Morrow reflex is likely not needed. The final segment, courtesy of Peter Schofield, demonstrates a newborn Poll-Hereford calf with hereditary hyperekplexia, displaying the same features of lack of habituation to tap and marked sustained axial hypertonic contractions (MP4 139237 kb)
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Stahl, C.M. (2022). Startle Disorders. In: Frucht, S.J. (eds) Movement Disorder Emergencies. Current Clinical Neurology. Humana, Cham. https://doi.org/10.1007/978-3-030-75898-1_19
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DOI: https://doi.org/10.1007/978-3-030-75898-1_19
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