Startle Disorders

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.KeywordsStartle reflexHyperekplexiaGlycine receptorStartle-induced disordersNeuropsychiatric startle disorders