The startle syndromes: physiology and treatment

Startle syndromes are paroxysmal and show stimulus sensitivity, placing them in the differential diagnosis of epileptic seizures. Startle syndromes form a heterogeneous group of disorders with three categories: hyperekplexia (HPX), stimulus-induced disorders, and neuropsychiatric syndromes. HPX is characterized by an exaggerated motor startle reflex combined with stiffness and is caused by mutations in different parts of the inhibitory glycine receptor, leading to brainstem pathology. The preserved consciousness distinguishes it from epileptic seizures. Clonazepam is the first-choice therapy. The stimulus-induced disorders cover a broad range of epileptic and nonepileptic disorders, and distinguishing the two can be difficult. Additional information from electroencephalography (EEG) and video registration can help. Many stimulus-induced disorders now have an identified gene defect. Antiepileptic drugs, including benzodiazepines, are frequently mentioned as the best treatment option. Neuropsychiatric syndromes are on the borderland of neurology and psychiatry, and their etiology is poorly understood. These syndromes include startle-induced tics, culture-specific disorders such as Latah, and functional startle syndromes. The electromyography (EMG) startle reflex in these syndromes is characterized by variable recruitment patterns and the presence of a second "orienting" response. Treatment options are limited, but urgently required. In the clinical setting, the patient's history and a (home) video recording together with genetic and electrophysiologic testing help to classify these challenging disorders.