Startle Syndromes

The startle syndromes are characterized by an exaggerated motor response to unexpected auditory, and at times somesthetic or visual, stimuli. Confusion has overtaken the classification of these syndromes as various terms have been introduced. Hypere kplexia has often been used synonymously with any startle disorder; however, the word hyperexplexia (to jump excessively) was used to describe the autosomal dominantly inherited exaggerated startle reflexes in a large Dutch kindred. Matsamuto and Hallett have advocated the use of the proper Greek term hyperexplexia to denote any movement disorder in which there is a physiological demonstration of exaggerated startle reflexes and have incorporated this terminology in dividing the startle syndromes into five main types ( ,T§.ble,,34i1.0. ) [84]

The pathophysiology of hyperexplexia has been recently elucidated, and now there is general agreement that the motor response in patients with hereditary and symptomatic hyperexplexia is a pathological exaggeration of the normal startle reflex. y To elicit a startle response in normal humans, an acoustic stimulus must have a volume of approximately 100 dB and a rise time of less than 5 msec. The response to such an unexpected stimulus varies in normals, and eye blinking may be the only visible reaction. Typically, however, facial grimacing, head flexion with shoulder abduction, elbow flexion, pronation of the forearms, and fist clenching occur. In normals, habituation (a decrease in magnitude of response with repetition of the stimulus) occurs with four to five repeated stimuli. Electrophysiologically, surface EMG recordings demonstrate a bilaterally symmetrical response composed of a blink response and activation of other craniocervical muscles with variable limb muscle recruitment. The overall pattern of muscle recruitment suggests that the normal human auditory startle reflex originates in the caudal brain stem. The audiogenic jumps seen in hyperexplexias are believed to be generated by the normal startle circuit under heightened excitability, perhaps of cerebral origin. Hereditary hyperexplexia is inherited as an autosomal dominant trait caused by




Common Initial Dose

Usual Maximum Dose

Clinical Uses


25-50 mg IV

50 mg po tid



0.5-1.5 mg/day

20 mg po qd

Most forms of myoclonus

L-5-Hydoxytryptophan with carbidopa

25 mg po qid

500 mg po qid

Posthypoxic myoclonus

25 mg po qid

50 mg po qid


10 mg IV

10 mg po tid



7.2 g/day po

16 g/day po

Cortical myoclonus


25-50 mg po qd

200 mg po qd

Segmental myoclonus

Valproic acid

15 mg/kg/day po

2000 mg/day

Most forms of myoclonus

IV, Intravenously; po, by mouth; qd, each day; qid, four times daily; tid, three times daily

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