Sixteen patients, including 1 neonate, in a large 5 generation family with hyperekplexia (startle disease or congenital stiff-man syndrome) were treated with clonazepam at the University of Texas Health Science Center, San Antonio, TX. All showed dramatic and sustained improvement. Clonazepam was introduced at a dose of 0.5 mg (0.125 - 0.25 mg for children at bedtime). The total daily dose in children did not exceed 1.5 mg and increases of .125 to .25 mg were made every third day until the symptoms improved. [1]
COMMENT. Hyperekplexia is a distinctive clinical syndrome of hypertonia and exaggerated startle responses that is inherited as an autosomal dominant. This familial form of hyperekplexia responds to clonazepam whereas the less frequent nonfamilial form sometimes fails to respond. A late onset variant with symptoms arising in adolescence responded dramatically to valproate and was not benefited by clonazepam. [2]
Of 15 patients with hyperekplexia identified in 3 families at the Departments of Neurology and Pediatrics, Wayne State University School of Medicine, Detroit, MI, 3 infants died unexpectedly during the neonatal period. Patients treated with clonazepam (0.1-0.2 mg/kg/day) had no serious apneic episodes and startle reflexes induced by nose tapping were diminished [3]. Hyperekplexia is included in the differential diagnosis of neonates with apnea, aspiration pneumonia, episodic muscular rigidity and near miss SIDS. Nose tapping to elicit the hyperekplexic startle response should be included in the routine examination of newborns. See Ped Neurol Briefs May 1991; 5:36. The primary pysiological abnormality in hyperekplexia involves spinal as well as brainstem hyperexcitability. [4]