Hyperekplexia: pedigree studies in two families.

We report on 2 unrelated Japanese families, each with several individuals affected with hyperekplexia, a rare autosomal dominant form of exaggerated startle response of neonatal onset. In the first family, affected relatives included a 4-week-old boy, his mother, grandmother, a maternal uncle, and 2 maternal cousins. In the second family, affected were a 4-week-old boy, his father, and an elder brother. These 9 individuals had various combinations of transient infantile hypertonia and hypokinesia, exaggerated startle response with falling episodes, nocturnal myoclonus and an easily elicited head retraction reflex, hip dislocation, and umbilical hernia. Treatment with clonazepam was effective in relieving these manifestations in the affected infants and children. Genetic analysis of these 2 families and 4 others in the literature suggests autosomal dominant inheritance with considerable variability but complete penetrance. Another 3 families in the literature were reported, suggesting the existence of startle disorder with an autosomal recessive inheritance. A sporadic case is also known, presumably representing a fresh mutation of a dominantly inherited trait.