- A genetic disorder also known as hyperexplexia in which babies have an exaggerated startle reflex (reaction). This disorder was not recognized until 1962 when it was described by Drs. Kok and Bruyn as a disease with the onset at birth of hypertonia (stiffness), exaggerated startle response, strong brain-stem reflexes (especially head-retraction reflex) and, in some cases, epilepsy. The hypertonia (stiffness) was evident with flexion of limbs, disappeared during sleep and diminished over the first year of life. The startle reflex was sometimes accompanied by acute generalized hypertonia (sudden stiffness) causing the person to fall like a log to the ground. There were 29 affected males and females in 6 generations, indicating that the disorder is an autosomal (non-sexlinked) dominant trait. A number of other families have since been found with this disease. Additional findings include a tendency to umbilical and inguinal hernias (presumably due to increased intraabdominal pressure) and congenital dislocation of the hip. The exaggerated startle response persists throughout life; startles can be elicited by lightly touching the person's nose, clapping or making other noises, or suddenly jolting the person's chair. The gene responsible for this disease has been found on chromosome number 5. (It is in bands 5q33.2-q33.3 and is a mutation in the gene for the alpha-1 subunit of the glycine receptor). Treatment is with medications. The neurologic features can usually be controlled with clonazepam (KLONOPIN). In some cases, phenobarbital, diazepam (VALIUM), and valproic acid (DEPAKENE) have also been found useful. Hyperexplexia is also called Kok disease, startle disease, exaggerated startle reflex, and stiff baby syndrome.
* * *A hereditary disorder in which there are pathologic startle responses, i.e., protective reactions to unanticipated, potentially threatening, stimuli of any type, particularly auditory; the stimuli induce often widespread and violent sudden contractions of the head, neck, spinal, and sometimes limb musculature, resulting in involuntary shouting, jerking, jumping, and falling; autosomal dominant and recessive inheritance forms, with the responsible gene localized to chromosome 5q; probably the result of lack of inhibitory neurotransmitters, glycine, or GABA. SYN: kok disease, startle disease. [hyper- + G. ekplexia, sudden shock, fr. ekplesso, to startle]
* * *hy·per·ek·plex·ia (hi″pər-ek-plekґse-ə) [hyper- + Gr. ekplēxia panic] a congenital condition of exaggerated startle reactions. In infants it has been observed in a syndrome accompanied by hypertonia, hypokinesia, and brisk cerebral bulbar reflexes, and in adults it is similar to jumping disease. Called also startle disease and hyperexplexia.
Medical dictionary. 2011.