Definition of Hyperexplexia

Hyperexplexia: A genetic disorder 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 (DEPAKOTE) have also been found useful.

Hyperexplexia is also called Kok disease, startle disease, exaggerated startle disease, hyperekplexia, and stiff baby syndrome.