Hereditary Hyperekplexia

Hereditary hyperekplexia is a nervous system disorder (neurological disorder), that is usually noticed shortly after birth. Symptoms in a newborn include generalized muscle stiffness while awake (hypertonia) and exaggerated startle reflex to unexpected loud noises, visual stimuli, or touch. Following the startle reaction, infants experience a brief period in which they are very rigid and are unable to move. During these rigid periods, some infants may stop breathing (apnea), which can be fatal and has been associated with sudden infant death syndrome (SIDS). Muscle stiffness usually fades completely during the first few years of life. However the exaggerated startle reflex and periods of rigidity may continue throughout a person's life. Depending on the severity of the continued symptoms, a child or adult with hereditary hyperekplexia may have an increased risk of falling or otherwise injuring themselves. In some cases, children with hereditary hyperekplexia may have mild developmental delays. Abdominal hernias and congenital dislocation of the hip have also been associated with hereditary hyperekplexia.

Diagnosis of hereditary hyperekplexia requires the three main features: generalized stiffness immediately after birth, excessive startle reflex to unexpected stimuli, and a short period of generalized stiffness following the startle response. Of note, the results of routine blood tests, urinalysis, brain imaging studies, or EEG are usually normal. Hereditary hyperekplexia has different inheritance patterns and is associated with changes (mutations) in at least five genes. Genetic testing is available to detect the genetic changes. Clonazepam is the most commonly used treatment and successfully reduces symptoms in most people.