Hereditary hyperekplexia
Hereditary hyperekplexia is a condition characterized by increased muscle tone and an exaggerated startle reaction to unexpected stimuli, such as loud noises.
Overview
Affected infants have increased muscle tone (hypertonia) and an exaggerated startle reaction. Following the startle reaction, infants experience a brief period of rigidity where they are unable to move. During these rigid periods, some infants may stop breathing, which can be fatal if prolonged. Infants typically exhibit hypertonia at all times, except when sleeping.
Symptoms
- Exaggerated startle reaction to unexpected stimuli.
- Brief periods of rigidity following a startle.
- Muscle twitches when falling asleep (hypnagogic myoclonus).
- Movements of the arms or legs while asleep.
- Spasms of limb and neck muscles when the nose is tapped.
- Recurrent seizures (epilepsy) in rare cases.
Symptoms typically fade by age 1, though older individuals may still startle easily, experience rigidity, or have a low tolerance for crowded or noisy environments.
Causes
Mutations in multiple genes, most notably GLRA1, cause hereditary hyperekplexia. These genes provide instructions for proteins in nerve cells involved in the response to glycine, a chemical messenger that inhibits nerve cell signaling. Mutations disrupt this signaling in the spinal cord and brainstem, leading to abnormal muscle movements and the characteristic startle response.
Inheritance
Hereditary hyperekplexia has different inheritance patterns:
- Autosomal dominant: A mutation in one copy of an associated gene is sufficient to cause the disorder. This may be inherited from an affected parent or result from a new mutation in the gene.
- Autosomal recessive: Both copies of an associated gene have mutations. Parents typically carry one copy of the altered gene without showing symptoms.
Other Names
- Congenital stiff-man syndrome
- Congenital stiff-person syndrome
- Familial hyperekplexia
- Hyperekplexia
- Startle syndrome
- STHE
- Stiff-baby syndrome
