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Hereditary hyperekplexia
Reviewed by Medicinsk Expert•Last updated: 2026-10-22•Conditions

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

Frequently Asked Questions

If an infant experiences apnea (stopping breathing) during a rigidity episode, it is a medical emergency. Caregivers should immediately stimulate the infant to restart breathing, such as by gently rubbing the back or tapping the feet, and ensure the airway is clear. If breathing does not resume quickly, call emergency services. In the long term, parents should be trained in infant CPR and may be advised to use a pulse oximeter at home to monitor oxygen levels during sleep, as episodes can occur during hypnagogic myoclonus. Can hereditary hyperekplexia be treated, and does it go away completely?::Hereditary hyperekplexia is often managed with medication, specifically clonazepam, which can significantly reduce the severity of the startle response and rigidity. While the underlying genetic cause cannot be cured, symptoms typically improve substantially with age. Most children see a marked reduction in symptoms by age one to two, and many adults have only mild residual symptoms, such as a heightened startle reflex in noisy environments, which may not require ongoing medication. How can I manage the condition in a school or daycare setting to prevent distress?::Inform teachers and caregivers about the condition and the specific triggers, such as sudden loud noises or unexpected touches. Creating a predictable environment and warning the child before sudden changes in noise or activity can help reduce anxiety and the frequency of startle responses. If the child is still experiencing significant rigidity, a quiet, low-stimulation space should be available for them to retreat to. Collaboration with the school nurse to ensure they are aware of the condition and any necessary interventions is also crucial. Is hereditary hyperekplexia related to epilepsy, and could my child have both conditions?::Hereditary hyperekplexia is distinct from epilepsy, although they can sometimes co-occur. The startle response and rigidity in hyperekplexia are not seizures, but some individuals with the condition may also have epilepsy. It is important to differentiate between the two because the treatments differ. If a child has episodes that look like seizures (such as loss of consciousness or rhythmic jerking not triggered by a startle), a neurologist should evaluate them to rule out or confirm a concurrent diagnosis of epilepsy.
Source: MedlinePlus, National Library of Medicine

MedlinePlus brings together authoritative health information from the National Library of Medicine (NLM), the National Institutes of Health (NIH), and other government agencies and health-related organizations.

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