Also known as

  • Familial Startle Disease Hereditary Hyperekplexia Startle Syndrome

Hyperekplexia is a rare hereditary, neurological disorder that may affect infants as newborns (neonatal) or prior to birth (in utero). It may also affect children and adults. Individuals with this disorder have an excessive startle reaction (eye blinking or body spasms) to sudden unexpected noise, movement, or touch.

Condition details

hyperekplexia is characterized by an unusually extreme startle reaction to sudden unexpected noise, movement, or touch. Arching of the head (exaggerated head-retraction reflex or HRR), spastic jerking movements (myoclonic jerks) or falling stiffly to the ground (without losing consciousness) tend to occur when the individual is startled. The frequency and severity of the startle response can be increased by emotional tension, stress, or fatigue. Jerking movements can also occur when the patient is trying to fall asleep (hypnagogic myoclonic jerks). Extreme muscle tension or stiffness (hypertonia) is common in infants with hyperexplexia, especially at birth. Affected babies may not move around much, and when they do, they tend to move slowly (hypokinesia). Other symptoms presented may include exaggeration of reflexes (hyperreflexia), interrupted breathing (intermittent apnea) and/or unstable walking (gait), usually with a mild wide-based stance. Some patients have hip dislocation.

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Photos of affected individuals

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  • No baby imageBaby
  • No toddler imageToddler
  • No child 4-10 imageChild 4-10
  • No child 10-18 imageChild 10-18
  • No adult imageAdult

Typical age range of first manifestation

  • Can manifest in Baby
  • Does not manifest in Toddler
  • Does not manifest in Child 4-10
  • Does not manifest in Child 10-18
  • Does not manifest in Adult


Lay terms Clinical terms
  • Startle easily Arching of the head Extreme muscle tension Jerking movements Jerking movements when trying to fall asleep
  • Excessive startle reaction Exaggerated head-retraction reflex Hypertonia Myoclonic jerks hypnagogic myoclonic jerks

Affected genes identified to date

  • GLRA1
  • SLC6A5
  • GLRB
  • GPHN
  • and ARHGEF9 (X-linked).

Inheritance patterns

Is X-linked Is Autosomal recessive Is Autosomal dominant

Rarely, when caused by a change in the ARHGEF9 gene, hyperekplexia is passed on in an X-linked pattern of inheritance. Women who are carriers have a 1 in 2 (50%) chance of passing the condition on to each SON they have.

Hyperekplexia is sometimes inherited in an autosomal recessive pattern. When both parents are carriers, there is a 1 in 4 (25%) chance of a child inheriting the condition.

Hyperekplexia is normally inherited in an Autosomal Dominant pattern. Each child of someone with the condition has a 1 in 2 (50%) chance of inheriting the condition.

Are carriers affected?

No, when the condition is inherited in a recessive or X-linked pattern.

How many are affected?

Less than 1 in 100,000.

Support groups and organisations

The Hyperekplexia Society

Known experts

No experts found

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