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Genetic and Rare Diseases Information Center (GARD)

Other Names for this Disease
  • Exaggerated startle reaction
  • Hyperexplexia hereditary
  • Kok disease
  • Startle disease, familial
  • Startle reaction, exaggerated
More Names
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Hereditary hyperekplexia is an inherited condition that is usually evident in infants. Symptoms include increased muscle tone (hypertonia) and an exaggerated startle reaction to unexpected stimuli, especially loud noises. Following the startle reaction, infants experience a brief period in which they are very rigid and unable to move. During these rigid periods, some infants stop breathing, which can be fatal. This may explain some cases of sudden infant death syndrome (SIDS). Symptoms typically fade by age 1. However, older individuals with this condition may still startle easily and have periods of rigidity. Others may have a low tolerance for crowded places and loud noises. This condition has different inheritance patterns and is associated with mutations in at least five genes.[1]
Last updated: 11/30/2011


  1. Hereditary hyperekplexia. Genetics Home Reference. April 2010 ; Accessed 11/30/2011.
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Basic Information

  • Genetics Home Reference (GHR) contains information on Hereditary hyperekplexia. This website is maintained by the National Library of Medicine.
  • The National Organization for Rare Disorders (NORD) is a federation of more than 130 nonprofit voluntary health organizations serving people with rare disorders. Click on the link to view information on this topic.

In Depth Information

  • The Online Mendelian Inheritance in Man (OMIM) is an catalog of human genes and genetic disorders. Each entry has a summary of related medical articles. It is meant for health care professionals and researchers. OMIM is maintained by Johns Hopkins University School of Medicine. 
  • Orphanet is a European reference portal for information on rare diseases and orphan drugs.  Access to this database is free of charge.
  • PubMed is a searchable database of medical literature and lists journal articles that discuss Hereditary hyperekplexia. Click on the link to view a sample search on this topic.