If an unborn baby is affected, this may first be noticed by the mother as abnormal intrauterine (within the womb) movements. When an affected baby is born, he or she is often stiff. For those unfamiliar with hyperekplexia, this feature may easily be misdiagnosed as spastic cerebral palsy (see entry Cerebral Palsy). When a baby startles, there may be a tendency for severe non-epileptic convulsions with increased stiffness and fast quivering. Even daily activities, such as bathing, may bring this on. These convulsions are potentially dangerous because they induce a severe syncope (loss of consciousness) owing to a lack of oxygen reaching the brain; these episodes can be recurrent and are also known as 'breathing attacks' or 'apnoea attacks'. Most individuals with hyperekplexia are of normal intelligence and may live as fulfilling and productive lives as other people. In some children however, there may be a delay in reaching developmental milestones, such as delayed speech or walking.

Hyperekplexia is caused by a mutation or change in genes involved in the action of a neurotransmitter called glycine. Glycine is called an inhibitory transmitter because it dampens down the action of nerve cells, particularly within the spinal cord and brain stem. When glycine receptors are impaired, the nerve cells are too easily excited. When strychnine poisoning was more prevalent, something very similar would happen, since strychnine also targets glycine receptors. So much so, that scientists call the receptors involved in hyperekplexia 'strychnine-sensitive glycine receptors'. It is now possible for mutations in the genes controlling these receptors to be detected; screening is carried out at Swansea University.

Diagnosis of hyperekplexia can be carried out using the 'nose tap' test. Tapping the tip of the nose of an unaffected baby will elicit no response, but in babies with hyperekplexia, there is an obvious startle response, which is repeated each time the nose is tapped. Diagnosis can be confirmed by screening for known gene mutations.

Hyperekplexia is often misdiagnosed as epilepsy. Children with hyperekplexia must be monitored closely and treated appropriately, so that abnormal startle reactions do not lead to convulsions and falls.

There are two effective treatments for hyperekplexia. First, if the baby is flexed, that is made to curl up by pressing the head towards the knees, the convulsion will stop. Secondly, if the doctor starts the baby on clonazepam, the excessive startle will diminish and the condition will improve as the baby grows older. Although doctors may have some concerns about addiction, benzodiazepine drugs do not appear to be a significant risk in this condition. Some sufferers develop minor complications, such as a hernia.

Inheritance patterns
In approximately 75 per cent of cases, hyperekplexia is inherited as an autosomal recessive trait. This means that there may be no known family history in most cases, although, in a quarter of cases, the condition is transmitted in an autosomal dominant manner. Families are advised to seek genetic counseling. In some cases, hyperekplexia occurs sporadically.

Prenatal diagnosis
This is possible in some cases.

There is no support group for Hyperekplexia in the UK. Cross referrals to other medical information entries on our website are intended to provide relevant support for those particular features of the disorder. Organisations identified in those entries do not provide support specifically for Hyperekplexia.

Families can use Contact a Family's freephone helpline for advice, information and, where possible, links to other families. Contact a Family's web-based linking service MakingContact.org can be accessed at www.makingcontact.org