An Overview of Hyperekplexia

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Hyperekplexia (HPX), as defined by the National Organization of Rare Diseases (NORD), is a hereditary neurological disorder. The condition is considered rare, impacting approximately 150 individuals throughout the world. HPX may occur in utero or young children, such as newborns and infants. Both children and adults can be affected by the disorder as well. HPX may be known as other names, including:

  • Familial startle disease
  • Hereditary hyperekplexia
  • Startle syndrome
  • Siff-baby syndrome

People with HPX demonstrate a magnified startle response to loud sounds or other surprising stimuli. Infants show increased muscle tone, also called hypertonia. After a startle response has occurred, infants will go through a period of rigidity (extreme muscle tension) and are unable to move. Some infants may stop breathing during the episode of rigidity, which can potentially lead to a life-threatening situation.   

Furthermore, the periods of rigidity may cause some individuals to fall without a loss of consciousness. People with HPX may demonstrate gait abnormalities, overactive reflexes, and other signs and symptoms, too.            

Symptoms

The overall severity of the condition can vary widely among individuals. Although symptoms can continue throughout the adult years, they are most prevalent in infants and children.

The symptom that is most characteristic of the disorder is the exaggerated startle reaction following an abrupt stimulus.

The stimuli can include sounds, an unexpected touch, a surprise movement like a bump, and the unanticipated sightings of people or other objects. Once the startle response is activated, an individual experiences rigidity or muscle stiffness and is likely to fall. There is a risk of injury because they are unable to use their arms to catch themselves and break the fall.

Other symptoms that may accompany HPX include:

  • arching of the head in conjunction with the startle reaction
  • jerking movements after the startle reaction or when trying to fall asleep
  • severe muscle tension, which may be most noticeable in infants
  • lack of movement or slower than normal movements in babies
  • overactive reflexes
  • intermittent apnea
  • unsteady gait
  • some infants may present with seizures, and HPX may be mistaken as epilepsy
  • dislocated hip at birth
  • inguinal hernia    

For many people, the symptoms of HPX will recede by age one, reports Genetics Home Reference, a branch of the U.S. National Library of Medicine. But this isn’t the case for everyone, and some older individuals will continue to exhibit a hyper-reactive startle response, episodes of rigidity, falls, and movements while sleeping. They may also experience overstimulation in crowds or places with loud noises.

Causes 

HPX affects both males and females. Although it is most often seen in babies at birth, it can present later in life such as during adolescence or adulthood.

The majority of HPX cases are caused by inheriting an autosomal dominant trait, which means you only need to receive one mutated gene from a parent to develop the condition. The genes that are most likely to develop mutations, according to NORD, include:

  • GLRA1 (The most prevalent mutation in HPX)
  • SLC6A5
  • GLRB
  • GPHN
  • ARHGEF9

These genes play a role in the body’s production of glycine, an inhibitory amino acid that lessens the response to stimuli in the brain and the nervous system. If glycine production is diminished or the receptors sustain damage, the nerve cells can’t regulate their reactivity, and consequently, they respond in an exaggerated way to stimuli.    

Other potential causes of HPX include an autosomal recessive pattern and a new mutation. With an autosomal recessive pattern, you inherit two mutated genes—one from each parent—and your parents may not show any signs or symptoms of the disorder. With new mutations, the genetic variant spontaneously arises in the early stages of an individual's development. 

Diagnosis

If an infant presents with an overactive startle response, increased stiffness, decreased ability to engage in voluntary movements, or seizures, your healthcare provider may consider a diagnosis of HPX. Since the disorder is most often an inherited one, the physician will take a detailed account of your family’s health history looking for clues to possible genetic diseases. An initial diagnosis of HPX may be based on the clinical presentation of the condition. 

To confirm the diagnosis, the doctor may order tests such as an electromyography (EMG), which looks at how well the nerves of the body are controlling the muscles, and an electroencephalography (EEG), which evaluates the electrical activity happening in various parts of the brain.

Additionally, genetic testing is an option to solidify a diagnosis and help find the cause. The field of genetic testing is always evolving, so new testing could emerge later in life that may help answer questions you or your family may have.

Treatment

Treatments are often aimed at managing the symptoms of HPX, most notably the overactive startle response and muscle systems. Currently, there is no cure for the disorder. The medications that might be used include anti-anxiety and anti-spastic drugs such as clonazepam, carbamazepine, phenobarbital, diazepam, and others.

Physical or occupational therapy may be used as adjunct therapies to ease symptoms, evaluate the need for assistive devices and adaptive equipment (strollers, walkers, seating and positioning devices) to improve quality of life. Cognitive behavioral therapy (CBT) can also be a beneficial therapy to lessen anxiety.

When an infant has significant breathing challenges, the doctor may suggest a breathing or heart rate monitor to keep a watchful eye on the child. Parents may also decide to learn infant CPR so that they can help their child should the need arise.     

Coping

An unexpected diagnosis of any kind can be overwhelming, but one involving a rare neurological disease can be downright scary. Finding support from others who are going through the same things you and your family are can be a huge help.

For online HPX support forums, you could try visiting The Hyperekplexia Society on Facebook, for one example.

Another way to cope and find support is to connect with people with related disorders and similar symptoms. Those conditions include epilepsy, generalized anxiety, and TIC disorders (like Tourette syndrome).

Ultimately, the more you know about the condition and the treatment options available to you, the more in control you’ll feel about your child and your family’s health.

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