Hyperekplexia

What is hyperekplexia?

Hyperekplexia is a rare nervous system disorder in which your baby has too much muscle tone so they have trouble moving their muscles (hypertonia). They also have an exaggerated startle reaction — body spasms or eye blinking — to sudden, unexpected stimuli (movement, noise or touch). After the startle reaction, babies experience a short period in which they’re very rigid and unable to move. During these periods, some babies stop breathing, which can be fatal if prolonged. The condition is sometimes associated with sudden infant death syndrome (SIDS).

Healthcare providers typically notice the condition prior to or shortly after birth in newborns. But hyperekplexia can also affect children and adults.

Other names for the condition include:

• Congenital stiff-person syndrome.
• Hereditary hyperekplexia.
• Startle disease.
• Startle syndrome.
• Stiff-baby syndrome.

How common is hyperekplexia?

Hyperekplexia affects about 1 in every 40,000 people in the United States.

What are the symptoms of hyperekplexia?

There are two forms of hyperekplexia: major and minor.

In the major form, hyperekplexia causes an atypical startle reflex to sudden unexpected touch, movement or noise. When startled, a person may arch their head, have spastic jerking movements or fall stiffly to the ground.

When a baby is falling asleep, they may have jerking movements. In addition, extreme muscle tension and muscle stiffness (hypertonia) are common in babies with the disorder. They may also move slowly (hypokinesia) and have mild intellectual disability.

Other major hyperekplexia symptoms that may occur in babies, children and adults include:

• Exaggeration of reflexes (hyperreflexia).
• Interrupted breathing (intermittent apnea).
• Unstable walking (gait).
• Hip dislocation present at birth.
• Inguinal hernias.

In the minor form, people with hyperekplexia typically only experience an exaggerated startle reaction infrequently with few or no other symptoms. In babies, the reaction may be brought on by fever. In children and adults, stress or anxiety may increase the intensity of the startle response.

Onset of both forms of the condition is usually present from birth. But in some people, it doesn’t develop until childhood or adulthood.

What causes hyperekplexia?

Genetic changes (mutations) in at least five different genes have been found to cause hyperekplexia. These genes include:

• ATAD1.
• GLRA1.
• GLRB.
• GPHN.
• SLC6A5.

These genes have an important role in producing proteins found in nerve cells (neurons) that affect how the cells respond to a molecule called glycine. Glycine is an amino acid, the building blocks of protein. It’s also a neurotransmitter, in charge of sending signals in your nervous system.

Usually, glycine works to control muscle fiber stimulation. A genetic mutation in any of the responsible genes can reduce the effects of glycine on your nervous system. The neurons then send uncontrolled messages to your brain and muscles. This causes your muscles to overreact and leads to the other symptoms of hyperekplexia.

You can inherit hyperekplexia in one of two ways: autosomal dominant or autosomal recessive.

What are the complications of this condition?

In infants, hyperekplexia causes a brief period in which they’re very rigid and unable to move. During these rigid periods, some babies may stop breathing, which can lead to sudden infant death syndrome (SIDS). SIDS is a major cause of unexplained death in infants younger than 1 year old.

Rarely, infants with hyperekplexia experience seizures (epilepsy) as well.

The symptoms of hyperekplexia typically fade by age 1. But children and adults with the condition may continue to startle easily and have periods of rigidity. This can lead to frequent falls. As people get older, they may develop a low tolerance for loud noises and crowded places. People who have epilepsy will have seizures throughout their lives.

How is hyperekplexia diagnosed?

To diagnose hyperekplexia, healthcare providers look for three main features:

• Generalized stiffness immediately after birth.
• An exaggerated startle response to unexpected stimuli.
• A short period of generalized stiffness following the startle response.

Routine tests — including blood tests, urinalysis, brain imaging studies and EEG — all typically show normal results. Genetic testing can detect the five genetic mutations that have been linked to the condition.

Is hyperekplexia treatable?

Yes, hyperekplexia is treatable. Hyperekplexia treatment in both infants and adults includes the use of an anti-anxiety and antispastic medication called clonazepam. Clonazepam is a tranquilizer in the benzodiazepine family that can help reduce the symptoms of hyperekplexia, including muscle stiffness. Healthcare providers prescribe a dose of 0.01 to 0.1 milligrams (mg) per kilogram (kg) for children and 0.8 mg for adults each day.

Your healthcare provider may recommend a range of other medications to treat the condition, including:

• Carbamazepine.
• Phenobarbital.
• Phenytoin.
• Diazepam.
• Sodium valproate.

Other treatments may include physical therapy to help with walking and cognitive-behavioral therapy to help reduce anxiety.

Can hyperekplexia be prevented?

Hyperekplexia is a genetic condition that can’t be prevented. If you’re concerned about passing on the condition to your child, you may wish to speak to a genetic counselor.

What is the prognosis for hyperekplexia?

In infants, the signs and symptoms of hyperekplexia usually fade by the age of 1. In children and adults, the effects of the condition can be lifelong. But there are medication options that can help lessen your symptoms.

How do I take care of myself?

If you have hyperekplexia, take your medications as prescribed by your healthcare provider. Your provider has recommended the best options for you based on your condition. These medications should help manage your symptoms.

When should I see my healthcare provider?

If your newborn has hyperekplexia, their healthcare provider will usually notice the condition at birth. They can sometimes diagnose the condition before birth. Your child’s provider will start treatment right away.

If you or your child have unexplained seizures, it’s important to see a healthcare provider right away. Hyperekplexia may be the cause of seizures.

What questions should I ask my healthcare provider?

If you or your child have hyperekplexia, you may want to ask your provider the following questions:

• What caused this condition?
• What treatment option do you recommend?
• Will my future children have hyperekplexia?