Everyone has a response when startled. People jump at loud noises or sudden movement. This is a startle response. Reacting strongly to normal stimuli that most people would not find startling or reacting especially strongly to startling stimuli is known as a hyperstartle response or hyperekplexia (literally, "exaggerated surprise").
Hyperekplexia is usually caused by a genetic disorder and is most often diagnosed in early infancy. Infants with hyperekplexia demonstrate an abnormally strong startle response, which causes their eyes to close and their limbs to extend. They are left unable to move while this occurs. These babies also will exhibit body stiffness while awake. The stiffness becomes worse when the baby is held or touched and disappears during sleep.
Hyperekplexia was first officially recognized in 1962. Two doctors noted the classic signs of the disorder in several infants, including stiffness, strong brain stem reflexes and exaggerated startle response. Since then, scientists have discovered the mutated gene that causes the hereditary disorder. Advances in genetic testing and treatments are helping to diagnose and treat this condition.
Infants with hyperekplexia are at risk of complications. Hip malformations, hernias and even sudden infant death syndrome (SIDS) have been linked to hyperekplexia. Fortunately, hyperekplexia can be diagnosed soon after birth, which allows for swift and early treatment. This may prevent some complications and also allows for identification and treatment of problems as they arise. In the case of hyperekplexia, treatment is used to mitigate symptoms because the root cause cannot be treated or cured.
The hyperekplexia seen in infants is the result of a genetic mutation. This mutation prevents glycine receptors from functioning properly. Because the glycine receptors are so insensitive to glycine, spinal inhibition decreases, resulting in an exaggerated startle response. While this response does become more subdued over time, there is no cure for the genetic mutation.
Disorders presenting with an exaggerated startle response often can be misdiagnosed. Infants with hyperekplexia have long been diagnosed with epilepsy and other seizure disorders because of their stiffness and strong immobilization when exhibiting a startle response.
Post-Traumatic Stress Disorder
Rarely, a hyperstartle response can be acquired later in life. This has been seen most often in post-traumatic stress disorder. However, the cause is different from the cause of hyperekplexia and is psychological rather than neurological or genetic. Rather than an uncontrollable physical response to stimuli, PTSD sufferers may experience a strong startle response in inappropriate situations that trigger memories of trauma. There is treatment for this condition, and unlike those with hyperekplexia, PTSD sufferers may eventually be rid of their hyperstartle response.
Many other neurological and psychological conditions, including Tourette's syndrome and "Mad Cow Disease," sometimes present with a hyperstartle response as a symptom. Psychiatrists may have a difficult time diagnosing their patients because a hyperstartle response is not indicative of any particular psychological condition. They would need to look at the patient's condition on the whole, rather than focusing on a few symptoms.