Hyperekplexia, or startle disease, is a rare neurological disorder characterized by an exaggerated response to unexpected stimuli. The response is typically accompanied by a transient but complete muscular rigidity. Suspectibility to startle responses is increased by emotional tension, nervousness, fatigue and the expectation of being frightened. Symptoms of this disease are present from birth, a reason why it is also named stiff baby syndrome, with infants displaying severe muscular rigidity. This disorder has a history of being misdiagnosed as epilepsy, although hyperekplexia can be readily distinguished by an absence of fits and retention of consciousness during the startle episodes. The symptoms are successfully treated by benzodiazepines, with clonazepam being the current drug of choice.
Dominant and recessive forms of this rare disease result from missense mutations of the a1 GlyR subunit gene on chromosome 5q. The most common mutation is the R271L substitution which leads to an autosomally dominant form. This mutation and other substitutions at the R271 site cause reduced glycine sensitivity and single-channel conductance. The reduced efficiency of gly-cinergic neurotransmission explains the human phenotype which exhibits an increasing level of excitability of motor neurons at the spinal cord and brain stem level.
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