Hyperekplexia
| Hyperekplexia | |
|---|---|
| Classification and external resources | |
| Specialty | neurology |
| ICD-10 | G25.8 |
| ICD-9-CM | 759.89 |
| OMIM | 149400 138491 138492 300429 300607 603930 604159 |
| DiseasesDB | 7208 |
| GeneReviews | |
Hyperekplexia ("exaggerated surprise") is a neurologic disorder classically characterised by pronounced startle responses to tactile or acoustic stimuli and hypertonia. The hypertonia may be predominantly truncal, attenuated during sleep and less prominent after a year of age. Classic hyperekplexia is caused by genetic mutations in a number of different genes, all of which play an important role in glycine neurotransmission. Glycine is used by the central nervous system as an inhibitory neurotransmitter. Hyperekplexia is generally classified as a genetic disease, but some disorders can mimic the exaggerated startle of hyperekplexia.
The three main signs of hyperekplexia are generalized stiffness, excessive startle beginning at birth and a short period of generalised stiffness following the startle reflex. Affected individuals are fully conscious during episodes of stiffness, which consist of forced closure of the eyes and an extension of the extremities followed by a period of generalised stiffness and uncontrolled falling at times. Initially, the disease was classified into a "major" and a "minor" form, with the minor form being characterized by an excessive startle reflex, but lacking stiffness. There is only genetic evidence for the existence of the major form.
Other signs and symptoms of hyperekplexia may include episodic neonatal apnea, excessive movement during sleep and the head-retraction reflex. The link to some cases of Sudden Infant Death remains controversial.
Hyperekplexia is known to be caused by a variety of genes, encoding both pre- and postsynaptic proteins. The symptoms displayed, as well as the forms of heritance, vary based on which gene is affected.
The first gene linked conclusively to hyperekplexia was GLRA1. The GLRA1 gene encodes the glycine receptor alpha-1 subunit, which, together with the glycine receptor beta subunit, forms synaptic glycine receptors. Inhibitory glycine receptors are ligand-gated chloride channels that facilitate fast responses in the brainstem and spinal-cord. Homomeric glycine receptors composed exclusively of alpha-1 subunits exhibit normal ion channel electrophysiology but are not sequestered at the synaptic junction. Native glycine receptors are thus supposed to be heteromers of the alpha-1 and beta subunits, in either a 3:2 or 2:3 ratio.
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