Startle and its disorders.

Exaggerated startle is an uncommon feature of various neurological diseases, but is still lacking precise analysis in many of them. So far, electrophysiologic and cinematographic analyses allow discriminating two main subtypes. The prototype of primary exaggerated startle is hereditary hyperekplexia, a well-studied disorder of the inhibitory glycine receptor and thus of the neuronal Cl- channel. The involuntary jerking in hereditary hyperekplexia is considered a reticular reflex myoclonus. The prototype of primary normal startle with secondary abnormalities is startle epilepsy where a surprise stimulus typically provokes a normal startle, which in turn initiates a focal (most often frontal lobe) seizure with tonic posturing of the limbs. Clinical differential diagnosis between both subtypes may be difficult in individual cases, but there are abnormalities in clinical and neurophysiologic reflex testing, which need, however, broad validation.