Hyperekplexia

Hyperekplexia is a rare disorder (or group of disorders) that may include dramatic neonatal onset (68) with nonepileptic convulsive syncopes that may prove fatal. Insofar as effective treatment is possible by repeatedly flexing the baby (69), diagnostic awareness should be high. An early major paper on this topic (70) described a dominantly inherited disorder in which there were stiff hypertonic neonates with later pathologic startles. Some confusion has been engendered by the title of this first paper, which referred to hyperexplexia, whereas the proper Greek term is hyperekplexia (71). The consistent specific diagnostic sign of hyperekplexia is elicited by tapping the infant's nose (72). In a normal infant, nose-tapping produces a minimal response, whereas in affected children there is an obvious and reproducible startle response including head retraction. This startle may be induced over and over again. The diagnosis in sporadic cases in which the baby is stiff and tends to startle is not too difficult. More difficult is the situation in which the baby is not stiff but does have neonatal onset convulsions with severe syncope. These dramatic nonepileptic seizures may be induced by bathing but the nose-tap test is clearly positive. Also of diagnostic value is the EEG recording during a seizure. A series of what superficially may appear to be spikes appears on the EEG, but these are actually rapidly recurring muscle potentials from scalp muscle (synchronous potentials also are seen on the ECG channel) whose fire rate decreases pari passu with slowing of both EEG and ECG in the resultant severe syncope. The genetic basis for both the usual dominantly inherited variety of hyperekplexia and apparently sporadic cases is a defect in either the alpha(1) (73) or beta (74) subunits of the strychnine-sensitive glycine receptor. Whatever the variety, clonazepam remains the prophylaxis of choice (see also Chapter 13).

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