Sensory and cognitive evoked potentials in a case of congenital hydrocephalus

We studied auditory and visual evoked potentials in D.W., a patient with congenital stenosis of the cerebral aqueduct. Head CT scans revealed marked hydrocephalus with expanded ventricles filling more than 80% of the cranium and compressing brain tissue to less than 1 cm in thickness. Despite the striking neuroanatomical abnormalities, however, the patient functioned well in daily life and was attending a local community college at the time of testing. Evoked potentials provided evidence of preserved sensory processing at cortical levels. Pattern reversal visual evoked potentials had normal latencies and amplitudes. Brain-stem auditory evoked potentials (BAEPs) showed normal wave V latencies. Na and Pa components of middle-latency AEP had normal amplitudes and latencies at the vertex, although amplitudes at lateral electrodes were larger than at the midline. In contrast to the normal sensory responses, long-latency auditory evoked potentials to standard and target tones showed abnormal P3 components. Standard tones (probability 85%), evoked N1 components with normal amplitudes (-3.7 microV) and latencies (103 msec), but also elicited large P3 components (17 microV, latency 305 msec) that were never observed following frequent stimuli in control subjects. Target stimuli (probability 15%) elicited P3s in D.W. and controls, but P3 amplitudes were enhanced in D.W. (to more than 40 microV) and the P3 showed an unusual, frontal distribution. The results are consistent with a subcortical source of the P300. Moreover, they suggest that the substitution of controlled for automatic processes may help high-functioning hydrocephalics compensate for abnormalities in cerebral structure.