CALLOSAL AND CORTICOSPINAL TRACT FUNCTION IN PATIENTS WITH HYDROCEPHALUS - A MORPHOMETRIC AND TRANSCRANIAL MAGNETIC STIMULATION STUDY

In 15 patients with symptomatic hydrocephalus, pressure-induced morpho logical changes of the brain and the function of callosal and corticos pinal fibres were studied. Morphometry of the corpus callosum (CC) was performed on midsagittal MR images. Focal transcranial magnetic stimu lation of the motor cortex was used to assess simultaneously excitator y motor responses in contralateral hand muscle (corticospinally mediat ed effect) and inhibition of tonic EMG activity in ipsilateral hand mu scles (transcallosal inhibition (TI) of the contralateral motor cortex ). Before a shunt operation, the midsagittal area of the CC was reduce d by 34% on average. The height and. to a lesser degree the length. of the CC were increased before the shuns operation. Thresholds and cent ral motor latencies of corticospinally mediated responses were normal. response amplitudes were smaller than in normal subjects. Motor thres holds increased from 38, SD 5 to 52, SD 8% (P<0.01) within 7 days afte r ventricular drainage, reflecting the increase in the distance betwee n stimulation coil and brain, The threshold increase paralleled a rest oration of normal anatomical conditions within 7 days after shunt oper ation and the improvement of motor symptoms and might be a predictor o f successful decompression. Transcallosal inhibition could be elicited in all patients. The measurements of TI lay within the normal range e xcept the duration. which was prolonged in 73% of 15 patients before s hunt operation as a probable indicator of an increased dispersion of c allosal conduction. The normalization of the area and shape of the CC after shunt operation and the normal corticospinal and callosal conduc tion times exclude degeneration. demyelination or functional block of a large proportion of callosal or corticospinal tract fibres or a subs tantial loss of nerve cells in motor cortex.