Motor evoked potentials in rats with congenital hydrocephalus.

Motor evoked potential (MEP) by focal transcranial magnetic stimulation was used to test the functional integrity of the motor cortex in congenital hydrocephalic rats. Magnetic MEPs, using a figure-eight coil above the head, were recorded in the tibialis anterior muscle. The latency of transcranial magnetic MEP was 3.4 msec in nonhydrocephalic rats. In the hydrocephalic rats, the MEP had a lower threshold than in nonhydrocephalic rats, and showed two peaks. Latencies of early and late peaks were 3.9 msec and from 5.4 msec to 10.0 msec, respectively. Our findings suggest that hydrocephalus in rats is associated with changes in pyramidal cell excitability in the motor cortical area, probably induced by the fluctuations in cortical excitability and synaptic interaction in hydrocephalic rats.