Effect of multiple subpial transection on motor cortical excitability in cortical dysgenesis

We report here a 12-year-old patient with unilateral cortical dysgenesis an d intractable simple partial seizure in his left arm, who underwent multipl e subpial transection (MST) in the right cerebral cortex including the prim ary motor cortex, We investigated motor cortical excitability using multimo dal transcranial magnetic stimulation (TMS) before and 1 month after MST, i n which surgical cortical incisions were made with strokes 5 mm apart and 4 mm deep. Preoperative TMS studies showed hyperexcitability in the affected motor cortex as abnormally prolonged muscle responses to TMS with a wide c ortical motor map, which were markedly reduced following the operation, The preoperative motor evoked potentials were large and polyphasic, and consis ted of early and late components. The late component was completely abolish ed after MST, suggesting that this component might be due to activation of the corticospinal tract neurones by long recurrent axon branches of dysplas tic excitatory pyramidal neurones, which were cut by MST, or by delayed, po lysynaptic intracortical conduction with marked temporal dispersion. Intrac ortical inhibition in the affected motor cortex was also disrupted preopera tively and improved after MST. Postoperative recruitment order of muscle re sponses to TMS was bilaterally symmetrical, indicating that MST did not int erfere with the function of the corticospinal tract neurones, The patient s howed fair motor recovery and good seizure control after the operation, The se results of TMS studies demonstrated the remarkable effectiveness of MST not only on intractable seizure but also on abnormal motor cortical organiz ation and hyperexcitability in cortical dysgenesis.