Predicting hand function after hemispherotomy: TMS versus fMRI in hemispheric polymicrogyria.

Patients with hemispheric malformations of cortical development (such as polymicrogyria) often develop medically intractable epilepsies for which hemispherotomy can be an excellent treatment option. Transcranial magnetic stimulation (TMS) and functional magnetic resonance imaging (fMRI) are noninvasive methods used to evaluate the sensorimotor system in adults and children before surgery. Preoperative results of both methods and their predictive values regarding hand function after hemispherotomy are described in four boys with hemispheric polymicrogyria, pharmacoresistent epilepsy, and hemiparesis with preserved grasp function of the paretic hand. TMS showing ipsilateral projections from the contralesional hemisphere but no evidence of crossed corticospinal projections from the lesioned hemisphere correctly predicted preserved postoperative grasp function in all four patients. In contrast, the interpretation of sensorimotor fMRI in patients with congenital hemiparesis is more difficult, as ipsilesional activation can occur as it was the case in three of four patients in the current study. This activation might represent contralaterally preserved primary somatosensory (S1) and not primary motor (M1) representation and is apparently not necessary for the paretic hand to still perform grasp movements.