Abstracts

Rationale: Per Penfield’s homunculus, cortical motor representation follows a well-defined topography constrained to the precentral gyrus (PrG). Yet, intraoperative and preoperative corticospinal maps reveal a broad spatial primary motor cortex distribution, particularly for the intrinsic hand muscles. While focal brain lesions in the PrG region can alter cortical motor organization, little is known about the effects of focal cortical lesions outside the PrG, with or without epilepsy, on the cortical motor map. We therefore test, by navigated transcranial magnetic stimulation (nTMS), whether isolated lesions or lesional focal epilepsy, outside the PrG, distort the cortical motor map. Methods: 682 patients with focal brain lesions and/or epilepsy underwent motor mapping by nTMS. Maps were generated for the abductor pollicis brevis (APB) muscle in 175 patients (age 4-71y). Only patients who underwent nTMS mapping of both healthy and affected hemispheres and had no radiographic evidence of motor cortex or corticospinal abnormality were included in this analysis. APB representation was designated by the anatomic distribution of the resultant map as overlapping with ≥1 of the following structures: the PrG, superior or middle frontal gyrus (SFG, MFG), or postcentral gyrus (PoG). We then tested whether APB distribution was determined by the location of the patient’s lesion differentially between lesional and contralesional hemispheres. Results: Sixty-one patients with focal brain lesions (all tumor) without epilepsy had topographically similar spatial distribution for APB motor sites (n.s.) in both hemispheres. However, in patients with epileptogenic focal brain lesions (n=114; tumor, cortical dysplasia and stroke), cortical APB distribution was dictated by seizure focus location in the affected hemisphere. Patients with anterior lesions (n=57) were more likely to have APB representation in the SFG and MFG (anterior to PrG) in the affected hemisphere than in the healthy hemisphere (ChiSq p<0.001). Patients with posterior lesions (n=57) were more likely to have PoG APB representation in the affected hemisphere than in the healthy hemisphere (ChiSq p<0.05). Conclusions: Focal neocortical epilepsy corresponds to an extension of cortical representation of intrinsic hand muscles towards seizure focus. Funding: No funding