gms | German Medical Science

Objective: Navigated magnetic brain stimulation (NBS) has become a valuable tool for mapping the motor cortex. The premotor cortex is known to modulate motor executions and has not been regarded as a target for NBS mapping. We wanted to clarify the usefulness of premotor NBS for mapping of the hand motor cortex.

Method: 10 human right handed adults were studied, 9 of which harbored a unilateral brain tumor. We used NBS for mapping of the precentral gyrus and premotor frontal cortex. 3D surface segmentation of high resolution T1 Dicom MR images at 25 mm from the scalp at the region of the hand motor knob were used as mapping surface. During NBS, muscle activity was monitored online by electromyography. The motor threshold, the amplitudes and latencies for the abductor digiti minimi (ADM) and abductor pollicis brevis (APB) muscles were determined. To visualize the cortical areas representing the hand muscles, stimulation sites that elicited motor evoked potentials were marked to the cortex surface at a depth of 25 mm. DTI fiber tracking was performed bilaterally in one patient with a precentral tumor.

Results: Mapping of the precentral gyrus showed the typical representation of the APB and ADM muscles in the hand knob of the precentral gyrus as described in Penfield’s homunculus. We found in 14 out of 20 hemispheres an additional hand motor area rostral to the hand knob of the precentral gyrus. This cortex area was mapped positive for both APB and ADM stimulation and had a smaller size of the hand knob itself and a maximum length of 2 cm. This premotor hand area reacted to NBS by hand muscle activation as fast as the hand knob of the precentral gyrus. The motor threshold and latencies were similar in both cortical areas. This premotor hand motor cortex could also be detected in those hemispheres with a tumor residing in the central cortex. DTI fiber tracking in one patient revealed direct corticospinal fiber connection of the NBS positive premotor area.

Conclusions: The present study indicates that there might be a distinct cortical representation for hand motor executions rostral to the well known hand knob of the precentral gyrus. This premotor hand area can be detected by NBS in healthy hemispheres and also in cortices altered by a space-occupying lesions and seems to have direct corticospinal connections, Therefore the NBS mapping of the premotor cortex should be performed in addition to the precentral gyrus on a routine basis.