Transcranial magnetic stimulation (TMS) is an established neurophysiological tool to examine the integrity of the fast-conducting corticomotor pathways in a wide range of diseases associated with motor dysfunction. This includes but is not limited to patients with multiple sclerosis, amyotrophic lateral sclerosis, stroke, movement disorders, disorders affecting the spinal cord, facial and other cranial nerves. These guidelines cover practical aspects of TMS in a clinical setting. We first discuss the technical and physiological aspects of TMS that are relevant for the diagnostic use of TMS. We then lay out the general principles that apply to a standardized clinical examination of the fast-conducting corticomotor pathways with single-pulse TMS. This is followed by a detailed description of how to examine corticomotor conduction to the hand, leg, trunk and facial muscles in patients. Additional sections cover safety issues, the triple stimulation technique, and neuropediatric aspects of TMS.

The topography of the cortical motor projection to the sternocleidomastoid (SCM) muscles was investigated in 15 normal subjects using the technique of transcranial magnetic stimulation. Contrary to the long-held view that the representation of the neck muscles in the motor strip is close to that of the face, our findings indicate that the projection to both the ipsilateral and contralateral SCM muscles arises from an area of cortex high up on the cerebral convexity close to the trunk representation and at a comparable level to the sensory representation of the neck in the post-central cortex. The use of shielded intramuscular recordings showed that surface recordings detect activity not only from the SCM muscle but also from the overlying platysma muscle whose cortical representation is lateral to that of the hand muscles. Our findings confirm that the corticomotor projection to the SCM muscle follows both a contralateral monosynaptic pathway, and an ipsilateral pathway, and indicate that the latter pathway may be disynaptic.

Corticomotor conduction and cortical topography were appreciably normal despite only liminal activation of the target muscle with voluntary effort. Muscles with these characteristics may benefit from a targeted rehabilitation program even in the chronic phase after SCI.

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