• Jerry Ku, Daniel Mendelsohn, Jason Chew, Jason Shewchuk, Charles Dong, and Ryojo Akagami.
• Division of Neurosurgery, Department of Surgery, Faculty of Medicine, Univer-sity of British Columbia, Vancouver, British Columbia, Canada.
• Neurosurgery. 2017 Mar 1; 80 (3): E194-E200.

Background And ImportanceLesions in the corticospinal tract above the decussation at the medullary pyramids almost universally produce contralateral deficits. Rare cases of supratentorial lesions causing ipsilateral motor deficits have been reported previously, but only ever found secondary to stroke or congenital pyramidal tract malformations.Clinical PresentationHerein, we report a case of ipsilateral corticospinal tract innervation discovered incidentally with intraoperative monitoring during a microsurgical resection of a vestibular schwannoma. Intraoperative monitoring with electrical transcranial stimulation of the frontal scalp triggered motor-evoked potentials in the ipsilateral arms. The uncrossed pathways were later confirmed with MRI tractography using diffusion tensor imaging.ConclusionTo the best of our knowledge, this is the first case of isolated ipsilateral motor innervation of the corticospinal tract discovered incidentally during a neurosurgical procedure. Given the increasing use of intraoperative monitoring, this case underscores the importance of cautious interpretation of seemingly discordant neurophysiological findings. Once technical issues have been ruled out, ipsilateral motor innervation may be considered as a possible explanation and neurosurgeons should be aware of the existence of this rare anatomic variant.

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