title: Ipsilateral Motor Innervation Discovered Incidentally on Intraoperative Monitoring: A Case Report.
abstract: Lesions 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.
  
  
  
  Herein, 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.
  
  
  
  To 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.
authors.affiliation: Division of Neurosurgery, Department of Surgery, Faculty of Medicine, Univer-sity of British Columbia, Vancouver, British Columbia, Canada.
authors.forename: Jerry
authors.initials: J
authors.lastname: Ku
authors.affiliation: Division of Neurosurgery, Department of Surgery, Faculty of Medicine, Univer-sity of British Columbia, Vancouver, British Columbia, Canada.
authors.forename: Daniel
authors.initials: D
authors.lastname: Mendelsohn
authors.affiliation: Division of Neuroradiology, Department of Radi-ology, Faculty of Medicine, University of British Columbia, Vancouver, British Columbia, Canada.
authors.forename: Jason
authors.initials: J
authors.lastname: Chew
authors.affiliation: Division of Neuroradiology, Department of Radi-ology, Faculty of Medicine, University of British Columbia, Vancouver, British Columbia, Canada.
authors.forename: Jason
authors.initials: J
authors.lastname: Shewchuk
authors.affiliation: Division of Neurosurgery, Department of Surgery, Faculty of Medicine, Univer-sity of British Columbia, Vancouver, British Columbia, Canada.
authors.forename: Charles
authors.initials: C
authors.lastname: Dong
authors.affiliation: Division of Neurosurgery, Department of Surgery, Faculty of Medicine, Univer-sity of British Columbia, Vancouver, British Columbia, Canada.
authors.forename: Ryojo
authors.initials: R
authors.lastname: Akagami
mesh_terms: D000328:Adult; D056324:Diffusion Tensor Imaging; D019054:Evoked Potentials, Motor / Q000502:physiology*; D005260:Female; D006801:Humans; D033162:Incidental Findings; D008279:Magnetic Resonance Imaging; D016343:Monitoring, Intraoperative / Q000379:methods; D009464:Neuroma, Acoustic / Q000473:pathology / Q000601:surgery; D019635:Neurosurgical Procedures / Q000379:methods; D011712:Pyramidal Tracts / Q000000981:diagnostic imaging / Q000473:pathology*
keywords: Intraoperative monitoring, Diffusion-weighted imaging,
                      Tractography; Ipsilateral motor innervation, Corticospinal tract