• Alfredo Quiñones-Hinojosa, Russ Lyon, Rose Du, and Michael T Lawton.
• Department of Neurological Surgery, University of California, San Francisco, San Francisco, California 94143-0112, USA. quinones@post.harvard.edu
• Neurosurgery. 2005 Apr 1; 56 (2 Suppl): E439; discussion E439.

Objective And ImportanceBrainstem cavernous malformations that seem to come to a pial or ependymal surface on preoperative magnetic resonance imaging studies may, in fact, be covered by an intact layer of neural tissue. For cavernous malformations in the cerebral peduncle, intraoperative stimulation mapping with a miniaturized probe can determine whether this overlying tissue harbors fibers in the corticospinal tract. In addition, intermittent monitoring with transcranial motor evoked potentials (TcMEPs) helps to protect this vital pathway during resection of the lesion.Clinical PresentationA 20-year-old woman collapsed after a cavernous malformation in the left cerebral peduncle hemorrhaged into the pons, midbrain, and thalamus. She presented with right hemiparesis and left oculomotor palsy.InterventionThe cavernous malformation was completely resected through a left orbitozygomatic craniotomy and transsylvian approach. Stimulation mapping of the cerebral peduncle with a Kartush probe (Medtronic Xomed, Inc., Jacksonville, FL) identified the corticospinal tract lateral to the lesion, and a layer of tissue over the lesion harbored no motor fibers. TcMEP monitoring helped to guide the resection, with increased voltage thresholds and altered waveform morphologies indicating transient impaired motor conduction. All TcMEP changes returned to baseline by the end of the procedure, and the patient's hemiparesis improved after surgery.ConclusionStimulation mapping of the corticospinal tract and intermittent TcMEPs is a safe and simple surgical adjunct. Expanded monitoring of the motor pathway during the resection of cerebral peduncle cavernous malformations may improve the safety of these operations.

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