Fig. 4. Intraoperative microscope photograph (A) illustrating the location of the cavernoma in the left hemimedulla. Preoperative axial (B) and sagittal (C) T1-weighted MR images showed a hyperintense lesion in the left dorsal medulla in a 26-year-old patient. Monitoring (D) of the somatosensory-evoked potentials (SEPs) during surgical removal of the cavernoma disclosed a selective lesion in the left dorsal column nuclei. During the initial incision in the region of the dorsal column nuclei, SEPs after the left median nerve stimulation diminished (^*) and subsequently disappeared (not shown), whereas SEPs from the right median nerve remained intact. The patient woke up after surgery with significant sensory loss in the left arm. In this patient we could only document the lesion, being unable to remove it. Adapted from Deletis V, et al., Operative Tech Neurosurg 2000;2:109-113, with permission from Elsevier.¹⁴.