Intraoperative neurophysiologic discovery of uncrossed sensory and motor pathways in a patient with horizontal gaze palsy and scoliosis

Objective: To report the intraoperative neurophysiologic discovery of clinically unsuspected non-decussation of the somatosensory and motor pathways. Methods: We performed somatosensory evoked potential (SEP) and transcranial electric stimulation (TES) muscle motor evoked potential (MEP) monitoring during scoliosis surgery for a 16 year old patient with familial horizontal gaze palsy and progressive scoliosis. Our routine procedures included optimizing tibial cortical SEP monitoring derivations through saggital and coronal (C4′, C2′, Cz′, C1′, C3′-mastoid) P37 mapping, which surprisingly indicated non-decussation. Consequently, we also obtained coronal median nerve SEPs and simultaneous bilateral muscle recordings to lateralized TES (C3-Cz, C4-Cz) intraoperatively and focal hand area transcranial magnetic stimulation (TMS) postoperatively. Results: For each nerve, tibial P37/N37 distribution was contralateral/ipsilateral and median N20 ipsilateral. For each hemisphere, ipsilateral TES MEPs had lower thresholds and TMS MEPs were exclusively ipsilateral. Accurate monitoring required reversed montages. Reevaluation of an MRI (previously reported normal) disclosed a ventral midline cleft of the medulla. Conclusions: The results indicate uncrossed dorsal column-medial lemniscal and corticospinal pathways due to brain-stem malformation with absent internal arcuate and pyramidal decussations. Significance: Simultaneous bilateral recording to unilateral stimulation demonstrates SEP/MEP hemispheric origin and is important for accurate interpretation and monitoring because decussation anomalies exist.