SOMATOSENSORY-EVOKED POTENTIAL MONITORING DURING POSITIONING OF THE PATIENT FOR POSTERIOR-FOSSA SURGERY IN THE SEMISITTING POSITION

OBJECTIVE: Midcervical flexion myelopathy is a rare but well-known com plication of posterior fossa surgery. To reduce the risk, we routinely used somatosensory evoked potential (SSEP) monitoring during position ing of the patient. METHODS: Fifty-five consecutive patients were oper ated on for posterior fossa lesions in the semisitting position via a median (5 patients) or a lateral (50 patients) suboccipital approach. During positioning, monitoring of SSEPs by stimulation of the tibial n erve (T-SSEP) as well as by stimulation of the median nerve (M-SSEP) w as established. In the case of pronounced SSEP changes, the head was r epositioned. Surgery was started after SSEP recordings were unchanged as compared to the baseline investigation. RESULTS: Effective monitori ng was possible in all cases. Whereas M-SSEP recordings showed no chan ges while placing patients in the sitting position, T-SSEP recordings were altered in 14 cases (25%). In cases using the midline approach, S SEP changes were never so pronounced to require repositioning of the h ead. Head flexion and rotation resulted in significant changes of T-SS EP recordings in eight patients (14.5 %), requiring repositioning. In two cases, an amplitude loss was noted. In only two of these eight pat ients were M-SSEP recordings markedly changed. SSEP recordings after r epositioning disclosed recovery of spinal cord function. In no patient were clinical signs of myelopathy observed postoperatively. CONCLUSIO N: We observed a high incidence of pronounced changes of T-SSEP record ings when the patient's head was flexed and rotated for lateral subocc ipital craniotomy in the semisitting position. Despite the low specifi city monitoring of T-SSEPs during positioning of the patient for poste rior fossa surgery, the semisitting position is strongly recommended.