"The disappearing evoked potentials": a special problem of positioning patients with skeletal dysplasia: case report

A retrospective study of 3 patients with skeletal dysplasia, who had a loss of the evoked potentials during prone positioning before spine surgical intervention.

To bring attention to the potential hazard of neurologic compromise during the positioning of patients with skeletal dysplasia for spine surgery. Recommendations are suggested to prevent the disappearance of intraoperative evoked potentials and, therefore, possible neural injury in these patients.

In a very few published cases, loss or attenuation of monitored potentials has been observed at the time of initial patient positioning. Although patients with skeletal dysplasia might be considered particularly vulnerable to spinal cord injury caused by malpositioning of the head and neck, to our knowledge, no association with lost evoked potentials has previously been described.

Intraoperative transcranial electrical motor-evoked potential and/or somatosensory evoked potential baseline studies were performed after induction in the supine position. These studies were repeated as soon as practicable, after intubation and, again, after the patients were turned prone. The neurophysiologist informed the surgeon that evoked potential change in latency or amplitude met warning criteria. Alteration in the surgical plan resulted in successful spinal surgery in these cases.

In case No. 1, repositioning of the head in flexion was sufficient to return the evoked potentials to normal. In the other two cases, attempts to reposition the patients prone failed, and the procedures were abandoned. In case No. 2, four months after the initial surgery, a halo cast for immobilization and craniocervical decompression were needed before the corrective cervical spine surgery, and in case No. 3, two steps were taken after the initial surgery: 1) trial positioning awake on the surgical table before surgery; and 2) awake postintubation prone positioning on the actual surgery day.

Patients with skeletal dysplasia are susceptible to serious neurologic misadventure when turned to a prone position. Neurophysiologic and/or clinical monitoring of patient positioning should be undertaken, and a plan of intervention, should loss of signal or function occur, must be implemented.