Prone Position-Induced Quadriceps Transcranial Motor Evoked Potentials Signal Loss-A Case Report

Transcranial motor evoked potential (TcMEP) is widely used intraoperatively to monitor spinal cord and nerve root function. To our knowledge, there is no report regarding TcMEP signal loss purely caused by patient positioning during the spinal procedure.

The objective of this article is to report an intraoperative TcMEP signal loss of a patient with fixed sagittal imbalance posture along with mild hip contractures.

A retrospective case report.

A 57-year-old man had fixed sagittal imbalance and flexed hip contractures. For a reconstruction surgery of T10 to the sacrum/ilium and L5 pedicle subtraction osteotomy (PSO), he was put in a prone position on a Jackson table. In order to accommodate his fixed hip flexion contracture, thigh pads were not used and pillows were placed under his bilateral thighs for cushioning. TcMEPs were used to assess lumbar nerve root function. Ten minutes after incision, bilateral vastus medialis TcMEPs were lost during spine exposure whereas all other data remained normal at baseline. The bilateral lower extremities were repositioned, with the knees flexed into a sling position to increase hip flexion. Five minutes after repositioning, the bilateral vastus medialis TcMEPs gradually improved and maintained baseline amplitude during the remainder of the surgery.

No muscle weakness was detected immediately after surgery. The patient was discharged day 6 postoperatively with markedly improved posture and alignment.

Insufficient hip flexion in patients with fixed sagittal imbalance and hip flexion contractures may cause TcMEP signal changes in the quadriceps response. TcMEP monitoring of bilateral lower extremities is highly recommended for patients with sagittal imbalance and hip contractures, with consideration for lower extremity repositioning when data degradation does not correlate with the actual spinal procedure being performed.