False-negative transcranial motor evoked potentials (TcMEPs) during surgery for congenital lumbar kyphoscoliosis: a case report

Transcranial motor evoked potential (TcMEP) monitoring is the gold standard for intra-operative neurological monitoring (IOM) of motor pathways during complex spine surgery because of its high sensitivity and specificity. However, although it is very low, the rate of false-negatives in TcMEP monitoring is not zero. Therefore, over-reliance on TcMEP monitoring can cause potentially preventable motor deficits.

We report a case of motor deficits due to nerve root stretch after surgical correction of a congenital lumbar kyphoscoliosis in a 56-year-old woman. TcMEPs did not show any significant changes during surgery, whereas free-run electromyography (EMG) demonstrated a long-lasting train activity in the left quadriceps femoris muscle (QF) after correction at the osteotomy area. According to the normal findings on TcMEP monitoring, we did not release the correction. Postoperatively, a significant decrease (grade 2-) in the manual muscle test for the left QF and iliopsoas muscle and hypesthesia of the left anterior thigh was revealed. Fortunately, muscle strength was fully recovered without revision surgery at 6 months postoperatively; however, numbness in the left anterior thigh persisted at 2 years after surgery.

Our report suggests that the possibility of false-negative TcMEPs should be kept in mind, especially during surgery with a risk of nerve root injury. When abnormal findings in the free-run EMG, including long-lasting train activity, are observed, surgeons should consider performing appropriate responses, such as the release of the correction, even when no substantial changes are seen in the TcMEPs.