Diagnostic accuracy of multifidus muscle spontaneous activity by needle electromyography for the detection of lumbar foraminal and lateral exit-zone stenosis

Purpose The medial branch of the posterior ramus of the lumbar spinal nerve is well known to be innervated independently and to end in the multifidus muscle without anastomosis. This prospective cohort study aimed to determine the diagnostic specificity and sensitivity of multifidus muscle denervation (MMD) by needle electromyography (N-EMG) for lumbar foraminal and lateral exit-zone stenosis (LF/LEZS). Methods We enrolled 61 consecutive patients experiencing unilateral dysesthesia and/or leg pain in the L4 or L5 regions with suspicious LF/LEZS. The Japanese Orthopedic Association (JOA) score, Visual Analog Scale (VAS) for leg pain, and N-EMG were examined. In this study protocol, all patients received at least 3 months of conservative therapy. Surgery was performed on patients who experienced less than 50 % VAS pain relief compared with their initial score after confirming the responsible level by lumbar nerve root block. The specificity of N-EMG was the proportion of patients who improved with conservative therapies (non-surgery) after 3 months. The sensitivity of N-EMG was the proportion of patients who improved with surgical therapies (surgery) after more than 12 months. Results Twenty-three patients underwent surgery. The initial lower JOA, positive Kemp test and motor weakness were significantly higher in the surgery group. The MMD by N-EMG indicated that 34 of 38 patients were negative in the non-surgery group. In the surgery group, 21 of 23 patients were positive. The diagnostic sensitivity and specificity were 91.3 and 92.1 %, respectively. Conclusions Needle electromyography is a simple and available additional method for the diagnosis of LF/LEZS.