Broad and Asymmetric Lower Extremity Myotomes

Study Design. Retrospective review of prospectively collected data. Objective. This study aimed to accurately map the lower extremity muscles innervated by the lumbar spinal roots by directly stimulating the spinal roots during surgery. Summary of Background Data. Innervation of the spinal roots in the lower extremities has been estimated by clinical studies, anatomic studies, and animal experiments. However, there have been discrepancies between studies. Moreover, there are no studies that have studied the laterality of lower limb innervation. Materials and Methods. In 73 patients with lumbar degenerative disease, a total of 147 spinal roots were electrically stimulated and the electromyographic response was recorded at the vastus medialis (VM), gluteus medius (GM), tibialis anterior (TA), biceps femoris (BF), and gastrocnemius (GC). The asymmetry index (AI) was obtained using the following equation to represent the left-right asymmetry in the compound muscle action potential (CMAP) amplitude. Paired t tests were used to compare CMAP amplitudes on the right and left sides. Differences in the AI among the same spinal root groups were determined using one-way analysis of variance. Results. The frequency of CMAP elicitation in VM, GM, TA, BF, and GC were 100%, 75.0%, 50.0%, 83.3%, and 33.3% in L3 spinal root stimulation, 90.4%, 78.8%, 59.6%, 73.1%, and 59.6% in L4 spinal root stimulation, 32.2%, 78.0%, 93.2%, 69.5%, and 83.1% in L5 spinal root stimulation, and 40.0%, 100%, 80.0%, 70.0%, and 80.0% in S1 spinal root stimulation, respectively. The most frequent muscle with maximum amplitude of the CMAP in L3, L4, L5, and S1 spinal root stimulation was the VM, GM, TA, and GM, respectively. Unilateral innervation occurred at high rates in the TA in L4 root stimulation and the VM in L5 root stimulation in 37.5% and 42.3% of patients, respectively. Even in patients with bilateral innervation, a 20% to 38% AI of CMAP amplitude was observed. Conclusions. The spinal roots innervated a much larger range of muscles than what is indicated in general textbooks. Furthermore, a non-negligible number of patients showed asymmetric innervation of lower limb by the lumbar spinal roots.