Functional myotome mapping via triggered electromyography during intraoperative cervical rootlet stimulation

Abstract

Myotome mapping provides essential anatomical and physiological information in neurosurgical practice. However, current cervical myotome charts are based largely on heterogeneous sources with considerable inconsistency and limited electrophysiological validation. This study aimed to describe a cervical functional myotome map based on amplitude-based triggered electromyography (tEMG) during intraoperative rootlet stimulation in the intradural space in patients undergoing cervical spinal tumor surgery. We retrospectively analyzed 20 patients who underwent cervical spinal tumor resection with intraoperative cervical rootlet stimulation between September 2023 and January 2025. After tumor resection, exposed ventral rootlets were directly stimulated using a bipolar stimulator (0.5 mA) to assess the preserved motor function. tEMG was recorded from trapezius (Tr), deltoid (D), biceps (B), triceps (T), and abductor pollicis brevis (APB). For each rootlet, the maximum tEMG amplitude among repeated stimulations was recorded. Functional mapping was constructed by calculating the mean amplitude per muscle across rootlet levels. A total of 33 cervical rootlets were stimulated. The most frequently stimulated levels were C6 and C8 (each 21.2%), followed by C7, C5, and T1. High-amplitude tEMG responses were recorded in the D from both C4 and C5 rootlets, with C4 eliciting a greater response (1097.0 μV) than C5 (659.4 μV), suggesting that C4 may contribute more significantly to D innervation than traditionally recognized. The C6 rootlet elicited distributed responses across the D, B, T, and, notably, also in the APB. C7 predominantly activated the T and APB, while C8 and T1 produced the highest amplitudes in the APB (1350.6 μV and 2259.7 μV, respectively). Some atypical activation patterns were also observed, such as D activation in response to C8 stimulation. This study presents a quantitative, amplitude-based functional mapping of cervical myotomes using intraoperative rootlet stimulation. Our findings reveal discrepancies from conventional myotome charts. Since all patients in this study had cervical spinal tumors, tumor-related direct or indirect effects on rootlet function may have influenced the amplitude patterns observed. Nevertheless, amplitude-based tEMG mapping may serve as a valuable tool for refining cervical functional myotome charts and improving diagnostic and surgical accuracy in cervical spine disorders.

Keywords: electric stimulation; electromyography; innervation; muscle; spinal nerve root.

Figure 1.

The patient inclusion flow chart. Among patients who underwent surgery for cervical spinal tumors between September 2023 and January 2025, those aged ≥18 years with pathologically confirmed cervical spinal tumors and available intraoperative triggered electromyography (tEMG) recordings were included. After applying exclusion criteria, 20 patients were analyzed.

Figure 2.

Intraoperative cervical rootlet stimulation and triggered electromyography recording method. The ventral side of the exposed rootlet was stimulated using a bipolar stimulator following tumor resection. Each rootlet was divided into the upper side (A) and lower side (B), and 2 to 3 repeated stimulations were performed at each site. The highest amplitude obtained among the repeated stimulations was defined as the representative triggered electromyography (tEMG) response for that rootlet.

Figure 3.

Functional myotome map. This heatmap visualizes the mean triggered electromyography (tEMG) amplitudes recorded in the trapezius (Tr), deltoid (D), biceps (B), triceps (T), and abductor pollicis brevis (APB) following direct stimulation of cervical rootlets (C1–T1). The Tr was predominantly activated by C2 stimulation, while the D showed relatively higher responses with C4 stimulation. C6 elicited balanced responses across multiple muscles, including APB, whereas C7 primarily activated the T and APB. C8 and T1 stimulations resulted in the strongest responses in the APB. Atypical patterns, such as D activation with C8 stimulation, were also observed.