Corticospinal Excitability in Bilateral M1 Hand Areas: Association with Neuropathic Pain After Spinal Cord Injury

Neuropathic pain (NP) is a major and debilitating complication of spinal cord injury (SCI), frequently occurring bilaterally below the injury level. Repetitive transcranial magnetic stimulation (rTMS) targeting the primary motor cortex (M1) is suggested as a treatment for NP following SCI (SCI-NP). However, the specific changes in corticospinal excitability (CSE) within the bilateral M1 remain unclear, hindering the development of optimized rTMS parameters for SCI-NP.

This retrospective study analyzed data from 625 SCI patients and 131 healthy controls. Motor-evoked potential (MEP) was used to assess CSE in the bilateral M1 hand areas. The hemispheric asymmetry of bilateral M1 CSE was measured by calculating the natural logarithm of the MEP amplitude ratio between the dominant hemisphere (DH) and non-dominant hemisphere (NDH), expressed as ln(DH/NDH amplitude). The study utilized correlation analysis and multiple linear regression to examine the associations between hemispheric CSE asymmetry and the course, severity, and emotional disturbances of NP.

SCI patients experiencing NP exhibited lower MEP amplitude than those without NP in bilateral M1 hand areas, with a more pronounced decrease in NDH. Hemispheric CSE asymmetry was found to be elevated and positively correlated with NP course (r=0.213, P=0.034), severity (r=0.317, P=0.004), and emotional disturbances (r=0.294, P=0.009). Notably, hemispheric CSE asymmetry was independently associated with NP severity and emotional disturbances, particularly in younger individuals (under 52 years), those with traumatic injuries, and those with non-cervical SCI.

Hemispheric CSE asymmetry shows potential as a biomarker for assessing SCI-NP severity and emotional disturbances in SCI patients. High-frequency rTMS targeting bilateral M1 hand areas may provide improved analgesic effects. This finding could enhance neuropathic pain assessment and guide rTMS optimization, potentially improving the quality of life for individuals with SCI.