Electrophysiological evidence of severe marginal mandibular branch injury in Bell palsy: Implications for optimizing clinical assessment and treatment – a retrospective study

Bell palsy (BP) is an acute idiopathic peripheral facial neuropathy typically presenting with unilateral weakness. However, the differential electrophysiological involvement of its 6 main branches remains poorly defined, limiting branch-level diagnosis and targeted management. This retrospective study aimed to characterize nerve conduction parameters across these branches, compare injury severity, and refine electrophysiological protocols to enhance clinical decision-making. This retrospective study enrolled 56 patients with BP. Compound muscle action potential (CMAP) amplitude and distal motor latency (DML) were measured for each facial nerve branch on both the affected side and unaffected side. Side-to-side comparisons were performed using paired-samples t tests or Wilcoxon signed-rank tests, as appropriate. Data are expressed as mean ± standard deviation or median (interquartile range). A P value <.05 was considered statistically significant. Compared to the US, the affected side demonstrated significantly reduced CMAP amplitudes and prolonged DMLs across all branches (all P  < .001). The severity of axonal injury, indicated by CMAP amplitude reduction ratios, ranked as follows: marginal mandibular > zygomatic (nasal) > buccal > zygomatic (orbital) > temporal > cervical. The severity of demyelinating injury, reflected by DML prolongation ratios, ranked as: marginal mandibular > buccal > zygomatic (orbital) > zygomatic (nasal) > temporal > cervical. The marginal mandibular branch exhibits the most severe electrophysiological involvement in BP. These findings underscore its critical role in disease severity and support the prioritization of this branch in both diagnostic assessment and targeted therapeutic intervention.