Facial nerve CMAP acquisition in mice: protocol, normative data, reliability, and injury/repair model application.

Purpose This study aimed to establish and validate a minimally invasive method for percutaneous compound muscle action potential (CMAP) recordings of the murine facial nerve, including normative values, reliability metrics, and biological sensitivity to nerve injury.Methods Thirty adult C57BL/6 mice underwent bilateral facial nerve CMAP recordings using percutaneous needle electrodes placed at defined anatomical landmarks. Three repeated measurements (total n=180) were obtained to assess intra-session reliability, while inter-side and inter-session reproducibility were evaluated in a subgroup of 25 animals over 30 days. CMAP amplitude, latency, and duration were analyzed using intraclass correlation coefficients (ICC), coefficients of variation (CV%), and minimal detectable change (MDC). Biological validity was tested in a facial nerve transection and microsurgical repair model (n=10), with longitudinal recordings at baseline, 30, 60, and 90 days.Results Mean CMAP amplitude was 10.0±1.5mV, latency 1.5±0.2 ms, and duration 1.0±0.1 ms. Intra-session reliability for CMAP amplitude and latency was moderate to good (ICC 0.61-0.81), with low variability (CV<15%), whereas inter-side and inter-session reliability varied across parameters. Inter-session amplitude reliability was moderate (ICC=0.59), with an MDC of approximately 2mV. Facial nerve injury resulted in a significant reduction in CMAP amplitude on the injured side compared with contralateral control (main effect of side: F(1,18)=40.1, p<0.0001), followed by partial recovery over time (interaction p<0.0001).Conclusions This study establishes a standardized protocol for murine facial nerve CMAP recording with moderate reliability and low variability. The method is well suited for detecting group-level functional changes and enables longitudinal assessment of facial nerve injury and regeneration.