Insights into Cervical Phrenic Nerve Variations Based on Cadaveric Dissection and Intraoperative Electromyography.

Abstract

Background: In complex cervical nerve surgeries, ensuring the integrity of the phrenic nerve (PN) adjacent to the anterior scalene muscle is critical. However, we have encountered a clinical paradox: Postoperative diaphragmatic dysfunction occurred even when the main PN trunk was preserved intraoperatively. This suggests the potential involvement of PN variations (PNVs), such as the accessory PN, which merges with the main trunk, and separate branches that maintain an independent course.

Methods: Using contralateral C7 nerve transfer (CC7) as a surgical model, this study aimed to characterize the prevalence and electrophysiology of the cervical PNV by combining cadaveric dissection with intraoperative electromyography (EMG) monitoring.

Results: Anatomical dissection revealed a PNV prevalence of 31.8% (7/22 sides), whereas intraoperative EMG identified PNVs in 63.3% of cases (19/30 sides). Intraoperatively, the PNV was typically observed coursing lateral to the main PN (47.4%, 9/19), overlying the anterior scalene muscle. Electrophysiological analysis demonstrated a moderate correlation between PNV- and PN-evoked responses (latency ρ = 0.661; amplitude ρ = 0.673; p < 0.05). The PN generally exhibited larger amplitudes (p < 0.001), confirming its role as the primary motor supply to the diaphragm. Notably, in 44.4% of cases with recordable signals, the PNV amplitude reached ≥50% of the ipsilateral PN amplitude, suggesting that these PNVs may contribute to diaphragmatic innervation. All identified branches were successfully preserved, with no severe postoperative complications observed.

Conclusion: Conventional anatomical studies may underestimate the prevalence of the PNV relative to intraoperative electrophysiological assessment. Intraoperative EMG monitoring is not required for all cervical nerve procedures; however, when feasible, particularly in procedures that may involve bilateral PNs, it can help identify and preserve functionally important branches, thereby reducing the risk of iatrogenic injury.