Abstract
Spinal accessory nerve (SAN) dysfunction persists after nerve-preserving neck dissection. We characterized the longitudinal SAN conduction trajectory and its association with surgical extent and adjuvant therapy. This prospective study enrolled 43 patients with oral cavity squamous cell carcinoma undergoing neck dissection with SAN preservation - selective neck dissection (SND; n = 35) or modified radical neck dissection (MRND; n = 8). Compound motor action potential (CMAP) amplitude, motor latency, and a novel, exploratory Spinal Accessory Asymmetry Index (SAI) were assessed at baseline, 3 weeks, and 3 months using hierarchical linear mixed-effects (LME) models. In the unilateral cohort (n = 41), CMAP amplitude fell 45.2% from baseline (9.15 ± 2.04mV) to a 3-week nadir (5.01 ± 2.47mV; p < 0.0001), with partial recovery by 3 months (6.26 ± 2.23mV; p < 0.001). The SAI showed a numerically larger but non-significant asymmetry after MRND at 3 months (17.82%; p = 0.078, n = 6) versus significant SND normalization (29.04%→15.90%; p < 0.0001). Adjuvant therapy distribution was balanced between groups (p = 0.781) but was not modelled as a covariate. Patient-level heterogeneity accounted for 46.2% of longitudinal variance (R2c = 0.4616). Nerve-preserving neck dissection produced a substantial subacute decline in SAN conduction with only partial recovery by 3 months, consistent with combined neuropraxic and axonal injury, though axonal loss was unconfirmed. The exploratory SAI suggested more persistent MRND asymmetry, though non-significant in this small subgroup, and should not argue against oncologically indicated neck dissection. Larger, multicentre studies with validated shoulder outcomes are needed.