Distal course and variability of the spinal accessory nerve: a cadaveric study with surgical implications.

PURPOSE: The spinal accessory nerve (SAN) is most vulnerable to injury along its distal extracranial course in the posterior cervical triangle. While proximal anatomy is well described, distal branching patterns and cervical plexus contributions to trapezius innervation remain poorly defined. This study aimed to characterise distal extracranial SAN branching and cervical plexus contributions and to assess whether proximal configurations predict distal trapezial innervation. METHODS: Ten sides from five adult cadavers were dissected from the emergence of the SAN at the posterior border of the sternocleidomastoid muscle (SCM) to its intramuscular termination in the trapezius. Emergence level, SCM branching pattern (Shiozaki classification), cervical plexus communications, loop formations, and distal branching were recorded. Linear morphometric measurements were obtained where feasible. RESULTS: The SAN entered the posterior cervical triangle at variable levels (4.5-8.5 cm from the mastoid). Only Shiozaki Type A and B SCM patterns were identified. Cervical plexus contributions were present in all specimens. An independent cervical branch to the superior trapezius occurred in 6/10 sides, and SAN-cervical loop formation in 2/10 sides. Distal intramuscular branching ranged from two to five terminal branches, with no clear relationship to proximal configuration or measurements. CONCLUSION: Distal SAN anatomy is highly variable and cannot be predicted from proximal patterns or surface landmarks alone, underscoring the need for meticulous identification of both the SAN and cervical contributions during posterior cervical triangle and trapezius-based surgery.