Spinal Accessory Nerve-Sparing Neck Dissection: Anatomy and Outcomes

Neck dissection began historically with George Crile's classical radical neck dissection (RND, 1906) — en bloc resection of all neck nodes (levels I-V) sacrificing sternocleidomastoid, internal jugular vein and spinal accessory nerve.

Functional cost of the radical approach: loss of the spinal accessory nerve causes trapezius palsy → shoulder drop (scapular winging), weakness of shoulder flexion/abduction, chronic shoulder pain (myofascial), sleep disturbance, significant quality-of-life loss. SCM loss disturbs neck contour (cosmetic), unilateral neck weakness. Internal jugular vein loss is usually minor but bilateral causes cerebral venous congestion.

Modifications (1980s — Suarez, Bocca, Byers): "functional" neck dissection — preserving one, two or all three structures sacrificed in RND. Modified radical neck dissection (MRND) classification: type 1 — only spinal accessory preserved (most important functionally); type 2 — XI + IJ vein; type 3 — XI + IJ + SCM (most functional).

Selective neck dissection (SND): instead of all 5 levels (I-V), removes specific sublevels based on primary tumour drainage. Types: SND I-III (supraomohyoid — oral cavity), SND II-IV (lateral — oropharynx/larynx), SND II-V (posterolateral — skin, parotid), SND VI (central — thyroid). Less morbidity, equivalent oncologic outcome (in appropriately selected cases).

Bilateral and N0 prophylactic dissection: in oral and oropharyngeal tumours, prophylactic SND ensures local control and captures micrometastases; positive locoregional survival impact.

Modern paradigm: RND is rarely indicated — only in extensive extracapsular spread, nerve invasion, SCM involvement. Standard is "nerve-sparing" MRND or SND.

Evidence base: Bocca 1985, Spiro 1990s and other large series showed no oncologic difference (5-year survival, local recurrence) between MRND and RND; substantial functional advantage with nerve preservation. Related overview: our head and neck cancer surgery programme.

The spinal accessory nerve (CN XI) has two roots: cranial (from medulla oblongata) and spinal (from C1-C5 spinal roots). The cervical portion ascends through foramen magnum, exits the skull via jugular foramen to the neck.

Neck course: 1) After exiting jugular foramen it descends posteroinferiorly between internal carotid artery and internal jugular vein. 2) At C1 (atlas) it turns laterally. 3) It pierces SCM from the deep aspect to its anterior surface (on average 4-5 cm below the mastoid tip, within level II). At this point it gives off branches to SCM. 4) It exits the posterior border of SCM (entering the posterior triangle — near Erb's point, ~6-8 cm above the clavicle). 5) It descends on the surface of levator scapulae and reaches the anterior border of trapezius. 6) It innervates trapezius from its deep aspect (especially upper and middle fibres).

Anatomic risk points: SCM passage (border of level IIa-IIb) — the most frequent injury site. Posterior triangle — near Erb's point (adjacent to lesser occipital and great auricular nerves); risk during skin biopsy, level V dissection, lipoma removal. Entry into trapezius — at risk during selective level V dissection.

Anatomic variations: SCM-piercing point may be high (level IIa) or low (level IIb). Some cases bypass SCM (anterior to it). Posterior triangle course can vary 1-2 cm. Individual anatomy confirmed by neuromonitoring.

Accessory nerve connections: CN XI anastomoses with C2-C4 sensory cervical plexus fibres (communicating branch); sensory contribution to trapezius is minor.

Trapezius motor functions: upper trapezius — shoulder elevation, adduction (shrugging); middle — scapular retraction; lower — scapular rotation (for shoulder abduction). Nerve injury produces trapezius atrophy, shoulder dysfunction, painful shoulder syndrome and "scapular winging".

Preoperative: planning by tumour lymphatic drainage; imaging review (CT/MRI) of nerve course; anterior/posterior tumour involvement, extracapsular extension. Plan defines nerve preservation strategy (full preservation vs near contact vs sacrifice).

Position and incision: supine, head rotated to the opposite side, neck extended. Modified Schobinger or hockey-stick incision. Skin flap raised, deltopectoral fascia in view.

SCM approach (level II): dissection along the anterior border of SCM. The point where the accessory nerve enters SCM is identified — usually 4-5 cm below mastoid tip, at the IIa-IIb border. Nerve runs 1-2 cm within SCM.

Identification methods: 1) Anterior approach — nerve comes into view at the anterior border as it enters SCM. 2) Posterior approach — nerve exits SCM's posterior border into the posterior triangle. 3) Retrograde tracing — SCM branches followed back to main trunk. 4) Erb's point approach — nerve usually located ~1 cm above C2-3 root in the posterior triangle.

Neuromonitoring: NIM (Nerve Integrity Monitor) records trapezius EMG. Low-amplitude stimulation (0.5-2 mA) locates the nerve; dissection under audible feedback allows real-time protection. Critical in reoperation and fibrotic tissue.

Level II dissection — including IIb if planned, with the nerve preserved, fat/nodes are removed bilaterally (above and below). Trapezius branches (upper, middle, lower fasciculi) protected.

Level V (posterior triangle) — if included, follow the nerve's course (posterior to SCM, on levator scapulae, into trapezius) and clear the fat/nodes below. Caution near transverse cervical artery and dorsal scapular nerve.

Postoperative test: at end of surgery, nerve integrity confirmed by neuromonitoring. Clinical test at postop day 1-2 — shoulder shrugging (trapezius), head turning (SCM partially nerve-controlled). Step-by-step details: our neck dissection guide.

Immediate postop (1-7 days): shoulder function may be partially reduced — transient neurapraxia from nerve manipulation (traction, electrocautery proximity). Trapezius strength slightly down, range full but fatigue/discomfort possible. Early-week shoulder protection, no heavy lifting.

Transient nerve dysfunction (neurapraxia): reduced electrical function from surgical traction without structural disruption. Typical recovery 2-12 weeks. Early physiotherapy: passive → active-assisted → active range exercise; maintain full shoulder ROM.

Permanent nerve injury (axonotmesis or neurotmesis): from dissection cut, transection, thermal injury (cautery). In modern technique rate is low (2-5% in functional neck dissection); classical RND was 100%. Findings: trapezius atrophy (6-12 weeks), shoulder drop (scapular winging), shoulder abduction <90°, chronic pain.

Rehabilitation protocol: SLP and physiotherapist supervision. Early (postop 1-4 weeks) — passive range, wound priority. Middle (4-12 weeks) — passive-active assisted, no strength. Late (12+ weeks) — active strength, trapezius strengthening, compensatory muscle strengthening (levator scapulae, rhomboids).

Shoulder pain syndrome management: NSAIDs, physiotherapy, posture training, corticosteroid injection (subacromial, trigger point) if needed. Severe — surgical (scapular stabilisation — Eden-Lange) in selected.

Oncologic follow-up: 5-year survival is equivalent between nerve-sparing MRND and classical RND (60-80% by stage). Locoregional control similar. If tumour invades or surrounds nerve, sacrifice is mandatory — oncology priority.

Quality of life: shoulder function, pain, arm use, sleep, work/activity return are markedly better with nerve preservation (DASH, SPADI, EORTC QLQ-H&N35 confirm). Patient satisfaction high.

Late outcomes: at 1 year, >75% of those with early dysfunction recover to near baseline; remaining 20-25% mild residual. Full transection — permanent; nerve grafting (sural) in selected — variable outcomes.

Tumour features: surgical approach by primary and clinical N stage. Early (cT1-2 N0 — low metastasis risk): selective dissection (e.g. SND I-III or II-IV). Advanced (cN+): MRND types 1-3 or selectively RND. Extracapsular extension, nerve involvement, SCM involvement may force sacrifice.

Primary site guide: oral cavity (tongue, floor of mouth) — supraomohyoid (level I-III); oropharynx (tonsil, base of tongue) — lateral (II-IV); larynx and hypopharynx — lateral (II-IV) + often VI; thyroid cancer — central (VI) + therapeutic lateral (II-V) if lateral nodes positive; skin (melanoma, SCC) — superior (parotid, II-V).

N+ neck — choice of functional dissection: single-level disease — selective (those levels + adjacent) may suffice. Multi-level disease, extracapsular spread — MRND type 1 (nerve preservation) is the recommended standard. Extensive tumour + nerve invasion — accept function loss with RND.

Intraoperative assessment: frozen section guides positive/negative; observed local spread may broaden the operation; nerve/SCM proximity demands careful dissection.

Effect of adjuvant therapy: postoperative RT often planned (in high-risk features); RT can add trapezius fibrosis and shoulder dysfunction. Early rehabilitation (ideally before RT) mitigates this.

Robotic and minimally invasive neck dissection: TORS and transoral robotic dissections have advanced primary tumour surgery; the classical open neck dissection remains standard, but retro-auricular incision (scarless robotic neck dissection) is selectively used — particularly when cosmetic concern is strong, prophylactic, or small positive nodule.

Multidisciplinary approach: head and neck surgery, medical/radiation oncology, pathology, imaging, physiotherapy, dietitian joint review. Patient preference and lifestyle (occupation, hobby — shoulder function importance) inform decisions. Related reading: our multidisciplinary tumour board.