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AN29.1-AN30.5 | Posterior triangle of neck — Self-Directed Learning
CLINICAL SCENARIO
Two clinical scenarios: (1) A motorcyclist in Chennai is thrown from his bike, landing on his shoulder. His arm hangs limply by his side in the "waiter's tip" position — he cannot abduct his arm or flex his elbow. (2) A neonate delivered by forceps has his head tilted to one side with the chin rotated to the opposite side — his right sternocleidomastoid is hard and shortened.
Which roots were torn in the motorcyclist? What is the "waiter's tip" position? What causes the neonate's twisted neck?
The posterior triangle of the neck is where the brachial plexus, accessory nerve, and cervical plexus emerge — mastering this region explains both injuries.
WHY THIS MATTERS
The posterior triangle has enormous clinical yield:
- Brachial plexus injuries — Erb's and Klumpke's palsy are classic exam cases covering the entire range of upper limb nerve injury.
- Torticollis (wryneck) — SCM fibrosis after birth injury is the most common cause in children; sternocleidomastoid tumour of infancy is an important paediatric diagnosis.
- Accessory nerve (CN XI) injury — during lymph node biopsy in the posterior triangle; results in winging of scapula and shoulder drop. Surgeons must identify this nerve.
- Cervical rib syndrome — an extra rib from C7 (cervical rib) compresses the subclavian artery and lower brachial plexus → thoracic outlet syndrome.
- Dural venous sinus thrombosis — a life-threatening complication of middle ear infections, mastoiditis, and head trauma with implications for the AN30 content in this module.
RECALL
Before reading: What are the cervical vertebrae C1-7? How many roots make up the brachial plexus? What is the difference between a ventral ramus and a dorsal ramus of a spinal nerve?
Posterior Triangle — Boundaries, Subdivisions & Contents (AN29.1)
Boundaries:
- Anterior: Posterior border of sternocleidomastoid (SCM)
- Posterior: Anterior border of trapezius
- Inferior (base): Middle third of clavicle
- Apex: Meeting of SCM and trapezius at the superior nuchal line
- Roof: Investing layer of deep cervical fascia + platysma
- Floor: Prevertebral fascia (over splenius capitis, levator scapulae, scalenus medius, scalenus posterior)
Subdivisions:
The inferior belly of the omohyoid crosses the posterior triangle, dividing it into:
- Occipital triangle (larger, upper): contains brachial plexus roots, cervical plexus branches, accessory nerve
- Subclavian (supraclavicular) triangle (smaller, lower): contains subclavian artery (3rd part), brachial plexus trunks, and subclavian vein
Structures in the Occipital Triangle:
- Accessory nerve (CN XI): Crosses the posterior triangle obliquely; runs in the fascial floor from the posterior border of SCM to trapezius → motor to both SCM and trapezius. At risk in lymph node biopsy.
- Cervical plexus branches (cutaneous): Great auricular nerve (C2, C3), Lesser occipital nerve (C2), Transverse cervical nerve (C2, C3), Supraclavicular nerves (C3, C4) — all emerge at "Erb's point" on the posterior border of SCM at the midpoint.
- Brachial plexus roots: C5, C6, C7, C8, T1 emerge between scalenus anterior and medius
Structures in the Subclavian Triangle:
- Subclavian artery (3rd part): Emerges from behind scalenus anterior, arches over first rib — compressed against rib to stop bleeding from upper limb
- Brachial plexus trunks: Upper (C5, C6), Middle (C7), Lower (C8, T1)
- Subclavian vein: Anterior to scalenus anterior (most anterior structure)
Sternocleidomastoid & Wryneck (AN29.2, AN29.4)
Sternocleidomastoid (SCM):
Attachments:
- Origin: Two heads — sternal head (front of manubrium sterni) + clavicular head (medial 1/3 of clavicle)
- Insertion: Mastoid process + superior nuchal line (lateral third)
Actions:
- One side: Tilts head to the same side (lateral flexion) + rotates chin to the OPPOSITE side
- Both sides together: Flex the neck (draw chin towards sternum) or extend the head when neck is fixed
Nerve supply: Accessory nerve (CN XI) — motor; branches from C2-3 — proprioception
Relations: Contains the carotid sheath (IJV, ICA/CCA, vagus nerve) posteromedially. The sternocleidomastoid is a key landmark in the neck — it divides the neck into the anterior and posterior triangles.
Blood supply: Occipital artery (upper), superior thyroid artery (lower)
Torticollis (Wryneck) (AN29.4):
- Definition: Head tilted to one side, chin rotated to opposite side — abnormal tonic contraction or fibrosis of SCM.
- Causes:
1. Congenital muscular torticollis: Fibrosis of SCM after birth injury (forceps delivery, breech) → hard "sternocleidomastoid tumour of infancy" — actually a fibrous mass, not a neoplasm. Treated by physiotherapy; surgery for persistent cases.
2. Ocular torticollis: Head tilt to compensate for strabismus
3. Spasmodic torticollis (cervical dystonia): Involuntary muscle spasm; treated with botulinum toxin
4. Atlanto-axial rotatory subluxation: "Cock robin" deformity after Grisel's syndrome (post-inflammatory)
5. Cervical spine injuries/tumours
Brachial Plexus — Erb's & Klumpke's Palsy (AN29.3)
Brachial Plexus:
Formed by anterior rami of C5, C6, C7, C8, T1.
Structure: Roots → Trunks (3) → Divisions (6) → Cords (3) → Branches (terminal)
| Formation | Components |
|---|---|
| Upper trunk | C5 + C6 |
| Middle trunk | C7 |
| Lower trunk | C8 + T1 |
Erb's Palsy (AN29.3) — "Upper brachial plexus injury":
- Roots injured: C5, C6 (± C7) — at Erb's point (junction of C5-C6 roots, just lateral to SCM)
- Mechanism: Violent stretching of the neck away from the shoulder — RTA (adult), shoulder dystocia/forceps delivery (neonate)
- Deformity: "Waiter's tip" or "policeman accepting bribe" position:
- Shoulder adducted and medially rotated (deltoid + supraspinatus paralysed)
- Elbow extended (biceps paralysed — C5, C6)
- Forearm pronated (supinator paralysed — C6)
- Wrist flexed (extensors at C6-7 weakened)
- Muscles affected: Deltoid, supraspinatus, infraspinatus, biceps brachii, brachialis, brachioradialis, supinator (all C5-C6 dominant muscles)
Klumpke's Palsy — "Lower brachial plexus injury":
- Roots injured: C8, T1 (lower trunk)
- Mechanism: Violent upward traction — baby pulled out by arm, adult hanging from a height/grasping a branch during fall, cervical rib compression
- Deformity: Intrinsic minus hand ("claw hand"):
- Loss of all intrinsic muscles of the hand (supplied by T1 via ulnar nerve + median nerve)
- Finger flexion at MCP (FDP/FDS active) with hyperextension at MCP (lumbricals/interossei lost)
- Wrist extension preserved (C7-C8 extensors partly intact)
- If T1 preganglionic injury: Horner's syndrome (ptosis, miosis, anhidrosis) from sympathetic chain involvement
Cervical Plexus (AN29.5):
- Formed by anterior rami of C1-C4
- Important branches: Phrenic nerve (C3, C4, C5 — "C3, 4, 5 keep the diaphragm alive"), lesser occipital, great auricular, transverse cervical, supraclavicular nerves (all emerge at Erb's point on SCM)
- Scalenus anterior (C3-C8): inserts on scalene tubercle of 1st rib; divides subclavian vessels anteriorly
- Scalenus medius (C3-C8): inserts on upper surface of 1st rib; brachial plexus roots pass between scalenus anterior and medius
Dural Folds, Venous Sinuses & Pituitary (AN30.1-5)
Dural Folds (AN30.3):
The dura mater forms four inward folds that partially divide the cranial cavity:
| Fold | Position | Separates |
|---|---|---|
| Falx cerebri | Midline, vertical, between cerebral hemispheres | Left and right cerebral hemispheres |
| Tentorium cerebelli | Horizontal, tent-like | Cerebral hemispheres (above) from cerebellum (below) |
| Falx cerebelli | Vertical, between cerebellar hemispheres | Left and right cerebellar hemispheres |
| Diaphragma sellae | Horizontal roof over sella turcica | Covers pituitary fossa; has central opening for pituitary stalk |
Dural Venous Sinuses (AN30.3, AN30.4):
Endothelium-lined channels between layers of dura. No valves. Receive cerebral veins and CSF (via arachnoid villi/granulations).
| Sinus | Location | Drains to |
|---|---|---|
| Superior sagittal sinus | Upper border of falx cerebri | Confluence of sinuses (torcular Herophili) → transverse sinus |
| Inferior sagittal sinus | Lower border of falx cerebri | Straight sinus |
| Straight sinus | Junction of falx and tentorium | Confluence → transverse sinus |
| Transverse sinus | Posterior margin of tentorium | Sigmoid sinus |
| Sigmoid sinus | Posterior cranial fossa, S-shaped | Internal jugular vein at jugular foramen |
| Cavernous sinus | Each side of sella turcica | Superior/inferior petrosal sinuses |
Clinical importance (AN30.4):
- Superior sagittal sinus thrombosis: Headache, seizures, raised ICP; associated with dehydration, hypercoagulable states, puerperium
- Cavernous sinus thrombosis: Septic or aseptic; facial sepsis route (valveless facial vein → ophthalmic vein → cavernous sinus)
- Lateral sinus (transverse + sigmoid) thrombosis: Complication of otitis media/mastoiditis → headache, ear pain, papilloedema
- Arachnoid granulations: CSF reabsorbed into superior sagittal sinus; blockage → communicating hydrocephalus
Effect of Pituitary Tumours on Vision (AN30.5):
The pituitary gland lies in the sella turcica (middle cranial fossa), immediately below the optic chiasm.
- A pituitary macroadenoma expands superiorly and compresses the optic chiasm from below.
- The optic chiasm carries the crossing nasal fibres from each retina (which represent the temporal visual fields of each eye).
- Compression of the chiasm → bitemporal hemianopia (loss of both temporal fields = tunnel vision).
- Further lateral expansion: compresses CN III (pupillary dilatation), IV, VI, V1 in the cavernous sinus → diplopia and facial pain.
SELF-CHECK
A neonate's right arm hangs by the side in the "waiter's tip" position — shoulder adducted and internally rotated, elbow extended, forearm pronated. Which nerve roots are injured?
A. C8 and T1 (Klumpke's palsy)
B. C5 and C6 (Erb's palsy)
C. C7 alone
D. Entire brachial plexus (C5–T1)
Reveal Answer
Answer: B. C5 and C6 (Erb's palsy)
Erb's palsy affects C5 and C6 (upper brachial plexus). The classic "waiter's tip" posture results from: deltoid/supraspinatus paralysis (shoulder adducted), biceps paralysis (elbow extended), and supinator paralysis (forearm pronated). Common after shoulder dystocia in neonates.
SELF-CHECK
A patient has a pituitary macroadenoma pressing upward on the optic chiasm. The expected visual field defect is:
A. Right homonymous hemianopia
B. Bitemporal hemianopia (tunnel vision)
C. Left monocular blindness
D. Central scotoma
Reveal Answer
Answer: B. Bitemporal hemianopia (tunnel vision)
The optic chiasm carries crossing nasal fibres (representing the temporal visual fields). Compression from below (pituitary adenoma) interrupts both sets of crossing nasal fibres → loss of both temporal visual fields = bitemporal hemianopia. Patients describe this as "tunnel vision."
SELF-CHECK
During posterior triangle lymph node biopsy, the surgeon inadvertently cuts the accessory nerve (CN XI). The expected deficit is:
A. Inability to turn the head to the opposite side (SCM palsy)
B. Ipsilateral shoulder drop, difficulty abducting arm above 90°, winging of scapula
C. Loss of sensation over the shoulder (C4 dermatomal pattern)
D. Ipsilateral tongue deviation
Reveal Answer
Answer: B. Ipsilateral shoulder drop, difficulty abducting arm above 90°, winging of scapula
CN XI (accessory nerve) supplies both SCM and trapezius. When cut in the posterior triangle (after it has already entered and exited SCM), only the trapezius is denervated. Loss of trapezius → shoulder drop (cannot support shoulder girdle), inability to shrug, and difficulty abducting the arm above 90° (trapezius holds scapula in position for supraspinatus to work). The winging here affects the inferior angle of the scapula (unlike serratus anterior winging which affects the medial border).
CLINICAL PEARL
Thoracic Outlet Syndrome — A Posterior Triangle Compressive Syndrome:
The "thoracic outlet" is the space between the clavicle, first rib, and scalenus anterior muscle. Compression of the brachial plexus and/or subclavian vessels here causes thoracic outlet syndrome (TOS).
Three types:
1. Neurogenic TOS (most common): Lower trunk (C8-T1) compression → ulnar border pain, intrinsic muscle wasting, weakness of grip. May mimic carpal tunnel syndrome or cervical disc disease.
2. Arterial TOS: Subclavian artery compression → Raynaud's, ischaemia of hand, subclavian artery aneurysm with distal embolism.
3. Venous TOS (effort thrombosis): Subclavian vein compression → axillary vein thrombosis (Paget-Schroetter syndrome) after vigorous arm use.
Anatomical causes: Cervical rib (C7), fibromuscular band, anomalous scalenus anterior, callus from clavicle fracture.
Clinical test: Adson's test — neck extension + rotation to affected side while abducting arm → obliterates radial pulse. Roos' test (elevated arm stress test) — arms in "surrender" position + repeated finger clenching → reproduces symptoms.
Treatment: Physiotherapy first; surgical decompression (scalenectomy ± first rib resection) for refractory cases.
REFLECT
Compare and contrast Erb's palsy and Klumpke's palsy. For each: mechanism of injury, roots involved, muscles paralysed, clinical deformity, and associated findings (e.g., Horner's syndrome). Present this as a table from memory.
KEY TAKEAWAYS
Posterior Triangle of Neck — Key Points:
- Boundaries: SCM (anterior), trapezius (posterior), clavicle (inferior). Omohyoid subdivides it into occipital (upper) and subclavian (lower) triangles.
- Key contents: Accessory nerve (CN XI) — motor to SCM and trapezius; cervical plexus (C1-4) branches; brachial plexus roots and trunks; subclavian artery (3rd part).
- SCM: Origin — sternum + clavicle; Insertion — mastoid + superior nuchal line. Action: ipsilateral tilt + contralateral rotation. Nerve: CN XI.
- Torticollis: Head tilt with chin rotation to opposite side; most common cause in neonates = congenital muscular torticollis (SCM fibrosis after birth injury).
- Erb's palsy (C5, C6): Waiter's tip posture — from violent neck-shoulder separation. Common in RTA and shoulder dystocia.
- Klumpke's palsy (C8, T1): Claw hand + Horner's (if preganglionic T1) — from traction on abducted arm.
- Dural folds: Falx cerebri (separates hemispheres), tentorium cerebelli (separates cerebrum from cerebellum), diaphragma sellae (covers pituitary).
- Dural venous sinuses: No valves; superior sagittal, transverse, sigmoid → IJV. Cavernous sinus adjacent to sella.
- Pituitary macroadenoma: Compresses optic chiasm from below → bitemporal hemianopia (crossing nasal fibres lost).