Triangles of the neck
Neck: Fascia, Spaces, Triangles, Cervical Plexus and Accessory Nerve
Neck anatomy for FCPS Part 1: deep cervical fascia and spaces, anterior and posterior triangles, cervical plexus, accessory nerve, cervical joints and neck lumps. Explains spread of neck infection, tracheostomy anatomy, torticollis and carotid compression.
Deep cervical fascia
| Layer | Encloses | Notes |
|---|---|---|
| Investing | Sternocleidomastoid and trapezius; roofs the posterior triangle | Splits round the parotid and submandibular glands; above the sternum it splits into two layers enclosing the suprasternal space with the jugular venous arch |
| Pretracheal | Thyroid, trachea, oesophagus; infrahyoid muscles | Attached to the hyoid and thyroid cartilage, so the thyroid moves on swallowing; continues into the superior mediastinum and fibrous pericardium |
| Prevertebral | Prevertebral muscles, scalenes, deep neck muscles | Floor of the posterior triangle; extends into the axillary sheath |
| Carotid sheath | Common and internal carotid, internal jugular vein, vagus | Ansa cervicalis in its wall; sympathetic trunk behind it |
Spaces and spread of infection
| Space | Route |
|---|---|
| Pretracheal space (behind the pretracheal fascia, in front of the trachea) | Down into the superior (anterior) mediastinum: infection after tracheostomy or thyroid surgery causes fever, chest pain and a widened upper mediastinum |
| Retropharyngeal space (between buccopharyngeal and alar fascia) | To the superior and posterior mediastinum; abscess in a child (or after TB of the cervical nodes or spine) with fever, noisy breathing and dysphagia |
| "Danger space" (between alar and prevertebral fascia) | Down to the diaphragm in the posterior mediastinum |
| Prevertebral space | Along the length of the spine; cervical TB abscess can bulge behind the pharynx or track to the axilla |
- Ludwig angina: bilateral cellulitis of the submandibular and sublingual spaces, usually from a lower (second or third) molar, whose roots lie below the mylohyoid line so infection enters the submandibular space. Woody swelling under the jaw, raised floor of mouth pushing the tongue up, stridor. Early on there is no single drainable abscess.
Tracheostomy layers
- Low midline incision: skin, superficial fascia (anterior jugular veins), investing fascia (the jugular venous arch lies between its two layers in the suprasternal space and is met before the strap muscles), strap muscles separated in the midline, pretracheal fascia, thyroid isthmus (over rings 2–4, divided or retracted), trachea.
Anterior triangle
Bounded by the midline, the lower border of the mandible and the anterior border of sternocleidomastoid.
| Triangle | Contents |
|---|---|
| Submental | Submental nodes |
| Submandibular | Submandibular gland, facial artery and vein, hypoglossal and mylohyoid nerves, nodes |
| Carotid | Carotid bifurcation (upper border of thyroid cartilage, C3/C4), IJV, vagus, hypoglossal nerve, ansa cervicalis |
| Muscular | Infrahyoid muscles, thyroid |
- The common carotid can be compressed against the anterior (carotid) tubercle of C6 (Chassaignac tubercle) to control bleeding; the subclavian is compressed against the first rib.
- The retromandibular vein is formed in the parotid by the superficial temporal and maxillary veins.
- Stellate ganglion block (in front of C7): ipsilateral Horner syndrome with a warm, dry hand from loss of sympathetic tone.
Posterior triangle
- Boundaries: posterior border of sternocleidomastoid, anterior border of trapezius, middle third of the clavicle. Roof: investing fascia; floor: prevertebral fascia over splenius capitis, levator scapulae and scalenes.
- Contents: accessory nerve (XI), cervical plexus cutaneous branches, brachial plexus trunks, third part of the subclavian artery, external jugular vein, occipital and supraclavicular nodes.
- Accessory nerve: crosses the triangle superficially, just under the investing fascia, from about the junction of the upper and middle thirds of SCM to trapezius. Injury during node biopsy or abscess drainage paralyses trapezius: drooping shoulder, weak shrug, difficulty raising the arm above the head (combing hair), with SCM spared.
- Suprapleural membrane (Sibson fascia) covers the lung apex above the clavicle; a defect after injury allows a cervical lung hernia that bulges on coughing.
Cervical plexus (C1–C4)
- Cutaneous branches emerge at Erb's point (midpoint of posterior border of SCM): lesser occipital (C2), great auricular (C2–C3) to the angle of the jaw and parotid skin, transverse cervical (C2–C3), supraclavicular (C3–C4) to the shoulder (referred diaphragmatic pain).
- Motor: ansa cervicalis (C1–C3) to the infrahyoid muscles (thyrohyoid and geniohyoid by C1 travelling with XII); phrenic nerve (C3–C5) on scalenus anterior.
Sternocleidomastoid and torticollis
- Supplied by XI (C2–C3 proprioceptive). One SCM tilts the head to its own side and turns the face to the opposite side.
- Congenital muscular torticollis (often after breech delivery): fibrosis of SCM; a right SCM lesion tilts the head to the right with the chin turned left. Check the hips (association with developmental dysplasia).
Cervical joints and injuries
- Atlanto-occipital joint: nodding (flexion-extension). Atlanto-axial joint: a pivot joint around the dens, giving rotation (looking left and right).
- Whiplash: initial hyperextension strains the anterior longitudinal ligament; rebound flexion follows.
- Judicial hanging: fracture through the pedicles of C2 (hangman's fracture) with cord injury.
- Klippel–Feil syndrome: short neck, low hairline, fused cervical vertebrae.
Neck lumps and congenital tracts
| Midline | Lateral |
|---|---|
| Thyroglossal cyst (moves on protruding the tongue) | Branchial cyst (anterior border of SCM, upper third) |
| Thyroid isthmus nodule (moves on swallowing) | Lymph nodes (TB, lymphoma, metastases; left supraclavicular node from abdominal cancer) |
| Dermoid cyst | Carotid body tumour (moves sideways, not vertically) |
| Plunging ranula | Cystic hygroma (posterior triangle, transilluminates; Turner syndrome) |
- Branchial fistula (second arch/cleft): external opening at the lower anterior border of SCM; the tract passes between the internal and external carotid arteries, over the hypoglossal and glossopharyngeal nerves, to open internally in the tonsillar fossa.
- Eagle syndrome: an elongated styloid process irritates the glossopharyngeal nerve (pain on swallowing and turning the head, often after tonsillectomy).
Remember Investing fascia roofs the posterior triangle; XI runs just under it, so superficial dissection there risks trapezius.
Exam trap Infection behind the pretracheal fascia goes to the superior mediastinum; retropharyngeal infection goes to the posterior mediastinum.
Exam trap The common carotid is compressed against the C6 tubercle, not the first rib.
How FCPS asks about this
- Nerve injured at posterior triangle node biopsy: accessory nerve; shoulder droops, cannot raise arm overhead.
- Why the thyroid moves on swallowing: pretracheal fascia attached to the larynx.
- Widened mediastinum after infected tracheostomy: pus tracked to the superior mediastinum.
- Vein cut above the sternum in tracheostomy: jugular venous arch in the suprasternal space.
- Internal opening of a branchial fistula: tonsillar fossa.
- Ludwig angina source: lower third molar; roots below the mylohyoid line.
- Head tilted right, chin to the left in a breech baby: right sternocleidomastoid torticollis.
Test yourself on this topic
5 of the 88 questions in our bank on this topic. Choose an answer to see why every option is right or wrong.
A 3-month-old girl has a soft, painless, multiloculated swelling in the left posterior triangle of the neck that transilluminates brilliantly. What is the most likely diagnosis?
Answer: Cystic hygroma
A cystic hygroma is a macrocystic lymphatic malformation, most often in the posterior triangle of the neck in infancy. It is made of multiple thin-walled lymph-filled spaces, so it is soft, compressible and brilliantly transilluminant. It arises from failure of developing lymphatic sacs to join the venous system and is associated with Turner syndrome. Large lesions can compress the airway.
Cystic hygroma
- Infant, posterior triangle
- Lymphatic malformation
- Brilliantly transilluminant
- Link: Turner syndrome
Remember Posterior triangle plus brilliant transillumination: hygroma.
Exam trap Infection or bleeding into a hygroma can cause sudden enlargement and airway compromise.
Why the other options are wrong
- Branchial cyst Lies at the anterior border of the SCM, usually in young adults.
- Thyroglossal cyst Midline and moves with tongue protrusion.
- Sternomastoid tumour Firm lump within the SCM; does not transilluminate.
- Dermoid cyst Contains keratin; does not transilluminate brilliantly.
Two weeks after drainage of a posterior triangle abscess, a woman cannot comb her hair with her right hand. She can abduct the arm only to about 90 degrees, and the right shoulder droops. Weakness of which of the following muscles mainly explains this?
Answer: Trapezius
The spinal accessory nerve crosses the posterior triangle superficially on the levator scapulae and is easily cut during node biopsy or abscess drainage. It supplies the trapezius, which, with the serratus anterior, rotates the scapula upward so the arm can be raised above the horizontal. Trapezius paralysis causes a drooping shoulder and inability to abduct beyond about 90 degrees, so tasks like combing the hair become difficult.
Accessory nerve injury
- Crosses posterior triangle superficially
- Supplies trapezius
- Shoulder droops
- No abduction above 90 degrees
Remember Posterior triangle surgery, drooping shoulder: accessory nerve.
Exam trap Sternocleidomastoid is often spared because its branch leaves above the posterior triangle.
Why the other options are wrong
- Deltoid Supplied by the axillary nerve; would weaken abduction from the start.
- Supraspinatus Starts abduction; supplied by the suprascapular nerve.
- Serratus anterior Long thoracic nerve lies deep, behind the brachial plexus, and is rarely hurt by superficial drainage; palsy causes winging rather than shoulder droop.
- Levator scapulae Weakness does not stop overhead abduction or cause droop this way.
A child has had a mucus-discharging pit at the lower anterior border of sternocleidomastoid since birth. Where does the tract open internally?
Answer: Tonsillar fossa
Most branchial fistulae come from the second pharyngeal cleft and pouch. The external opening lies at the anterior border of the lower third of the sternocleidomastoid. The tract runs up through the carotid bifurcation, between the internal and external carotid arteries, above the hypoglossal and glossopharyngeal nerves, to open internally in the tonsillar fossa, the remnant of the second pouch. Third and fourth arch fistulae, which open into the piriform fossa, are much rarer.
Branchial fistula
- Usually second arch
- Outside: lower third of anterior SCM
- Passes between ICA and ECA
- Inside: tonsillar fossa
Remember Second arch fistula ends in the tonsil.
Exam trap The tract runs close to the hypoglossal nerve, which can be injured at excision.
Why the other options are wrong
- Foramen caecum of the tongue Origin of the thyroglossal duct, not a branchial fistula.
- Piriform fossa Internal opening of rare third or fourth pouch sinuses.
- Pharyngeal recess Part of the nasopharynx; not related to branchial fistulae.
- External acoustic meatus Related to first cleft anomalies, which open near the ear.
During thyroidectomy for a very large goitre, the surgeon needs to divide the sternothyroid and sternohyoid to gain access. Why are these strap muscles cut in their upper part rather than near the sternum?
Answer: Ansa cervicalis enters them low down
The infrahyoid strap muscles are supplied by branches of the ansa cervicalis (C1 to C3), which lies on the carotid sheath. These branches enter the sternohyoid, sternothyroid and inferior belly of omohyoid in their lower parts. Dividing the muscles high up, near their upper attachments, leaves most of each muscle still connected to its nerve, so the muscle is not denervated and wasted after repair.
Dividing strap muscles
- Supplied by ansa cervicalis
- Nerves enter the lower part
- Cut high to preserve nerve supply
Remember Cut the straps high; the ansa comes in low.
Exam trap Thyrohyoid is supplied by C1 fibres travelling with the hypoglossal, not by the ansa loop itself.
Why the other options are wrong
- Their blood supply enters from above Blood supply is not the reason for the level of division.
- Upper division avoids the external jugular vein The external jugular vein lies more laterally on the SCM.
- The recurrent laryngeal nerve lies below them The nerve lies deep to the thyroid lobe, not the strap muscles.
- The lower part is attached to the pleura The strap muscles are not attached to the pleura.
After an interscalene block for shoulder surgery, a man is breathless with a raised hemidiaphragm on that side. What is the most likely cause?
Answer: Phrenic nerve on scalenus anterior blocked
The phrenic nerve (C3 to C5) runs downward on the anterior surface of scalenus anterior, deep to the prevertebral fascia, only a short distance from the interscalene groove. Local anaesthetic injected for an interscalene block almost always spreads to it, causing ipsilateral diaphragmatic paralysis, which may be poorly tolerated in patients with lung disease. A pneumothorax would show lung collapse rather than a raised hemidiaphragm.
Interscalene block and phrenic
- Phrenic on scalenus anterior
- Very close to interscalene groove
- Block gives hemidiaphragm palsy
Remember Avoid interscalene blocks in severe lung disease.
Exam trap Horner syndrome after this block is a side effect, not a complication needing treatment.
Why the other options are wrong
- Pneumothorax from pleural puncture Would show a collapsed lung, not a high diaphragm.
- Recurrent laryngeal nerve blocked Causes hoarseness, not diaphragm palsy.
- Spread to the stellate ganglion Causes Horner syndrome, not breathlessness.
- Intravascular injection into the vertebral artery Causes seizures or collapse, not a raised hemidiaphragm.
Where this sits in the CPSP syllabus
- Paper 1: Head, neck and back, Neck: fascia, triangles, cervical plexus and accessory nerve
- Surgery & Allied, Paper 2: Head and neck, Neck: fascia, triangles, cervical plexus and accessory nerve