17.3 Neck: Cervical Vertebrae & Hyoid, Fascial Layers, Superficial Muscles, Triangles, Root of the Neck, Visceral Layers & Lymphatics

Why this area matters

The TOS lists:

• bones of the neck (cervical vertebrae, hyoid bone);
• fascia of the neck (superficial and deep cervical fascia);
• superficial and lateral muscles (platysma, sternocleidomastoid, trapezius);
• the anterior and posterior triangles;
• deep structures (prevertebral muscles, root of the neck);
• the visceral layers (endocrine: thyroid and parathyroid; respiratory: larynx and trachea; alimentary: pharynx and esophagus);
• lymphatics of the neck.

Thyroid disease, neck masses, deep neck infections and airway emergencies are common in Philippine practice.

Bones and joints of the neck

• Cervical vertebrae:
  • transverse foramina carry the vertebral arteries from C6 upward;
  • C1 (atlas) has no body and C2 (axis) has the dens;
  • C7 has the prominent spinous process.
  • Hangman's fracture is a fracture through the pedicles of C2 in hyperextension. Jefferson fracture is a burst fracture of C1.
• Hyoid bone: a U-shaped bone at C3 that anchors the tongue and the supra- and infrahyoid muscles. It does not articulate with any other bone. A fractured hyoid suggests manual strangulation.

Fascial layers and spaces

• Superficial cervical fascia contains the platysma (CN VII) and the external jugular vein.
• Deep cervical fascia:
  • The investing layer surrounds the neck and splits to enclose the sternocleidomastoid, trapezius, parotid and submandibular glands.
  • The pretracheal layer encloses the thyroid, trachea, esophagus and the infrahyoid muscles. It attaches the thyroid to the larynx, so the thyroid moves with swallowing.
  • The prevertebral layer covers the vertebral column and deep muscles, and its extension forms the axillary sheath.
  • The carotid sheath is described below.
• Spaces:
  • The retropharyngeal space lies between the pretracheal (buccopharyngeal) and prevertebral layers. Retropharyngeal abscesses in children cause neck stiffness and dysphagia.
  • The "danger space" lies within the prevertebral fascia and runs from the skull base to the diaphragm, so infection there can reach the mediastinum.
  • The submandibular space is the site of Ludwig angina.

Superficial and lateral muscles

• Sternocleidomastoid (CN XI): turns the face to the opposite side and flexes the neck. Congenital muscular torticollis follows birth injury or fibrosis of the muscle, and the head tilts toward the affected side.
• Trapezius (CN XI): elevates and rotates the scapula. Injury to CN XI in the posterior triangle causes shoulder droop.
• Platysma (CN VII): tenses the neck skin.

Cervical Triangles & The Carotid Sheath

The neck is anatomically bifurcated by the sternocleidomastoid (SCM) muscle into anterior and posterior triangles:

The Anterior Triangle

Bounded by the anterior border of the SCM, the inferior border of the mandible, and the anterior midline of the neck. Subdivided by the digastric and omohyoid muscles into:

1. Carotid Triangle: Bounded by the superior belly of the omohyoid, the posterior belly of the digastric, and the anterior border of the SCM. Contains the bifurcation of the common carotid artery, internal and external carotid arteries, internal jugular vein, CN X (vagus), CN XII (hypoglossal), and the ansa cervicalis.
2. Submandibular (Digastric) Triangle: Bounded by the inferior border of the mandible and the anterior and posterior bellies of the digastric. Contains the submandibular gland, facial artery and vein, marginal mandibular branch of CN VII, lingual nerve, and hypoglossal nerve.
3. Submental Triangle: Bounded by the anterior bellies of the digastric and the hyoid bone. Contains submental lymph nodes.
4. Muscular (Omotracheal) Triangle: Bounded by the superior belly of the omohyoid, the anterior border of the SCM, and the anterior midline. Contains the infrahyoid strap muscles (sternohyoid, sternothyroid, thyrohyoid, omohyoid), thyroid gland, parathyroid glands, trachea, and larynx.

The Posterior Triangle

Bounded by the posterior border of the SCM, the anterior border of the trapezius, and the middle third of the clavicle. The floor is formed by the splenius capitis, levator scapulae, and scalene muscles. Subdivided by the inferior belly of the omohyoid into the large occipital triangle and the small supraclavicular (omoclavicular) triangle.

• High-Yield Neuroanatomy: The Spinal Accessory Nerve (CN XI) emerges from behind the middle of the posterior border of the SCM (Erb's point) and runs superficially across the floor of the posterior triangle to enter the anterior surface of the trapezius. Its subcutaneous trajectory renders CN XI exceptionally vulnerable to iatrogenic transection during cervical lymph node biopsies or radical neck dissections, resulting in trapezius atrophy, shoulder droop, and inability to abduct the arm past 90 degrees.

The Carotid Sheath

A dense tubular fascial column derived from all three layers of deep cervical fascia (investing, pretracheal, and prevertebral layers). Contents:

• Common Carotid Artery and Internal Carotid Artery: Located medially.
• Internal Jugular Vein (IJV): Located laterally.
• Vagus Nerve (CN X): Located posteriorly in the groove between the artery and vein.
• Ansa Cervicalis: Embedded within or directly on the anterior wall of the sheath; derived from $C_1\text{--}C_3$ spinal roots, innervates the infrahyoid strap muscles (omohyoid, sternohyoid, sternothyroid).
• Critical distinction: The cervical sympathetic trunk does not travel inside the carotid sheath; it lies directly posterior to the carotid sheath, embedded within the prevertebral fascia.

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Thyroid Surgical Anatomy & Laryngeal Innervation

Thyroidectomy is among the most common cervical operative procedures. Meticulous identification of the laryngeal nerves is mandatory to prevent devastating postoperative airway and vocal complications:

1. Recurrent Laryngeal Nerve (RLN):

   • Branch of the vagus nerve (CN X). Exhibits asymmetrical anatomical pathways:
     • Right RLN: Loops around the right subclavian artery at the root of the neck before ascending in the tracheoesophageal groove.
     • Left RLN: Loops under the aortic arch, immediately distal to the ligamentum arteriosum, before ascending through the superior mediastinum and tracheoesophageal groove.
   • In the neck, both RLNs ascend in intimate contact with the branches of the inferior thyroid artery and pass through the suspensory ligament of Berry before entering the larynx.
   • Motor function: Innervates all intrinsic laryngeal muscles EXCEPT the cricothyroid.
   • Sensory function: Infraglottic laryngeal mucosa below the true vocal folds.
   • Unilateral injury: Vocal cord is paralyzed in the paramedian position; manifests as hoarseness, a breathy voice, and mild vocal fatigue.
   • Bilateral injury: Both vocal cords assume the paramedian position, causing severe inspiratory stridor, acute airway obstruction, and life-threatening asphyxiation requiring immediate emergency intubation or tracheostomy.

2. Superior Laryngeal Nerve (SLN):

   • Originates high from the inferior ganglion of CN X and bifurcates into two branches:
   • External Branch of SLN: Accompanies the superior thyroid artery as it approaches the superior pole of the thyroid gland. Innervates the cricothyroid muscle, which tenses and elongates the vocal cords to generate high-pitch vocal tones. Iatrogenic injury during ligation of the superior thyroid vessels produces vocal fatigue, loss of high-register singing voice, and a monotone voice.
   • Internal Branch of SLN: Pierces the thyrohyoid membrane in company with the superior laryngeal artery. Provides pure sensory innervation to the supraglottic laryngeal mucosa down to the vocal cords. Injury abolishes the cough reflex, placing the patient at high risk for silent aspiration.

Root of the neck

• The anterior scalene is the key landmark:
  • the phrenic nerve descends on its front;
  • the subclavian vein passes in front of it;
  • the subclavian artery and the brachial plexus pass between the anterior and middle scalenes (the interscalene triangle). A cervical rib or tight scalenes here cause thoracic outlet syndrome.
• Subclavian artery branches: the vertebral artery, the internal thoracic artery, the thyrocervical trunk (including the inferior thyroid artery), the costocervical trunk and the dorsal scapular artery.
• The thoracic duct arches over to enter the junction of the left subclavian and internal jugular veins. Injury causes a chyle leak.
• The cervical pleura rises above the clavicle, so the lung can be injured by procedures or wounds here.
• The stellate ganglion (inferior cervical and first thoracic ganglia) lies in front of the neck of the first rib. Damage causes Horner syndrome.

Visceral layers

• Endocrine layer:
  • The thyroid gland has two lobes and an isthmus over tracheal rings 2 to 4. Its blood comes from the superior thyroid artery (external carotid; with the external laryngeal nerve) and the inferior thyroid artery (thyrocervical trunk; with the recurrent laryngeal nerve). It drains through the superior, middle and inferior thyroid veins.
  • Thyroglossal duct cysts are midline and move upward when the tongue is protruded, because the duct descended from the foramen cecum.
  • The parathyroids: the superior pair comes from the fourth pharyngeal pouch, and the inferior pair comes from the third pouch with the thymus, so the inferior glands may lie low, even in the mediastinum. Accidental removal during thyroidectomy causes hypocalcemic tetany.
• Respiratory layer:
  • Larynx: the thyroid, cricoid (the only complete ring) and epiglottic cartilages, plus the paired arytenoids.
  • The vocal folds are the narrowest part of the adult airway. In infants, the subglottic region at the cricoid is narrowest, which explains croup.
  • Cricothyroidotomy goes through the cricothyroid membrane in an emergency. Tracheostomy is usually done below the isthmus.
  • The posterior cricoarytenoid is the only abductor of the vocal folds.
• Alimentary layer:
  • The pharynx has nasopharynx, oropharynx and laryngopharynx parts, and three constrictor muscles (CN X).
  • Zenker diverticulum pushes out through a weak area of the inferior constrictor (Killian dehiscence).
  • The esophagus begins at C6, at the level of the cricoid, which is its narrowest point and a site where foreign bodies lodge.

Lymphatics of the neck

• Levels I to VI guide neck dissection:
  • I: submental and submandibular;
  • II, III and IV: upper, middle and lower deep cervical (jugular chain);
  • V: posterior triangle;
  • VI: central compartment, around the thyroid and trachea.
• All head and neck lymph ultimately drains through the deep cervical chain to the jugular trunk.
• A hard left supraclavicular node (Virchow node, the signal node) suggests abdominal cancer, especially gastric, draining through the thoracic duct.
• Tuberculous lymphadenitis (scrofula) commonly affects the cervical nodes in the Philippines. Nasopharyngeal carcinoma often presents with a level II or V neck mass.