Ch13: Neck II — Anterior Region (Thyroid, Carotid Sheath)（颈前区）

# Ch13: Neck II — Anterior Region (Triangles, Thyroid, Carotid Sheath)

Preparatory Mindset

The anterior region of the neck is dominated by the thyroid gland (its blood supply & the nerves in surgical relation — the lecture's key point) and the carotid sheath (its contents). The submandibular triangle and the structures around scalenus anterior are also flagged as must-know.

Exam mindset: The lecture's key/difficult points: (1) contents of the submandibular triangle, (2) the arteries of the thyroid gland & nerves in relation to it, (3) contents of the carotid sheath, (4) structures around the scalenus anterior. Thyroid surgery anatomy (recurrent laryngeal & superior laryngeal nerves) is an LMCHK surgical favorite.

Core Concepts

1. Anterior triangle — subdivisions (recap with detail)

Bounded by the midline, inferior border of the mandible, and anterior border of SCM:

• Submental triangle: between the anterior bellies of digastric & the hyoid bone — submental lymph nodes, small veins → anterior jugular vein
• Submandibular (digastric) triangle: between the mandible & both bellies of digastric — submandibular gland, submandibular lymph nodes, hypoglossal nerve (CN XII), mylohyoid nerve, facial artery & vein, lingual artery
• Carotid triangle: bounded by SCM, posterior belly of digastric, superior belly of omohyoid — carotid sheath contents (CCA/ICA, IJV, vagus), ECA & branches (superior thyroid, lingual, facial), hypoglossal n., ansa cervicalis, thyroid, larynx, pharynx
• Muscular triangle: between midline, omohyoid, SCM — infrahyoid (strap) muscles, thyroid & parathyroid glands

2. Carotid sheath (concept 98 — MUST KNOW)

1. Common carotid artery (medially) → divides into internal & external carotids at the level of the upper border of the thyroid cartilage (C4) 2. Internal jugular vein (laterally — anterolateral to the artery) 3. Vagus nerve (CN X) (posterior, between artery & vein) 4. (Deep cervical lymph nodes along it)

• Formed by contributions from all three cervical fascial layers
• Contents:
• Carotid sinus & body (page 9 of lecture): carotid sinus = pressoreceptor (baroreceptor) — at the origin of the ICA; carotid body = chemoreceptor — at the bifurcation (sensitive to O2/CO2/pH)
• Clinical: carotid sinus hypersensitivity → syncope (carotid sinus massage); carotid body tumor (chemodectoma) at the bifurcation; carotid endarterectomy in the carotid triangle

3. External carotid artery & its branches

• Arises from the CCA at the upper border of thyroid cartilage; lies medial to the ICA (then becomes superficial)
• Branches (mnemonic "SALFOPMS" / "Some Anatomists Like Freaking Out Poor Medical Students"): Superior thyroid, Ascending pharyngeal, Lingual, Facial, Occipital, Posterior auricular, Maxillary, Superficial temporal (terminal)
• First branch = superior thyroid artery (with the external laryngeal nerve) — surgical landmark
• Ligation is done between the superior thyroid & lingual branches

4. Thyroid gland (MUST KNOW — concepts 94 + lecture 13)

- Superior thyroid artery (from ECA) — accompanies the external laryngeal nerve (branch of superior laryngeal n.) → relation: ligate the superior thyroid artery close to the gland to avoid the external laryngeal nerve (superior laryngeal nerve injury → voice weakness) - Inferior thyroid artery (from thyrocervical trunk) — loops behind the carotid sheath; the recurrent laryngeal nerve crosses it (usually posterior to the artery or between its branches) → relation: ligate the inferior thyroid artery away from the gland to avoid the recurrent laryngeal nerve - Thyroid ima artery (from brachiocephalic, inconstant) — ascends in front of the trachea

- External laryngeal nerve (from superior laryngeal n., CN X): runs with the superior thyroid artery to the cricothyroid muscle — injury → hoarse, weak voice (can't tense vocal cords) - Recurrent laryngeal nerve (RLN, CN X): on the LEFT loops under the aortic arch; on the RIGHT loops under the right subclavian artery; ascends in the tracheo-esophageal groove; enters the larynx behind the cricothyroid joint; innervates all intrinsic laryngeal muscles except cricothyroid (which is external laryngeal) - RLN injury: unilateral → hoarseness, vocal cord paralysis (paramedian); bilateral → respiratory obstruction (stridor) — surgical emergency

• Location: front of the neck, below the larynx; butterfly-shaped, two lobes (right & left, dorsolateral to the trachea) connected by the isthmus at tracheal rings 2-3
• Weight/size: one of the largest endocrine glands
• Capsule: true capsule + false (pretracheal) capsule; the recurrent laryngeal nerve & parathyroid glands lie between the two (behind the false capsule) — surgical importance
• Blood supply:
• Nerves in relation to the gland:
• Parathyroid glands: usually 4, on the posterior surface of the thyroid (behind the false capsule); supplied by the inferior thyroid artery; injury during thyroidectomy → hypocalcemia (tetany) — the clinical importance the summary lecture stresses

5. Structures around the scalenus anterior (difficult point — MUST KNOW)

• Anterior (in front): phrenic nerve (C3-C5, descends on the muscle from lateral to medial), subclavian vein (crosses in front of the muscle... actually the vein lies in front of the first rib & muscle)
• Posterior (behind): subclavian artery (2nd part) & brachial plexus (roots/trunks) — through the interscalene fissure (scalene gap) between scalenus anterior & medius
• Medial: vertebral artery (behind the carotid sheath, enters the foramen transversarium of C6)
• Clinical: scalenus anterior syndrome; thoracic outlet syndrome (cervical rib) compressing the subclavian artery/lower trunk

6. Phrenic nerve (C3-C5)

• Formed mainly from C4, with contributions from C3 & C5 ("3-4-5 keeps the diaphragm alive")
• Descends on the anterior surface of scalenus anterior (from lateral to medial), passes between the subclavian artery (behind) & subclavian vein (in front), enters the thorax anterior to the lung root → innervates the diaphragm
• Clinical: phrenic nerve injury (neck surgery, scalene block) → paralyzed hemidiaphragm (elevated on CXR, paradoxical movement on sniff test)

7. Subclavian artery branches (recap "VIT C")

Vertebral, Internal thoracic, Thyrocervical trunk (inferior thyroid, suprascapular, transverse cervical), Costocervical trunk

High-Yield Points

- Superior thyroid artery + external laryngeal nerve (ligate close to gland → protect external laryngeal → cricothyroid → voice) - Inferior thyroid artery + recurrent laryngeal nerve (ligate away from gland → protect RLN)

• Carotid bifurcation: at the upper border of the thyroid cartilage (C4); ICA has no branches in the neck, ECA has 8
• ECA branches mnemonic: "SALFOPMS" — Superior thyroid, Ascending pharyngeal, Lingual, Facial, Occipital, Posterior auricular, Maxillary, Superficial temporal
• Thyroid surgery nerves:
• RLN: left loops under aorta; right under subclavian; in the tracheo-esophageal groove; supplies all intrinsics except cricothyroid; bilateral injury → airway obstruction
• Parathyroids: 4 glands, behind the thyroid, inferior thyroid artery supply; removal → hypocalcemia/tetany
• Carotid sheath: "CCA/ICA + IJV + vagus" (IJV is the most lateral/large; vagus posterior)
• Carotid sinus = baroreceptor; carotid body = chemoreceptor
• Phrenic: C3-C5 on scalenus anterior → diaphragm
• RLN vs external laryngeal (mnemonic): "External goes with the Superior; Recurrent goes with the Inferior" — ELS with STA, RLS with ITA

Topic Summary

The anterior neck's key content = the carotid sheath (CCA/ICA, IJV, vagus; sinus = baroreceptor, body = chemoreceptor), the ECA branches (SALFOPMS), the thyroid gland (isthmus at rings 2-3; superior thyroid a. + external laryngeal n. vs inferior thyroid a. + recurrent laryngeal n. — the critical surgical relations; 4 parathyroids behind), and the structures around scalenus anterior (phrenic nerve in front, subclavian artery & brachial plexus behind through the scalene fissure). Thyroid surgery requires protecting the external laryngeal & recurrent laryngeal nerves and the parathyroid glands — the single most tested surgical-anatomy theme of the head & neck.