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Brachial cyst

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I now have comprehensive information from multiple authoritative sources. Here is a complete overview of branchial (cleft) cysts:

Branchial Cleft Cyst

Also called a lateral cervical cyst or lymphoepithelial cyst, a branchial cleft cyst is a congenital developmental anomaly of the neck arising from incomplete obliteration of the branchial apparatus during embryogenesis.

Embryological Origin

During embryonic development, the pharyngeal pouches form "out-pocketing" structures that normally give rise to glandular elements (thymus, parathyroids, etc.). Their connections with the pharyngeal lumen — the pharyngobranchial ducts — should obliterate. Failure of obliteration results in a branchial sinus or cyst.
There are two competing theories:
  1. Cysts are remnants of patent pharyngobranchial ducts
  2. Cysts are remnants of the cervical sinus of His (an ectodermal-lined structure formed by overgrowth of the 2nd arch over arches 3–5)
Pharyngobranchial ducts diagram showing branchial arch development and the cervical sinus of His
Figure: Development of the branchial arches in a 4-week embryo, showing the ectodermal-lined cervical sinus of His — KJ Lee's Essential Otolaryngology

Types by Arch of Origin

The vast majority (~90%) arise from the 2nd branchial arch. The cutaneous opening, when present, is always anterior to the anterior border of the sternocleidomastoid (SCM), deep to the platysma (a 2nd arch derivative).
TypeOriginCourseOpens Into
2nd arch (most common, 90%)Cervical sinus / 2nd pharyngobranchial ductDeep to 2nd arch derivatives, superficial to 3rd arch derivatives; superficial to CN XII, anterior to SCM; between internal and external carotid arteries; deep to stylohyoid ligamentTonsillar fossa
3rd arch3rd pharyngobranchial ductSubplatysmal; superficial to CN XII, deep to internal carotid & CN IX; pierces thyrohyoid membrane above internal branch of superior laryngeal nervePyriform fossa
4th arch4th pharyngobranchial ductRight: loops around subclavian artery; Left: may be intrathoracic, medial to ligamentum arteriosumLower pyriform sinus or larynx

Clinical Features

  • Most commonly seen in young adults (20–40 years), though can present at any age
  • Presents as a smooth, painless, compressible mass along the anterior border of the SCM, frequently at the junction of the superior and middle thirds, just below the angle of the mandible
  • May swell and become tender during upper respiratory tract infections (classic history)
  • Slow-growing; typically 2–5 cm in diameter
  • Usually no sinus tract is visible externally (distinguishing pure cyst from sinus)
  • Infection is a frequent cause of presentation

Pathology

  • Lining epithelium: stratified squamous or pseudostratified ciliated columnar epithelium
  • Cyst wall: fibrous with prominent lymphoid tissue and germinal centers (characteristic feature — also why it is called a lymphoepithelial cyst)
  • Contents: clear/watery or mucinous fluid; may contain desquamated cells and granular debris

Anatomical Relationships (2nd arch cyst — surgical importance)

When a 2nd arch cyst passes superiorly through the bifurcation of the common carotid artery (at the level of vertebrae CIII/CIV), it passes:
  • Between the internal and external carotid arteries
  • Between the stylohyoid ligament and the glossopharyngeal (CN IX) and hypoglossal (CN XII) nerves
  • The vagus (CN X) and spinal accessory (CN XI) nerves lie posterior to the cyst
  • Terminates at the medial border of the pharyngeal constrictor muscles
These cranial nerves are vulnerable to injury during surgical excision due to their anatomic intimacy with the cyst tract.

Diagnosis

  • CT or MRI — to delineate the full extent of the cyst and tract prior to surgery
  • Clinically: characteristic location + age + enlargement with infections

Treatment

Surgical excision of the cyst and its complete associated tract. Incomplete excision leads to recurrence.

Important Distinction: Malignancy

  • Branchial cleft cysts rarely undergo malignant transformation
  • When a cystic neck mass is found in an older patient, especially smoker/drinker, metastatic squamous cell carcinoma (from the oropharynx, tonsil, or base of tongue — especially HPV-related) must be excluded before labeling it a branchial cyst
  • "Most cystic SCCs in the neck are metastases from cancers of the upper aerodigestive tract" — Robbins Pathologic Basis of Disease
Sources: Robbins, Cotran & Kumar Pathologic Basis of Disease | KJ Lee's Essential Otolaryngology | Gray's Anatomy for Students | Dermatology 2-Volume Set 5e
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