Branchial Cleft Cyst

Embryological Origin

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)

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

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)

• 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

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

Embryology of the Pharyngeal (Branchial) Arches

Overview & Timing

Structural Composition of Each Arch

"Each pharyngeal arch is characterized by its own muscular components. The muscular components of each arch have their own cranial nerve, and wherever the muscle cells migrate, they carry their nerve component with them." — Langman's Medical Embryology

Derivatives of Each Pharyngeal Arch

Arch 1 — Mandibular Arch

Arch 2 — Hyoid Arch (Reichert's Cartilage)

Arch 3

Arch 4 & 6 (combined in humans)

Pharyngeal Pouches (Endodermal — internal)

Fig 9.7: Derivatives of the pharyngeal pouches — The Developing Human (Moore)

Clinical pearl: Because parathyroid III descends with the thymus (which migrates far caudally), parathyroid III ends up inferior to parathyroid IV in the adult — hence "inferior parathyroid" comes from the 3rd pouch and "superior parathyroid" from the 4th pouch.

Pharyngeal Clefts / Grooves (Ectodermal — external)

Failure of obliteration of the cervical sinus gives rise to branchial cysts, sinuses, and fistulas.

Pharyngeal Arch Nerves — Summary

"The nerve of the fourth arch becomes the superior laryngeal nerve, which supplies the cricothyroid muscle and constrictor muscles of the pharynx. The nerve of the sixth arch becomes the recurrent laryngeal nerve, which supplies various laryngeal muscles." — The Developing Human

Aortic Arch Arteries

Clinical Significance