A 3-year-old boy from Mumbai presents with a painless swelling in the anterior triangle of the neck, just below the angle of the mandible. On examination, the swelling is soft, compressible, and transilluminates. The child has a history of recurrent upper respiratory tract infections. Imaging confirms a cystic lesion. Which embryological structure failed to obliterate, resulting in this presentation?

Explanation

## Diagnosis: Cystic Hygroma (Lymphangioma) from Second Pharyngeal Pouch Remnant ### Clinical Presentation Analysis The clinical triad of **anterior neck swelling + transillumination + recurrent infections** is pathognomonic for a cystic hygroma arising from remnants of the second pharyngeal pouch. The second pouch gives rise to the **palatine tonsil fossa**, and incomplete obliteration of the second pharyngeal cleft (external) leaves behind the **cervical lymph sac**, which can accumulate lymphatic fluid. ### Embryological Basis **Key Point:** The second pharyngeal pouch endoderm forms the **epithelial lining of the palatine tonsil fossa**. Failure of the second pharyngeal cleft to obliterate completely leaves behind the **cervical lymph sac** (derived from jugular lymph sacs), which becomes a cystic hygroma. **High-Yield:** Cystic hygroma classically occurs in the **posterior triangle of the neck** (75% of cases) or anterior triangle (as in this case), and transillumination is a key diagnostic sign due to the fluid-filled nature of the lesion. ### Differential Embryological Origins | Pharyngeal Pouch | Derivative | Clinical Remnant If Incomplete Obliteration | |---|---|---| | **1st** | Tubotympanic recess → middle ear, Eustachian tube | Branchial cyst (anterior to sternocleidomastoid) | | **2nd** | Palatine tonsil fossa, stylohyoid ligament | **Cystic hygroma** (lymph sac remnant) | | **3rd** | Inferior parathyroid, thymus | Thymic cyst (lower neck/mediastinum) | | **4th** | Superior parathyroid, ultimobranchial body | Ultimobranchial cyst (rare, lateral neck) | **Clinical Pearl:** The second pharyngeal cleft (ectodermal) normally obliterates by fusion of the hillocks of His. Failure of this process → **branchial cleft cyst**. However, the lymphatic component (cystic hygroma) arises from the jugular lymph sac, which is intimately associated with second pouch derivatives. ### Why Transillumination Occurs The cyst is filled with **clear lymphatic fluid**, allowing light to pass through — this is the hallmark distinguishing feature from solid masses or branchial cysts (which may be mucoid but less reliably transilluminate). **Mnemonic:** **CYSTIC HYGROMA = Lymph Sac + Second Pouch Failure** - **C**lear fluid (transilluminates) - **Y**oung children (present early) - **S**oft, compressible mass - **T**ransilluminates brilliantly - **I**nfections (recurrent, if infected → abscess) - **C**ervical location (anterior or posterior triangle) [cite:Langman's Embryology 14e Ch 10]