An infant is brought to the clinic by the parents with the history of a swelling in the neck since birth. On examination, the swelling is found to be soft, partially compressible and brilliantly trans-illuminant. The most likely diagnosis is
cystic hygroma
The correct answer is cystic hygroma, a macrocystic lymphatic malformation caused by failure of the jugular lymph sacs to connect with the venous system. About 65-75 percent arise in the posterior triangle of the neck and roughly half to two-thirds are evident at birth. Because the loculi contain clear lymph, the swelling is soft, fluctuant, partially compressible and brilliantly trans-illuminant, the key bedside sign. It may swell on crying and large lesions threaten the airway; associations include Turner and Noonan syndromes. Treatment is sclerotherapy with OK-432 (picibanil) or bleomycin, or excision.
A sternomastoid tumour is a firm, fusiform, non-transilluminant swelling in the sternocleidomastoid appearing at 2-3 weeks with torticollis. A branchial cyst lies at the anterior border of the upper sternocleidomastoid, contains turbid cholesterol-rich fluid so does not transilluminate, and presents in young adults. A thyroglossal cyst is midline and moves on deglutition and tongue protrusion.
Key point: A brilliantly trans-illuminant, compressible posterior triangle swelling present since birth is a cystic hygroma until proved otherwise.