A two year old boy presented to the emergency department with new-onset fever. The boy's mother mentioned that a mass in the boy's left arm known since birth had grown larger several days previously. Of note, the boy had never had workup for the mass.
The boy was examined by a surgeon, who noted a bulge just above the elbow with focal warmth and fluctuation, and referred him for soft tissue ultrasound of the arm.
On commencement of the ultrasound examination, the arm was noted to be uniformly swollen, without any skin discoloration or a well-defined mass.
The findings are consistent with a cystic hygroma of the arm. The single hypoechoic structure is probably infected, which could explain the acute clinical findings, or hemorrhagic.
Cystic hygroma is a type of congenital lymphangioma. The vast majority of cases occur in the neck or the axilla. Barring the axilla, cystic hygroma of the upper extremity is extremely rare; a Pubmed search for cystic hygroma of the arm/upper extermity yielded a single report 1 and another article which listed 7 cases of cystic hygroma of the upper extremity 2.