A CT was performed which showed a cystic superior mediastinal mass, and hypertrophy of the left upper limb.

Cystic hygroma (lymphatic vascular malformation) is a rare benign lesion, accounting for 1-5% of mediastinal tumors. Most are detected by age two, presenting with pain, dyspnea, infection, hemorrhage or respiratory compromise. Cystic hygroma is more common in the neck and axilla, with only 5% of cases limited to the mediastinum. Lymphatic vascular malformations may be mixed with other forms of vascular malformation, including capillary or venous. There is an association with limb hypertrophy.

Usually well-circumscribed and of fluid density, cystic hygromas may also have an infiltrative appearance, and may be uni- or multilocular. The density can also be variable with a combination of fluid, soft tissue density and fat.

Management may be by surgical excision or by injection with OK-432, a preparation containing Streptococcus pyogenes antigens, which induces an inflammatory response and subsequent obliteration of the abnormal cavities. OK-432 injection was performed in this case, with good initial results.