FIGURE 120.12 Lateral neck cystic hygroma (lymphangioma) in an infant Cystic hygromas (lymphangiomas) represent malformations of the lymphatic system They consist of dilated lymphatic channels and may be unilocular or multilocular They occur most often in the posterior triangle of the neck ( Fig 120.12 ) but may be found in the axillae, groin, popliteal fossae, or on the chest or abdominal wall ( Fig 120.13 ) When found in the neck, extension of the mass into the anterior triangle, sublingual space, retropharyngeal space, or mediastinum is possible Such infiltration can result in airway compromise and/or compression of vascular and neural structures Most cystic hygromas are present at birth or become apparent shortly thereafter Patients usually present with a slow-growing, painless neck mass that is soft and compressible, although some patients are brought for care because of sudden enlargement caused by secondary infection or hemorrhage within the lesion Anatomic delineation of the mass is best performed