True regarding cystic hygroma is -

Cystic hygroma Cystic hygroma is a swelling usually occuring in the lower third of the neck It is most commonly seen in posterior triangle of the neck, but may also occur in axilla, groin & mediastinum It results due to sequestration of a poion of the jugular lymph sac from the lymphatic system. It usually manifests in the neonate or in early infancy (occasionally present at bih) The swelling is soft and paially compressible and invariably increases in size when the child coughs or cries. The characteristic that distinguishes it from all other neck swellings is that it is brilliantly translucent. The cysts are filled with clear lymph and are lined by endothelium. Ref : Bailey and love surgery text book 27th Ed.