This is a tiny hole in the lower part of the neck formed by incomplete closure of the second branchial arch when it grows down over the third and fourth branchial arches in the embryo. The sinus is frequently associated with a track running up the neck, often as high as the posterior pillar of the fauces in the pharynx (so forming a branchial fistula).
The condition is normally observed in children, most often in the first year of life. It presents as a small orifice discharging mucous in the anterior border of the sternomastoid, one-third of the way up from the muscle's origin.
Treatment is by surgical excision, dissecting the fistula out as high as possible. The patient or his/her parents should be informed that more than one incision may be necessary despite the single, small hole observable.