A branchial cleft cyst, also known as a cleft sinus, is a birth defect that appears on the sides of the neck or below the collarbone. It occurs when the embryonic tissues of the collarbone and neck areas do not develop in a normal fashion. There is usually a lesion that is formed that is similar to slits found in the gills in fish; this is where the cyst forms. The branchial cleft cyst can collect drainage fluid from the sinuses that make it prone to infection. Although the cyst forms during the embryonic stage, it may not present itself until early adulthood.

In many cases, a branchial cleft cyst will not present any symptoms. In other cases, it can manifest itself as a small lump, a pit, or a skin tag on either side of the neck or beneath the collarbone area. If a pit is formed, fluid from the sinuses may drain from it. Sometimes the cyst may become tender to the touch. Other times, it can become inflamed, enlarged, or even become abscessed, if the affected person suffers from an upper respiratory tract infection. If the lump is large, symptoms may include difficulty swallowing, difficulty breathing, or hoarseness.

There are two different kinds of branchial cleft cyst lesions: the primary branchial cleft cyst lesion and the secondary branchial cleft cyst lesion. The primary lesion is smooth and not tender to the touch, while the secondary is tender, especially if it is infected. If the cyst becomes infected, it must be treated with antibiotics immediately.

Besides treating the associated infection with a round of antibiotics, one way to cure a person affected with a branchial cleft cyst is surgery. Usually a surgeon will perform a set of horizontal incisions to remove the cyst. Surgery is not recommended for patients under the age of three months; however, there are always medical exceptions. If the person is suffering from a severe infection or an abscess, surgery should not be performed until the infection or abscess is cleared up.

In some cases, surgery is insufficient and other forms of treatment are needed. A few people have opted against surgery for sclerotherapy with a specific sclerosing agent, OK-432. Sclerotherapy is most commonly used to treat varicose veins. For treatment of a branchial cleft cyst, the fluid in the cyst is drained. Then, the cyst is injected with the sclerosing agent. The cyst will disappear completely nearly half of the time.