The human body reflects centuries of genetic variation and evolutionary history. Some unusual traits, such as heterochromia or extra fingers, are immediately visible, while others are so subtle that people may never notice them.

One such feature is the preauricular sinus (P.A.S.), a tiny hole, pit, or dimple located near where the face meets the upper ear cartilage. It may look like a faint mole, scar, or old piercing mark, but it is actually a congenital anomaly linked to early embryonic development of the ear.

Medically, a preauricular sinus is defined as a small congenital fistula—an epithelial-lined tract beneath the skin. Although called a fistula, it usually ends blindly rather than connecting to another structure. The visible opening is known as a pit or dimple, while the sinus refers to the hidden tunnel that may extend several millimeters or even centimeters through nearby tissue. The depth and shape of this tract vary widely and influence the likelihood of infection.

The condition originates during early fetal development, typically between the fifth and seventh weeks of pregnancy. At this stage, the external ear forms from six small tissue swellings around the first branchial cleft, known as the Hillocks of His. These structures must fuse and migrate precisely to create the final ear shape. If the hillocks do not merge completely, a tiny cleft may remain, forming a narrow tunnel beneath the skin. In this sense, the preauricular sinus is essentially a small developmental remnant of an incomplete fusion process.

Globally, the trait is relatively uncommon but varies greatly among populations. In the United States and Europe it occurs in less than 1% of people, while in some Asian populations the rate can reach 4–10%, and around 4% in parts of Africa. It usually appears on only one ear, though 25–50% of individuals have it on both sides. The condition also has a strong hereditary component and is often inherited as an autosomal dominant trait with incomplete penetrance, meaning a person may carry the gene without showing the visible pit.

In most cases, the preauricular sinus causes no symptoms and requires no treatment. However, because the tract is lined with skin cells that produce oils and shed debris, the opening can occasionally become blocked. When this happens, bacteria may grow inside the pocket, leading to redness, swelling, pain, or abscess formation. Chronic buildup can also produce a cyst, and incomplete treatment of infections may cause recurring problems.

Medical management focuses on treating infections and preventing recurrence. Acute infections are typically handled with antibiotics and sometimes incision and drainage if pus accumulates. For individuals who experience repeated infections, surgical removal of the entire sinus tract may be recommended after inflammation subsides. Surgeons aim to excise the tract completely to prevent recurrence while avoiding nearby structures such as facial nerve branches, though the procedure generally carries low risk when performed carefully.