Pilonidal Sinus

A pilonidal sinus (PNS) is a small hole or tunnel in the skin. It may fill with fluid or pus, causing the formation of a cyst or abscess. It occurs in the cleft at the top of the buttocks. A pilonidal cyst usually contains hair, dirt, and debris. It can cause severe pain and can often become infected. If it becomes infected, it may ooze pus and blood and have a foul odor.

The most common cause of an anal fistula is a perianal abscess, which usually forms over an infected anal gland. An abscess is a pocket of pus that develops at the site of an infection.