What is a Pilonidal Sinus?
Pilonidal means a ‘nest of hairs’.
A sinus tract is a small abnormal channel (like a narrow tunnel) in the body. A sinus tract typically goes between a focus of infection in deeper tissues to the skin surface. So the tract may discharge pus from time to time onto the skin.
A pilonidal sinus is tract which commonly contains hairs. It occurs under the skin between the buttocks (the natal cleft) a short distance above the anus. The sinus track goes in a vertical direction between the buttocks. Rarely, a pilonidal sinus occurs in other sites of the body.
What causes a Pilonidal Sinus?
The exact cause is not clear. There are various theories. For example, one theory is that the problem may develop from a minor congenital or hereditary abnormality in the skin of the natal cleft. This may explain why the condition tends to run in some families. Part of the abnormality in this part of the skin may be that the hairs grow into the skin rather than outwards.
Another theory is that you develop skin dimples (skin pits) in the skin between the buttocks. These may develop from damaged hair follicles (the small structures under the skin that make the hairs) due to local pressure or friction. Because of local pressure, growing hair in the natal cleft may get pushed into the skin pits.
Whatever the cause, once hair fragments become ‘stuck’ in the skin they irritate the skin and cause inflammation. Inflamed skin quickly becomes infected and so a recurring or persistent infection tends to develop in the affected area. The infection causes the sinus to develop which often contains broken pieces of hair.