- What is a pilonidal sinus?
Pilonidal means a ‘nest of hairs’.
A narrow tunnel (a small abnormal channel) in the body is what we call a sinus tract. It typically goes between a focus of infection in deeper tissues to the skin surface. This means that the tract may discharge pus from time to time on to the skin.
A sinus tract which commonly contains hairs is called a pilonidal sinus. It occurs under the skin between the buttocks (the natal cleft) a short distance above the back passage (anus). It goes in a vertical direction between the buttocks. Nevertheless, there are some rare cases when a pilonidal sinus occurs in other sites of the body.
- What causes a pilonidal sinus?
There are various theories about what can cause a pilonidal sinus and the exact cause is not clear. For example, one theory is that the problem may develop from a minor abnormality, which you were born with, in the skin between the buttocks. This explains 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.
The development of skin dimples (skin pits) in the skin between the buttocks is another theory. They may develop as a result of local pressure or friction causing damage to the small structures below the skin which are responsible for making hairs (the hair follicles). Growing hair in the natal cleft may get pushed into the skin pits because of local pressure.
No matter what the cause is, once hair fragments become ‘stuck’ in the skin they irritate it and cause inflammation. Inflamed skin quickly becomes infected and so a recurring or persistent infection tends to develop in the affected area. This kind of infection causes the sinus to develop which often contains broken pieces of hair.
(A similar condition can occur between the fingers of hairdressers caused by customers’ hair entering moist, damaged skin.)