Pilonidal Sinus

Pilonidal means (pilus – hair and nidus – nest) a ‘nest of hairs’. A pilonidal sinus is a sinus (tract under the skin) which commonly contains hairs. It most commonly occurs under the skin between the buttocks (the natal cleft) a short distance above the anus. Rarely, a pilonidal sinus occurs in other sites of the body.


There are various theories about the cause. A congenital theory is that the problem develops 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.

The disease is rare in children and in people over the age of 40. It is four times more common in men (as they are hairier than women).

Certain factors increase the risk of developing the condition and include:

  • sedentary occupations where one is sitting for long durations

  • obesity

  • having a hairy, deep natal cleft

  • family history of the condition.


This condition used to be called ‘jeep bottom’ in the world wars as it was common in army jeep drivers. This was probably a result of many hours driving and ‘bouncing’ on a hard seat which caused irritation, minor injury and pressure around the natal cleft.


To know more about Pilonidal sinus, click here to download the pdf.

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