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Last reviewed dd mmm yyyy. Last edited dd mmm yyyy

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The vast majority of pilonidal sinuses are acquired anomalies. Rarely, a congenital sinus may be associated with a dermoid cyst.

They develop at the onset of puberty. This is when sex hormones start to act on pilosebaceous glands in the natal cleft. A hair follicle becomes distended with keratin and then becomes infected. Folliculitis ensues and an abscess extends down into the subcutaneous fat. Tracks form out of the cavity in the direction of neighbouring hair growth - in over 90% of cases towards the patient's head. Hairs are sucked or drilled into the cavity as a result of friction with the movement of the buttocks. The hairs on the barbs act to prevent expulsion. These hair then provoke a foreign body reaction and infection.

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