Pilonidal sinus has nothing to do with the anorectum, anatomically or embryologically. Pilonidal sinuses or cysts occur in the midline in the upper part of the natal cleft overlying the lower sacrum and coccyx. Because of their proximity to the anus, infected pilonidal cysts (abscesses) are sometimes mistakenly diagnosed as perirectal abscesses. An abscessed pilonidal sinus is always located in the midline (although there may be secondary fistulous openings on either side of the midline) and does not communicate with the anorectum. On the other hand, long, horseshoe-type fistulas emanating from a perirectal abscess may drain close to the location of a pilonidal sinus but not in the midline.
Although once thought to be congenital in nature, pilonidal sinus is now considered an acquired problem. The sinus is formed by the penetration of the skin by ingrowing hair, which causes a foreign body granuloma reaction. The sinus is perpetuated by the presence of the hair and repeated bouts of infection. Although pilonidal sinuses or infected pilonidal cysts occur most commonly before the fourth decade of life, a small portion of patients may develop this problem in their fourth decade. Pilonidal sinus and abscess formation should be considered a chronic and recurring disease.
Carcinoma is a rare complication of chronic, recurring pilonidal sinus disease. It is more frequent in men and is usually a well-differentiated dermal-type squamous cell carcinoma.
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