What is the pathophysiology of pilonidal cyst and sinus?

Updated: May 20, 2019
  • Author: Alex Koyfman, MD; Chief Editor: Gil Z Shlamovitz, MD, FACEP  more...
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It has been postulated that hair penetrates into the subcutaneous tissues through dilated hair follicles, which is thought to occur particularly in late adolescence, though follicles are not found in the walls of cysts. Upon sitting or bending, hair follicles can break and open a pit. Debris may collect in this pit, followed by development of a sinus with a short tract, with a not clearly understood suction mechanism involving local anatomy, eventually leading to further penetration of the hair into the subcutaneous tissue. [5, 3] This sinus tends to extend cephalad, likely owing to mechanical forces involved in sitting or bending. [5] A foreign body-type reaction may then lead to formation of an abscess. If given the opportunity to drain spontaneously, this may act as a portal of further invasion and eventually formation of a foreign body granuloma. Infection may result in abscess formation.

Microscopically, the sinus where the hair enters is lined with stratified squamous epithelium with slight cornification. Additional sinuses are frequent. Cyst cavities are lined with chronic granulation tissue and may contain hair, epithelial debris, and young granulation tissue. Cutaneous appendages are not seen in the wall of cysts, meaning the cysts lack epithelial lining, unlike the sinus. [5] Cellular infiltration consists of PMNs, lymphocytes, and plasma cells in varying proportions. Foreign body giant cells in association with dead hairs are a frequent finding.

In summary, 3 pieces are instrumental in this process: (1) the invader, hair; (2) the force, causing hair penetration; and (3) the vulnerability of the skin. [1, 3, 6] This process has been well characterized by Patey and Scarff as well as a number of other authors from the second half of the 20th century through today.

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