A pilonidal cyst is a skin condition involving a pocket that forms in the skin, containing hair and skin debris. This condition develops near the tailbone, at the top of the cleft of the buttocks. It is common in young adult men but can affect anyone, particularly individuals who sit for prolonged periods. While called a “cyst,” it is technically a sinus tract—a cavity below the skin’s surface. This condition can become a painful, infected abscess.
Anatomical Location and External Appearance
Pilonidal disease occurs in the natal cleft, the formal term for the crease between the buttocks. The initial external sign is a small dimple or pit on the surface of the skin in the midline of this crease.
These small openings are the external entrances to sinus tracts, which are channels that lead into the subcutaneous tissue. There may be a single pit or a series of openings. If the cyst becomes infected and forms an abscess, the skin over the area will become red, swollen, and tender. A painful lump may form, and the openings may drain clear, cloudy, or bloody fluid, sometimes with a foul odor.
Internal Structure of a Pilonidal Cyst
Beneath the skin’s surface, a pilonidal cyst is more complex than its external appearance suggests. The visible pit on the skin is the opening of a sinus tract, a tube that extends downward into the deeper tissue. This tract connects the outside environment to a subcutaneous cavity. This cavity is not a true cyst, as it is lined with granulation tissue, which is part of the body’s inflammatory response.
The cavity itself is often filled with a collection of debris, with embedded hair being the most characteristic component. The term “pilonidal” translates to “nest of hair,” which accurately describes the contents. Along with hair, the cavity can contain skin debris and other foreign material. The body recognizes this embedded hair as a foreign substance, triggering an inflammatory reaction that leads to the formation of the cavity and surrounding granulation tissue.
When the contents of this cavity become infected by bacteria, it evolves into a pilonidal abscess. This infection causes the cavity to fill with pus, a thick fluid made of dead white blood cells, tissue debris, and bacteria. The pressure from the accumulating pus causes the swelling and pain associated with an acute infection. The pressure can cause the abscess to spontaneously drain through the sinus opening or create new openings.
Visualizing the Formation Process
The development of a pilonidal cyst is an acquired condition, not something a person is born with. The process is initiated by loose hairs that fall and gather in the natal cleft. Friction and pressure, created by sitting, walking, or tight clothing, cause these hairs to puncture and burrow into the skin. The anatomy of the buttock cleft creates a suction-like effect during movement, which can draw these loose hairs into the skin.
Once a hair has penetrated the skin, the body perceives it as a foreign object. This triggers an immune and inflammatory response at the site of entry. The body attempts to wall off the foreign hair, leading to the formation of a pit at the skin’s surface and a subsequent tract around the hair shaft. This process establishes the initial pilonidal sinus.
Over time, more hairs and skin debris can become trapped in this initial opening, being drawn deeper into the subcutaneous tissue. The body’s continued reaction to this accumulating foreign material results in the formation of the larger subcutaneous cavity or cyst. If bacteria are introduced into this environment, an infection can take hold, leading to the formation of a painful abscess filled with pus and escalating the condition from a chronic sinus into an acute infection.