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A pilonidal cyst is an abnormal pocket in the skin that usually contains hair, skin debris and other abnormal tissue. A pilonidal cyst is almost always located near the tailbone at the top of the cleft of the buttocks. The term "pilonidal" comes from the Latin words for hair ("pilus") and nest ("nidus").

Pilonidal cysts occur more often in men than in women, and they are most common in people between the ages of 15 and 24.

A pilonidal cyst may remain harmless. If it becomes infected, however, it will cause pain, inflammation and usually drainage of fluids.

A pilonidal cyst may cause no noticeable symptoms (asymptomatic). The only sign of its presence may be a small pit on the surface of the skin.

When it's infected, a pilonidal cyst becomes a swollen mass (abscess). Signs and symptoms of an infected pilonidal cyst include: 

• Pain
• Localized swelling
• Reddening of the skin
• Drainage of pus or blood from an opening in the skin (pilonidal sinus)
• Foul smell from draining pus
• Hair protruding from a pilonidal sinus
• Fever (uncommon)

A pilonidal sinus may appear as a tract, a passage below the surface of the skin that connects the infected pilonidal cyst to the opening on the skin's surface. More than one sinus tract may form to drain a single infected cyst.

There is some disagreement about what causes pilonidal cysts. Most pilonidal cysts appear to be caused by loose hairs that penetrate the skin. Friction and pressure — skin rubbing against skin, tight clothing, long periods of sitting, bicycling or similar factors — force the hair down into skin. Responding to the hair as a foreign substance, the body creates a cyst around the hair.

This explanation accounts for rare cases of pilonidal cysts that occur in other parts of the body. For example, barbers, dog groomers and sheep shearers have developed pilonidal cysts in the skin between fingers.

Another possible explanation is that normal stretching or motion of deep layers of skin causes the enlargement and rupture of a hair follicle, the structure from which a hair grows. A cyst then forms around the ruptured follicle.

Some children are born with a small indentation in the skin (sacral dimple) near the top of the cleft of the buttocks. Rarely, a sacral dimple may become infected, essentially forming a pilonidal abscess.

Certain factors can make you more susceptible to developing pilonidal cysts. These include:

• Obesity
• Inactive lifestyle
• Occupation or sports requiring sitting
• Excess body hair
• Stiff or coarse hair
• Poor hygiene
• Excess sweating

If you notice any signs or symptoms of a pilonidal cyst, see your doctor. He or she can diagnose the condition by examining the lesion.

If a chronically infected pilonidal cyst isn't treated properly, there may be an increased risk of developing a type of skin cancer called squamous cell carcinoma.

Treatment for an infected pilonidal cyst may include:

Incision and drainage
The initial treatment for an infected pilonidal cyst, or pilonidal abscess, is usually a procedure performed by your primary care doctor or a surgeon. You're given a local anesthetic to numb the area before your doctor opens the abscess, drains the fluids and removes hair and other debris. The wound is packed with dressing so that it can heal from the inside out. For about 50 to 60 percent of people with an infected pilonidal cyst, this treatment is effective and no further treatment is needed.

Surgery
A surgical procedure is necessary if you have a recurring infected pilonidal cyst or if one or more sinus tracts are present. A surgeon will expose the cysts and all sinus tracts in order to clean out hairs, pus and other debris. The preferred surgical procedures generally fall into two categories:

• Open wound. The surgical wound is left open and packed with dressing to allow it to heal from the inside out. This process results in a longer healing time but usually a lower risk of a recurring pilonidal cyst infection.
• Closed wound. After cleaning out the cyst and sinus tracts, the surgeon stitches the wound closed. The healing time is shorter with these procedures, but there is a greater risk of recurrence. To improve healing time and lower the risk of recurrence, the surgeon may make the incision off to one side and create a flap of skin. Therefore, when he or she stitches the wound, it is not in the cleft of the buttocks, where healing is particularly difficult.

Wound care is extremely important after surgery. Your doctor or nurse will give you detailed instructions on how to change dressings, what to expect of a normal healing process and when to call the doctor. Postoperative care will also include shaving around the surgical site to prevent hairs from entering the wound.

To prevent future pilonidal cysts from developing:

• Remove any hair from this area on a regular basis. Shaving or depilatory creams work well, but you may need laser treatments for hair removal.
• Clean the area daily with glycerin soap, which tends to be less irritating. Rinse the area thoroughly to remove any soapy residue. Washing briskly with a washcloth helps keep the area free of hair accumulation.
• Keep the area clean and dry. Don't use powders, oils or herbal remedies on this area.
• Avoid sitting for long periods of time.