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Pilonidal sinus

A pilonidal sinus is an infected tract under the skin between the buttocks. Treatment commonly involves an operation. After the operation it is advised that the area should be kept the area free of growing hair by regular shaving or other means of hair removal.

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What is a pilonidal sinus?

Diagram detailing the natal cleft

Diagram detailing the natal cleft
  • Pilonidal means a 'nest of hairs'.

  • A sinus tract is a narrow tunnel (a small abnormal channel) in your body. A sinus tract typically goes between a focus of infection in deeper tissues to the skin surface. This means that the tract may discharge pus from time to time on to the skin.

A pilonidal sinus is a sinus tract which commonly contains hairs. It occurs under the skin between the buttocks (the natal cleft) a short distance above the back passage (anus). The sinus tract goes in a vertical direction between the buttocks. Rarely, a pilonidal sinus occurs in other sites of the body.

The pilonidal sinus usually starts as a pilonidal cyst. This is the lump before the sinus develops. However in practice the terms tend to be used interchangeably.

What causes a pilonidal sinus?

The exact cause is not clear. There are various theories including:

Birth abnormality

One theory is that the problem may develop from a minor abnormality in the skin between the buttocks that was present at birth. This may explain why the condition tends to run in some families. Part of the abnormality in this part of the skin may be that the hairs grow into the skin rather than outwards.

Skin dimples

Another theory is that skin dimples (skin pits) may develop in the skin between the buttocks as a result of local pressure or friction causing damage to the small structures below the skin, responsible for making hairs (the hair follicles). Because of local pressure, hair growing in the natal cleft may become pushed into the skin pits.

Whatever the cause, once hair fragments become 'stuck' in the skin they irritate it and cause inflammation. Inflamed skin quickly becomes infected and so a repeated (recurrent) or persistent infection tends to develop in the affected area. The infection causes the sinus (which often contains broken pieces of hair) to develop .

(A similar condition can occur between the fingers of hairdressers which is caused by customers' hairs entering moist, damaged skin.)

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How common is pilonidal sinus?

This condition affects around 26 in 100,000 people each year in the UK. It is rare in children and in people over the age of 40. It is four times more common in men than in women (as men tend to have more body hair than women).

Certain factors increase the risk of developing the condition and include:

  • Having a job involving a lot of sitting (a sedentary occupation).

  • Being overweight or obese.

  • Having a previous persistent irritation or injury to the affected area.

  • Having a hairy, deep natal cleft.

  • Having a family history of the condition.

This condition used to be called 'jeep seat' as it was common in army jeep drivers. This was probably a result of many hours of driving and 'bouncing' on a hard seat, which caused irritation, minor injury and pressure around the natal cleft.

It used to be thought that excessive sweating was one of the causes but recent studies have not shown this.

Pilonidal sinus symptoms

A pilonidal sinus may not cause any symptoms at first. Some people notice a painless lump at first in the affected area when washing. However, in most cases, symptoms develop at some stage and can be 'acute' or 'chronic'.

Rapid-onset (acute) symptoms

Pain and swelling may develop over a number of days as a ball of pus with surrounding skin infection (an infected abscess) develops in and around the sinus. This can become very painful and tender.

Persistent (chronic) symptoms

Around 4 in 10 people have a repeated (recurrent) pilonidal sinus. Some pain may develop which is less intense than the acute symptoms. Usually the sinus discharges some pus which releases the pressure and so the pain tends to ease off and not become severe. However, the infection does not clear completely. This can mean that the symptoms of pain and discharge can last long-term, or flare up from time to time, until the sinus is treated by an operation.

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Is pilonidal sinus dangerous?

Pilonidal sinuses are not dangerous in themselves. Recurrent infections can be painful and require multiple courses of antibiotics. There is some evidence that untreated, recurrently infected pilonidal sinuses can develop into a squamous cell carcinoma of the skin. This is a very rare complication.

Pilonidal sinus treatment

If you have no symptoms

If there are no symptoms then keeping the affected area clear of hairs (by shaving, etc) and keeping the area clean with good personal hygiene may be all that is required.

If you have rapid-onset (acute) symptoms

If there is an infection then antibiotics will usually be prescribed. Painkillers (such as paracetamol and/or ibuprofen) may be very helpful to improve the pain. An emergency operation is sometimes required. This procedure punctures (incises) and drains the ball of pus with the surrounding skin infection (abscess).

If you have persistent (chronic) symptoms

In most cases, an operation will be advised. There are various operations which are done to cure this problem. A surgeon will be able to give the details and the pros and cons of each operation. The options include the following:

Wide excision and healing by secondary intention.

This operation involves cutting out (excision of) the sinus but also cutting out a wide margin of skin which surrounds the sinus. The wound is not closed but just left open to heal by natural healing processes (healing by 'secondary intention'). This usually requires several weeks of regular dressing changes until it heals fully. The advantage of this method is that all inflamed tissue is removed and the chance of the condition coming back (a recurrence) is low.

Excision and primary closure.

This means taking out the section of skin which contains the sinus. This is done by cutting out an oval-shaped (ellipse) flap of skin either side of the sinus, which takes out the sinus. The two sides of the ellipse are then stitched together.

The advantage for this is that, if successful, the wound heals quite quickly. The risk of a recurrence or of developing a wound infection after the operation is higher than with the above procedure. This risk may be reduced by using a wound technique in which the line of stitches is moved away from between the buttocks.

A plastic surgery technique

In some cases, where the sinus recurs or is extensive, plastic surgery may be advised to remove the sinus and refashion the nearby skin.

There are variations on the above procedures, depending on the circumstances, the size and extent of the sinus, and whether it is a first or recurrent problem. A surgeon will be able to discuss the most suitable type of operation.

New techniques are being researched to try to improve the recovery after having an operation.

After your operation

The surgeon will usually advise that the wound should be kept clean and any hair growing near it be shaved or removed by other means. Some surgeons recommend that, even when the wound is healed, the area should be kept free of hair growing by shaving the area every few weeks, or by other methods to remove the hair. This reduces the chance of the problem coming back (recurring).

Further reading and references

Article history

The information on this page is written and peer reviewed by qualified clinicians.

  • Next review due: 8 Aug 2028
  • 10 Aug 2023 | Latest version

    Last updated by

    Dr Pippa Vincent, MRCGP

    Peer reviewed by

    Dr Surangi Mendis
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