A pilonidal sinus is a small hole or "tunnel" in the skin. It usually develops in the cleft of the buttocks where the buttocks separate.

More than one hole may develop, and often these are linked by tunnels under the skin.

Most people associate the word sinus with the nose, but sinuses can occur anywhere in the body. Sinus is simply a medical term for a channel or cavity.

A pilonidal sinus will not usually cause any noticeable symptoms unless it becomes infected. This can cause a pus-filled abscess to develop. Signs that you may have an infection include pain, redness and swelling in the affected area.

See your GP as soon as possible if you think you have a pilonidal sinus. Without treatment, any pain and infection may get worse.

Your GP should be able to diagnose a pilonidal sinus after looking at the affected area of skin. Further testing is not usually required.

Read more about the symptoms of pilonidal sinus.

Why do pilonidal sinuses happen?

The exact cause of a pilonidal sinus is unclear, although it's thought to be caused by loose hair piercing the skin.

Certain things can increase your chances of developing a pilonidal sinus, such as being obese, having a large amount of body hair, and having a job that involves a lot of sitting or driving.

Read more about the causes of pilonidal sinus.

How are pilonidal sinuses treated?

If you have a pilonidal sinus, it's important to keep the area clean and dry. It may also help to remove any hair near the sinus. These steps can reduce the risk of infection.

Showering at the end of the day to remove stray hairs from the cleft between your buttocks may also help.

If a pilonidal sinus becomes infected, it should be treated as soon as possible, as it's likely to get worse.

Treatment usually involves taking antibiotics and having the pus drained from the abscess during a minor operation called incision and drainage.

If the sinus keeps becoming infected, it may have to be surgically removed. Several techniques can be used, including:

• cutting out the sinus and leaving the wound open so it can heal, and packing it with a dressing daily
• closing the wound with a flap of skin and stitches
• scraping out the sinuses and filling them with a special glue

Read more about treating pilonidal sinus.

Who is affected?

Pilonidal sinuses are rare, affecting about 26 in 100,000 people each year.

They affect at least twice as many men as women. The average age for a pilonidal sinus is 21 in men and 19 in women.

They are less common in children and people over the age of 45.