Pilonidal sinus (pilonidal cyst): what it is and what hair has to do with it

A pilonidal sinus is a small hole or tunnel in the skin at the top of your bottom, between your buttocks. Some people call it a pilonidal cyst, and NHS Highland's guideline describes pilonidal disease forming cysts, sinuses or a network of tunnels under the skin. Most people don't notice theirs unless it becomes infected.

Men get them more often. NICE's guidance for GPs says pilonidal sinus is 2 to 4 times more common in men than in women, probably because men are hairier, and more common between the ages of 15 and 30. Even the name points to hair, since 'pilonidal' comes from the Latin words for hair (pilo) and nest (nidus).

What are the symptoms of a pilonidal cyst?

A pilonidal cyst usually causes no symptoms until it becomes infected. The NHS says an infection causes pain and swelling at the top of the cleft in your buttocks, and a pus-filled abscess can develop, sometimes suddenly, making it hard to sit down. If infections keep coming back, you may have ongoing discomfort or pain and regularly seeping pus or blood.

A pilonidal abscess, as NICE's guidance describes it, is a painful, tender lump in or near the buttock crease that may leak pus, sometimes with a fever.

What does an early-stage pilonidal sinus look like?

Before any infection, NICE describes one or more small, painless pits in the midline of the buttock crease, sometimes with hairs poking out of them. Some people find a painless lump or swelling by chance, for example while washing. A sinus like this needs no treatment: the NHS recommends a "watch and wait" approach.

The sinus sits at the top of the crease, not at the anus, which sets it apart from piles, an anal fissure or an itchy bottom.

What does hair have to do with a pilonidal sinus?

Hair is the leading explanation, though nobody is completely sure. The NHS says it isn't clear what causes a pilonidal sinus, but a skin problem, pressure or friction may push hair between the buttocks inwards. That may be hair growing around the buttocks, or loose hair shed from the buttocks or elsewhere that gathers in the crease.

NICE's guidance sets out the main theory. Loose hairs in the crease are driven into the skin by the rolling movement of the buttocks, the body reacts to them as a foreign body, and a small pit forms that collects more hair, dead skin and debris. Long-term inflammation turns it into a tunnel under the skin, which may become infected, and the cavity underneath usually contains a nest of hairs.

Pilonidal disease has also been reported on the scalp, in the armpit and groin and between the fingers, NICE notes, and theories that don't involve stray hairs don't adequately explain those rare cases.

NICE's risk factors include:

• being male, and aged 15 to 30
• having a lot of body hair, or coarse hair
• a deep buttock crease
• a higher body mass index (BMI)
• a family history of pilonidal sinus
• poor hygiene
• a job or lifestyle that involves sitting for more than 6 hours a day
• repeated injury to the area at the base of the spine

The NHS also says sitting for long periods can increase your chances.

Does hair removal help with a pilonidal sinus?

Possibly, though the evidence is limited and the sources don't agree on the method. NICE tells GPs to advise everyone with pilonidal sinus disease on long-term measures that may lower the chance of it coming back: careful hygiene with regular baths or showers, and removing buttock hair, for example by laser, waxing or shaving.

• Laser had the lowest recurrence rate in one review, but the evidence is mixed. A 2018 review of 14 studies with 963 people who'd had surgery (Pronk and colleagues) compared how often the sinus came back. It did in 9.3% of those who had laser hair removal, 23.4% of those who shaved or used hair removal cream, and 19.7% of those who removed no hair. The authors say the studies were small and of limited quality, and that a high-quality trial is needed. The German national guideline, which also cites that review, adds a randomised study in which people who had laser hair removal after one type of operation (a Karydakis flap) had a significantly higher risk of the sinus coming back. NHS Highland adds that there isn't yet enough evidence to recommend laser for everyone.
• Shaving comes with warnings. NHS Highland's guideline tells GPs to warn patients that shaving has been associated with the sinus coming back. NICE cites the German national guideline, which warns that shaving after surgery may raise that risk: sharp-ended hairs with no roots have been found inside sinus cavities shortly after a haircut. It suggests permanent hair removal, such as laser, instead of shaving, at least in theory.
• Don't shave a sinus without advice. The NHS says not to shave the affected area unless a GP advises you to.

If you're weighing up methods in general, our guide to pubic hair removal compares trimming, shaving, waxing and laser, and the risks of each. With a pilonidal sinus, ask your GP or surgeon which they'd recommend.

How is a pilonidal sinus treated?

Pilonidal cyst treatment depends on whether it's infected. One that isn't infected doesn't need treating: the NHS recommends watch and wait, keeping the area clean by showering or bathing regularly. For an infected one, treatment depends on your symptoms, the size of the sinus and whether it keeps coming back. A painful, swollen abscess usually needs to be cut and drained, and you may also need antibiotics.

The NHS says you'll usually be offered painkillers, such as paracetamol and non-steroidal anti-inflammatory drugs (NSAIDs), to help with pain and swelling. For a large or repeatedly infected sinus you may need surgery, and pilonidal sinus surgery comes in several forms.

Pilonidal sinus treatments compared

	What happens.	Anaesthetic and hospital stay.	Worth knowing.
Watch and wait	For a sinus with no signs of infection: no treatment, keep the area clean	None	The NHS says not to shave the area unless a GP advises you to
Incision and drainage	A small cut lets the pus out of an abscess	Local or general anaesthetic; usually arranged the same day you see your GP, and you can leave the same day	You may need antibiotics, and you will need daily dressing changes
Wide excision and open healing	The sinus and some surrounding skin are cut out, and the wound heals on its own	Local or general anaesthetic; usually home the same day	Lowest risk of it coming back; regular dressing changes
Excision and wound closure	The sinus is removed and the two sides are stitched together	Local or general anaesthetic; usually home the same day	Quicker recovery; higher risk of infection; stitches out after about 10 days
Endoscopic ablation	A thin tube with a camera guides removal of hair and infected tissue, then heat seals the sinus	Spinal or local anaesthetic; usually home the same day	No cut needed; about a month to heal completely, sometimes less

From the NHS. Your doctor will talk you through which suits you.

For a large area, the NHS says plastic surgery is sometimes used, and less invasive options such as fibrin glue injections may be available in some places. There is no ideal treatment yet. The German national guideline, updated in 2020, says no method meets every aim of being simple, painless, quick to heal and unlikely to come back. NICE says the choice depends on how far the disease has spread, the surgeon's experience and what you want.

What is recovery like after pilonidal sinus surgery?

Most people are back at work within 2 weeks of surgery, the NHS says, depending on the procedure, how quickly you recover and the job you do. While you heal, the NHS suggests you:

• keep the affected area clean
• wear comfortable, loose-fitting cotton underwear
• eat plenty of fibre to make going to the toilet easier and avoid straining
• don't lift heavy objects or do strenuous exercise for the first week or so
• don't ride a bike for 6 to 8 weeks
• don't go swimming until your wound has completely healed

You may have a follow-up appointment with your specialist, usually about 6 weeks later. NICE says the disease often comes back after surgery, which is why its advice on hygiene and hair removal is for the long term.

When should you get help for a pilonidal sinus?

Ask for an urgent GP appointment or contact NHS 111 if you have a small lump at the top of your bottom, between your buttocks, that's painful, bleeding or leaking pus. The NHS says these symptoms can develop quickly, often over a few days, and they're signs of infection that need treating.

NICE lists sepsis among the possible complications. The NHS says to call 999 or go to A&E if an adult:

• is breathing very fast
• is confused, has slurred speech, or is not making sense
• has blue, pale or blotchy skin (on black or brown skin, this may be easier to see on the palms of the hands or soles of the feet)
• has a very high or very low temperature, feels hot or cold to the touch, or is shivery
• has a rash that does not fade when you press it
• has symptoms you're worried might be sepsis

A pilonidal sinus that isn't bothering you doesn't need treatment. NICE's guidance is to watch and wait, and to get medical help if symptoms start.