Pilonidal Sinus

The exact cause of pilonidal sinuses is unclear. It is generally thought they are caused by loose hairs pushing into the skin.

They could also be caused by deep layers of skin being stretched and moved, leading to a hair follicle rupturing.

Hair follicles

Hair follicles are the small holes that an individual hair grows out of. Pressure and friction on a hair follicle could somehow damage it, causing a pilonidal sinus.

Ifahair follicle becomes blocked, itcan become enlarged and then burst. Abroken hair maypush into the skin, leading to an infection.

This may explain why pilonidal sinuses are common around the buttocks, assitting and driving will cause pressure and friction in this area.

There have been reports of hairdressers developing pilonidal sinuses on their hands. This may be caused by hair becoming trapped in moist, damaged skin betweena hairdresser's fingers.

Infection of the pilonidal sinus

Ifa broken hair pushes into the skin,the skin becomesirritated, red and swollen. Bacteria can quickly infectthis skin. The cleft between the buttocks is an ideal place for bacteria tospread, asit is often moist and warm.

When theskin is infected, the immune system (the body's natural defence against infection and illness)tries tofight the bacteria, which resultsin a collection of pus known asan Abscess .

Increased risk

Some things are known to increase your risk of pilonidal sinuses, including:

• obesity having a body mass index (BMI) of 30 or above
• age pilonidal sinuses can occur at any age, but are more common in young adults between the ages of15 and 40
• having an above-average amount of body hair, which may be why more men are affected than women
• having coarse and curly body hair
• a previous injury to the affected area of skin for example, from a fall
• having a deep cleft between your buttocks
• having a family history of the conditionmore thanone-third of people have a family member with the condition
• having a job involving a lot of driving or sitting down for long periods

Jeep seat

During the Second World War, thousands of army jeep drivers developed pilonidal sinuses. The condition became so widespread it was nicknamed "jeep seat", or "jeep disease".

Asso many men doing the same sort of job developed the condition, thissuggests environmental factors must play a role inits development. These factors could include:

• wearing restrictive clothingsuch asarmy uniforms
• repetitive motionsuch as bouncing around in the seat of a jeep
• poor personal hygiene