The exact cause of pilonidal sinuses is unclear. It is generally thought that they are caused by loose hairs that get pushed into the skin. There are several theories as to how this could happen.
Hair follicles
Hair follicles are the small holes that an individual hair grows out of. It is thought that pressure and friction on a hair follicle could somehow damage it, causing a pilonidal sinus.
There are two suggestions for how a hair or a hair follicle causes a sinus:
- A hair follicle may become blocked, causing the follicle to become enlarged and then burst.
- A broken hair may be inserted into the skin, leading to an infection.
The cleft between the buttocks is an ideal place for bacteria to spread because it is often moist and warm. The pressure that is applied to this area - such as from sitting - can also push more hairs or debris into the pit that forms.
There have been reports of hairdressers developing pilonidal sinuses on their hands. This may be caused by hair from their clients becoming trapped in moist, damaged skin between a hairdresser’s fingers.
Infection of the pilonidal sinus
If a broken hair is pushed into the skin, this irritates the skin and causes it to become red and swollen. Bacteria can quickly infect this skin.
The immune system (the body’s natural defence against infection and illness) will send white blood cells to fight the infection. A collection of dead white blood cells and bacteria will then build up, resulting in pockets of pus, known as an abscess.
Risk factors
Factors that are known to increase your risk of pilonidal sinuses include:
- obesity - having a body mass index (BMI) of 30 or above
- having an above-average amount of body hair
- having a deep cleft between your buttocks
- having a family history of the condition - over one-third of people have a family member with the condition
- having a job that involves a lot of driving or sitting down for long periods
Jeep seat
During World War Two, thousands of army jeep drivers developed pilonidal sinuses. The condition became so widespread that it was nicknamed jeep seat or jeep disease.
The fact that so many men doing the same sort of job developed pilonidal sinuses suggests that environmental factors must play an important role in the development of the condition. Examples of such factors include:
- wearing restrictive clothing, such as army uniforms
- repetitive motion, such as bouncing around on the seat of a jeep
