Embarrassing Illnesses

Exactly what causes a pilonidal sinus to develop is unclear. The consensus of opinion is that they are caused by loose hairs which become pushed into the skin. There are a number of theories about how this could occur:

Congenital theory

The congenital theory of how pilonidal sinuses occur is that some people are born with a minor birth defect that causes them to develop an abnormality in the skin between their buttocks. The skin may be dimpled to some extent, making it is easier for a loose hair to penetrate into the skin and begin forming a sinus.

There is some evidence that cases of pilonidal sinuses run in families, which would seem to support the congenital theory.

One of the weaknesses of the congenital theory is that during World War Two thousands of army jeep drivers developed pilonidal sinuses. The condition became so widespread that it was nicknamed ‘jeep seat’.

The fact that so many men doing the same sort of job developed pilonidal sinuses suggests that environmental factors, as well as genetic ones, must play an important role in the development of the condition.

Acquisition theory

The acquisition theory is that environmental factors place pressure and friction on a hair follicle, damaging it and causing an in-growing hair. Follicles are the small holes that hairs grow out of. The in-growing hair pushes into the skin producing the pilonidal sinus.

Examples of environmental factors which may be responsible for damaging a hair follicle, leading to the development of a pilonidal sinus, include:

• restrictive clothing (such as an army uniform), and
• repetitive motion, such as bouncing around on the seat of a jeep. 

Risk factors

Currently, the known risk factors for pilonidal sinuses include:

• obesity: a body mass index (BMI) of 30, or more,
• having an above-average amount of body hair,
• having a family history of the condition, and
• having a job which involves doing a lot of driving, or sitting down for long periods.