Origin of the Pilonidal Sinus

Origin of the Pilonidal Sinus

Hair under the skin is the cause!

The Pilonidal Sinus is not the result of a malformation during embryonic development.

These fistulas in the gluteal fold are caused by the body's reaction to hair that has penetrated the subcutaneous tissue (subcutis).

Hair consists of keratin. The body has no enzymes to break down keratin. As a result, hair is treated as a foreign body and a capsule of scar tissueis formed.  

Can a Pilonidal Sinus become dangerous? Don't worry, a Pilonidal Sinus never connects to intestines, bones or spinal cord.

Similar to a Pilonidal Sinus , fistulas in acne can look inverse and anal fistulas with a starting point in the rectum.

Origin of the Pilonidal Sinus - All important facts

Origin of the Pilonidal Sinus schematic drawing
Schematic drawing of a (1 Pilonidal Sinusnormal hair root, 2 enlarged hair root with broken hair, 3 inflammation by impaled hair, 4 large pit, 5 small pit, 6 secondary fistula opening, 7 impaled loose hair, 8 fistula cavity with embedded hair, 9 granulation tissue)

The "pit" - cause of the pilonidal sinus

Where does the hair in the Pilonidal Sinus?

Sinus pilonidalis with numerous pits of different sizes
Sinus pilonidalis with numerous pits of different sizes

In each Pilonidal Sinus one, they are found exactly in the middle of the gluteal fold, often hidden in the depth of the gluteal fold, which has been Lord called the Pit. They are also called porus or primary fistula.

In some patients you can find only a single, in others a multitude of these pearl-like openings.

The size of these pits ranges from barely visible black dots to holes several millimetres in size. Sometimes there are broken off, loose hairs in them, from some of them a whitish, pasty secretion can be expressed like a blackhead (comedo).

The pits are lined with skin and therefore cannot close by themselves. They become the entry point for bacteria and thus the cause of recurring inflammation. Ardelt found out that primarily anaerobic and Gram-negative bacteria play a role, and increasingly aerobic and Gram-positive bacteria play a role in recurrence.

Thus, the sacrococcygeal pilonidals sinus is pathologically an infected foreign body granuloma, regardless of different views on its origin.

Patey, D. (1970). The Principles of Treatment of Sacrococcygeal Pilonidal Sinus. Proceedings of the Royal Society of Medicine, 63(9), 939-940.

Development of a pit from a hair root according to BASCOM

spearing of broken hair
The most frequent mechanism of the formation is certainly the breaking off and impaling of the hair fragments in the hair root (follicle)

"Contrary to common belief, most pilonidal (fistulas) are apparently not caused by (impaled) hair shafts. Instead, the hair follicles appear to be the source." (Bascom 1980)

Under the microscope, Bascom saw a gradual development from a normal hair root (follicle) to a pit. He described pits of different stages side by side in the same patient. Often all hairs in a fistula were of equal length and had a terminal end. Broken hairs as well as keratin and skin scales filled the hair root.

Furthermore, suction forces when sitting down and standing up could be measured ("the pit sucks").

Pit formation according to Bascom
Emergence of the pit from a hair root (redrawn according to Bascom, 1 normal, 2 dilated, 3 infected follicle, 5 acute, 6 chronic abscess, 7 final stage of a duct lined with skin

Development of a pit from a skewered loose hair according to KARYDAKIS

Implanting loose hair in the gluteal fold
More rarely, hairs from the back or head can also impale themselves into the skin
Hair Ultrastructure
Grooves and barbs through the horny scales favour the penetration of a hair (drawing after electron microscopic preparation by DOLL et al.)

For Karydakis the matter was clear. According to his conviction, the "pits" are created by spearing broken hairs. Scanning electron microscopic examinations of Dahl (1992) support this theory that hairs split and drill into the skin.

Needle-like, sharp ends, such as those created during haircutting, allow penetration into intact skin. The scales of keratin look like barbs. With the side formerly facing the root, the hair works its way deeper and deeper into the skin.

Page was able to prove experimentally in 1969 that a hair can work its way forward several centimetres in a pit after 30 minutes while sitting, but not if it penetrates with the tip first. The movement towards the tip of the hair is prevented, the hair cannot loosen itself.

Recent investigations of Doll and Bosche at least 20-30% of patients seem to support this theory. Especially hairs from the neck area were often found in the fistula. Therefore, frequent visits to the hairdresser and haircuts with short shaved neck hair would be a risk factor for a Pilonidal Sinus.

We may find loose hair from the neck or back inside the fistula tube in 10-20% of our patients. At each performance, loose hair of varying length lies in the gluteal fold. We have also seen unusual sources of penetrated hair, such as over 10 cm long curls (from the girlfriend) in a young man with a brush haircut and grey-black short hair (from the sled dog) in a blonde woman with long hair.

Formation of a sinus pilonidalis from hair growth under the skin?

malformation of the hair follicles
An orderly growth of tender hairs inside the fistula tract is rarely observed

In individual cases, one finds an orderly, brush-like image of a lawn of tender, short hair in a cut open fistula tube. 

I have not found this form described in the medical literature and have no explanation for this form of appearance.

Risk factors for the Pilonidal Sinus

Probable risk factors are:

  • Strong hairiness: The majority of patients Pilonidal Sinus affected by the disease have above-average dense and strong hair.
  • Male gender: Men are affected by coccyx fistulas about twice as often as women
  • Family history: Relatives of Pilonidal Sinus patients seem to have a slightly higher risk. However, a genetic factor could not be found.
  • Sitting activity: Who does not have it? We spend a large part of our lives sitting, at school and university, in the office or in the car. Nevertheless, statistics from the armed forces show that drivers and soldiers of lower rank have a significantly higher risk of developing a Pilonidal Sinus "black belt" than officers.

Questionable risk factors are

  • Overweight? Some studies found overweight as measured by BMI > 25 more frequently in patients with Pilonidal Sinus than in the control group (e.g. corresponds to a body weight of over 90 kg in a 185 cm tall, 20-year-old man)
  • Lack of hygiene? You don'Pilonidal Sinus t get one just from not washing enough. Studies have found an increased risk of developing a Pilonidal Sinus, if you shower or bathe less than three times a week
  • Smoking? Hair-related Pilonidal Sinus is not more common among smokers than among non-smokers. Mixed forms with acne inversa are found almost exclusively among smokers and are characterized by complicated fistula courses and more frequent relapses.

Do not seem to have any significance

  • Sweating
  • Type of underwear
  • occupational dust exposure

Forms of a Pilonidal Sinus

But how Pilonidal Sinus exactly do you recognize one? Basically there are three manifestations of inflammation, which are accompanied by different symptoms:

Blande Form: The initial stage of one Pilonidal Sinus often goes unnoticed or the complaints are minor and unspecific (pain when sitting on hard chairs, feeling like a "pimple" or bruise). Sometimes there is no discomfort except for a visible opening in the gluteal fold. Strictly speaking, a doctor rarely sees the bare form. Who goes to the doctor without complaints?

Acute form: A pilonidal abscess can develop within a few days, the underlying Pilonidal Sinus one existed unnoticed before. The typical form with a reddened, painful bump on the buttocks or in the coccyx region can be recognized at a glance. The externally visible symptoms may be minor. The patient is in severe pain, although not much is visible. On closer examination, a hardening can be felt and the skin appears conspicuous. The pits are often not visible due to swelling. Sometimes patients report that the pain started after a fall. A causal connection cannot be explained. It is not uncommon for complaints to occur after prolonged sitting, as is the case with schoolchildren and office workers or after long flights.

Chronic form: Some patients have only mild pain and notice the presence of the fistula only accidentally or due to the secretion of blood or pus. The colonisation of the pilonid fistula by the bacteria always present in this region can cause a very unpleasant odour development. The capsule often contains many collagen fibres and feels hard like cartilage, so that irritations occur when sitting for long periods without any actual pain. Sometimes the patients themselves discover small openings in the area of the gluteal fold.