Inflammation of the gluteal fold due to ingrown hairs

Pilonidal Sinus Cause, origin and forms

Pilonidal Sinus Cause and origin

Initial stage of the pilonidal sinus

Emergence, risk factors and courses

Most people will probably not encounter this diagnosis until they themselves or a loved one is affected. An inconspicuous small opening in the buttock fold (the pit) is often the first sign. Through this hole, hairs enter the subcutaneous tissue (subcutis). Pilonidal Sinus: Sometimes several of these openings can be seen, all leading into the same fistula cavity, making Pilonidal Sinus recognizable. 

Hair is made of keratin. Although the body is able to produce keratin, it is not able to break it down. This leads to hair being treated as a foreign body in the body. Experts agree that ingrown hairs are the cause of coccyx fistulas.

A sheath of scar tissue forms, which forms the fistula capsule. This capsule prevents the chronic inflammatory process from spreading. Experts refer to this as a foreign body granuloma.

Can a Pilonidal Sinus become dangerous? Don't worry, a fistula on the coccyx is never the cause of fistulas on the intestine, bones or spinal cord. Congenital malformations during embryonic development are also not the cause of coccygeal fistulas.

Acne inversa and anal fistulas can be confused with coccygeal fistulas due to their appearance. Smoking is often the cause of acne inversa, while inflammation of a proctodeal gland is the cause of anal f istulas.

All about Pilonidal Sinus Cause and origin

Schematic drawing: Emergence Pilonidal Sinus

Pilonidal Sinus Cause and origin explained in the schematic diagram
Schematic drawing of a Pilonidal Sinus (1 normal hair root, 2 dilated hair root with broken hairs, 3 inflammation due to impaled hairs, 4 large pit, 5 small pit, 6 secondary fistula opening, 7 impaled loose hair, 8 fistula cavity with embedded hairs, 9 granulation tissue). Sites 1 and 7 correspond to the initial stage of the fistula
Where does the hair in the Pilonidal Sinus come from ?

The "pit" - cause of the pilonidal sinus

In the removal of the pits lies the key to healing
Numerous Pits - the Pilonidal Sinus cause
Sinus pilonidalis with numerous pits of different sizes

Lord called the openings or entry ports found in the middle of the gluteal fold at each Pilonidal Sinus pit. They are also called porus (lat. passage, gate) or primary fistula. They like to hide in the depth of the fold. The presence of pits reliably identifies the Pilonidal Sinus.

Some patients have only single, others a multitude of these openings strung like pearls. Removal of the pits is the decisive measure for healing the Pilonidal Sinus.

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 ... pilonidal sinus, despite differing opinions... is an infected foreign body granuloma.

BASCOM: Pits arise from hair roots

Pilonidal Sinus Origin Theory I:Impaling Broken Hair
The most frequent mechanism of the development is certainly the breaking off and impaling of the hair fragments in the hair root (follicle)

"Contrary to popular belief, the origin of most pilonidal (fistulas) is apparently not due to (impaled) hair shafts. Instead, hair follicles appear to be the source." (Bascom 1980)

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

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

Stages of Pilonidal Sinus Emergence: 1 Normal, 2 and 3 Pilonidal Sinus Early stage, 4 - 6 Pilonidal Sinus Emergence
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 terminal stage of a duct lined with skin

KARYDAKIS: Pits are created by impaling loose hairs.

Pilonidal Sinus Emergence Theory II:Impaling loose hair in the buttock crease
More rarely, hair from the back or head can also impale itself into the skin
Hair Ultrastructure
Grooves and barbs through the horny scales favour the penetration of a hair (drawing after electron microscope preparation by DOLL et al.)

Arrowheads and barbs

For Karydakis the matter was clear. He is convinced that the pits are formed by impaling broken hairs. Scanning electron microscopic studies by Dahl (1992) support the theory that split hair fragments with needle-like, sharp ends bore into intact skin.

The scales of keratin act like barbs. With the side formerly facing the root ahead, the hair works its way deeper and deeper into the skin.

Page was able to prove experimentally in 1969 that a hair can penetrate several centimeters into a minimal opening after only 30 minutes when sitting with the end near the root first. The structure of the hair scales causes a screw-like anchorage. Consequently, once a hair has been impaled, it can no longer detach itself.

Searching for clues with a microscope and a forensic scientist

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 hairwould 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 presentation, loose hairs of varying length lie in the gluteal fold. We have also seen unusual sources of penetrating hair, such as curls of more than 10 cm length (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.

Hair growth in the fistula tunnel?

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

In isolated cases, an orderly, brush-like image of a lawn of delicate, short hairs is found in an incised fistula tube. 

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

What factors actually increase the risk of a Pilonidal Sinus?

Risk factors for the Pilonidal Sinus

Facts and myths

With regard to our own patients we have the impression that the cliché of the very hairy, overweight and much sweating Pilonidal Sinus patient does not have much to do with reality. In most cases we see an average hair density.

We would therefore like to take a look at medical publications to find out which circumstances actually increase the risk of Pilonidal Sinus . 

In 1992, Karydakis postulated that soft and softened skin was more prone to hair impaction. He saw 3 categories of potential risk factors:

  • H - (Hair) factors
    • H1 Number of loose hairs that collect in the butt fold.
    • H2 The more or less pronounced sharpness of the hair root tip.
    • H3 Type of hair (hard or silky)
    • H4 Shape of hair (straight, not curly hair is the type that tends to impale).
    • H5 Dandruff of the hair - more pronounced at the age of 10-22 years.
  • F (Force) factors
    • F1 Depth 
    • F2 Tightness of the gluteal fold
    • F3Friction during movements between the sides of the buttress fold
  • V (Vulnerability) factors
    • V1 Softness
    • V2 Maceration
    • V3 Erosion
    • V4 shafts
    • V5 Large pores
    • V6 Wounds
    • V7 Scars
m : w
2,2 : 1
21 %
38 %
7 %
39 % / 29 %
44 %
3 : 1
- –
67 % / 37 %

Probable risk factors

(Strong) hairiness

Lots of hair = high risk? There is no Pilonidal Sinus before puberty. Many of the patients affected by a Pilonidal Sinus are said to have a denser and stronger than average hair. In a Norwegian study, on the other hand, the number of patients with low hairiness was greater.

It is possible that the percentage of intensely hairy patients depends more on the latitude at which the study is conducted. Consequently, a study from the north of Europe would find less intense hairiness among the patients with Pilonidal Sinus than a study from the Mediterranean region. The reliability of the classification also seems questionable to me, since hardly any study indicates the methodology with which the density of hairiness was measured.

Dark and strong hair is simply perceived more than hair that is equally strong in number but less conspicuous with a lower pigment content and diameter of the hair.

Male gender

Men are affected by coccygeal fistula 2-3 times more often than women. Therefore, the diagnosis is often made late in women. We have quite a few female patients who have been diagnosed with Pilonidal Sinus despite having delicate, barely visible hairs. In the follow-up treatment one has to be especially careful that these regrowing hairs do not disturb the healing process.

Family history

Relatives of Pilonidal Sinus patients seem to have a slightly higher risk (Yildiz). It is not uncommon for several siblings to have Pilonidal Sinus. Familial clusters are also repeatedly reported over several generations. However, research has not yet found a molecular, genetic factor that would be responsible for this.

Sitting activity

Who doesn't have them? Much of our lives are spent sitting down, at school and university, in the office or in the car. Yet statistics from the armed forces show that motorists and low-ranking soldiers are at a significantly higher risk of developing Pilonidal Sinus than officers. Especially driving on bumpy roads or off-road seems to promote the development or even the inflammation of an existing Pilonidal Sinus ("Jeep disease").