A pilonidal sinus is a clump (or nest) of hair under the skin - first described in 1833.

The natal cleft (bum crack), combined with stiff shed hairs and rolling movement of buttocks push the hair through the skin like a pin - thus forming a clump of hair under the skin!




  • acute: abscess (boil) - if this does not settle with antibiotics, surgical drainage of the infection may be required.
  • longstanding: pain, discharge (pus and blood) and bad smell
  • recurrence of symptoms after surgery can be frustrating and is not uncommon



If you look carefully, there will be one or more small openings (pits) in the depths of the natal cleft. Hair may protrude from these. Typically there is a small lump above these pits and slightly to the left. This is where hair accumulates (and sometimes exits - secondary sinus).



The condition starts in adolescence.

While some researches propose a sucking mechanism to draw hairs in, there is ample evidence that a lose hair is able to penetrate intact skin - helped along by scales which ensure hair always rolls/advances in one direction.


Medical and Surgical Information

No special tests are required to make the diagnosis but occasionally Magnetic Resonance Imaging may be necessary to distinguish a pilonidal sinus from a fistula-in-ano.

A pilonidal sinus will only heal if the causative actors are eliminated: hair, cleft, force.


The modified Karydakis flap is an elliptical semilateral excision of midline pits and the nearby secondary sinus with the ellipse displaced 2cm from the midline. A 1cm thick flap is sutured in two layers and the skin closed with a dissolving stitch.

The operation is performed as a day case under General Anaesthesia.

For the majority, healing takes approximately 2 - 3 weeks.


15% of patients (1 in 7) will develop a minor wound infection near the anus but this typically heals itself within a few weeks.

Recurrence of the pilonidal sinus occurs in less than 2 % of patients.




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