Pilonidal sinus came to notoriety during WWII as the leading cause of non-traumatic morbidity in soldiers — so much so that it ended being called ‘Jeep disease’. On average it lead to 100 days off per soldier treated.
However, the condition was first described in 1833 with the case of a woman with a sinus containing hair, and acquired its modern name of pilo (hair) nidal (nest) sinus in 1880. More recently it has been defined as an epithelially-lined tract that usually contains hair.
Its incidence is 26:100,000 with a 4:1 male-to-female ratio. The peak age of incidence is 20–30 years of age, but occurs in patients from puberty to around the age of 40 years.
Pilonidal sinus is now widely believed to be an acquired disease in the majority of patients.
The aetiology is disputed. One theory suggests that follicular occlusion is the cause. Changes at puberty are thought to cause keratin to swell and block midline hair follicles, which then causes the follicle to become infected and rupture inwards leading to a subcutaneous abscess that discharges on to the skin. The discharging tracts attract further hair and a foreign body reaction takes hold. The original tract from the follicle to subcutaneous cavity becomes epithelialised, while the discharging sinus acquires granulation tissue.
The alternative theory is that the primary agent is loose hair, which enters root-first, sets up an inflammatory reaction and attracts other hair to also enter root-first through the same entry point.
These hairs then make their way through the skin in a circular fashion and eventually work their way out.
The tracts they form are the sinuses that are observed, and where infection has set in then subcutaneous cysts appear.
The bacteria that infect pilonidal sinuses tend to be either Staphylococcus aureus or anaerobes. Where the reaction is severe enough it will form a subcutaneous abscess. The term pilonidal cyst is a misnomer, as the abscess cavities are not lined with epithelium.
The most common site for these processes is the midline of the inter-gluteal region. This is thought to be due to the anatomy in this area, which attracts the deposition and concentration of foreign bodies such as loose hair.
Other sites can give rise to a pilonidal sinus, including interdigital disease seen in barbers. Umbilicus, axillae and even penile disease have been reported.
The sinus may be asymptomatic at the beginning of the disease, but as it progresses and a deeper abscess forms it presents as a fluctuant mass. These present either as acute pilonidal abscesses or as chronically painful and discharging.
Risk factors include being white, male, obese, hirsute, having a hairy, deep natal cleft and a positive family history.
Differential diagnoses include anal fistula, hidradenitis suppurativa, perirectal abscess, TB, syphilis and rarely squamous cell carcinoma.
Diagnosis is primarily clinical but MRI may be used and this is particularly helpful in differentiating anal fistulae.
Early cases may be managed conservatively with antibiotics if evidence of infection is present, and by depilation.
Where an abscess is present, the primary management is surgical and will require incision and drainage.
A simple incision and drainage without primary closure will result in a recurrence rate of 60 per cent, whereas identification and excision of the sinus gives a 15 per cent recurrence rate.
Primary closure results in decreased morbidity but higher recurrence rates.
Incisions should be made lateral to the midline, as midline incisions heal poorly due to constant movement.
Scrupulous hygiene is important to stop hair penetrating the healing scar.
Where disease is recurrent a host of non-definitive surgical techniques exist.
Depending on which aetiological theory is preferred, the aim is to either remove all potentially precipitating hair-follicles or change the local anatomy to prevent aggregation of causative loose hair follicles. Other techniques include the use of phenol, electrocautery and laser. In very complex cases plastic surgery may be required.
It is worth noting that the incidence of pilonidal sinus after the age of 40 is very low.
Dr Hashmi is a GP and part-time tutor at St George’s Hospital, south London