Chapter 19 — Pilonidal Sinus Disease

Pilonidal sinus disease affects the natal cleft, at the top of the buttock crease over the tailbone. It is not a disease of the anal canal itself. But it sits close enough that it is often confused with the fistula-in-ano and abscess described in Chapters 12 and 13. This chapter explains pilonidal sinus disease in full, and explains clearly how to tell it apart from those two conditions. This is the same pattern used for every disease chapter in this manual.

19.1 What Is Pilonidal Sinus Disease

Pilonidal sinus disease is a chronic condition of the skin and soft tissue over the sacrococcygeal region, at the top of the natal cleft. Small pits form in the midline skin. Loose hair fragments become trapped beneath the skin through these pits, and the body reacts to this trapped hair as a foreign body.

This foreign body reaction leads to chronic inflammation, and often to a sinus tract, and sometimes to an abscess. The word pilonidal itself means "nest of hair", which describes the finding well.

The condition is entirely separate from the anal canal. It does not communicate with the anal canal, and it is not caused by anal gland infection, unlike the fistula-in-ano described in Chapter 12.

19.2 Why It Happens

Loose hair fragments, shed from the surrounding skin, work their way beneath the surface through small natural pits in the midline of the natal cleft. Once trapped beneath the skin, this hair triggers a persistent foreign body reaction.

Several factors increase the risk. A deep natal cleft allows hair to collect and penetrate more easily. Coarse or abundant body hair increases the amount of loose hair available to become trapped. Prolonged sitting, common in certain occupations, increases pressure and friction in the natal cleft. Obesity adds to the depth of the cleft and to friction. Poor local hygiene allows loose hair and skin debris to accumulate.

The condition is most common in young men, particularly those with dense body hair, though it also occurs in women.

19.3 Classification

Pilonidal sinus disease presents along a spectrum, from mild to severe.

Asymptomatic pits — small midline openings, found incidentally, without pain, swelling, or discharge.

Acute pilonidal abscess — sudden, painful swelling in the natal cleft, often needing urgent drainage.

Chronic pilonidal sinus — a persistent tract, with intermittent or continuous discharge, and repeated episodes of swelling.

Complex or recurrent disease — multiple sinus tracts, often after one or more previous operations that have not fully resolved the problem.

19.4 Presentation

Follow the history-taking method described in Chapter 2.

Pain and swelling in the midline, over the sacrococcygeal region, at the top of the buttock crease.

Discharge, which may be purulent, blood-stained, or simply persistent moisture staining the underwear.

A history of one or more similar episodes in the past, sometimes already treated elsewhere with incision and drainage.

Occasionally, the patient notices hair protruding from a small opening, which is a distinctive and useful clue.

19.5 Examination

Follow the sequence described in Chapter 3. Position the patient prone, or in the left lateral position. Gently part the buttocks for a clear view of the natal cleft.

Inspection, described in Chapter 4, should specifically look for midline pits, secondary openings away from the midline, visible hair tufts, and any discharge. In an acute abscess, look for redness, swelling, and tenderness over the affected area.

The key examination finding that separates pilonidal disease from the conditions in Chapters 12 and 13 is location and depth. Pilonidal openings sit in the midline over the sacrococcygeal region, well above and behind the anus itself. The tract does not run down toward the anal canal. It does not communicate with the anal canal either.

A general examination of the perianal region should still be performed, as part of the standard sequence. A digital rectal examination, described in Chapter 5, should also be performed. This excludes any coexisting anorectal condition.

19.6 Differential Diagnosis

Fistula-in-ano — described in Chapter 12. A fistula tract communicates with the anal canal itself, at the dentate line. A pilonidal tract does not. This distinction is the single most important one to make correctly.

Perianal or perirectal abscess — described in Chapter 13. These arise from infection around the anal canal, and sit closer to the anus itself, rather than over the sacrococcygeal region.

Hidradenitis suppurativa is a separate condition affecting apocrine sweat glands. It typically produces multiple, scattered openings over a wider area. It is not confined to the midline natal cleft.

A sacrococcygeal dermoid or teratoma is rare. It should still be considered in an unusual presentation, particularly in a child or young person. These are congenital conditions, not acquired ones.

19.7 Investigations

Pilonidal sinus disease is usually a clinical diagnosis, made on inspection alone, and does not routinely need imaging.

MRI, described in Chapter 8, is reserved for complex or recurrent disease. It maps the full extent of the tracts before repeat surgery. It is also useful when the distinction from a fistula-in-ano is genuinely unclear on examination.

19.8 Principles of Management

An acute pilonidal abscess needs incision and drainage first, in the same way as any other abscess. Definitive treatment of the underlying sinus is planned once the acute infection has settled.

For chronic or recurrent disease, definitive surgical treatment is usually needed, since a sinus tract containing trapped hair rarely resolves permanently on its own.

Lifestyle measures reduce the risk of recurrence, and should be advised in every patient regardless of which surgical technique is chosen. These include regular hair removal from the natal cleft, weight loss where relevant, and avoiding prolonged sitting where practical.

Minimally invasive options suit a patient with limited disease and a strong preference to avoid a larger wound. One example is careful removal of the midline pits alone, combined with drainage of any lateral tracts.

19.9 Surgical Techniques

Excision with midline closure — the sinus and surrounding tissue are excised, and the wound is closed directly in the midline. This technique is simple. It carries a comparatively high rate of wound breakdown and recurrence, since the natal cleft remains deep. The deep cleft continues to collect hair and moisture after healing.

Excision with open healing — the sinus is excised, and the wound is left open to heal by secondary intention. Recurrence rates are lower than with midline closure, at the cost of a longer healing time, often several weeks, with regular dressing changes.

Off-midline flap techniques excise the sinus and close the wound to one side of the midline. Examples include the Karydakis procedure and the Bascom cleft-lift procedure. This flattens and shallows the natal cleft in the process. This directly addresses the underlying anatomical problem, and offers a genuinely lower recurrence rate than either midline closure or open healing.

Minimally invasive pit-picking is sometimes called the Bascom I procedure. It removes only the midline pits themselves, together with drainage of any lateral tracts. This leaves the bulk of the surrounding tissue undisturbed. It suits early or limited disease particularly well.

19.10 Special Situations

Acute abscess at first presentation — drain the abscess first. Definitive treatment of the underlying sinus is planned as a separate, later step, once acute inflammation has settled.

Recurrent disease after previous surgery — favour an off-midline flap technique over repeating a midline closure. The same anatomical problem that led to the first recurrence will otherwise persist.

An obese patient — weight loss should be discussed as part of the treatment plan. It directly reduces the depth of the natal cleft. It also reduces the risk of further recurrence.

19.11 Complications of Treatment

Wound infection or wound breakdown, particularly after midline closure.

Recurrence of the sinus, which is more likely after midline closure than after an off-midline flap technique.

A slow-healing or non-healing wound, particularly after open healing by secondary intention, needing patience and regular follow-up.

19.12 Common Mistakes to Avoid

Choosing a simple midline closure without discussing the higher recurrence rate compared with an off-midline technique.

Mistaking a pilonidal sinus for a fistula-in-ano, or the reverse, without carefully checking whether the tract communicates with the anal canal.

Proceeding straight to definitive surgery on an acutely infected abscess, rather than draining it first.

Failing to advise the patient on hair removal and hygiene after surgery, which meaningfully reduces the risk of recurrence.

19.13 Recording and Follow-Up

Record the site, the number and position of any pits or openings, and whether the tract communicates with the anal canal. For example: "Pilonidal sinus, midline pit over sacrococcygeal region with one lateral opening, no communication with anal canal. Plan: off- midline (Karydakis) excision, hair removal advice given. Review in 2 weeks." Arrange follow-up to confirm wound healing, and to reinforce hair removal and hygiene advice.

19.14 Why This Chapter Matters

Pilonidal sinus disease sits close enough to the anorectal region to be genuinely confused with the fistula and abscess described in Chapters 12 and 13. Yet it is a distinct condition. It has its own cause, its own examination findings, and its own surgical solutions. Distinguishing it correctly avoids an unnecessary or wrongly planned operation. The next chapter turns to a different problem entirely: anal incontinence.