Perianal abscess and fistula

Perianal abscess and perianal fistula are the two most common perianal disorders. Perianal abscess is the acute manifestation and perianal fistula is the chronic manifestation of purulent perianal infection. Fistulas may connect the abscess to the anal canal or to the perianal skin.

These disorders more commonly affect men, and they usually affect people in the age between 20 and 60.

Etiology

These disorders are primary in almost all cases, due to idiopathic obstruction and infection of the glands of the anal crypt. Rare causes include Crohn disease, ulcerative colitis, and colorectal cancer.

Clinical features

Perianal abscess presents with local discomfort, pruritus, and pain. An erythematous subcutaneous mass is present. The pain and discomfort worsen with sitting and defecation. Some may present with signs of systemic inflammation like fever.

Perianal fistula may present similarly, with the addition of purulent discharge from the anal canal or from perianal skin. Multiple fistulas are suspicious for Crohn disease.

Goodsall’s rule state that fistulas which open anteriorly tend to follow a simple, direct course while fistulas that open posteriorly tend to follow a more curving path. However, this rule is not always true and is, according to some, best forgotten altogether.

Diagnosis and evaluation

Clinical examination and inspection are usually enough to give the diagnosis. Digital rectal examination is usually impossible due to pain.

Very small abscesses and fistulas may be detected by transanal ultrasound.

Treatment

Perianal abscess is treated with incision and drainage, under general anaesthesia or regional anaesthesia. In the acute inflamed stage, we don’t look for fistulas because 2/3 of the fistulas heal spontaneously after opening the abscess, and the risk for complications is high if fistulas are treated while there is inflammation.

If the fistula persists over weeks/months, treatment is necessary. Superficial fistulas may be excised. Deeper fistulas are treated with loose setons. A loose seton (a nonabsorbable suture) is guided through the fistula tract and tied together. The seton allows any fluid to drain alongside it, and it prevents the fistula from closing prematurely, potentially forming an abscess. Inflammation close to the anal sphincter may lead to incontinence and is important to avoid. After some weeks/months, the seton may be removed and the fistula allowed to close, if deemed peaceful. If not, fistulotomy may be necessary.

Cutting setons are still used but should be avoided due to injury to the sphincter, yielding a higher complication rate.

A technique known as “advanced mucosal flap” may also be used. A tissue flap is used to cover the internal opening of the fistula, allowing it to heal while preserving faecal continence.