An anal fistula is an abnormal communication that develops between the skin around the anus and the anal canal.
The vast majority of anal fistulas develop after a perianal abscess (boil). They occur less commonly in other bowel diseases such as Crohns disease. In very rare cases, they are secondary to other conditions like TB, HIV, radiation and cancer.
Anal fistulas can be superficial (involving no muscle), can involve the internal anal sphincter (intersphincteric) or can involve both internal and external sphincters (transphincteric,suprasphincteric).
These are anal fistulas not involving a large amount of the sphincters (< 30 – 50% of muscle length).
These are anal fistula involving a large amount of the sphincters (>30 – 50 % of muscle length) , anterior fistulas in women, recurrent fistulas and fistulas associated with Crohns disease or radiation.
Anal fistulas can cause intermittent symptoms of pain, swelling and a pus-like discharge.
The diagnosis can usually be made clinically (ie history and examination). Occasionally, an examination under anaesthetic (EUA) is required. In recurrent and complex fistulas an MRI can be useful to plan surgery.
Anal fistulas almost never resolve without surgery. Abscesses must be drained before definitive repair if present.
Simple fistulas are treated by:
Fistulotomy – laying the fistula open (success rate > 95%)
Complex fistulas usually require insertion of a seton (a rubber band drain) to allow infection and inflammation to settle. A seton usually stays in a minimum of 6 weeks. The options of management subsequently are: