The so-called 'cryptoglandular hypothesis' ascribes the aetiology of fistula-in-ano to the glands that sit in the inter-sphincteric space around the anal canal. Spread of sepsis from an infected gland leads to perianal abscess which usually presents acutely (see above). Epithelialisation of the track leads to establishment of a fistula-in-ano. A classification of fistulas by the late Sir Alan Parks in 1976, described four main groups; intersphincteric, transsphincteric, supras-phincteric and extrasphincteric. A full assessment of a fistula-in-ano requires the identification of the internal and external openings, the primary track, any secondary extension and any diseases complicating the situation. Extensions occur in approximately 10-15% of patients, and are more prevalent in recurrent or Crohn's fistulas.

The goal of treatment of fistula-in-ano is to eradicate the fistula while maintaining continence.

Up to 25% of fistulas recur, and recurrence is usually due to sepsis missed at surgery and left untreated. MRI is effective at identifying the cause for recurrent sepsis and surgery guided by MRI can reduce further episodes of recurrence by up to 75%.

The best way to cure fistula is to lay open the track and allow it to heal by secondary intention, thus obliterating the fistula; however, in order to achieve this, a varying amount of sphincter muscle must be divided. The majority of fistulas will be amenable to this technique, but laying open of high or transphincteric fistulas may result in incontinence. There are various options for the treatment of high or complex fistulas, from the use of permanent seton sutures, to mucosal advancement flaps or the use of tissue glues.

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