Anal sepsis
Endoanal ultrasound of a superficial intersphincteric fistula with the internal opening (arrow) at 7o’clock
Perianal abscess and anal fistula are often straightforward and do not require further investigation. However, there may be doubt as to whether any sepsis is present, or the exact extent of any track that has formed. Clinical assessment is more difficult following previous fistula surgery, and there may also be the need to assess the extent of any sphincterotomy. Endoanal ultrasound and MRI are the main imaging tools, and are to some extent complementary.
Endoanal ultrasound is a simple quick study that gives high quality images of the main sphincteric components: the internal and external anal sphincters and the longitudinal layer. Fistula are seen as hypoechoic tracks, which may be traced through the sphincter to the internal opening (1), with any branching or abscess noted. The extent of any division of the internal sphincter may be assessed, but one of the problems with ultrasound is that fibrosis is hypoechoic and therefore difficult to differentiate from inflammation.
MRI is really the gold standard as inflammatory tissue is clearly distinguished from normal or fibrosis on fat suppressed sequences. These are taken axially at right angles to the longitudinal axis of the canal, and coronally in line with that axis. The high signal from inflammatory tissue has to be distinguished from slow moving venous blood, but provides an immediately obvious indicator of sepsis. MRI is much better than ultrasound in demonstrating tracks and abscesses outside the external sphincter (2), particularly supralevator extensions, but only slightly better within the sphincter.
Endoanal ultrasound is therefore an appropriate examination in primary fistula where there is some clinical concern, but MRI is the choice for complex fistula (all fistulas due to Crohn's disease) and for recurrence fistula (3).
Reference List
(1) Bartram C, Buchanan G. Imaging anal fistula. Radiol Clin North Am 2003; 41(2):443-457.
(2) Buchanan GN, Halligan S, Bartram CI, Williams AB, Tarroni D, Cohen CR. Clinical examination, endosonography, and MR imaging in preoperative assessment of fistula in ano: comparison with outcome-based reference standard. Radiology 2004; 233(3):674-681.
(3) Buchanan G, Halligan S, Williams A, Cohen CR, Tarroni D, Phillips RK et al. Effect of MRI on clinical outcome of recurrent fistula-in-ano. Lancet 2002; 360(9346):1661-1662.