Imaging with EAUS has been shown to be a useful method (accuracy ranging from 63% to 100%) for evaluation of perianal abscesses and fistulae in CD [7-11,13,18]. Van Outryve et al.  reported that the routine proctological examination showed Crohn's lesions in 30% of patients and EAUS performed with a linear probe detected anorectal anomalies in 75% of patients. Moreover, EAUS was superior to CT scan in the study of pararectal and para-anal abscesses and fistulae. Schratter-Sehn et al.  demonstrated 46 fistulae, verified by surgery, in 36 patients with CD using EAUS performed with a 5-MHz sector (270°) scanner. Tio et al.  evaluated 36 patients with CD and suspected perianorectal disease. In the 17
Fig. 29. Posterior transsphincteric fistula with horseshoe secondary extension in the ischioanal space (arrows)
Fig. 31. Posterior transsphincteric fistula, after hydrogen peroxide injection, appearing as a hyperechoic tract extending through the external sphincter, with a secondary hyperechoic tract in the right ischioanal space t
patients that had surgically confirmed fistulae, EAUS identified fistula in 14 (82.4%). Solomon  reported that patients with CD often have high and complex perianal fistulae. Using EAUS performed with a 7- to 10-MHz radial probe, they found that 40% of fistulae involved the upper half of the anal sphincter, 30% of patients had supralevator chronic abscess cavities and 29% of women had rectovaginal fistulae.
In the literature, several studies have reported on ultrasonographic findings after treatment. West et al.  evaluated the effect of combined ciprofloxacin and infliximab in perianal CD using HP-enhanced 3-
Fig. 31. Posterior transsphincteric fistula, after hydrogen peroxide injection, appearing as a hyperechoic tract extending through the external sphincter, with a secondary hyperechoic tract in the right ischioanal space
D ultrasound (3-D HPUS). Only three of the 13 patients with a clinical response at week 18 also showed improvement on 3-D HPUS. This result is in agreement with van Bodegraven et al. , who reported that although clinical improvement was seen after short-term infliximab treatment, most fistulae still persisted on EAUS. These persistent lesions may lead to relapse of symptoms or abscess formation. In a long-term study, Rasul et al.  evaluated whether the clinical improvement after infliximab in 35 patients with Crohn's disease perianal fistulae was associated with endosonographic closure of fistula
tracts. TPUS identified more fistulae than appreciated clinically, as well as the presence of unsuspected fluid collections. Short-term clinical response was higher than radiographic response, and fistulous tracts on TPUS were still present in many clinical responders at week 8 (49% vs 14%, respectively). At week 56, complete clinical closure of fistulae remained in approximately one half of patients; however, ultrasonographic healing occurred in 46% of patients, and this correlated well with clinical healing. The main conclusion of this study is that even though fistulae clinically may have healed in the short term after initial treatment, further long-term therapy may be required if defects persist radiographically, even to a point that radiological improvement is seen so that symptom relapse and abscess formation can be prohibited.
A number of comparative studies have been performed to assess the efficacy of EAUS and MRI in evaluating perianal CD [8-11]. Some studies have shown superiority for MRI  whereas others have shown little difference [10, 11]. MRI scans can involve use of an endoanal coil or phased-array coil. Endoanal coil provides excellent spatial resolution but is unable to detect deeper fistula tracts and distant abscess collections, which are better assessed using phased-array coil . MRI is useful for assessing the integrity of the IAS and EAS as well as identifying complex fistula tracts and abscesses ; however, pelvic MRI was shown to have a tendency to miss short or superficial fistula tracts. Orsoni et al.  conducted a prospective study comparing EAUS performed with a linear probe, pelvic MRI performed with body coil and examination under anaesthesia
(EUA) in 22 patients and found EAUS to be the most sensitive modality. Agreement for fistulae with EAUS and MRI when compared with surgical findings was 82% and 50%, respectively. Schwartz et al.  compared the accuracy of MRI performed with phasedarray coil, EAUS performed with both radial and linear probe and EUA in 34 patients with Crohn's perianal fistulae. All three methods demonstrated good agreement with the "consensus gold standard" (MRI, 87%; EAUS, 91%; EUA, 91%). In addition, when any two of the three methods were combined, accuracy was 100%. Maier et al.  showed that MRI was superior to EAUS performed with a radial probe in the assessment of fistulae in CD. Overall sensitivity of EAUS and MRI was 73% and 91% for primary fistu-lae and 69% and 88% for recurrent fistulae, respectively. Wedemeyer et al.  showed an excellent agreement (k >0.83) between MRI performed with a phased-array surface coil and TPUS in 25 patients with CD and clinical signs of perianal inflammatory disease. They recommended TPUS as a screening tool in acute perianal disorders in CD and to evaluate treatment outcomes.
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