The fistulas in UC may be difficult to assess, especially in patients with IPAA, in whom the diagnosis and classification of these lesions are hampered by deformation of the pouch-anal anastomosis, defects of the anal sphincters, and the presence of sinuses and folds that may simulate the presence of incomplete perianal fistulas. However, to date, no study has assessed the accuracy of transanal ultrasound in detecting fistulas and abscesses in patients with IPAA.

Transanal ultrasound may reveal defects in the anal sphincters and assess wall and peri-pouch changes. A study that quantified the anatomical changes of the anal sphincters after IPAA correlated these findings with anal manometry and fecal incontinence in 23 patients [9]. The results showed that IPAA leads to a reduction in the thickness of the internal anal sphincter (mean 1.16 mm), with tapering or gaps probably caused by direct trauma to the sphincter because of mucosectomy and/or denervation (Fig. 13.2); however, the reduction in the internal anal sphincter thickness did not correlate with continence. In patients with pouchitis, transanal ultrasound may show a variably increased wall thickening, hypertrophy of the peri-pouch fat, and enlarged lymphnodes (Fig. 13.3).

With the high resolution of transanal ultrasound in imaging the rectal walls and assessing the depth of intestinal inflammation, this technique has been proposed to differentiate UC from CD and infectious colitis, to assess disease activity, and to predict the response to medical treatment and the need for surgery, both in patients with active UC and in those with IPAA.

Preliminary results showed that in UC, rectal wall thickness as assessed by intraluminal sonography using transanal or miniaturized probes in combination with endoscopy correlates with disease activity and may predict both the response to medical therapy and the occurrence of relapse [10,11].

Furthermore, since CD tends to be transmural while UC is a superficial mucosal inflammatory process, hopes were raised that trans-rectal ultrasound would be effective in discriminating cases of otherwise indeterminate colitis. However, such efforts have been largely disappointing [12], although a recent pilot study showed that transrectal ultrasound elastography may be a promising diagnostic tool to differentiate UC from CD. Specifically, the stiffness of the bowel wall, as assessed according to the strain ratio, was shown to better differentiate inflammatory bowel disease than a simple measurement of rectal wall thickness [13], while in a small study the detection of perirectal lymph nodes in patients with nonspecific or suspected infectious colitis was shown to suggest a diagnosis of associated UC or to predict the development of a chronic colitis [14].