MRI scanning in perianal Crohn's disease: An important diagnostic adjunct

Magnetic Resonance Imaging (MRI) is now a familiar tool in the colorectal diagnostic armamentarium. With the advent of newer generation scanners and endorectal coils the application of MRI to the diagnosis and management of perianal sepsis and the complications of perianal Crohn's disease (CD) is rapidly progressing. MRI has been shown to offer several important technical advantages over other forms of imaging in the evaluation of the anorectum and pelvic floor especially in the context of inflammatory disease and sepsis. In particular, MRI, using conventional external phased array coils with or without additional endorectal coils, offers extremely good definition of the intersphincteric plain and the level of the puborectalis sling as well as the other elements of the pelvic floor and the perirectal tissues. However, questions remain as to the precise place of MRI in the management of perianal CD. There have been many reports of the use of MRI in perianal sepsis, both sporadic (1,2) and sepsis in association with CD (3,4). The characteristic scan appearances of acute inflammatory change, fistula track formation, and localized sepsis together with the elucidation of the optimum imaging planes (5) have led to an increased use of MRI in the diagnosis of perianal sepsis. These reports have led to the suggestion that it may have a role in the routine management of suspected anal sepsis even in place of the conventional surgical Examination Under Anaesthetic (EUA). Few studies have undertaken any kind of direct comparison between the diagnostic accuracy of MRI and EUA in perianal sepsis. In the study from St. Mark's Hospital prospectively evaluating 35 patients with presumptive diagnoses of fistula in ano of any origin, Lunniss and colleagues demonstrated a high diagnostic accuracy of both MRI and EUA. However, two fistulae not found at EUA were demonstrated on preoperative MRI and two extensions of the track not suspected at EUA were identified by MRI and subsequently confirmed by the clinical course (6). Data from the same series of patients demonstrated 9% had failure of primary treatment, which could be related to the pathology missed at EUA but present on initial MRI scan (7). In another study of mixed perianal pathology, evaluated by MRI and EUA, all 10 fistulae in the series were correctly diagnosed by both MRI and EUA although more accurate information regarding extent was returned by the MRI in one case, giving rise to altered clinical management (8). Studies looking specifically at perianal sepsis in CD are even less common. The prospective study by Haggett and colleagues from Oxford (9) looked at 25 consecutive patients with suspected perianal CD. They demonstrated that MRI accurately represented the EUA findings in all patients and correctly identified a colovesical fistula in one patient missed by EUA and endoscopy. The key to evaluating the role of MRI in perianal CD is to focus on the difficult problems of management. There is no question of the high sensitivity and specificity of EUA performed by experienced coloproctologists, and the few prospective studies largely confirm this. The issue is not of MRI replacing or rendering EUA obsolete. It is, however, clear from these same studies that in certain cases MRI can offer important information not necessarily identified at EUA. MRI may be used to extend diagnosis and management in cases where EUA is either unhelpful, equivocal, or does not otherwise concur with the clinical diagnosis. This aspect has not been separately addressed but these limited data from the published studies would suggest that between 4 and 9% of cases might have benefitted from the increased diagnostic accuracy offered by MRI scan. In our experience MRI can provide information above and beyond EUA on several key fronts. Despite a history suggestive of the presence of fistulation, even repeated EUA may fail to demonstrate the track. It is our experience that these cases can be readily confirmed by MRI scanning, as in the case of a 34-year-old female with clinically mild perianal disease and a history suggestive of rectovaginal fistulation. EUA by an experienced coloproctologist failed to reveal any evidence of a fistula but MRI scanning confirmed the presence of the track (Fig. 1). Transverse MRI demonstrating the presence of a rectovaginal fistula (white arrow). With complex or extensive sepsis, EUA may fail to identify all collections present. Continuing symptoms or clinical suspicion of a persistent collection despite apparent appropriate surgical drainage can be confirmed using MRI scanning. Occasionally, despite clinical suspicion, proximal extension or suprasphincteric involvement of the complex is not evident at EUA. MRI is extremely useful in identifying all the proximal elements of the sepsis as demonstrated by the case of a 15-year-old girl with known CD. On presentation with symptoms of perianal disease, surgical assessment suggested only unilateral superficial sepsis. MRI scan revealed extension of the collection across the midline with a horseshoe configuration accompanied by supralevator extension (Fig. 2). MRI scan demonstrating posterior horseshoe perianal abscess with anterior and superior extension into the supralevator space (white arrow). Thus there is both anecdotal and prospective evidence that MRI can be used to demonstrate key aspects of pathology in perianal disease not found at EUA even in the hands of experienced coloproctologists. Although not necessarily indicated in all patients with perianal CD, MRI is of considerable value in difficult cases where the clinical course does not conform to surgical findings. It will never be a substitute for the EUA, which has the additional advantage that undrained sepsis can be treated under the same anaesthetic, but MRI scanning can lay to rest uncertain diagnoses and allows more precise management particularly in complex cases. In perianal CD, despite the value of the experienced surgical finger, MRI scanning offers a diagnostic hand of great accuracy and sensitivity in selected cases.