Indirect CT and MRI signs
At MRI, the normal vagina is a collapsed fibromuscular structure situated between the urinary bladder and the rectum with an H-shaped configuration in axial sections and elongated J-shape in sagittal views [17, 18].
At both CT and MRI, the identification of abnormal vaginal distension by intraluminal stool (Figs. 5 and 6) or air (Fig. 7) should alert the radiologist to suggest the presence of an entero-VF Alternatively, a distended vagina with intraluminal simple fluid is seen in both entero- and urinary VF [6, 17, 18].
Furthermore, MRI may well depict the associated inflammation of the vagina, seen as diffuse mural thickening with oedematous hypersignal, obliteration of the normally T2-hypointense submucosa layer and prominent CM enhancement (Fig. 7) [6, 17, 18]. Alternatively to a VF, similar cross-sectional inflammatory changes may result from infectious vaginitis from various pathogens, albeit this condition rarely requires directed imaging. Furthermore, a similar appearance that reflects diffuse vaginal oedema is normally observed after recent vaginal hysterectomy (Fig. 8) [25].
Direct CT and MRI signs
The cross-sectional imaging hallmark of an entero-VF is represented by a variably oriented track that interconnects the vagina with the sigmoid colon, rectum or anus. Radiologists should focus on which part of the vagina is affected, since this information is particularly relevant for choosing the appropriate surgical approach. Broadly, in sagittal viewing at both CT and MRI, vagina can be subdivided in upper (at the level of the lateral vaginal fornices), middle (at the level of the urinary bladder base) and lower third (at the level of the urethra) [2, 6].
The entero-VF may be stool- (Figs. 5 and 6), air- or fluid-filled (Figs. 9, 10 and 11). At MRI, active ano- and recto-VF generally appear as short tubular structures with high T2-weighted signal intensity reflecting a combination of intraluminal fluid and oedematous walls. Compared with the T1-hypointense fluid content, after CM administration, VF margins show intense inflammatory enhancement corresponding to the inflamed granulation tissue (Figs. 6, 7, 9 and 10). Conversely, chronic fistulous tracks show scar-like low T2-weighted signal and usually do not enhance [33]. When present, associated abscesses appear as fluid-filled cavities with internal near-water CT hypoattenuation, high T2-weighted MRI signal intensity and restricted diffusion reflecting the presence of pus, demarcated by an intensely enhancing peripheral wall (Fig. 11) [6, 17, 18].
Colovaginal fistulas
The typical diverticulitis-related colo-VF communicate between the inferior aspect of the sigmoid colon to the midline or left lateral aspect of the vaginal dome, and is identified in approximately 60% of cases as a short vertical tract on coronal and sagittal CT images. An abscess cavity may be interposed between the two structures (Fig. 12a) [34, 35].
In patients with consistent clinical findings, the detection of adhesion between the thickened wall of the sigmoid colon and the vaginal dome and the associated presence of air into the vaginal lumen (Fig. 12) are sufficient to confirm the diagnosis. Alternatively, the same findings should be reported as suspicious of VF and warrants focused gynaecological examination [2, 6, 7].
Surgical repair of colo- and high recto-VF is performed via abdominal approach to remove the diseased colonic segment, and frequently requires a temporary diverting stoma and interposition of omental or epiploic fat [2, 11].
Rectovaginal fistulas
Typically anteriorly oriented and located on or near the midline, recto-VF are recognizable on sagittal CT and MRI images and connect the posterior aspect of vagina and anterior wall of rectum (Figs. 5 and 6). Compared with colo- and ano-VF, recto-VF are often wider and associated with identifiable discontinuity or cleft in the facing vaginal and rectal walls. In patients with post-surgical VF, CT with optional contrast enema (Fig. 3) represents the preferred technique and the anastomotic site is readily identified by the presence of surgical staples. Whereas CT clearly shows metal, on MRI (Fig. 6), the presence of suture materials is suggested by susceptibility artefacts, more prominent on gradient-echo T2-weighted images [2, 6, 7].
The detection of solid pelvic tissue is the hallmark of a neoplastic aetiology of the recto-VF (Fig. 7) [2, 6, 7, 36].
Depending on fistula site, cause and features, surgical management may be performed via transabdominal, transperineal, endorectal or vaginal (in low recto- and ano-VF) approach. Excision may be completed with interposition of autologous tissue (such as Martius flap or gracilis muscle). If present, prosthetic mesh should be removed. Alternatively, biomaterials such as fistula plugs or fibrin adhesive glues may be positioned [11, 37].
Anovaginal fistulas
Ano-VF depart at or below the dentate line and are often challenging to diagnose at imaging, particularly in lean women with very thin adipose tissue between the vagina and anus. The characteristic MRI appearance is a short midline or paramedian T2-hyperintense and enhancing band, best seen on FS sequences, which cross anteriorly the thin anovaginal septum (Figs. 9 and 10). Radiologists should report the site and direction of ano-VF described using the surgical “anal clock” scheme with the patient in the lithotomy position, in which the vagina is located between 11 and 1 o’clock and the natal cleft is at 6 o’clock position. Abscesses (Fig. 11) and signs of vaginal inflammation may coexist. In our experience in CD patients, perianal MRI has adequate accuracy (over 90%) for ano-VF, despite the fact that they are often short, small and collapsed [20, 21, 38,39,40].
Although explanation of the anatomical Parks’ [41] and St. James’ hospital [42] classifications lies beyond the scope of this article, MRI allows classification of perianal inflammatory disease according to the relationships with the internal and external sphincter muscles and therefore differentiation between inter-, trans-, supra- and extra-sphincteric fistulas; therefore, MRI contributes to the distinction between simple or complex perianal disease according to the American Gastroenterological Association (AGA) criteria [20, 21].
Patients with CD tend to have complex perianal disease including wide ano-VF, multiple or branching fistulas, intersphincteric “horseshoe” and levator ani abscesses (Figs. 11 and 13), and generally warrant MRI to detect abscesses and guide surgical examination under anaesthesia before starting anti-TNFα therapies such as infliximab. Furthermore, MRI provides a consistent follow-up following surgery and seton placement (Figs. 10d and 11d) [39, 40].