Utility of MRI in perianal disease

 

Perianal fistulas are observed in 14-38% of all patients with Crohn’s disease. Correct classification of the fistulas helps predict their course and may modify the management approach.

The classification of the AGA, contemplating simple and complex fistulas, is widely used thanks to its simplicity and easy application. Simple fistulas are superficial or low (located close to the linea dentata), with a single external orifice. They are not painful or fluctuating, and rectoscopy reveals no mucosal involvement. Complex fistulas in turn are high lesions, with multiple orifices. They may be abscessified and associated to rectovaginal fistulas or active disease as evidenced by rectoscopy.

 
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Figure 1.
 

Complex fistulas easily recur and respond poorly to conventional treatments. In this context, early anti-TNFα therapy may be considered in such cases.

The management strategy in patients with fistulizing disease addresses different aspects that are determined at the time of diagnosis, such as the number and complexity of the fistular tract, the origin of the fistula, and the presence of abscesses. Magnetic resonance imaging (pelvic MRI) is recommended as the initial diagnostic technique in patients with fistular disease, since it is noninvasive and very precise – improving the visualization of small inflammatory foci and modifying the surgical approach in up to 15% of all cases. It is also advisable to determine the inflammatory activity of the rectal mucosa via endoscopy (ECCO consensus 2006).

 
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Figure 2. Pelvic MRI in a patient with a complex fistula showing drainage and anal pain. A horseshoe-shaped fistula is observed, with abscessification of the tract.
 

 
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Figure 3. Left trans-sphincter fistula. Abscessification of the fistular tract. MRI characterizes the type of fistula, assessing the existence of an abscessified tract (hyperintensity in T2-weighted imaging). In these cases combined medical and surgical treatment is advisable.
 

In the algorithm for perianal disease it is necessary to consider the use of an imaging technique (MRI or echoendoscopy) for due evaluation, and seton drainage in the case of complex fistulas. When these strategies are associated to the most effective medical treatment, the short- (86%) and long-term (76%) are the highest responses, preventing the formation of abscesses during treatment.

 
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Figure 4. Seton drainage in abscessified complex fistulas, combined with the early use of anti-TNF drugs, favors complete closure of the fistula. (Reference 1)
 

Seton removal should be carried out when the data relating to the abscessification / inflammation of the fistular tract have disappeared. An imaging technique (MRI or echoendoscopy) prior to seton removal is recommended. A fistula without external drainage may exhibit activity along the internal fistular trajectory that can give rise to future recurrences. “Fistular closure” is a term reserved for fistulas without external drainage and with no evidence of activity according to the imaging findings.

 
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Figure 5. Seton removal once no evidence of inflammation is observed at MRI favors the maintenance of fistular disease remission.
 

 
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Figure 6. Residual perianal lesions following seton removal.
 

 
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Figure 7. Current appearance. The patient is subjected to maintenance treatment with adalimumab, and has been in complete fistular remission for over two years.

 
 

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