In an earlier study of 104 patients by Buchanan et al [28], comparing endoanal US with MRI, endoanal US correctly classified 81% of perianal fistulae compared with 90% by MRI. Their accuracy for detection of the internal opening was 91% by endoanal US and 97% by MRI. Endoanal US is limited by the field of view and depth of penetration. Accuracy for identification of extrasphincteric and suprasphincteric tracts (50% and 67%, respectively) is lower compared with transsphincteric and intersphincteric tracts (93% and 88%, respectively) [27]. Other limitations include obscuration of the tract or secondary extensions by gas in the tract or gas in an associated abscess. In patients with recurrent disease, it may be impossible to distinguish tracts with active inflammation from those with fibrosis and granulation tissue, which is often a clinical question in patients with CD. Hydrogen peroxide may be injected into the external opening of the fistula during endoanal US to enhance visualization of fistula tracts [29]. A tract filled with hydrogen peroxide is brightly hyperechoic on endoanal US, improving its visualization and connection to abscess cavities as well as differentiating it from scar tissue [25]. Variant 2: Suspected rectal fistula. Rectovesicular or rectovaginal. Initial imaging. Rectovaginal and rectovesicular fistulae are uncommon. The most common cause of a rectovaginal fistula is obstetric or vaginal trauma (88% of cases) [32], followed by CD, which accounts for approximately 9% of cases [33]. Other causes include radiation; pelvic infections (diverticulitis, tuberculosis, lymphogranuloma venereum, human papilloma virus, HIV, cytomegalovirus, and schistosomiasis); malignancies of the anorectum, perineum, and gynecologic organs; and iatrogenic injury and postoperative complications. Upon initial imaging, the organ of origin may be unknown. Rectovaginal fistulae are subclassified as high or low fistulae.
Anorectal Disease. In an earlier study of 104 patients by Buchanan et al [28], comparing endoanal US with MRI, endoanal US correctly classified 81% of perianal fistulae compared with 90% by MRI. Their accuracy for detection of the internal opening was 91% by endoanal US and 97% by MRI. Endoanal US is limited by the field of view and depth of penetration. Accuracy for identification of extrasphincteric and suprasphincteric tracts (50% and 67%, respectively) is lower compared with transsphincteric and intersphincteric tracts (93% and 88%, respectively) [27]. Other limitations include obscuration of the tract or secondary extensions by gas in the tract or gas in an associated abscess. In patients with recurrent disease, it may be impossible to distinguish tracts with active inflammation from those with fibrosis and granulation tissue, which is often a clinical question in patients with CD. Hydrogen peroxide may be injected into the external opening of the fistula during endoanal US to enhance visualization of fistula tracts [29]. A tract filled with hydrogen peroxide is brightly hyperechoic on endoanal US, improving its visualization and connection to abscess cavities as well as differentiating it from scar tissue [25]. Variant 2: Suspected rectal fistula. Rectovesicular or rectovaginal. Initial imaging. Rectovaginal and rectovesicular fistulae are uncommon. The most common cause of a rectovaginal fistula is obstetric or vaginal trauma (88% of cases) [32], followed by CD, which accounts for approximately 9% of cases [33]. Other causes include radiation; pelvic infections (diverticulitis, tuberculosis, lymphogranuloma venereum, human papilloma virus, HIV, cytomegalovirus, and schistosomiasis); malignancies of the anorectum, perineum, and gynecologic organs; and iatrogenic injury and postoperative complications. Upon initial imaging, the organ of origin may be unknown. Rectovaginal fistulae are subclassified as high or low fistulae.