The Value of Transrectal Ultrasound in the Preoperative Diagnosis of Complex Anal Fistula (CAF): Based on a Retrospective Cohort Study

OBJECTIVE: A case-control study was employed to retrospectively analyze the value of transrectal ultrasound in the preoperative diagnosis of complex anal fistula (CAF). METHODS: The clinical data of 128 patients with CAF treated in our hospital from March 2019 to June 2021 were analyzed retrospectively. All patients were examined by transrectal ultrasound and MRI with Hitachi HI Vision Ascendus ultrasound diagnostic apparatus and MRI. The general data of the patients (age, sex, course of disease, complications, and previous operation history) and ultrasonic image characteristics were recorded. The consistency of internal orifice, head, branch/abscess, and abscess detected by ultrasound, MRI, and ultrasound combined with MRI were compared, and the sensitivity, accuracy, and specificity of ultrasound, MRI, and the combination of ultrasound and MRI (ultrasound+MRI) in the diagnosis of different Parks classification of anal fistula (AF) were compared. RESULTS: The ultrasound images of the rectal probe in typical cases were compared with the MRI images. The characteristics of the ultrasound images were as follows: the outer orifice of AF was a thin strip of mixed echo or low echo leading to the skin side, and the inner orifice showed local dilated low echo, mixed echo, or interruption of mucosal continuity. The following are the MRI image features: abnormal long bar signal shadow from the dorsal side of the end of the coccyx to the S5 plane, low signal on T1WI, high signal on T2WI, blurred boundary, uneven signal, bifurcation in the lower end of the tail for “Y” shape, one branch opening at the body surface at about 6 o'clock, the other walking horizontally, passing through the levator ani muscle to the right posterior position of the rectum at about 6:00 o'clock, and penetrating the inner mouth of the rectum at 6 o'clock. The detection of internal orifice, head, branch/abscess, and abscess were compared by three examination methods. There was significant difference in the detection rate of internal orifice and branch/purulent cavity among the three methods (P < 0.05). The detection rates of internal mouth and branch/abscess cavity by ultrasound and MRI (94.77% and 94.94%) were higher than those by single ultrasound (75.16% and 79.78%) and MRI (81.05% and 83.15%) (P < 0.05). There was no significant difference in the detection rate of ultrasound, MRI internal orifice, and branch/purulent cavity (P > 0.05). There was no significant difference in the detection rate of supervisor and abscess among the three methods (P > 0.05). The results of operation included transsphincter type (n = 53), intersphincter type (n = 45), and superior sphincter type (n = 30). Analysis of transsphincter type AF detected by three methods: 42 cases of transsphincter type AF and 86 cases of nonsphincter type AF were detected by ultrasound, 36 cases of transsphincter type AF and 92 cases of nontranssphincter type AF were detected by MRI, 57 cases of transsphincter type AF and 71 cases of nonsphincter type AF were detected by ultrasound and MRI. The comparison of the efficacy of the three methods in the diagnosis of transsphincter AF and the sensitivity of the three methods in the diagnosis of transsphincter AF showed significant difference (P < 0.05). The sensitivity of ultrasound and MRI in the diagnosis of transsphincter AF (96.23%) was higher than those of single ultrasound (67.92%) and MRI (64.15%) (P < 0.05). There was no significant difference in the accuracy and specificity of the three methods in the diagnosis of transsphincter AF (P > 0.05). There were 41 cases of intersphincter type AF and 87 cases of nonsphincter type AF detected by ultrasound, 38 cases of intersphincter type AF and 90 cases of nonsphincter intersphincter type AF detected by MRI, and 45 cases of intersphincter type AF and 83 cases of nonsphincter intersphincter type AF detected by ultrasound and MRI. The sensitivity and accuracy of the three methods in the diagnosis of intersphincter AF were statistically significant (P < 0.05). The sensitivity and accuracy (100.00% and 100.00%) of ultrasound and MRI in the diagnosis of intersphincter AF were higher than those of single ultrasound (66.67% and 79.69%) and MRI (71.11% and 85.16%) (P < 0.05). There was no significant difference in the specificity of the three methods in the diagnosis of intersphincter AF (P > 0.05). The results of three methods were compared, including 24 cases of superior sphincter type AF and 89 cases of nonsuperior sphincter type AF, 21 cases of superior sphincter type AF, and 107 cases of nonsuperior sphincter type AF detected by MRI and 93 cases of superior sphincter type AF and 128cases of nonsuperior sphincter type AF detected by ultrasound and MRI. There was no significant difference in the sensitivity, accuracy, and specificity of the three methods in the diagnosis of superior sphincter AF (P > 0.05). CONCLUSION: The sphincter, anorectal, and surrounding tissues were clearly demonstrated by transrectal ultrasound. The internal orifice, head, branch/abscess, abscess, and the relationship between abscess and sphincter in the diagnosis of CAF were in good agreement with the surgical results. Ultrasound+MRI can take into account the advantages of ultrasound and MRI, make up for each other, and improve the detection rate of internal orifice and branch/abscess. It can improve the sensitivity of diagnosis of transsphincter AF and the sensitivity and accuracy of intersphincter AF, which can provide intuitive and valuable imaging information for surgical intervention.