Correlation of the Anatomy of the Intersphincteric Anal Fistula with Sex: An Analysis through Anorectal Three-Dimensional Ultrasound

Abstract

Introduction Anal fistula surgery is often associated with continence disorders due to transection of the anal sphincter muscles. A comprehensive understanding of the anatomy of the anal canal and fistula can help prevent this outcome.

Objective To correlate the anatomy of the intersphincteric anal fistula with the patient's sex using three-dimensional endoanal ultrasound (3D-EAUS).

Materials and Methods The present is a retrospective observational study, involving an analysis of the medical records of patients seen at the Coloproctology Service of a Public Tertiary Hospital in the state of Maranhão, Brazil, from July 2016 to December 2022. Patients were categorized by sex and assessed for the position of the internal opening (IO), distance from the IO to the anal margin, and amount and percentage of internal anal sphincter (IAS) muscle compromised by the fistulous tract.

Results Intersphincteric fistulae were more common in men. The average age among men was of 46.46 years, and, among women, it was of 38.17 years. There was a difference between the sexes in terms of the duration of compromised IAS, which was longer among men. The percentage of compromised IAS was higher in males. The IO was located at a greater distance from the anal margin in male patients compared to female ones.

Conclusion Male patients with intersphincteric anal fistula had the internal fistulous opening positioned more distant from the anal margin, with a greater length and percentage of the IAS muscle compromised by the fistulous tract compared to female patients with the same condition.

Keywords

anal fistula - 3D endoanal ultrasound - anatomy

Introduction

The anal canal represents the last anatomical part of the digestive tract. This short segment is significant, not only due to its essential contribution to the physiological mechanism of continence, but also because it is susceptible to the development of various conditions, both benign and malignant. Within the scope of benign conditions, perianal fistula stands out, a frequent occurrence in proctological practice, characterized by causing discomfort, pain and the release of pyosanguineous secretion. These symptoms compromise patients' quality of life, making an appropriate therapeutic approach imperative.[1]

Most fistulas have a cryptoglandular origin, which means that they arise from acute inflammation of the anal glands, arranged circumferentially within the anal canal, at the level of the dentate line, accumulating purulent secretion and posteriorly draining through an external orifice in the anal margin. This drainage path originates from different types of anal fistulas, depending on their relationship with the anal sphincter muscles. Other etiologies are less prevalent, but of significant importance, such as those secondary to Crohn's disease, obstetric injury, radiation proctitis, foreign bodies, and infections, among others.[2] [3]

The first step in diagnosing an anal fistula is to take an anamnesis and perform a physical examination. In general terms, patients usually report the previous drainage of an abscess, either by surgery or spontaneously. Over time, this process culminates in the formation of an orifice that expels purulent or pyosanguineous secretion, either intermittently or continuously. During the physical examination, an external orifice (EO) is often identified on the anal margin and, on touch, the presence of a fovea can be noted, corresponding to the internal orifice (IO). If the course of the fistula is superficial, it can also be detected by palpating the anal margin.[4]

To complement the diagnosis, imaging tests are used to identify the components of the fistula (EO, fistulous tract, and IO) and document their relationship with the structures of the anal canal. It is essential to correlate its path with the anal sphincter muscles. This set of exams includes pelvic magnetic resonance imaging (MRI) and endoanal ultrasound, both of which allow the anatomy of the anal canal and the fistula to be described.[4] [5]

Three-dimensional endoanal ultrasound (3D-EAUS) enables the acquisition of multiplanar images (sagittal, coronal, and oblique planes), allowing precise measurement of the length of the anal sphincters. This enables the identification of important anatomical parameters for patients' surgical planning, such as the percentage of affected muscle and the distance from the internal orifice to the anal margin.[5]

Anal fistula is a condition historically documented in medical history and, to this day, remains a challenging pathology in terms of treatment. The management of anal fistulas is predominantly surgical, with a variety of techniques available, requiring in-depth knowledge. Surgical complications are diverse, ranging from recurrence to incontinence of varying degrees, due to the sectioning of the compromised sphincter muscle in the path of the fistula.[6] Therefore, it is of great importance to carry out a thorough analysis of the anatomy of the anal canal and assess the patient's state of continence before deciding on a surgical approach. Depending on the number of sphincters affected, it is possible to predict those patients who are at greater risk of incontinence and, thus, sphincter saving techniques would be more appropriate.[6] [7]

Therefore, by properly documenting the anatomy of the anal fistula using imaging exams, together with knowledge of the anal sphincter's involvement caused by the fistula, it is possible to identify patients at risk of postsurgical complications. The current study's objective is to prevent these complications by choosing the most effective surgical treatment, adapted to the specific characteristics of each patient, including the relationship between their sex and anal involvement. As well as to correlate the anatomy of the intersphincteric anal fistula based on the sex of the patient, using 3D-EAUS.

Materials and Methods

This is a retrospective cross-sectional study, with data obtained by evaluating the electronic medical records of patients treated at the Coloproctology Service at Hospital Universitário da Universidade Federal do Maranhão (HU-UFMA). The study was conducted in accordance with the ethical standards required and approved by the institutional Ethics in Research Committee, in agreement with the attributions defined in Resolution no. 466/2012 and Operational Standard no. 001 of 2013 of the Brazilian National Health Council (Conselho Nacional de Saúde, CNS, in Portuguese), under CAAE: 75976623.6.0000.5086.

All the patients evaluated presented with an anal fistula and underwent a 3D anorectal US examination between July 2016 and December 2022. Classification was subsequently carried out according to the type of anal fistula. The inclusion criteria were patients with intersphincteric anal fistula in whom it was possible to identify all the components of the fistula (IO, fistulous tract, and EO) through ultrasound analysis.

The Flex-Focus (BK Medical, Burlington, MA, USA) equipment was used. The patients were examined in the left lateral decubitus position and had undergone a rectal enema 2 hours before the examination. After rectal examination, the probe was introduced up to the anorectal junction. Two scans were carried out. The first scan was without contrast and the second scan was performed after the injection of 0.5 to 1.0 mL of 10.0% H2O2 through the EO using the plastic portion of an 18-or 21-G Jelco catheter (ICU Medical, Inc., San Clemente, CA, USA). The IO appears in the image as a hypoechoic rupture in the internal anal sphincter (IAS) and subepithelial tissue. When, however, H2O2 is applied, a hyperechoic area is clearly observed crossing the IAS.

To homogenize the study population and reduce bias, we excluded patients under the age of 18 years; those with sexually transmitted diseases in the anal region; those with associated anorectal conditions (inflammatory bowel disease, cancer); patients who had undergone previous pelvic radiation; those with previous sphincter lesions related to vaginal delivery or previous anorectal surgery; as well as those with more than one internal orifice.

The data was grouped according to sex, age and anatomical characteristics of the anal canal and anal fistula, these included the position of the internal orifice, the distance from the internal orifice to the anal margin, the amount and percentage of IAS muscle compromised by the fistulous tract.

The sample consisted of 37 patients with intersphincteric anal fistula, and the parameters assessed by 3D-EAUS were: length (cm) of the IAS, corresponding to the distance between the proximal and distal edge of the musculature; position of the IO in relation to the anal margin, corresponding to the distance between IO and the distal edge of the external anal sphincter (EAS); total length of the compromised sphincter, distance from the distal edge of the EAS to the point where the fistulous tract crosses the musculature; percentage of the EAS that would be sectioned during the operative procedure, calculated by dividing the size of the compromised sphincter by its total length. All measurements were made on multiplanar images. Two planes were used simultaneously to minimize measurement error.

Statistical analyses were performed using the GraphPad Prism (GraphPad Software Inc., La Jolla, CA, USA) software. Descriptive statistical methods (average, median, standard deviation, and interquartile range) were used to evaluate the data. As for the analytical methods, Student's t test was used to compare the measurements. The confidence interval (CI) adopted was 95% with a significance level of p < 0.05. The database and spreadsheets for statistical analysis, tables and graphs were prepared using GraphPad Prism and Excel (Microsoft Corp., Redmond, WA, USA).

Results

This study evaluated 37 patients with anal fistulas of cryptoglandular origin, classified as intersphincteric, with males being more prevalent, corresponding to 31 patients.

In males, the average age was 46.46 (18–70) years, and 6 patients had a secondary tract. Among women, the average age was 38.17 (20–52) years (p = 0.13), as shown in [Fig. 1]. Only one female patient had a secondary tract.

Fig. 1 Distribution of patients with intersphincteric anal fistula according to sex and age.

In terms of the positioning of the intersphincteric fistulas in the anal hemi-circumference, there was no significant difference in the positioning of the tract among men (15 in the anterior region and 16 in the posterior region). In women, however, the fistulous tracts were predominantly anterior (5 anterior and 1 posterior). There was no difference between the length of the IAS between the sexes (p = 0.085). In men, the average length of the IAS was 3.847 ± 0.471 (range: 3.200–4.780) cm. Among women, the average length of the IAS was 3.475 ± 0.456 (range: 2.960–4.070) cm ([Fig. 2]).

Fig. 2 Ratio of the length (in centimeters) of the internal anal sphincter (IAS) according to sex.

The average length of the impaired IAS was 1.327 ± 0.5594 cm, ranging from 0.400 to 2.700 cm in males. In women, the average length of the compromised IAS was 0.660 ± 0.262 (range: 0.310–0.930) cm, with a statistical difference between the sexes, with it being more compromised in men (p = 0.0077), as shown in [Fig. 3].

Fig. 3 Ratio of the length (in centimeters) of compromised IAS according to sex.

The percentage of compromised IAS was also higher in men (p = 0.025). The average percentage of compromised IAS in men was 34.50 ± 15.72% (range: 10.81–79.00%). In women, the average percentage of compromised EAI was 19.25 ± 8.142% (range: 9.09–30.06%), as shown in [Fig. 4].

Fig. 4 Relationship between sex and the percentage of compromised IAS.

The distance of the internal fistulous orifice from the anal margin was greater in men (p = 0.0151). In this sample, the internal orifice (IO) in men was positioned in a more proximal region of the anal canal when compared to the IO of fistulas in women. In males, the mean distance from the IO to the anal margin was 1.916 ± 0.6055 cm, ranging from 0.9 to 3.6 cm. In females, the average distance from the IO to the anal margin was 1.247 ± 0.3926 (range: 0.820–1.700) cm ([Fig. 5]).

Fig. 5 Relationship between sex and the distance from the internal orifice to the anal margin.

Discussion

Anorectal fistulas are common conditions in the field of coloproctology. From an epidemiologic point of view, intersphincteric fistulas are the most prevalent in the general population, with a higher incidence in males compared to females, as observed in this study.[8] According to Sahnan et al.,[9] adult men are twice as likely to develop an abscess and/or fistula compared to women. These findings corroborate the results of the study conducted by Murad-Regadas et al.,[10] which investigated the anatomical characteristics of anal fistulas using three-dimensional endorectal ultrasound (3DUS). This study included 37 (28 male and 9 female) patients with intersphincteric anal fistulas.

Anorectal fistulas are more common between 30 and 50-years old.[11] [12] The incidence of anal fistula developing from an anal abscess varies between 15 and 38%. The average age for the presentation of abscesses and fistulas is 40-years-old, with an age range from 20 to 60 years.[8] The sample of this study presented an average age similar to that described in the literature for the higher prevalence of anal fistula: 46.46 years among male patients, and 38.17 years among female subjects.

The diagnosis of anal fistulas is possible through a detailed anamnesis and a well-conducted physical examination. However, many fistulous tracts, especially the deeper ones, might not be identified during the physical examination. In these circumstances, for more appropriate and effective surgical planning, it is crucial to use imaging tests to highlight the relationship between them and the anatomical components of the patient's anal canal, especially the sphincters. In this context, 3D-EAUS plays a crucial role, allowing precise visualization of anatomical parameters that will influence the choice of surgical technique, as well as orienting patients regarding the therapeutic plan, aligning expectations in relation to the risk of recurrence and possible surgical complications, including fecal incontinence.[13]

The present study investigated the anatomical variations in intersphincteric anal fistulas between the sexes. The research is based on the premise that the anatomy of the anal canal differs between men and women, with it being shorter in females and longer in males, as indicated in the study by Regadas et al.[14] The pectineal line cannot be precisely identified by 3D-EAUS, but we can infer its position by the height of the crypts and where the internal orifice can be identified.[5]

We found no significant difference in the position of intersphincteric fistulas in the hemicircumference among men (15 anterior and 16 posterior). However, in women, there was a predominance of fistulous tracts in the anterior hemicircumference (5 anterior and 1 posterior). In contrast, in the study conducted by Murad-Regadas et al.,[10] there was a predominance of tracts in the posterior hemicircumference among women, diverging from the results found in the present study. Nonetheless, in a study by Alshiek et al.[7] evaluating 30 women with intersphincteric anal fistulas using 3D-EAUS, it was observed that the position of the tract and the internal orifice were similar between the hemicircumferences of the anal canal.

This imaging test can clearly identify the entire anatomy of the anal canal in different planes, showing the size and position of all anatomical structures and accurately depicting the configuration of the anal canal. The IAS is more proximal than the EAS, being formed by the thickening of the circular muscular layer of the rectal wall extending into the middle canal.[13]

Using 3D-EAUS, the study by Regadas et al.[14] described the distribution of the sphincter muscles in detail and highlighted sex differences, especially in the anterior quadrant. In the present study, the average length of the IAS was greater in men than in women, but this difference did not reach statistical significance (p = 0.085). In contrast, both the anterior and posterior IASs were significantly shorter in women than in men in the studies by Regadas et al.[14] and Murad-Regadas et al.[10]

The IAS was more affected by the fistulous tract in males than in females (1.32 versus 0.66 cm), and the percentage of compromised IAS was also higher in males (34.5 versus 19.25%). These results are consistent with the findings of Murad-Regadas et al.,[10] who also indicated that the fistulous tract crosses the IAS more proximally and compromises a higher percentage of the muscle in men, due to differences in the position of the pectineal line between the sexes. In the study by Murad-Regadas et al.,[6] which evaluated only women with intersphincteric anal fistulas, the average value of the IAS compromised by the fistulous tract (1.3 cm) was discrepant with the present study, as was the average percentage of the IAS that was cut (47%).

Regarding the distance of the internal orifice (IO) from the anal edge, Murad-Regadas et al.[15] in their study of anatomical differences in anterior transphincteric fistulas found that the length of the anterior IAS was significantly greater, and the IO was positioned significantly higher in men. This last finding is consistent with the results of the present study, in which a greater distance of the IO from the anal margin (more proximal IO) was also observed in men than in women.

Conclusion

Significant anatomical differences exist between the sexes regarding the anatomy and tracts of intersphincteric anal fistulas. In women, the internal orifice (IO) is closer to the anal margin due to the anatomically smaller anal canal compared to men. Male patients have a more proximal IO, a longer IAS, and consequently, present a higher degree of impairment of the IAS when compared to female patients These findings are relevant to the development of therapeutic strategies and the treatment of these patients.

Conflict of Interests

The authors have no conflict of interests to declare.

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Eingereicht: 20. Februar 2024

Angenommen: 06. September 2024

Artikel online veröffentlicht:
18. Dezember 2024