Preoperative magnetic resonance imaging of anal fistulas with scrotal extension

doi:10.21203/rs.3.rs-2450344/v1

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Preoperative magnetic resonance imaging of anal fistulas with scrotal extension

https://doi.org/10.21203/rs.3.rs-2450344/v1

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The aim of this study was to elucidate the magnetic resonance (MR) characteristics of anal fistulas extending to the scrotum, the rules applicable to them, and to correlate MR features with surgical findings. We conducted a retrospective study in 150 consecutive patients with anal fistulas extending into the scrotum, who were diagnosed and underwent surgery at University Medical Center Ho Chi Minh City between January 2017 and April 2022. MR findings were evaluated and compared with surgical findings using the Cohen’s kappa coefficient (k) with a 95% confidence interval. 150 patients (mean age 37.6 ± 10.9 years) with 166 fistulas, including 150 anal fistulas with scrotal extension. Most fistulas were low transsphincteric (80.0%, 120/150 patients). There was strong agreement for primary tract classification and detecting the location of internal openings between MRI and surgical findings with k = 0.83 (0.78–0.87) and k = 0.89 (0.85 – 0.93) (p<0.001), respectively. There is a significant correlation between the location of internal openings and the type of fistula (p<0.05). Low transsphincteric fistulas were predominant in the anterior group (103/122 patients vs. 10/19 patients), while in the posterior group it was more common in the high transsphincteric fistulas (7/19 patients vs. 14/122 patients), and the intersphincteric fistulas (1/19 patients vs. 5/122 patients); and the suprasphincteric fistulas were only seen in posterior group (1 patient). Anal fistulas with scrotal extension are exceptions to Goodsall’s rule. Albeit being long-tract fistulas, most of them are low transsphincteric and have anterior internal openings.

Health sciences/Gastroenterology/Gastrointestinal diseases/Anal diseases

Health sciences/Anatomy/Gastrointestinal system/Large intestine/Anus

Health sciences/Diseases/Gastrointestinal diseases/Anal diseases

anal fistula

scrotal extension

magnetic resonance imaging

Goodsall’s rule.

Anal fistula is a common disease in the anorectal region, defined as an abnormal tract connecting the anorectal canal and the perineal skin. Anal fistulas affect approximately 0.01% of the population. They are more common in men in their fourth decade, which is two- to four-fold compared to women [1–3].

In some cases of males with anal fistulas, the fistulas can extend into the scrotum. The presentations are variable in those cases. The scrotum usually presents with redness, swelling, pain, and pus discharge from the external opening, while anal symptoms may be absent or minimal [4–6]. Such presentations may be confusing with acute scrotal diseases. With a high index of suspicion, careful palpation can reveal the fistulous tract between the external opening into the scrotum and the internal opening into the anal canal. Involvement of the scrotum in anal fistulas can lead to diagnostic confusion, aggressive interventions, and reconstructive challenges [6–8].

Goodsall’s rule for predicting the trajectory of anal fistulas is popular; however, the long-tract anterior fistula, which may include fistulas with scrotal extension, is an exception to this rule [7]. Magnetic resonance (MR) is extremely useful in these cases thanks to its capability to provide precise anal canal anatomy and the relation of the fistula to perineal structures. It allows precise definition of the fistulous track and identification of secondary fistulas or abscesses, helping the surgeons avoid side effects (such as fecal incontinence) and decrease the incidence of recurrence [9, 10].

According to our best knowledge, this is the first study about the role of MR for preoperative evaluation of anal fistulas with scrotal extension. We conducted this study to elucidate the MR characteristics of anal fistulas extending to the scrotum, the rules applicable to them, and to correlate MR features with surgical findings.

2.1. Patients

A total of 150 consecutive patients with anal fistulas extending into the scrotum who were diagnosed and underwent surgery at University Medical Center, Ho Chi Minh City between January 2017 and April 2022 were included in this study. All included patients underwent preoperative MRI examination with intravenous contrast. The exclusion criteria were (a): patients having recurrent fistula, or (b) their operative records were not detailed (Fig. 1).

2.2. Magnetic resonance protocol

MR imaging was performed on either a 1.5 Tesla (MAGNETOM Avanto, Siemens Healthineers, Germany) or 3 Tesla (MAGNETOM Verio, Siemens Healthineers, Germany) system using six-channel body phased-array coil for pelvic imaging and no special patient preparation was required.

The MR protocol consisted of the following pulse sequences: sagittal T2-weighted (T2W) turbo spin-echo (TSE), oblique coronal T2W TSE with fat saturation (FS), and oblique axial T2W TSE with the imaging planes oriented orthogonal and parallel to the axis of anal canal, respectively.

The parameters for these T2W sequences were repetition time (TR) 3000–5000 ms, echo time (TE) 75–85 ms, field of view (FOV) 20–25 cm, turbo factor (TF) 17, slice thickness 3–4 mm, intersection gap 0.6–0.7mm, matrix 320x256 (for sagittal and coronal plane) or 320x320 (for axial plane), number of signals acquired (NSA) 2. The T1-weighted (T1W) TSE FS with gadolinium enhancement (post-contrast) (0.2ml/kg) were obtained in three planes (axial, coronal, and sagittal). The parameters for post-contrast T1W TSE FS are TR 600–700, TE 10–12 ms, FOV 20–25 cm, TF 3, slice thickness 3–4 mm, intersection gap 0.6–0.7mm, matrix 320x256 (for coronal plane) or 320x320 (for sagittal and axial plane), NSA 2.

2.3. Image analysis

We retrospectively reviewed the patient’s medical records, including their operative records and MR findings. The MR images were analyzed by two radiologists who had more than five years of experience in MR fistulography. We classified anal fistulas in the study according to Park’s classification, which includes intersphincteric, transsphincteric, suprasphincteric, and extrasphincteric fistulas. We further classified transsphincteric fistulas as high or low as follows: high transsphincteric fistulas pass through the upper half of the external anal sphincter, whereas low transsphincteric fistulas traverse the lower half [11, 12].

We defined the scrotal extension of the anal fistula (with or without external openings as well as the distance between external opening and anal verge) using the physical examination results reported in the operation records. On MR, internal openings were detected when the tract terminated in the intersphincteric space [11, 13]. The location of the internal openings was described according to anal clock positions (Fig. 2). Openings at the 10, 11, 12, 1, and 2 o’clock positions were classified as anterior, while those at the 3, 4, 5, 6, 7, 8, and 9 o’clock positions were classified as posterior. A one-hour difference between MR images and surgery was considered acceptable.

2.4. Statistical analysis

Statistical analysis was conducted using STATA version 16 (STATA Corp., Texas, USA). Obtained data were presented as mean and standard deviation (with normal distribution) or median and interquartile range (with skewed distribution), ranges, numbers, and ratios. Numbers and ratios were analyzed using the Fisher’s exact test. A p-value < 0.05 was considered statistically significant.

MR findings were compared with surgical findings using the Cohen’s kappa coefficient (k) with a 95% confidence interval. The degree of agreement was interpreted as follows: ≤ 0 as no agreement, 0.01–0.20 as none to slight, 0.21–0.40 as fair, 0.41– 0.60 as moderate, 0.61–0.80 as substantial, and 0.81–1.00 as almost perfect agreement.

3.1. Patient characteristics

150 male patients having anal fistulas with scrotal extension were eligible for the study. The mean age at surgery was 37.6 ± 10.9 years (range 14–79 years). The age group of 30–50 was the most common (62.0%), followed by the age group of under 30 (26.0%) and over 50 (12.0%). Most fistulas had histological findings of non-specific inflammation (98.7%, 148/150), only 2 patients were due to tuberculosis. The mean duration from MR acquisition to surgery was 1.5 ± 1.8 days (range 0–15 days).

3.2. MR images analysis

There was a total of 150 patients with 166 fistulas, including 136 patients with one fistula (with scrotal extension), 12 patients with two fistulas (one fistula with scrotal extension and another fistula), and 2 patients with three fistulas (one fistula with scrotal extension and two other fistulas).

3.2.1. Scrotal extension (with and without external opening in the scrotum)

The external opening was detected in 142/150 (94.7%) patients with 159 external openings. 129 patients (86.0%) had one, 10 patients (6.7%) had two, 2 patients (1.3%) had three, and 1 patient (0.7%) had four external openings. All external openings were more than 3 cm from the anal verge. In the remaining 8 patients without external openings detected, the presence of scrotal involvement was recorded based on surgical reports. Including these 8 cases, the total number of locations where scrotal involvement took place was 167. Among the aforementioned 167 locations, we noticed a left-side predominance (38.9%, 65/167), followed by midline (33.5%, 56/167) and right-side (27.6%, 46/167).

3.2.2. Primary tract

Each of 150 patients had only one fistula-in-ano extending to the scrotum. According to Parks classification, most of the fistulas were classified as low transsphincteric (80.0%, 120/150 patients), followed by high transsphincteric (14.7%, 22/150 patients), intersphincteric (4.0%, 6/150 patients), and suprasphincteric fistula (1.3%, 2/150 patients). There was strong agreement for primary tract classification between MR and surgical findings with a kappa value of 0.83 (95% CI: 0.78–0.87) (p < 0.001), which is demonstrated in Table 1.

Table 1

Agreement between MR and surgery in the classification of primary tract according to the Parks classification. Data are presented as numbers of primary tracts. Kappa value: 0.83 (95% CI: 0.78–0.87), p < 0.001.
Surgery MRI	Low trans	High trans	Inter	Supra	Total
Low trans	114	5	1	0	120
High trans	1	19	1	1	22
Inter	0	0	6	0	6
Supra	0	0	0	2	2
Total	115	24	8	3	150
Low trans: low transsphincteric fistula, High trans: high transsphincteric fistula, Inter: intersphincteric fistula, Supra: suprasphincteric fistula.

3.2.3. Internal opening

141 internal openings were detected in 150 patients. The most common location of the internal openings was from 11 o’clock to 1 o’clock (73.3%) at the level of dentate line, followed by 6 o’clock (9.3%). The remaining positions were less common. There was good agreement in detecting the location of internal openings between MR and surgical findings with a kappa value of 0.89 (95% CI: 0.85–0.93) (p < 0.001), which is shown in Table 2.

Table 2

Agreement between MR and surgery in detecting the location of internal openings. Data are presented as numbers of internal openings. Kappa value: 0.89 (95% CI: 0.85–0.93), p < 0.001.
Surgery MRI	1h	2h	3h	6h	7h	8h	9h	10h	11h	12h	No i.n	Total
1h	12	0	0	0	0	0	0	0	0	4	1	17
2h	0	3	1	0	0	0	0	0	0	0	0	4
3h	0	0	1	0	0	0	0	0	0	0	0	1
6h	0	0	0	13	0	0	0	0	0	0	3	16
7h	0	0	0	0	1	0	0	0	0	0	0	1
8h	0	0	0	0	0	0	0	0	0	0	0	0
9h	0	0	0	0	0	0	1	0	0	0	0	1
10h	0	0	0	0	0	0	3	3	0	0	0	6
11h	0	0	0	1	1	0	0	1	12	1	0	16
12h	5	0	0	0	0	0	0	0	2	71	1	79
No i.n	1	0	0	0	0	1	0	0	1	1	5	9
Total	18	3	2	14	2	1	4	4	15	77	10	150
i.n = internal opening

We observed a significant correlation between the location of internal opening (anterior or posterior to the transverse anal line) and the fistula type (Table 3). Low transsphincteric fistulas accounted for the majority of fistulas having anterior internal openings (84.4%). In the posterior group, the percentages of low transsphincteric and non-low-transsphincteric fistulas (including intersphincteric, high transsphincteric and suprasphincteric fistulas) were roughly comparable (52.6% and 47.4%, respectively).

Table 3

The relation between the location of internal openings and the fistula type. Data are presented as numbers of patients. There was a significant correlation (p < 0.05).
Fistula type Location of i.n	Low trans	High trans	Inter	Supra	Total
Anterior group	103	14	5	0	122
Posterior group	10	7	1	1	19
No i.n	7	1	0	1	9
Total	120	22	6	2	150
Low trans: low transsphincteric fistula, High trans: high transsphincteric fistula, Inter: intersphincteric fistula, Supra: suprasphincteric fistula, i.n: internal opening

Occasionally, an anal fistula can extend into the scrotum, causing pain, edema, pus disharge from the external opening and making clinicians confused with other scrotal disorders. Our study found that anal fistulas with scrotal extension occurred mainly in men around the age of 40, which is consistent with previous studies [2, 3].

We observed that patients with anal fistulas extending into the scrotum had one to three fistulas, zero to four external openings, but the majority finding was one fistula extending into the scrotum with one external opening.

According to Parks classification, anal fistulas can be classified into four types based on their relationship to anal sphincters: intersphincteric, transsphincteric, suprasphincteric, and extrasphincteric in which the intersphincteric fistulas were the most common (70%) [14]. In our study, the most prominent fistula type was low transsphincteric (76.7%). This is in accordance with the results of Araki’s study, which reported that low transsphincteric was the most prevalent categorization (82.6%) among anal fistulas extending to the scrotum. Because the Colles’ fascia which is composed of loose connective tissue overlays the anterior portion of the superficial external sphincter muscle, anterior low transsphincteric fistulas can penetrate the external sphincter and directly enter this fascia. Meanwhile, high transsphincteric and suprasphincteric fistulas with posterior internal openings often curve anteriorly in the the subcutaneous fat tissue of ischiorectal fossa instead of penetrating the Colles’ fascia.

In the present study, there was strong agreement for primary tract classification between MR imaging and surgical findings, qualifying MR as a valuable modality in the preoperative classification of anal fistulas. On MR, high transsphincteric fistulas passed through the upper half of the external sphincter, whereas low transsphincteric fistulas traversed the lower half (Figs. 3 and 4). The above-mentioned MR classification of transsphincteric fistulas is acceptable and simple to apply, especially when each part of the external sphincter is not clearly defined. The proposed classification has been used for deciding between non-sphincter-preserving and sphincter-preserving procedures [12, 18]. There were instances when surgery and MR disagreed on categorizing transsphincteric fistulas as high or low. This may be due to the difference between surgery and MRI in the selection of anatomical landmarks. Surgeons divided transsphincteric fistulas into high and low based on the parts of the external sphincter being penetrated: a fistula was considered high transsphincteric if it penetrated the deep external sphincter, while one penetrating the superficial or subcutaneous external sphincter was deemed low transsphincteric [7].

The location of scrotal involvement in our study was most common in the left scrotum (38.9%), followed by the midline (33.5%) and the right scrotum (27.6%). This is consistent with the study of Araki et al. [7], which also reported that the left scrotum was the most common site of scrotal extension. A study of 367 patients [11] demonstrated that most intersphincteric and low transsphincteric fistulas had external openings located less than 3 cm from the anal verge, whereas the majority of high transsphincteric, suprasphincteric and extrasphincteric fistulas had their external orifices situated more than 3 cm from the anal verge. By contrast, in our work, although all external openings were more than 3 cm away from the anal verge, the fistulas were mostly low transsphincteric.

Proper identification of the internal opening is an integral part of fistula surgery to avoid recurrence [15]. On MR, we may not be able to visualize the location of the fistula opening into the anal canal mucosa, where its signal is as high as that of the fistula. As a result, some authors have suggested looking for the area of maximal intersphincteric inflammation to detect the internal opening [10, 11]. The good agreement between MR and surgery in detecting the location of internal openings illustrated in our study supports the notion that MR is a reliable modality for the preoperative assessment of internal openings in anal fistulas with scrotal extension. The most common location of the internal openings was from 11 o’clock to 1 o’clock (74.7%) at the mid-portion of the anal canal, corresponding to the level of the dentate line on surgery followed by 6 o’clock (10.7%). This is in accordance with the Midline rule mentioned in previous reports, which stated the midline crypt was the primary internal origin of all anal fistulas regardless of the external opening location. In comparison with Goodsall’s rule, the Midline rule more accurately predicted the natural course of anal fistulas, especially those with anterior-based external openings, including fistulas with scrotal extension [16, 17].

We observed a significant correlation between the location of internal opening (anterior or posterior to the transverse anal line) and the fistula type (p < 0.05). This is consistent with the previous reported [7, 18] which showed that the majority of anal fistulas extending into the scrotum were low transsphincteric fistulas with the anterior internal opening. Also, high transsphincteric and suprasphincteric fistulas usually had posterior internal openings. The Goodsall’s rule states that if the external opening is anterior to the transverse anal line, the fistula will penetrate radially and open into the anterior wall of the anal canal provided it is less than 3 cm from the anal verge, or else it will open in the midline posteriorly. When lying more than 3 cm from the anal verge, anterior fistulas may have a curved track, which is similar to posterior fistulas [4, 19] (Fig. 5). This study shows that the anal fistula with scrotal extension was an exception to Goodsall’s rule: though the external openings were more than 3 cm from the anal verge, most fistulas had an anterior internal opening (86.5%, 122/141 patients) (Fig. 3). Anal fistulas that do not comply with Goodsall’s rule have the risk of creating iatrogenic false tracks and openings when being probed [16, 20]. Accordingly, it is not secure for the surgeons to solely rely on Goodsall’s rule to identify the internal openings in cases where the tracts extend to the scrotum. In these instances, MR is a valuable and reliable modality for preoperative evaluation, which eventually helps reduce the risk of complications (fecal incontinence, urinary tract damage, and sexual dysfunction) as well as recurrence. This is consistent with the previous reported [7, 18] which showed that the majority of anal fistulas extending into the scrotum were low transsphincteric fistulas with the anterior internal opening. Also, high transsphincteric and suprasphincteric fistulas usually had posterior internal openings.

Though the external openings are more than 3 cm from the anal verge, most anal fistulas with scrotal extension are low transsphincteric and have anterior internal openings, making them exceptions to Goodsall’s rule. On the other hand, they are consistent with the Midline rule, so we suggest surgeons utilize the Midline rule when dealing with fistulas extending to the scrotum. We recommend performing magnetic resonance in fistulas with scrotal extension thanks to its excellent performance in classifying the primary tract and detecting the internal opening, which are major contributors to surgical prognosis.

Funding: This research received no external funding.

Acknowledgments: The authors wish to thank Dr. P.H.T. Phuoc who helps us in the design of illustrations.

Author Contributions: Conceptualization, Duc Vo and Chien Phan; Methodology, Truc Nguyen; Formal Analysis, Truc Nguyen and Chien Phan; Investigation, Truc Nguyen and Nam Nguyen; Data Curation, Truc Nguyen and Chien Phan.; Writing – Original Draft Preparation, Truc Nguyen and Nam Nguyen; Writing – Review & Editing, Duc Vo and Chien Phan and Linh Nguyen and Thien Nguyen; and all the authors revised and commented on the paper and approved the final version of the manuscript. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement: The study was conducted according to the guidelines of the Declaration of Helsinki, and approved by the Ethics Committee of the University of Medicine and Pharmacy in Ho Chi Minh City (IRB-VN01002/IORG0008603/FWA00023448, dated 11 November 2022).

Informed Consent Statement: All the patients, or their guardians, provided written informed consent prior to their involvement in the study.

Data Availability Statement: The data presented in this study are available on request from the corresponding author. The data are not publicly available in order to limit the amount of publicly-available patient personal information.

Conflicts of Interest: The authors declare no conflict of interest.

de Miguel Criado, J., et al. MR imaging evaluation of perianal fistulas: spectrum of imaging features. Radiographics 2012, 32, 175–194.
Torkzad, M.R.,Karlbom, U. MRI for assessment of anal fistula. Insights Imaging 2010, 1, 62–71, doi:10.1007/s13244-010-0022-y.
Domínguez, A.,Pitrella, A.,Noceti, M. Perianal Fistulas: Magnetic Resonance Characterization. Rev. Argent. Radiol 2017, 81, 129–134.
Dr K. Sri Bhargavi,Dr A. Ramya,Dr V. Mohan Rao. A Rare Case of Long Complex Ano-Scrotal Fistula, Review of Literature and Its Management with LIFT. International Journal of Science and Research (IJSR) 2019, 8, 270–272.
Kumar, T.S.,Naresh, G.,Akther, M.J. Lift procedure for long complex ano-scrotal fistula and review of literature. International Surgery Journal 2016, 4, 423–426.
Alahmari, A.,Mansouri, M.,Alwaal, A. Complex anal fistula involving male genital tract: a new diagnostic entity. BMJ Case Reports CP 2019, 12, e231702.
Araki, Y.,Kagawa, R.,Yasui, H.,Tomoi, M. Rules for anal fistulas with scrotal extension. Journal of the Anus, Rectum and Colon 2017, 1, 22–28.
Shindhe, P.S. Management of rare, low anal anterior fistula exception to Goodsall's rule with Kṣārasūtra. Ancient Science of Life 2014, 33, 182.
Sharma, G.,Khandige, G.,Mohan, M. Magnetic resonance imaging in perianal fistulas—A pictorial atlas. Indian Journal of Gastroenterology 2016, 35, 337–342.
Halligan, S.,Stoker, J. Imaging of fistula in ano. Radiology 2006, 239, 18–33.
Vo, D., et al. The role of magnetic resonance imaging in the preoperative evaluation of anal fistulas. Scientific reports 2019, 9, 17947, doi:10.1038/s41598-019-54441-2.
van Onkelen, R.S.,Gosselink, M.P.,van Rosmalen, J.,Thijsse, S.,Schouten, W.R. Different characteristics of high and low transsphincteric fistulae. Colorectal disease: the official journal of the Association of Coloproctology of Great Britain and Ireland 2014, 16, 471–475, doi:10.1111/codi.12578.
Beets-Tan, R.G., et al. Preoperative MR imaging of anal fistulas: does it really help the surgeon? Radiology 2001, 218, 75–84.
Parks, A.G.,Gordon, P.H.,Hardcastle, J.D. A classification of fistula-in-ano. The British journal of surgery 1976, 63, 1–12, doi:10.1002/bjs.1800630102.
Abou-Zeid, A.A. Anal fistula: intraoperative difficulties and unexpected findings. World journal of gastroenterology: WJG 2011, 17, 3272.
Cirocco, W.C.,Reilly, J.C. It is time to retire Goodsall's Rule: the Midline Rule is a more accurate predictor of the true and natural course of anal fistulas. Techniques in coloproctology 2020, 24, 317–321, doi:10.1007/s10151-020-02167-z.
Cirocco, W.C.,Reilly, J.C. Challenging the predictive accuracy of Goodsall's rule for anal fistulas. Diseases of the colon and rectum 1992, 35, 537–542, doi:10.1007/bf02050532.
Araki, Y., et al. Analysis of Deep Posterior Anal Fistulas by Magnetic Resonance Imaging: Site of Primary Abscess and Extension Patterns According to the Primary Abscess Depth. Journal of the anus, rectum and colon 2018, 2, 130–138, doi:10.23922/jarc.2018-002.
Muralidhar, V.,R, G.S.,Krishnamurthy, A. Long anterior anal fistula with an external opening in the medial raphe of the scrotum is likely to open radially inside the anal canal: an addition to the Goodsall's rule. BMJ case reports 2018, 2018, doi:10.1136/bcr-2018-227560.
Coremans, G.,Dockx, S.,Wyndaele, J.,Hendrickx, A. Do anal fistulas in Crohn's disease behave differently and defy Goodsall's rule more frequently than fistulas that are cryptoglandular in origin? The American journal of gastroenterology 2003, 98, 2732–2735, doi:10.1111/j.1572-0241.2003.08716.x.

No competing interests reported.

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Preoperative magnetic resonance imaging of anal fistulas with scrotal extension

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Abstract

Figures

1. Introduction

2. Materials And Methods

2.1. Patients

2.2. Magnetic resonance protocol

2.3. Image analysis

2.4. Statistical analysis

3. Results

3.1. Patient characteristics

3.2. MR images analysis

3.2.1. Scrotal extension (with and without external opening in the scrotum)

3.2.2. Primary tract

3.2.3. Internal opening

4. Discussion

5. Conclusion

Declarations

References

Additional Declarations

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