Unraveling the Causes of Perianal Itching: Magnetic Resonance Imaging as a Diagnostic Tool for Perianal Fistulas and Its Mimics

Abstract

Background:

Perianal itching is a common complaint associated with a variety of anorectal conditions such as fistulas, abscesses, hemorrhoids and pilonidal disease. Accurate diagnosis is essential for effective treatment and magnetic resonance imaging (MRI) plays a pivotal role due to its excellent soft-tissue resolution and multiplanar imaging.

Objective:

The objective of this study was to evaluate the diagnostic utility of MRI in patients with perianal itching, focusing on the identification and classification of perianal fistulas and their mimics, using St. James and Parks classification systems.

Materials and Methods:

This prospective observational study was conducted in the department of radiodiagnosis over 6 months (December 2023–May 2024) and included 59 patients aged ≥ 18 years with perianal itching. MRI (1.5T) pelvis scans were obtained using T1, T2, short tau inversion recovery, diffusion-weighted imaging, apparent diffusion coefficient and postcontrast sequences. Fistulas were classified using the St. James and Parks systems. Chi-square test assessed the correlation between the classifications.

Results:

Among 59 cases, perianal fistula was most common (57.6%), followed by abscess (33.9%), pilonidal sinus (5.1%) and hemorrhoids (3.4%). The mean age was 45.8 ± 10.6 years with male predominance. Grade 2 fistulas ( 47.06%) and intersphincteric type (64.70%) were most frequent. A strong correlation (P < 0.001) was observed between St. James and Parks classifications. Common complications included abscesses (70.6%) and horseshoe tracts (14.7%). MRI findings showed high concordance with clinical and surgical data.

Conclusion:

MRI is a sensitive, noninvasive tool for evaluating perianal itching. It effectively characterizes underlying pathology, guides treatment planning, shows strong classification correlation and enhancing clinical decision-making.

INTRODUCTION

Perianal disorders do give considerable discomfort, morbidity and often manifest symptoms of pruritus, pain or discharge.[1] Perianal fistulas constitute a very troublesome pathological condition as they are an abnormal communication of a tract between the anal canal and perianal skin.[2,3] Usually, these fistulae develop due to an infection in the cryptoglandular glands, wherein abscess formation takes place, followed thereafter by the fistulous formation.[3] If not treated and well managed, perianal fistulas can give rise to symptom persistence, repeated infections and a host of complications including anal incontinence and septic condition of the perianal region.[4] It is well worth differentiating perianal fistulas from similar conditions such as perianal abscess, pilonidal sinus and hemorrhoidal disease for the purpose of proper management and treatment options. The imaging techniques comprise computed tomography (CT), fistulography, anal endosonography and magnetic resonance imaging (MRI).[5] MRI remains the most favored imaging modality for the study of perianal fistulas, offering great soft tissue contrast for exact delineation of the fistulous tract, its extensions and any complications involved.[5,6] MRI is instrumental while doing a preoperative evaluation as it helps in identifying secondary tracts, internal openings and abscesses of a perianal fistula and their relationship with the sphincter complex.[7] Traditional imaging modalities, such as fistulography and CT scans are limited by their inability to accurately visualize the complex anatomy of the perianal region and detect secondary fistulous extensions.[5,8]

There are several classification systems for perianal fistulas with the St. James University Hospital classification being most widely accepted. This system categorizes fistulas based on complexity, sphincter involvement and the presence of secondary extensions, aiding in both treatment planning and prognostication.[9] Proper classification and assessment of perianal fistula using MRI techniques are concurred in maximizing surgical success; thus, recurrence is less likely and reduces the chance of sphincter damage.[5] Since perianal fistulas are commonly encountered and seriously affect the quality of life of the patient, the present study was designed to evaluate the role of MRI in the diagnosis and differentiation of perianal fistulas from mimics. By analyzing anatomical features, complications and associated conditions in a series of patients who presented with perianal itching, the work highlights the role of MRI in guiding clinical decision-making.

MATERIALS AND METHODS

Study design and setting

A prospective observational study was conducted in the department of radiodiagnosis over 6 months, from December 2023 to May 2024. Approval was obtained from the Institutional Ethics Committee (Ref. No. IEC/2023/RA/05), and written informed consent was acquired from all enrolled patients.

Patients presenting with perianal itching and suspected perianal fistula or its mimics were screened based on predefined inclusion and exclusion criteria. Based on the inclusion and exclusion criteria, 60 patients were included in the study out of which one patient was excluded from the study due to a prior history of perianal surgery.

Inclusion criteria

1. Patients aged 18 years and above presenting with perianal itching and symptoms suggestive of perianal fistula, abscess or other perianal pathologies

2. Patients who provided written informed consent for participation in the study

3. Patients without contraindications to MRI imaging.

Exclusion criteria

1. Patients with known malignancy or history of prior perianal surgery

2. Patients with contraindications to MRI, including pacemakers and metallic implants

3. Patients who refused to provide informed consent.

Magnetic resonance imaging protocol

All patients underwent MRI using a 1.5T scanner with a phased-array coil. The following sequences were obtained:

1. T1-weighted (T1W) images – axial, coronal and sagittal

2. T2-weighted images – axial

3. Short tau inversion recovery (STIR) sequences – axial, coronal and sagittal

4. Diffusion-weighted imaging (DWI) and apparent diffusion coefficient – axial

5. Postcontrast T1W images (where necessary).

Image analysis

MR images were evaluated for internal and external openings, fistulous tract location and extent, scarring, sphincter muscle connection and transmural inflammation.

Classification of fistula was categorized based on Park’s classification, whereas grading was done based on St. James’s University Hospital classification.

Data collection and statistical analysis

Demographic and clinical data, including age, gender and imaging findings, were recorded. Statistical analysis was performed using SPSS software (version 27.0, IBM Corporation, Armonk, NY, USA). Descriptive statistics were used to summarize patient characteristics, fistula classification and associated complications. Continuous variables were expressed as mean ± standard deviation, whereas categorical data were presented as frequencies and percentages. The Chi-square test was applied to assess associations between categorical variables. Statistical analyses were performed at a 95% confidence level and a P < 0.05* was considered statistically significant.

RESULTS

The study included 59 patients out of which 34 cases were of perianal fistula, 20 cases were perianal abscess, 3 cases of pilonidal sinus and 2 cases of hemorrhoids. The age distribution of patients varied from 21 to 70 years, with a male predominance. The mean age of the study population was 45.80 ± 10.55 years.

Among the 20 cases of abscesses, the highest occurrence was in the 31–40 age group (35%), followed by 41–50 years (25%) and 51–60 years (20%). The condition was more common in males (65%) than in females (35%). Fistula cases were most frequently observed in the 31–40 age group (37.5%), followed by 51–60 years (28.1%) and 41–50 years (25%). Males (78.1%) were more commonly affected than females (21.9%). Hemorrhoids were reported in only two cases, one in the 41–50 age group (male) and one in the 61–70 age group (female), each contributing 50% of the total cases. The pilonidal disease was noted in three patients, with the majority of males in the 31–40 age group (66.7%) followed by a single female case in the 41–50 age group [Table 1].

Table 1.

Age and gender distribution of patients with perianal itching (n=59)

Age groups (years)	Sex
	Male	Female	Total
Abscess (years) (n=20)
21–30	0	1 (14.30)	1 (5)
31–40	3 (23.10)	4 (57.10)	7 (35)
41–50	4 (30.80)	1 (14.30)	5 (25)
51–60	3 (23.10)	1 (14.30)	4 (20)
61–70	3 (23.10)	0	3 (15)
Total	13 (100)	7 (100)	20 (100)
Fistula (years) (n=34)
21–30	2 (5.9)	0	2 (5.9)
31–40	10 (38.5)	2 (25.0)	12 (35.3)
41–50	6 (23.1)	3 (37.5)	9 (26.5)
51–60	7 (26.9)	3 (37.5)	10 (29.4)
61–70	1 (3.8)	0	1 (3.8)
Total	26 (100)	8 (100)	34 (100)
Hemorrhoids (years) (n=2)
41–50	1 (100)	0	1 (50)
61–70	0	1 (100)	1 (50)
Total	1 (100)	1 (100)	2 (100)
Pilonidal (years) (n=3)
31–40	2 (100)	0	2 (66.70)
41–50	0	1 (100)	1 (33.30)
Total	2 (100)	1(1000)	3 (100)

Data expressed in frequency and percentage, n (%)

The sphincter involvement was observed in 20% of abscess cases with the external sphincter being affected in 15% and the internal sphincter in 5% [Graph 1].

Graph 1.

Patient of perianal abscess with spincter muscle involvement

Fistula classification and distribution

As far as comorbidities were recorded, the majority of the cases (82.40%) had no comorbidities, followed by diabetes mellitus in 11.80% of cases, followed by inflammatory bowel disease and tuberculosis (2.9% each) [Graph 2], St. James University Hospital criteria (an MRI-based grading system) and the Parks classification (a surgical system based on anatomical location) were used to classify the fistulas. Under the St. James criteria, most fistulas were Grade 2, as seen in (47.06%) patients, followed by Grade 4 (20.59%), Grade 1 (17.65%), Grade 3, (8.82%), and Grade 5 (5.88). For the Parks classification, the majority of cases were intersphincteric (64.70%), with transsphincteric fistulas representing 29.41%, and both extrasphincteric and suprasphincteric types accounting for 2.94% each [Table 2].

Graph 2.

Comorbidities associated with perianal fistula (n = 34)

Table 2.

Classification of perianal fistulas based on St. James criteria and parks classification (n=34)

Classification	n (%)
St. James University Hospital criteria
Grade 1	6 (17.65)
Grade 2	16 (47.06)
Grade 3	3 (8.82)
Grade 4	7 (20.59)
Grade 5	2 (5.88)
Parks classification
Extrasphinteric	1 (2.94)
Intersphinteric	22 (64.70)
Suprasphinteric	1 (2.94)
Transsphinteric	10 (29.41)
Total	34 (100)

Data expressed in frequency and percentage, n (%)

The correlation between the St. James MRI-based classification and the Parks anatomical classification revealed that Grade 1 and Grade 2 fistulas were exclusively intersphincteric. Grade 3 and Grade 4 fistulas were entirely transsphincteric, indicating increasing complexity. Grade 5 fistulas were classified as either extrasphincteric or suprasphincteric. A statistically significant association was observed between the two classifications (P < 0.001), suggesting that higher MRI grades are associated with more complex anatomical fistula types [Table 3].

Table 3.

Association between St. James grade and Parks type (n=34)

ST. James classification	Type (parks classification)					P
	Extrasphinteric	Intersphinteric	Suprasphinteric	Transsphinteric	Total
Grade 1	0	6 (27.27)	0	0	6 (17.65)	<0.001*
Grade 2	0	16 (72.73)	0	0	16 (47.06)
Grade 3	0	0	0	3 (30.00)	3 (8.82)
Grade 4	0	0	0	7 (70.00)	7 (20.59)
Grade 5	1 (100)	0	1 (100)	0	2 (5.88)
Total	1 (100)	22 (100)	1 (100)	10 (100)	34 (100)

Data expressed in frequency and percentage, n (%), using Chi-square test at a 95% confidence level, with a P<0.05 (*) considered statistically significant

Complications and associated findings

MRI analysis revealed the presence of secondary abscesses, supralevator extension and horseshoe collections [Graph 3].

Graph 3.

Magnetic resonance imaging findings of complications in perianal fistula cases (n = 34)

MRI successfully identified internal openings, secondary abscesses and tract extensions. DWI shows restricted diffusion in all cases of perianal abscesses. The imaging findings correlated well with intraoperative observations, reinforcing the diagnostic accuracy of MRI.

The majority of pilonidal sinus cases were observed in males (66.7%), with an average lesion length ranging from 17 mm to 61 mm. All cases had external openings and abscess formation was noted in all patients. No cases showed anal sphincter involvement [Table 4].

Table 4.

Distribution of pilonidal cases (n=3)

Length (mm)	External Opening	Cranial extension (mm)	Caudal extension (mm)	Distance from coccyx (mm)	Anal sphincter involved	Abscess
17	Yes	5	3	3	No	Yes
61	Yes	10	6	6	No	Yes
20	Yes	4	5	5	No	Yes

Overall, these results highlight the effectiveness of MRI in diagnosing perianal fistulas, detecting associated complications, and distinguishing them from other perianal conditions.

DISCUSSION

In our study, MRI not only identified perianal fistulas and their complications but also effectively differentiated them from mimicking conditions such as perianal abscesses, pilonidal sinus and hemorrhoids. Accurate distinction is essential for appropriate management as treatment strategies vary significantly among these entities.

In our cohort, perianal fistulas were the most frequent pathology (57.6%) among patients presenting with perianal itching, followed by perianal abscesses (33.9%), pilonidal sinus (5.1%) and hemorrhoids (3.4%). These results align with Chail et al., who reported perianal fistulas in 77% of their cases, followed by sinuses (12%), abscesses (7%) and pilonidal sinus (1.7%).[10] Similarly, Agha et al. emphasized the predominance of fistulas in perianal pathology and underlined the necessity of MRI for preoperative evaluation.[9]

Our study observed a mean age of 45.80 ± 10.55 years with a clear male predominance. This is consistent with studies by Kakani et al. (mean age ~ 43 years),[11] Terzi et al. (median age 43 years)[12] and Joyce et al., who also noted a higher incidence in middle-aged males.[7]

Perianal abscess

Perianal abscesses constituted 33.9% of our study population. These are typically caused by infection of the anal glands and can present acutely with pain, swelling and occasionally discharge. MRI proved particularly valuable in identifying these abscesses, especially when clinical findings were equivocal. DWI sequences were instrumental, demonstrating restricted diffusion with rim enhancement, confirming abscess formation [Figure 1]. This finding is in agreement with Daabis et al.[13] and Zhao et al.[14] who emphasized the role of MRI in identifying deep-seated or multiloculated abscesses that may be missed on ultrasound or clinical examination. In our study, abscesses were most frequently noted in the 31–40 age group, with a male predominance of 65%. Importantly, sphincter involvement was observed in 20% of abscess cases, highlighting the need for careful evaluation to prevent postoperative incontinence.

Figure 1.

Short tau inversion recovery (STIR) axial (a), diffusion-weighted imaging axial (b) and apparent diffusion coefficient axial (c) images show well-defined heterogeneous collection with diffusion restriction with thick rim seen in perianal region anteriorly extending from 12 to 3’o clock position and extending inferiorly on the left side along the natal cleft reaching just up to the skin surface with mild surrounding edema and inflammatory changes suggestive of abscess

Pilonidal sinus

Pilonidal sinus was noted in 5.1% of our patients, mostly affecting males in the 31–40 age group. All cases showed external openings and associated abscess formation. MRI was effective in identifying the sinus tracts and their cranial/caudal extensions, providing detailed measurement of tract length and relation to the coccyx [Figure 2]. No sphincter involvement was observed in any case, which differentiates it from fistulous disease. Arkenbosch et al.[6] and Liang et al.[8] also demonstrated that MRI can accurately distinguish pilonidal sinuses from posterior midline fistulas. The detailed imaging provided by STIR sequences enabled confident diagnosis and preoperative planning in these cases.

Figure 2.

Saggital STIR (a) and axial STIR (b) images show a hyperintense pilonidal sinus in the right side of the natal cleft region at the level of the lower sacrum and coccyx with surrounding edema. There is no evidence of STIR hyperintensity in the coccyx

Hemorrhoids

Hemorrhoids were identified in 3.4% of patients, with one case each in the 41–50-and 61–70-year age groups. MRI findings included ill-defined hyperintense lesions on STIR sequences in the lower anal canal, suggestive of inflamed hemorrhoidal tissue [Figure 3]. Although not the primary modality for diagnosing hemorrhoids, MRI can incidentally detect thrombosed or prolapsed hemorrhoids, especially when patients are referred for other perianal complaints. These incidental findings are crucial in differentiating hemorrhoids from more complex pathologies. Gage et al.[5] and Joyce et al.[7] also noted the utility of MRI in identifying such secondary findings, which may contribute to perianal discomfort or pruritus.

Figure 3.

STIR coronal (a) and STIR axial (b) images show an Ill-defined heterogeneously hyperintense signal area in the lower anal canal in the anterior aspect and extending in the perianal region on the left side lesion just below the anal verge no evidence of any obvious internal/external opening suggestive of hemorrhoids

Fistula classification

According to the St. James classification, Grade 2 fistulas were the most prevalent (47.06%), followed by Grade 4 (20.59%) and Grade 1 (17.65%). Similar trends were found in studies by Chail et al.[10] and Madany et al.[15] In contrast, Singh et al. observed a higher incidence of Grade 1 lesions (42%).[16]

For the Parks classification, intersphincteric fistulas (64.70%) were most common [Figures 4 and 5], followed by transsphincteric types (29.41%). This contrasts with Kakani et al., who reported transsphincteric (51.6%) as more common.[11] Our findings are consistent with those of Arkenbosch et al.[6] who emphasized the diagnostic precision of MRI in classifying fistula types, directly influencing surgical strategy.

Figure 4.

STIR axial (a and c) and STIR coronal (b and d) images show an inter-sphincteric STIR hyperintense tract of approximately 18 mm in length runs in the left ischiorectal fossa with an external opening in the gluteal cleft on the left side. The fistulous tract is running between the left internal sphincter (green line) and the left external sphincter (red line)

Figure 5.

STIR sagittal (a) STIR axial (b and d) and STIR coronal (c and e) images show linear fistulous STIR hyperintense tract between the right internal sphincter (green line) and right external sphincter (red line) suggestive of intersphinceteric fistula with an external opening in the perianal region at 9’o clock position. The tract is seen coursing superiorly and piercing the internal sphincter at the 10 o’clock position. No evidence of any abscess is seen. No evidence of any supralevater extension

Complications and associated findings

Secondary abscesses were present in 70.6% of cases, underscoring the value of MRI in identifying such complications early. Supralevator extension (2.9%) [Figure 6] and horseshoe collections (14.7%) were also detected, reinforcing MRI’s capacity to visualize complex anatomical involvement. The frequency of secondary tracts (2.9%) in our study was lower than the 20% reported by Zhao et al.,[14] which could reflect differences in disease severity. The presence of these complications significantly influences management and prognosis.

Figure 6.

STIR axial (a), T2 axial (b) and STIR coronal (c and d) shows trans-sphincteric hyperintense fistulous tract in the right ischiorectal fossa with extension of fistula (green highlight) above the levator ani muscles (red line)

Our findings showed strong concordance between MRI findings and surgical observations, supporting MRI as the gold standard for perianal pathology evaluation. The use of contrast-enhanced and diffusion-weighted sequences improved differentiation between active inflammatory tracts and fibrotic ones.

The St. James University Hospital classification system showed a clear correlation with clinical outcomes and surgical findings, consistent with previous research by Morris et al.,[16] and Madany et al.[15] This reinforces the value of MRI-based grading in treatment planning and risk stratification.

CONCLUSION

MRI stands out as a powerful, noninvasive diagnostic tool in the evaluation of patients with perianal itching, a symptom that often masks a spectrum of underlying anorectal conditions. In this study, MRI not only identified perianal fistulas with high accuracy but also differentiating them from mimicking conditions such as abscesses, pilonidal sinuses and hemorrhoids. This distinction is crucial, as each pathology demands a different clinical and surgical approach.

Incorporating MRI into the diagnostic algorithm for patients presenting with perianal symptoms ensures a more accurate, comprehensive and individualized approach to care. Its detailed anatomical insights support better treatment decisions, reduce the risk of recurrence or complications and ultimately enhance patient outcomes.

Conflicts of interest

There are no conflicts of interest.

Funding Statement

Nil.