Management of Complex Anal Fistula in Recurrent Perianal Abscess: A Case Report

Introduction

Perianal abscesses primarily result from anal gland infections41]. Bacteria infiltration of perianal tissues through the openings of the anal glands leads to localized abscess formation. Standard treatment involves surgical incision and drainage [2,3]. However, disease progression may be complicated by the formation of an anal fistula, particularly an occult anal fistula, predisposing patients to recurrent abscesses and complicating etiological assessment.

This report presents the case of a 32-year-old woman with recurrent perianal abscesses following incision and drainage. This recurrent perianal abscess may be originated from anal gland infection, evidenced by polymicrobial flora (gram-positive cocci, gram-negative bacilli) [1,4,5]. The patient was overweight (BMI 28.23 kg/m2) and her diabetes was not well controlled, which may have exacerbated recurrence via impaired immunity and bacterial proliferation [6,7]. MRI confirmed a high complex-type anal fistula undetected by initial imaging, explaining the atypical thigh abscesses [8,9]. The fistula’s trajectory through pelvic muscles sustained the chronic infection, while MRI revealed persistent inflammation and fluid signals. Anatomic complexity and intermittent fistula closure can limit early detection, underscoring the need for MRI in refractory cases with atypical presentations. This case provides clinical insights into the diagnostic and therapeutic challenges associated with this condition.

Case Report

A 32-year-old woman (height: 163 cm, weight: 75 kg, BMI: 28.23 kg/m2) presented on June 15, 2023 with swelling and pain in the right buttock and thigh, without identifiable predisposing factors. Initial treatment with oral amoxicillin (0.5 g, tid) and ibuprofen (0.3 g, bid) proved ineffective. Twelve days after treatment, her symptoms worsened, with the onset of fever, chills, and hypotension (75/52 mmHg). Laboratory evaluation revealed the following: WBC 38.11×109/L, RBC 3.08×1012/L, HGB 80 g/L, LY% 1.8%, MONO% 4.2%, NEUT% 93.6%, PLT 361×109/L, and hsCRP 141.4 mg/L. Based on these findings, she was diagnosed with septic shock and admitted to the Emergency Department. She had a 5-year history of diabetes and self-administered oral metformin extended-release tablets (0.5 g, bid), with no regular monitoring of blood glucose levels and other significant comorbidities.

Computed tomography (CT) imaging revealed encapsulated fluid and gas collection adjacent to the rectum and right cervix, with mild wall thickening. The infection extended posteriorly to the subcutaneous tissue near the coccyx, the right gluteus maximus, and the posterolateral right thigh muscles. Swelling and multiple gas-density shadows observed in the right gluteus maximus suggested an ongoing infection. Multidisciplinary consultation found no definitive evidence of an anal fistula. CT-guided percutaneous drainage of the abscess was performed, and Gram staining of the aspirate identified numerous gram-positive cocci and a moderate number of gram-negative bacilli. Targeted anti-infective therapy was initiated based on bacterial culture and susceptibility testing, resulting in a gradual decrease in drainage output and resolution of clinical symptoms. The drainage tube was removed on July 14, 2023.

Three days after drainage tube removal, the patient experienced recurrent fever and chills (maximum temperature of 38.9°C) with no apparent cause. Right-thigh tenderness recurred, extending 3 cm below the right knee, with associated redness, induration, and elevated skin temperature. Laboratory tests on July 17, 2023 showed the following: PLT 613×109/L, WBC 13.41×109/L, NEUT% 86.7%, RBC 3.59×1012/L, HGB 92 g/L, hsCRP 238.10 mg/L, and PCT 0.37 ng/mL. By July 19, 2023, her symptoms had further worsened. CT imaging (Video 1) revealed abscess formation in the right thigh, prompting incision and drainage (Figure 1). Intraoperative exploration found no definitive rectal or anal fistula. Debridement and vacuum sealing drainage (VSD) were then performed, leading to the patient’s recovery and discharge on August 24, 2023.

By October 2023, the incision had properly healed, and a follow-up ultrasound revealed heterogeneous hypoechoic areas in the right buttock (2.2×1.8 cm) and right gluteal-subcutaneous region (4.6×0.8 cm). However, on November 30, 2023, the incision site became raised and ulcerated (Figure 2), with malodorous and purulent discharge. A sinus tract measuring approximately 7 cm in depth was identified, raising suspicion of an anal fistula. A second ultrasound showed a heterogeneous hypoechoic area in the perianal region at the 8–9 o’clock position, measuring 5.8×2.9 cm, with indistinct boundaries relative to the anal canal. Additionally, a cord-like hypoechoic tract measuring 7.1×0.9 cm extended to the surface drainage site. Laboratory tests showed WBC 6.42×109/L, NEUT% 70.9%, and hsCRP <0.50 mg/L. Given these findings, the incision was extended to facilitate drainage.

On December 16, 2023, the patient was admitted for surgical wound repair. Sinus tract radiography revealed no clear communication between the sinus tract and the anal canal. Intraoperative exploration confirmed that the sinus tract terminated approximately 2 cm from the rectum. Digital rectal examination and methylene blue testing showed no evidence of an anal fistula (Figure 3). Debridement and sinus tract drainage were successfully performed, resulting in the patient’s recovery and discharge on January 22, 2024.

On February 21, 2024, the mid-portion of the incision ruptured again (Figure 2), with purulent discharge and a 7-cm-deep sinus tract. These findings raised renewed suspicion of an anal fistula, prompting patient referral to a specialized colorectal hospital for further evaluation. MRI revealed an abnormal signal at the 6 o’clock position of the anal canal at the dentate line (internal opening), approximately 25 mm from the anal verge. The fistula tract extended upward between the sphincter muscles to the right levator ani at the 7–11 o’clock position. The tract traversed the puborectalis muscle at the 5 o’clock position, descending into the right ischiorectal fossa and reaching the right gluteus maximus, with a maximum width of approximately 8 mm. A cord-like T2WI hypointense signal observed in the right gluteus maximus demonstrated high signal intensity on fat-suppressed images, extending downward to the posterior right thigh and connecting with the identified lesions. Fluid signals were observed within the fistula tract, with localized thickening of the right levator ani, external anal sphincter, and puborectalis muscles, while the left levator ani appeared normal. These imaging findings were consistent with a high complex-type anal fistula.

The patient subsequently underwent high anal fistulectomy and seton treatment twice for definitive repair of the anal fistula in March and May in 2024. One-year postoperative follow-up showed no evidence of recurrence, and anal function was well preserved, with no incontinence (Table 1 shows the timeline of clinical events of this patient).

Discussion

This case provides valuable insights into the diagnostic challenges and clinical implications of managing refractory perianal infections. The patient’s clinical course highlights the interplay of anatomical, microbiological, and systemic risk factors, as well as the limitations of standard diagnostic approaches in detecting occult fistulas.

Initial imaging with CT and ultrasound failed to detect the fistula, demonstrating their limited sensitivity in detecting complex or occult fistulas. In contrast, MRI, with its superior soft-tissue resolution, emerged as the criterion standard for accurately delineating fistula anatomy [10]. This case underscores the importance of early MRI evaluation in recurrent abscesses with atypical presentations, particularly in high-risk individuals such as diabetics. The delayed diagnosis, along with repeated negative results from methylene blue testing and digital rectal examination, shows the limitations of isolated clinical assessments in detecting complex fistulas. This case highlights the need for a multidisciplinary approach involving colorectal surgeons, radiologists, and infectious disease specialists to improve diagnostic accuracy. Early referral to specialized centers with advanced imaging modalities could expedite diagnosis, facilitate timely intervention, and minimize morbidity. The need for 2 definitive fistula surgeries after multiple failed drainage emphasizes the challenges in managing complex perianal infections. Given the suprasphincteric trajectory of the fistula, sphincter-preserving techniques were crucial to prevent incontinence. This case reinforces the principle that definitive surgical repair of fistulas, rather than repeated drainage alone, is essential for preventing recurrence [11]. The patient’s obesity (BMI 28.23 kg/m2) and diabetes potentially contributed to impaired immune responses and delayed tissue repair, exacerbating disease progression [12]. Elevated inflammatory markers, including hsCRP 141.4 mg/L, and WBC 38.11×109/L, observed during septic episodes reflected systemic immune activation. However, transient normalization of hsCRP (<0.50 mg/L) during quiescent phases masked ongoing localized infection, reflecting the complexity of monitoring disease progression in immunocompromised individuals.

Recurrent abscess formation after adequate drainage should immediately raise suspicion of an underlying fistula, even in the absence of initial radiological or clinical evidence. This case demonstrates that occult fistulas may only become evident after multiple recurrences or through advanced imaging. Systemic anti-infective therapy is crucial in early management of complex perianal infections. In this case, conservative treatment without early surgical intervention led to significant improvements in local symptoms and inflammatory markers, highlighting the effectiveness of targeted anti-infective therapy. However, the presence of an occult fistula or inadequate drainage contributed to the disease progression, necessitating surgical intervention. Aggressive glycemic control and weight management are essential for reducing infection risk and improving surgical outcomes in diabetic patients [13].

Conclusions

This report highlights the diagnostic and therapeutic challenges associated with perianal abscesses complicated by occult fistulas, particularly in patients with diabetes and obesity. Key clinical insights include: (1) Occult fistulas should be strongly considered in high-risk individuals; (2) MRI should be prioritized in refractory or anatomically atypical cases; (3) Definitive surgical repair, rather than repeated drainage alone, is essential for long-term resolution; (4) Multidisciplinary collaboration and early referral to specialized centers can optimize patient outcomes.

Future research should focus on identifying biomarkers and refining imaging protocols to predict fistula formation in high-risk patients and to enable preemptive intervention. Clinicians should maintain a high index of suspicion for occult fistulas in recurrent perianal infections, utilizing advanced diagnostics and multidisciplinary approaches to improve diagnostic accuracy and minimize morbidity.