Kidney cysts affect approximately 5% of the general population and are categorized as simple or complex [1]. Simple kidney cysts are often found in normal kidneys, with incidence increasing with age [2]. The Bosniak classification system categorizes cysts into five groups (I, II, IIF, III, and IV) with malignancy risk ranging from nearly zero (Bosniak I) to over 85% (Bosniak IV) [3]. Benign simple renal cysts (Bosniak I and II) do not pose a malignancy risk and typically require intervention when symptomatic (such as infection, hemorrhage, and mass effect). Infection is a rare complication of simple benign renal cysts, with a prevalence of approximately 2.5% [4,5]. While a course of antibiotic therapy is generally the first-line treatment, definitive operative intervention may be necessary for chronic pain, recurrent urinary tract infection, hematuria, abscess formation, and/or impaired renal function [1].

A 74-year-old male patient presented at our hospital with worsening lower abdominal pain and fever. He reported a 1-month history of exacerbated lower abdominal pain that worsened during ambulation. The associated symptoms include fever, chills, frequent urination, urgency, and nocturia. Past history included hypertension and myocardial infarction. The patient had tenderness in the left costovertebral angle (CVA). Pelvic computed tomography (CT) revealed a 57 mm cyst located in the lower pole of the left kidney. Imaging revealed perirenal fat stranding and thickening of the perirenal fascia (Fig. 1).

The initial vital signs are recorded as blood pressure (BP) 153/92 mmHg, heart rate 125 bpm, respiratory rate 16 breaths per minute, and body temperature (BT) 38.0℃. Laboratory results include a white blood cell (WBC) count of 16,310 cells/μL (80% neutrophils), hemoglobin (Hb) of 16.3 g/dL, creatinine (Cr) of 1.06 mg/dL, C-reactive protein (CRP) of 10.92 mg/L, troponin-T of 0.027 ng/mL, and prostate-specific antigen of 1.83 ng/mL. Urinalysis reveals 5-9 red blood cells and 1-4 WBCs per high-power field, with negative bacteria. An infected renal cyst was diagnosed and ciprofloxacin was administered.

On the following day, the BT decreased from 37.2℃ to 36.5℃, and the hemodynamics remained stable. Laboratory findings included a WBC count of 12,870 cells/μL (81.8% neutrophils), Hb of 13.1 g/dL, Cr of 1.03 mg/dL and CRP of 16.49 mg/L. Despite a slight elevation in CRP levels, leukocytosis improved and the fever resolved. Blood and urine cultures were negative for bacteria. The BP persisted with a slight hypotensive tendency, and self-BP medications were discontinued.

On the fourth day, the fever abated spontaneously to 36.6℃ and left CVA tenderness improved significantly. Next day peak fever rose to 37.7℃ and then immediately decreased. Systolic BP remained stable, and symptoms improved. Cardiac arrest at night ventricular tachycardia was observed during intubation, leading to direct-current cardioversion. However, this results in a pulseless electrical activity. Return of spontaneous circulation was achieved after cardiac massage, and the patient was transferred to the intensive care unit. Laboratory analysis revealed WBC of 14,360/μL (42.4% neutrophils), CRP of 9.09 mg/L, procalcitonin of 0.16 ng/mL, CK-MB of 1.57 ng/mL, and Troponin-T of 0.028 ng/mL. Electrocardiogram revealed sinus rhythm without ST-segment changes. The infection markers showed a decreasing trend. Although it is challenging to ascertain the failure of antibiotic treatment, the antibiotic was escalated to meropenem because of a suspected septic condition. CT revealed a cyst measuring approximately 5.6 cm with wall thickening and perilesional fat infiltration suggestive of a progressive infection (Fig. 2). The following day, percutaneous drainage (PCD) was performed, yielding approximately 50 mL of purulent fluid resembling pus. Subsequently, extubation was performed. Enterobacter cloacae identified in the PCD pus culture was susceptible to ciprofloxacin and ertapenem.

On the first week, the PCD volume decreased and there was serial improvement in vital signs and laboratory parameters. The antibiotic was changed to ciprofloxacin, a narrow-spectrum antibiotic selected based on PCD pus culture results. On the second week, a follow-up kidney ultrasonography revealed a reduction in the size of the complicated renal cyst from 6.00 to 3.27 cm. The PCD tubogram indicates a minimal residual cavity, leading to PCD removal. On the 16th day, the patient was discharged.

After discharge, residual flank pain persisted, despite a CRP of 0, and the infected renal cyst, typically requiring antibiotic treatment for 4-6 weeks, warranted the continuation of medication therapy [6,7]. CT follow-up revealed a significantly diminished size of the cyst with a persistently irregular thick rim in the lower pole of the left kidney (Fig. 3). Antibiotic therapy was discontinued on the 28 days after discharge, completing 6 weeks of antibiotic administration.

Six months prior, a follow-up renal ultrasonography was performed. The cyst in the lower pole of the left kidney, which decreased in size after the PCD insertion, showed further reduction.

Management protocols for infected renal cysts are primarily based on cases involving individuals with autosomal dominant polycystic kidney disease (ADPKD), where symptoms often arise because of the severity of the cystic condition [8]. In a review outlining the therapeutic approaches for patients with ADPKD and infected renal cysts, initial treatment typically involves antibacterial agents, particularly fluoroquinolones. This preference is attributed to the superior efficacy of fluoroquinolones in penetrating cysts, owing to their lipophilic properties and bactericidal activity against Gram-negative enteric pathogens [9]. Furthermore, the therapeutic regimen includes cyst puncture, drainage, cyst aspiration, and surgical intervention. Notably, patients experiencing treatment failure with initial antibiotic therapy commonly exhibited urinary stones (6%) or large cysts (diameter ≥ 5 cm, 27%) [9].

Physicians typically base their treatment decisions on symptoms and physical examination findings. However, in the case of the aforementioned patient, early PCD intervention for an infected renal cyst > 5 cm in size reduced the potential risk of precipitating cardiac arrest. This approach mirrors the management strategy for renal abscesses, where PCD, along with antibacterial treatment, is employed when the abscess surpasses 5 cm [1]. It is widely acknowledged that relying solely on antibiotic therapy may prove insufficient due to compromised antibiotic penetration into the cyst. Moreover, in the context of infected renal cysts, urine culture often yields negative results, whereas bacterial culture from cyst aspiration proves positive [10]. Employing active aspiration and drainage for infected renal cysts facilitates the identification of bacteria, aiding in the selection of appropriate antibiotics.

Fortunately, the cyst in this case was amenable to drainage through the PCD. If the cyst had been in a more challenging location, such as a parapelvic cyst or near the renal hilum, a surgical approach would have been necessary [5].

This case report highlights the essential requirement of active drainage in conjunction with antibacterial therapy for infected renal cysts > 5 cm in size.