Clinical History
Elderly male patient with comorbidities including ischaemic heart disease, atrial fibrillation, and obstructive lung disease, currently hospitalised because of relapsing sterile pericardial effusion after previous pericardiocentesis.
Clinical course complicated by onset of septic fever with severely increased acute phase reactants, negative haemocultures after empiric, intensive antibiotic treatment.
Imaging Findings
Four months earlier, ultrasound (Fig. 1) did not disclose abnormal findings. A sizeable (6 cm) simple renal cyst was noted at the middle third of right kidney.
At admission, body CT requested to investigate possible silent septic sources was performed without intravenous contrast because of renal dysfunction (40 ml/min estimated glomerular filtration rate). The previous site of the right-sided renal cyst was occupied by a larger (11x8 cm) fluid-attenuation collection with minimal, uniform wall thickening and internal gas bubbles (Fig. 2).
Clinical and imaging suspicion of renal cyst superinfection (probably resulting from bacteraemia) was confirmed by positioning of percutaneous drainage, yielding immediate aspiration of 350 ml sero-purulent fluid which tested positive for Escherichia coli infection. With improved renal function, performance status and laboratory tests during hospitalisation, follow-up contrast-enhanced CT (Fig. 3) and unenhanced CT acquisition eight days later (Fig. 4) showed progressive collapse of the abscess cavity. He was finally discharged after drainage removal.
Discussion
A common incidental finding in the general adult population, simple renal cysts are confidently diagnosed on imaging studies on the basis of their characteristic anechoic ultrasound appearance with posterior through-transmission, homogeneous fluid-like CT attenuation, markedly T1-hypointense and T2-hyperintense MRI signal, without mural thickening, irregularities, and contrast enhancement. Exceptionally associated with renal impairment or symptoms, sporadic cysts do not require further workup or treatment. One of the most common hereditary disorders, autosomal polycystic kidney disease (ADPKD) occurs in 1:400-1:1000 white individuals, and includes progressive development of cysts in both kidneys, liver, and other organs, ultimately leading to renal enlargement, decreased function, and hypertension [1].
Whereas at least 30% of patients with ADPKD experience at least one episode of infection during lifetime, renal cyst superinfection (RCS) is an exceptional occurrence in the general population. RCS may result from bacteraemia or from ascending urinary tract infection. In either case, cyst suppuration represents a serious complication, potentially leading to sepsis and death [2-4].
With RCS, the usual clinical manifestations include fever and flank pain. Alternatively, as in this patient RCS may underlie an unexplained sepsis. In most cases, superinfection requires aggressive treatment with drainage or nephrectomy to eradicate infection [3].
At imaging, RCS may be suggested by abrupt increase in size or changed mural or endoluminal features of a known simple cyst. In most cases, RCS mimics a intra- or extraparenchymal renal abscess, appearing as a variable-echogenicity or complex collection with through-transmission at ultrasound. As this case demonstrates, currently multidetector CT is the mainstay modality to comprehensively investigate potential sources of unexplained sepsis. At CT, RCS appears as a hypoattenuating collection, sometimes with intraluminal gas, without internal enhancement but with an enhancing rim representing a pseudocapsule with variable thickness. Similar appearances are detected by MRI, including inhomogeneous content, thickened and enhancing walls. Perirenal fluid and/or fat stranding may be associated as ancillary findings [4-6].
In conclusion, the uncommon possibility of RCS should be suspected in patients with clinical and laboratory signs of sepsis associated with pre-existent renal cysts, particularly if multiple or hereditary. In such instances, prompt imaging assessment with enhanced CT or MRI is warranted. Alternatively, renal cyst superinfection may be confirmed by ring-like increase of 18F-fluorodeoxyglucose (FDG) uptake at FDG positron emission tomography (PET) [2-4].
Differential Diagnosis List
Renal abscess from simple cyst superinfection.
Uncomplicated simple renal cyst
Haemorrhagic renal cyst
Complex renal cyst
Pyelonephritis
Pyonephrosis
Urinary tuberculosis
Renal carcinoma
Final Diagnosis
Renal abscess from simple cyst superinfection.
