Teaching Case

Elderly male patient with multiple comorbidities including pulmonary emphysema, cardiac arrhythmia, previous coronary bypass surgery, hospitalised with upper abdominal pain and fever (38.5°C) despite empiric antibiotic therapy. Stable vital signs, absent peritonism at physical examination.
Laboratory disclosed elevated C-Reactive protein (70 mg/L), creatinine (1.82 mg/dL), normal lipase, cholestasis and liver function.

Sixteen months earlier contrast-enhanced multidetector CT (MDCT) and MRI with MR-cholangiopancreatography (MRCP) were performed to investigate choledocholithiasis after recent cholecystectomy, then treated with endoscopic sphincterotomy. Simple liver cysts with fluid attenuation, imperceptible walls and absent enhancement were present, the largest (3.5 cm) in the 4th segment (Fig. 1).
Currently ultrasound (not shown) and repeated MRCP detected markedly enlarged dominant liver cyst compared to baseline, with fluid-like content and diffusely irregular, moderately thickened periphery with oedematous signal intensity. Gadolinium-enhanced MRI showed absent internal enhancement, early enhancement of the surrounding liver parenchyma, hyperenhancement of the minimally thickened peripheral “rim”, consistent with liver abscess. The intrahepatic and common bile ducts showed unchanged caliber, without mural enhancement suggesting infectious cholangitis (Fig. 2).
Percutaneous drainage (Fig. 3) removed 50 ml purulent fluid which tested positive for Klebsiella pneumoniae infection, allowing correct antibiotic choice. The drainage was removed following ultrasound finding of collapsed abscess cavity, and the patient recovered successfully.

In the general population, solitary or multiple liver cysts are commonly encountered on imaging studies. Representing fluid-containing benign developmental lesions, simple cysts are confidently diagnosed on the basis of their anechoic sonographic appearance with smooth thin walls and posterior acoustic transmission, homogeneous water attenuation with well-defined margins at CT, and markedly T1-hypointense and T2-hyperintense MRI signal, absent mural and internal contrast enhancement [1, 2].
In the vast majority of cases, liver cysts are asymptomatic, not associated with altered hepatic function, and do not require further workup, follow-up, or treatment. However, in exceptional cases complications may occur, including mass effect of large and/or multiple cysts causing portal and/or caval compression, biliary obstruction, intracystic bleeding, superinfection, or rupture [1, 2]. Among these, infection develops occasionally in sporadic cysts, in 8.4% of patients with autosomal dominant polycystic kidney disease (ADPKD) [3-8].
Clinically, cyst infection is suggested by persistent high-grade or subacute fever, flank or liver tenderness, leukocytosis and elevated acute phase reactants, normal or minimally altered liver function tests, and may be confirmed by detection of neutrophils and bacteria (most usually Escherichia coli or Enterobacteriaceae) in aspirated fluid. Unfortunately, cyst infection is usually unsuspected in patients with fever and abdominal pain, and represents a challenging diagnosis in patients with ADPKD [3-8].
As exemplified by this occurrence, cross-sectional imaging helps diagnosing superinfection by showing cyst enlargement, mural and/or internal modifications leading to a complex cystic lesion which may be confused with a cystic tumour. Most usually, an infected cyst appears as a purulent (abscess) collection such as those resulting from ascending cholangitis, portal phlebitis, or bacteraemia, with moderately increased and/or inhomogeneous CT attenuation, more or less homogeneous fluid-like MRI signal intensity, and irregular thickened walls with peripheral “rim” contrast enhancement. Other helpful features to further characterise a liver abscess include high-signal intensity oedematous halo, and transient hypervascularisation of the adjacent parenchyma, which are absent in uncomplicated cysts [1, 2, 6].
Although less available, positron-emission tomography (PET) may reveal increased 18-fluorodeoxyglucose uptake in infected cysts, and may be reserved for unclear or ADPKD cases [3, 5].
In conclusion, cyst superinfection should be suspected and appropriately investigated with cross-sectional imaging such as MRI when a patient with known liver cysts develops fever and abdominal pain [6]. Treatment of infected cysts requires combined antibiotic therapy plus drainage for collections over 5 cm [5, 8].