Teaching Case

A 68-year-old male presents with fatigue and 10kg weight loss over 3 weeks. Physical examination shows an adipose patient (BMI 30kg/m²) with fever (38°C). Laboratory work up reveals anaemia (haemoglobin 4 mmol/L) and elevated white blood cell count (16,400 cells/mcL), C-reactive protein (148mg/L) and Creatinine (246 μmol/L).

CT-imaging of thorax and abdomen was performed in search of malignant disease. Initial chest x-ray (not given here) had raised suspicion for the presence of lung nodules.

Thorax CT shows several nodular lung lesions, some with small caverns (Fig. 1). Mere image-based differential diagnosis between metastatic disease and septic embolism is challenging, yet strict subpleural localisation is more in favour of the latter. Discrete pleural effusion and numerous mediastinal lymph nodes are found, none relevantly enlarged (Fig. 2).

In the abdominal CT, there is pyelonephritis of the left kidney with reduction of corticomedullary differentiation, urinary retention and perirenal fat stranding (Fig. 3/4). The pyelon wall is thickened and ill-defined, with contrast media uptake. Perinephric abscess and renal vein thrombosis constitute local complications.

The bladder wall is thickened, indicating inflammation. Permanent vesical catheter due to prostate hyperplasia was installed just days before, presumably the cause of infection (Fig. 3). Final diagnosis could mainly be established in the combination of CT-imaging and positive blood and urine culture for Staphylococcus aureus, respectively (results that came in one day after CT).

Catheter associated urinary tract infection (UTI) make up 12.9% of health care associated infections in the U.S. [1]. Besides gram-negative bacteria, Staphylococcus aureus is a common pathogen. If not treated, ascending infection from the lower urinary tract can eventually affect both kidneys, causing pyelonephritis or advance into the blood stream. Perinephric abscess is a severe complication of UTI, either enabled through local spread, or —in 30% of the cases— secondary to haematogenous seeding [2]. Renal vein thrombosis as well as septic embolism of the lung from UTI are extremely rare conditions and only few cases are reported in the literature, respectively [3,4].

In the clinical perspective, presentation of UTI patients is usually unspecific with fever and generalised weakness. In the majority of cases laboratory work up of blood and urine samples lead the way to diagnosis and antibiotic treatment will start. Ultrasound and cross-sectional imaging play an important role in order to exclude complications or —like in our case— to rule out differential diagnoses like malignancy.

In CT renal affection of UTI most frequently presents with focal or multifocal wedge-shaped areas of hypoperfusion, perinephric fat stranding, hydronephrosis and kidney enlargement [5]. Differentiation of focal nephritis and pyelonephritis is challenging, yet in the latter case, pyelon wall will be thickened, ill-defined and take up contrast agent [6]. Renal abscesses can appear as complex cystic renal masses mimicking renal cell carcinoma (RCC) [7]. Associated inflammatory changes in and around the kidney and local invasion favour infection. In the lung embolic lesions can appear identical to metastatic tumour deposits and differential diagnosis might only be successful if sufficient clinical information is given. Yet imaging features in favour of embolic lesions will be strict localisation in subpleural space, presence of cavitation and low density due to potential liquid transformation. Renal vein thrombosis needs to be differentiated from vein infiltration by RCC, the latter showing contrast media uptake.

Patients with complex UTI involving shock or septicaemia require hospitalisation and immediate antibiotic treatment. In our case the patient recovered well after therapy with Flucloxacillin and renal function went back to normal. Other sites of infection or malignancy were ruled out by bronchoscopy, gastroscopy and colonoscopy, no malignant tumour was found. In follow-up ultrasound imaging six weeks after CT, perirenal abscess and the largest pulmonal lesion on the left were resolved.

Teaching Points

In case of strict subpleural localisation of multiple small nodular lung lesions in CT, septic embolism should be one differential diagnosis, depending on clinical patient presentation.

CT imaging plays an important role in the complication management of urinary tract infection, such as renal vein thrombosis and perinephric abscess.