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Introduction

Urinary tract infections (UTIs) include cystitis, lower urinary tract, upper urinary tract and pyelonephritis (PN).  PN develops when pathogens ascend the ureters to the kidneys, via hematic transmission from bacteremia and via lymphatic transmission [1–3].

These patients may also present with sepsis, multiple organ system dysfunction, shock, and/or acute renal failure, which likely occur in patients with urinary tract obstruction/urinary tract abnormalities and in older or diabetics patients.

Acute PN can be complicated by the formation of renal corticomedullary and perinephric abscess/es, and by emphysematous pyelonephritis, or papillary necrosis. Risk factors for such complications include urinary tract obstruction and diabetes (particularly for emphysematous pyelonephritis and papillary necrosis) [4]. Both urinalysis and urine culture with susceptibility testing result inform the diagnosis.

Imaging studies for diagnosis or management of acute complicated UTI are not usually warranted being generally reserved to patients with persistent clinical symptoms despite appropriate antimicrobial therapy, or there is the suspect of obstruction of the urinary tract with decrease of renal function or a fast decline in the urinary output, while imaging is performed in patients who have recurrent symptoms [5–8].

We report three cases in which renal ultrasound (US) and Computed Tomography (CT) without contrast failed to allow diagnosis of PN complications, while contrast CT showed renal abscesses in two patients and Nuclear Magnetic Resonance (MRI) without contrast a frank PN in one. The cases were selected from medical records of patients admitted to the Nephrology, Dialysis and Transplantation Unit at Padua University Hospital (Italy).

Ethical review and approval were waived for this study as required for a retrospective clinical investigation. The three patients of this study gave their consent to report their clinical cases. Irreversible anonymization of data prevented the exposure of the patients to any risk and subjects privacy was saved by the anonymization process which prevented any possible transmission of sensible data.

 

Cases Presentation

Case 1

The first case was a 20 year old man, who referred to the A&E Unit due to a bilateral lumbar ache since two days with fever 39°C and cough. Ceftizoxima and ciprofloxacin were prescribed; his clinical history was negative, normal blood pressure, 3 doses Covid-19 vaccine. No peritonism, no aches at the palpation of the kidneys. No anemia, creatinine 174 μmol, high C Reactive Protein (CRP) (45.6 mg/dl), hematuria and proteinuria 0.30 g/L. Renal US and plain abdomen CT were normal (Figure 1 A and B). The patient was hence admitted to our Nephrology Unit.

In our Unit the nephritic panel showed normal complement, ANA and ANCA. Given the severe clinical signs, an MRI was prescribed to further investigate the potential presence of PN signs, which showed multiple triangular zones of impaired diffusion compatible with PN (Figure 1 C). Together with antibiotics, the patient was hydrated, with subsequent normalization of the temperature curve and urinalysis serum and creatinine decreased to 102 μmol/L.

Case 2

The second case, a 71 year old female with diabetes on insulin, hypercholesterolemia and hypertension, referred to A&E with nausea, left flank pain, fever (up to 38.5°), anemia, acute renal failure (AKI) (Cr 291 μmol/L), normal urinalysis, CRP 509 mg/l. Left kidney US showed mild dilatation of the calyces and an oval area hypovascularized at the medium third (Figure 2 A). CT without contrast showed enlarged and globular left kidney, imbibition of the perirenal adipose tissue, dilatation of the calyces (Figure 2 B). She was hence admitted to our Nephrology Unit.

In our Unit the patient was treated with piperacilline-tazobactam and ceftazidime and due to the persistence of fever the patient was switched to meropenem and daptomicyn. Cultural tests (both urine and blood) resulted negative. Contrast CT showed an abscess at the medium third of the left kidney of 35×35×40 mm³ communicating with an area of 30×15 mm² at the upper pole and multiple small subcortical areas of the same significance (Figure 2 C).

The patient continued with meropenem and daptomicyn for 14 days until resolution. At discharge creatinine 109 μmol. The follow-up imaging showed a complete resolution of the previously detected abscess.

Case 3

The third case was a 55 year old female, who referred to A&E with fever, left flank pain, AKI (creatinine 145 μmol) and increased CRP 70 mg/L.  Renal US and CT without contrast were not resolutive (Figure 3 A and B). The patient was referred to our Unit.

In our Unit, the patient was hydrated and treated empirically with beta-lactams which were continued for 14 days with piperacillin-tazobactam, which led to the resolution of the fever and the flank pain. Contrast CT was prescribed, which showed multiple abscesses in the left kidney (Figure 3 C). The patient was discharged home with serum creatinine of 110 μmol/L. The follow-up imaging showed a complete resolution of the previously detected abscesses.

 

Discussion

The main objective of imaging in acute complicated UTIs is the appropriate evaluation of any possible underlying process, which might delay the response to therapy or intervention (stone or obstruction) or miss the diagnosis of an infective complication (renal or perinephric abscess/es).

In patients presenting with severe systemic symptoms such as sepsis or septic shock, imaging and a prompt appropriate antimicrobial treatment play a critical role.

Imaging should be promptly done in patients with sepsis or septic shock to reveal the obstruction or abscess that requires urgent treatment [9–11]. The timely detection of these conditions can significantly influence the clinical trajectory and improve patient outcomes.

To detect factors associated with acute complicated UTIs, CT scanning (with and without contrast) is the imaging analysis of choice; it is more sensitive than renal US for detecting abnormalities predisposing to or caused by infection and the extent of the disease [9].

CT without contrast is the standard for demonstrating stones, gas-forming infections, hemorrhage, obstruction, and abscesses. Contrast shows alterations in renal perfusion. CT findings in PN include hypodense lesions due to ischemia caused by neutrophilic infiltration and edema [10, 11].

Renal US is appropriate in patients when exposure to contrast or radiation is undesirable. Renal US may detect acute PN as a hypoechoic area of the renal parenchyma or sometime as hyperechoic area with or without loss of the normal corticomedullary differentiation, this latter caused by the presence in the renal tissue of interstitial edema [12]. This poor specificity and sensitivity of renal US in detecting PN and its complication could be overcome by the contrast enhancement ultrasound (CEUS), which has emerged as new alternative technique in the accurate detection of complicated acute pyelonephritis in order to avoid exposure to radiations. CEUS, in fact, compared to the B mode renal US imaging alone or implemented with Doppler or power Doppler techniques, has shown to improve the signal-to-noise ratio, thus increasing the diagnostic accuracy in case of complicated acute UTIs and complicated acute PN in particular [13–15]. In addition, Mitterberger et al. demonstrated very similar diagnostic performance between CEUS and CT, especially for the focal involvement [16]. However, CEUS suffers from some limitations mainly related both to the patient’s habitus and to the operator’s experience. In this regard, the image quality of parenchymal perfusion can be affected by patients’ factors such as obesity and bowel gas and awareness of these issues should help in addressing which patient would benefit more from CEUS or CT/MRI for the evaluation of acute PN and its complications.

Moreover, MRI has no advantage over CT while it is advantageous when it is required to avoid contrast dye or ionizing radiation [17].

The cases we have reported clearly show that renal US and CT without contrast would have missed the diagnosis both of renal abscesses and PN, which were confirmed by the third level imaging (contrast CT and MRI).

 

Conclusion

When is upper level imaging indicated in complicated acute PN? In all three cases we described, the patients presented with AKI, which can entail the presence of PN complications. The important feature which gives the main indication for an upper level imaging in PN is the presence of AKI. Therefore, it is suggested that renal function impairment is one major factor that should lead to the evaluation of an upper level imaging in the setting of PN, as standard-first line imaging (renal US and/or CT without contrast) might fail to identify PN complications [8].

 

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