Acute focal bacterial nephritis is associated with invasive diagnostic procedures - a cohort of 138 cases extracted through a systematic review

AFBN is commonly presented with fever (reported in 98% of the cases) and leukocytosis, indicating severe illness and sepsis. Similar to acute pyelonephritis, ipsilateral flank pain is commonly encountered (80%), while dysuria and/or symptoms of lower urinary tract infection (UTI) were reported only in 18% of the cases in our cohort. Non-specific symptoms such as nausea, vomiting and abdominal pain or guarding may be present complicating differential diagnosis (Fig. 3). Palpation of an abdominal mass was reported in 7% of cases (n = 10) [3, 6, 7].

The most frequent pathogen isolated in urinary cultures of patients with AFBN is Escherichia coli. In our cohort it was detected in 83% of all positive urine cultures. Sporadically, other gram negative bacteria such as Klebsiella species [1, 8, 9], Proteus mirabilis [10] and Serratia marcescens [11] have been isolated. Urine cultures were negative in 41% of cases. A summary of urine microbiologic findings is depicted in Fig. 4. Blood cultures were positive in 19% of cases and were negative or not specified for the rest. E. coli was also most commonly isolated in blood cultures representing 69% of positive blood cultures (Fig. 5). It is of worth mentioning that in 2.2% of the cases (n = 3) there was a strong indication of hematogenous spread. Staphylococcus aureus bacteremia was present in all these cases with concurrent AFBN and skin abscess [12], osteomyelitis [8] or glenohumeral septic arthritis [13]. One of the latter cases developed a renal abscess [12].

AFBN occurs more often in women than in men (ratio 2:1 in our cohort) and affects all ages from childhood to the elderly patient. Immune compromising diseases as well as other predisposing factors are associated with the formation of AFBN according to our cohort, such as Diabetes mellitus [1, 8, 9, 13‐18], pregnancy [4, 14, 19], urinary tract abnormalities [1, 3, 4, 8‐10, 14, 20, 21], prior or concurrent respiratory tract infection [3, 4, 16], former kidney transplantation [7, 22‐24], alcohol and drug abuse [2, 25, 26], autoimmune diseases [1, 6, 10] and AIDS [25] (Table 2). Yet it has to be emphasized that AFBN also affects previously healthy immunocompetent patients.

The diagnosis of AFBN is dependent on radiologic imaging. AFBN is characterized by hypoperfused wedged-shaped or round and space-occupying lesions in the kidney, exhibiting no capsule. Lesions can be uni- or multifocal. In our cohort 52% of patients received computed tomography (CT) and/or magnetic resonance imaging (MRI) during acute phase of AFBN all of them showing characteristic focal lesions. MRI was applied only in 2.9% of cases. Sonography in addition to CT and/or MRI was performed in 41% and had 91% sensitivity in this subgroup. In total, sonography was applied in 83% of cases (solely or combined with intravenous urography (IVU) and/or CT and/or MRI). Relating to the total amount of sonographies this diagnostic modality had 96% sensitivity in our cohort. 20% of the diagnoses were solely set by sonography. Radiologic features are summarized in Table 3.

In native CT lesions are hypodense, showing decreased enhancement after intravenous application of contrast agent in comparison to normal parenchyma [3, 4, 8, 9, 27, 28]. They do not show cortical rim sign and thus wedged-shaped lesions can be differentiated from renal infarction [29]. Associated perinephric fat stranding might be present [12, 30]. The absence of contrast enhancement at lesion margins and the non-liquefactive density may enable the differentiation to renal abscess formation.

MRI was not often employed for AFBN according to the literature. In MRI focal lesions are hypointense in T2w, showing decreased enhancement in post-contrast T1w images [19, 31‐33]. Relying data on diffusion weighted imaging is lacking, but lesions commonly show a certain degree of diffusion restriction in correlation to histopathological findings with high levels of cell-density in affected parenchyma.

In sonography focal lesions can be either hypo- or hyperechoic [10, 34, 35]. Doppler ultrasound with or without contrast enhancement confirms decreased focal blood flow [36]. In several cases there were no sonographic abnormalities while AFBN lesions were evident in CT [6, 18, 30, 34, 37, 38].

Arteriographic data on AFBN are available from publications from 1979 to 1992 confirming focal hypoperfusion. Application of this invasive technique was not mentioned beyond 1992. Similarly, employment of IVU for AFBN was last reported in 2002 [39]. IVU might suggest a renal mass with no or delayed contrast excretion or rather lack abnormalities [27, 33, 40].

All but one patient in our cohort received antibiotic treatment. The latter case was suspected for having a renal tumor and was directly treated with nephrectomy [3]. There is no standard regimen for choice of antibiotic agent and duration of antimicrobial therapy. The choice of antimicrobial agents was not specified in 82% of the cases. If specified, cephalosporins were most commonly used followed by broad-spectrum penicillins. Duration of treatment and hospitalization was very variable and not precised in most of the case reports. Duration of antibiotic treatment ranged up to 6 weeks. An uneventful course was reported after conservative management with antibiotics in the vast majority of cases. Still, treatment failure requiring additional measures was also reported. Four patients (2.9%) developed a renal abscess, three of which required drainage [11, 12, 26, 28]. One of the latter exhibited methicillin-resistant Staphylococcus aureus (MRSA) bacteremia. Two patients (1.5%) underwent percutaneous puncture due to persisting fever. Aspirates showed no drainable pus but grew Pseudomonas aeruginosa [23] and MRSA [30] respectively and adaptation of antibiotic therapy led to a favorable disease course for both patients. Two patients (1.5%) required ureteral stenting or stone extraction due to underlying obstructive lithiasis [4, 16] and three patients (2.2%) underwent nephrectomy because of reported non-response to antibiotic therapy or rather therapeutic mismanagement [1]. In addition to the above-named interventions further invasive procedures were applied in a significant number of cases due to uncertainty in diagnosis of AFBN, most commonly suspicion for a renal malignancy or abscess. A summary of all invasive procedures is shown in Table 4.

Clinical relapse after initial treatment was reported in 2.9% of cases (n = 4). Three out of the four patients recovered upon conservative re-treatment [1, 8]. One patient exhibiting poor general state of health and noncompliance with the ongoing oral medication after discharge returned septic and died [28].