4Discussion - Background
Acute pyelonephritis is a bacterial infection of the renal tract commonly affecting renal parenchyma (1). The most common causal bacteria for acute pyelonephritis is Escherichia coli which is complicated by the rising incidence of antibiotic resistance (1).
Pyelonephritis affects ~28/10,000 people (2) and is more common in females aged 18-49 (1-3). Approximately 7% of all cases of acute pyelonephritis require hospitalisation (2), however with increasing antibiotic resistance this figure could rise. Risk-factors for pyelonephritis include diabetes, sickle-cell, female gender, and pregnancy (2,3). Acute pyelonephritis affects up to 2% of pregnancies often occurring the second trimester (3). However poorly managed or antibiotic resistant pyelonephritis can progress to abscess formation, sepsis and renal failure (4).
5Discussion - Clinical Perspective
The diagnosis of UTI’s is often undertaken with clinical examination and a urine dipstick test. A urine dipstick test has a sensitivity and specificity of ~75-84% and 82-98% respectively (5) and the combination of clinical symptoms and positive urine cultures is often sufficient to achieve a diagnosis. In cases where pyelonephritis is suspected Computed Tomography is the modality of choice (2-4) however, in pregnant women ultrasound should be undertaken due to its ionising radiation free nature (4).
Ultrasound appearances of acute pyelonephritis are subtle and rarely present (4). One study documented positive ultrasound findings for pyelonephritis in only 24% of confirmed cases (4). When present, ultrasound appearances of pyelonephritis demonstrate parenchymal changes which may be isoechoic and heterogeneous with increased vascularity on colour doppler imaging (4). Other findings of pyelonephritis include renal enlargement, loss of corticomedullary differentiation and abscess formation (4).
When pyelonephritis progresses to a renal abscess, ultrasound demonstrates a progressive change over time (6). Early abscess formation demonstrates a focal area of reduced echogenicity and hypovascularity on colour doppler interrogation (6). As a renal abscess progresses it may undergo liquefaction resulting in a thick-walled, complex cystic lesion with internal debris and a hyperaemic rim on colour doppler (6).
MRI can confirm renal abscess formation within the pregnant population due to its ionising radiation free nature (6). MRI confirms renal abscess formation with Diffusion Weighted MRI (DW-MRI) (6) which demonstrates suppressed/ restricted diffusion within the abscess body (6).
UGPD of renal abscesses as treatment is often reserved for serious cases or cases which do not respond to antibiotic treatment (7). UGPD are often successful however, due to the complex appearances of abscesses on B-mode ultrasound; targeting purely cystic portions of the lesion can be challenging for the operator (8). The use of Contrast Enhanced Ultrasound (CEUS) can help delineate between the enhancing renal parenchyma and the avascular cystic regions of the collection (8). CEUS may also help improve targeting of the fluid components of the abscess and lead to better procedural success rates (8). However, the use of CEUS in pregnancy is not advocated owing to a lack of research into this area.
 Colgan, R., Williams, M and Johnson, J. (2011) ‘Diagnosis and Treatment of Acute Pyelonephritis in Women’, American Family Physician, 84(5), pp. 519-526 doi / PMID: 21888302
 Medina, M and Castillo-Pino, E. (2019) ‘An introduction to the epidemiology and burden of urinary tract infections’, Therapeutic Advances in Urology, 11(1), pp. 3-7 doi: 10.1177/1756287219832172
 Dawkins, JC., Fletcher, HM., Rattray, CA., Reid, M and Gordon-Strachan, G. (2012) ‘Acute Pyelonephritis in Pregnancy: A Retrospective Descriptive Hospital Based Study’, International Scholarly Research Network Obstetrics and Gynaecology, 2012(1), pp. 1-6 doi: 10.5402/2012/519321
 William, DC., Brent, JD and Mark, DT (2008) ‘From the archives of the AFIP Pyelonephritis: Radiologic-Pathologic Review’, RadioGraphics, 28(1), pp. 255-278 doi: https://doi.org/10.1148/rg.281075171
 Ramakrishnan, K and Scheid, DC (2005) ‘Diagnosis and Management of Acute Pyelonephritis in Adults’, American Family Physician, 71(5) pp. 933-942 doi/ PMID: 15768623
 Das, CJ., Ahmad, Z., Sharma, S and Gupta, AK (2014) ‘Multimodality imaging of renal inflammatory lesions’, World Journal of Radiology, 6(11) pp. 865-873 doi: 10.4329/wjr.v6.i11.865
 Rai, RS., Karan, SC and A Kayastha, SM (2007) ‘Renal and Perinephric Abscesses Revisited’, Medical Journal Armed Forces India, 63(3), pp. 223-225 doi: https://doi.org/10.1016/S0377-1237(07)80139-0
 Huang, DY. Yusuf, GT., Daneshi, M., Ramnarine, R., Deganello, A., Sellars, ME., Sidhu, PS (2018) ‘Contrast-enhanced ultrasound (CEUS) in abdominal intervention’, Abdominal Radiology, 43(4), pp. 960-976 doi: 10.1007/s00261-018-1473-8.