Clinical Case Scenario

A 37-year-old woman with a history of type 2 diabetes mellitus arrived at the emergency department with a history of five days of worsening severe left upper quadrant abdominal pain that radiated to her left flank. She also reported fever, chills, nausea, vomiting, and general malaise. Upon arrival, her vital signs were a heart rate of  110 bpm and a axillary temperature of 39 °C. Physical examination revealed tenderness in her left costovertebral area, but her abdomen was not tender. Lab tests confirmed a urinary tract infection with a white blood cell count of 12,510 cells/mm3 and an increase in immature white blood cells. Additionally, the serum creatinine level of 5.71 mg/dL prompted the physician to perform a bedside Point-of-Care Ultrasound (POCUS) of the left kidney (figure 1) to check for urinary tract obstruction.

Figure 1: POCUS of the left kidney

(Case courtesy of Maulik S Patel, Radiopaedia.org, rID: 17283)

Wrong Answer: A) Simple renal cyst with posterior enhancement: A simple renal cyst is a benign, fluid-filled sac that typically appears anechoic (black) on ultrasound with well-defined walls and often exhibits posterior acoustic enhancement (increased brightness behind the cyst due to sound passing easily through fluid). This finding is common and not indicative of a severe infection like emphysematous pyelonephritis (EPN). This describes a benign finding, not an acute, severe infection.

Wrong Answer: B) Hydronephrosis with hyperechoic renal parenchyma: Hydronephrosis (dilation of the renal collecting system) can be seen if there is an obstruction. They are not specific to the gas-producing nature of emphysematous pyelonephritis. While concerning, these findings are not pathognomonic for EPN.

Right Answer: C) Multiple echogenic foci within the renal parenchyma with dirty shadowing or reverberation artifact: This is the most characteristic POCUS finding for EPN, which is a severe, necrotizing infection of the kidney characterized by gas formation within the renal parenchyma, collecting system, or perinephric tissue, usually by gas-forming bacteria. On ultrasound, gas appears as highly echogenic (bright) foci. Unlike calcifications, which produce sharp, "clean" posterior shadowing (Figure 2), gas produces "dirty shadowing" (a poorly defined, smudgy shadow) or a "reverberation artifact" (multiple parallel bright lines-(comet-tail artifact) due to the sound waves bouncing back and forth within the gas pocket (Figure 3, white arrows). These artifacts are key to distinguishing gas from other hyperechoic structures. This describes the direct visualization of gas, which is the defining feature of EPN.

Figure 2: Right kidney with calculus (label 3) in renal pelvis (Case courtesy of Maulik S Patel, Radiopaedia.org, rID: 10079)

Figure 3:: On ultrasound, gas produces a "reverberation artifact" (white arrows) (Case courtesy of Maulik S Patel, Radiopaedia.org, rID: 17283)

Clinical pearl:
● Emphysematous Pyelonephritis (EP) is a severe, necrotizing kidney infection characterized by gas presence in the renal tissue, most commonly caused by E. coli (around 70% of cases), but also by other bacteria like Klebsiella pneumoniae.
● Though uncommon, EP carries a significant mortality risk (ranging from 12.5% to 78%) and is predominantly observed in women and individuals with diabetes.
● Patients typically present with upper urinary tract infection symptoms (fever, flank pain, dysuria, nausea, vomiting) and often have costovertebral tenderness; however, these symptoms cannot clinically differentiate EP from uncomplicated pyelonephritis, making early diagnosis challenging but vital.
● While CT is the imaging modality of choice due to its high sensitivity for gas and ability to assess for obstruction, renal POCUS can also detect intraparenchymal air as hyperechoic foci with "dirty" shadowing or reverberation artifacts, distinguishing it from the "clean" shadowing of renal stones.
● Treatment involves aggressive resuscitation, broad-spectrum antibiotics, and often percutaneous nephrostomy for abscesses or obstruction; timely diagnosis, particularly via POCUS as a screening tool, is crucial for risk stratification and expediting definitive management, given its high mortality and complications.

Wrong Answer: D) Unilateral kidney atrophy with increased cortical echogenicity: Unilateral kidney atrophy (shrinkage) and increased cortical echogenicity (brightness of the outer kidney layer) typically suggest chronic kidney disease, severe chronic pyelonephritis with scarring, or renal artery stenosis. While a patient with chronic diabetes might have these findings, they indicate a chronic process and do not specifically point to the acute, gas-forming infection of EPN.