Ultrasound Findings of Emphysematous Pyelonephritis: A Case Series

Abstract

Emphysematous pyelonephritis (EP) is a severe complication of acute pyelonephritis caused by gas-producing bacteria, with high morbidity and mortality. Early recognition is essential, but the symptoms of EP are similar to those of severe acute pyelonephritis, complicating the diagnosis. Although computed tomography (CT) is the preferred diagnostic tool, its use is often limited in resource-constrained settings. Point-of-care ultrasound (POCUS) offers a rapid, accessible alternative for identifying signs of complicated pyelonephritis, including EP and obstructive pyelonephritis. Renal POCUS can be used to screen and risk stratify patients with severe acute pyelonephritis, optimizing CT use and reducing delays in diagnosing complications. We present cases from a hospital in Lima, Peru, illustrating how POCUS effectively facilitated the timely diagnosis and management of EP.

1. Introduction

Emphysematous pyelonephritis (EP) is a rare but serious complication of acute pyelonephritis characterized by the destruction of renal tissue due to gas-producing bacteria. Early recognition is crucial because EP has a high morbidity and mortality rate, and requires timely management.^1,2 However, the symptoms of EP are similar to those observed in severe, acute pyelonephritis, making it difficult to differentiate between them based on clinical presentation alone. Although computed tomography (CT) is the imaging modality of choice for diagnosing EP, CT imaging is not typically performed on all patients presenting with features of severe, acute pyelonephritis, and typically reserved for those who are critically ill and/or failing to improve despite appropriate antimicrobial therapy. Furthermore, limited-resource or rural settings may encounter difficulties in accessing CT imaging because of equipment availability or cost limitations. Therefore, a targeted approach to imaging may be preferable in such settings. Additionally, patients with EP are more likely to be too unstable to be safely transported to the CT scanner, leading to additional delays in diagnosis and coordination of care.

Alternatively, point-of-care ultrasound (POCUS) is a rapid bedside diagnostic tool that can be employed to identify features suggestive of EP and other potential complications, including signs of obstructing pyelonephritis. Given the ease and increasing availability of POCUS, there may be a role for advocating for performing a renal POCUS on all patients presenting with acute pyelonephritis in the emergency department (ED) to help identify features concerning for potential complications from acute pyelonephritis such as EP or obstructing pyelonephritis. However, we acknowledge the operator-dependent nature of POCUS, which imposes inherent limitations on its use. Nonetheless, it remains a valuable tool, especially in settings with limited access to CT imaging. We present a series of cases from a large, public safety-net hospital in Lima, Peru where ultrasound helped identify key pathological findings suggestive of EP, enabling timely recognition and the need for additional confirmatory CT imaging.

2. Cases

2.1. Case 1

A 37-year-old woman with history of diabetes mellitus type 2 presented with severe right upper quadrant abdominal pain, radiating to the right flank, worsening over the past 5 days, associated with fevers, chills, nausea, vomiting, and malaise. Triage vital signs were notable for tachycardia to 110 beats per minute and fever to 39 °C. On examination she had right-sided costovertebral tenderness and a nontender abdomen. Laboratory results revealed a leukocytosis of 12,510 cells/mm³ with a left shift, and urinalysis with significant pyuria consistent with a urinary tract infection. The patient was found to be in acute renal failure with a creatinine of 5.71 mg/dL, which prompted the physician to perform a bedside renal ultrasound to rule out obstructive uropathy as the cause of acute renal failure. Renal POCUS did not show signs of hydronephrosis but was notable for an enlarged right kidney with multiple hyperechoic areas with associated reverberation artifact on the posterior wall and within the renal parenchyma, consistent with air due to a necrotizing infection (Fig 1). A presumptive diagnosis of EP was made.

Figure 1.

Long-axis view of the right kidney with multiple hyperechoic areas (→) within the renal parenchyma and pelvis with reverberation artifact suggestive of gas. There is abnormal echogenicity of the renal parenchyma (∗) with loss of corticomedullary differentiation.

Based on the ultrasound findings, an abdominal CT (Fig 2) was performed, confirming the diagnosis of EP. The patient’s ED course was complicated by refractory metabolic acidosis requiring temporary hemodialysis. Urine culture confirmed pan-sensitive Escherichia coli. Her clinical course improved with broad-spectrum antibiotics and she was discharged on hospital day 10.

Figure 2.

Axial view of abdominal CT revealing an enlarged right kidney with air within the renal parenchyma (→), compatible with EP.

2.2. Case 2

A 42-year-old woman patient with history of diabetes mellitus type 2 presented to the ED because of approximately 3 weeks of urinary frequency, urgency, and dysuria, followed by onset of nausea and vomiting, and now subjective fever and chills for the past 5 days. Triage vitals were notable for a heart rate of 106 beats per minute and fever to 39 °C. Physical examination was notable for right-sided abdominal pain to deep palpation, nonperitoneal, and right-sided costovertebral tenderness. The patient’s inflammatory markers were notably elevated, with a C-reactive protein level of 416.5 mg/L, leukocytosis of 27,880 mm³, and a procalcitonin level of 3.4 ng/mL. Laboratories were suggestive of prerenal azotemia with a creatinine of 2.58 mg/dL and a blood urea nitrogen (BUN) of 91.71 mg/dL. Urinalysis was consistent with a urinary tract infection due to significant pyuria. A renal POCUS was performed to assess for hydronephrosis given the patient’s impaired renal function. There was no evidence of obstructive uropathy on POCUS but there were multiple hyperechoic areas associated with reverberation artifact suggestive of EP (Fig 3).

Figure 3.

Long-axis view of the right kidney showing hyperechoic areas (→) within the renal pelvis and parenchyma associated with reverberation artifact suggestive of air. Note the similar appearance of air within the intestine, seen as a hyperechoic area with reverberation artifact (∗).

Based on the ultrasound findings a CT of the abdomen and pelvis with intravenous (IV) contrast was ordered, which confirmed the diagnosis of EP. The patient was treated with meropenem because of multiresistant E coli identified in the urine culture. She responded well to treatment and was discharged after completing a prolonged 21-day course of IV antibiotics.

2.3. Case 3

A 32-year-old woman with history of nephrolithiasis patient presented to the ED with 3 days of bilateral flank pain, nausea, and dysuria. On physical examination, tenderness was noted in diffusely in the upper abdominal quadrants with costovertebral tenderness bilaterally. Laboratory tests revealed normal renal function, and leukocytosis to 21,430 mm³. Urinalysis showed large pyuria suggestive of a urinary tract infection. Renal POCUS was performed because of the severe flank pain endorsed by the patient to assess for signs of obstructive pyelonephritis. Renal POCUS revealed bilateral staghorn calculi and renal enlargement, with hyperechoic lines and reverberation artifacts throughout the left kidney, suggestive of EP (Fig 4).

Figure 5.

Longitudinal view of the right kidney showing band-like hyperechoic images with reverberation artefact throughout the kidney (→) concerning for gas. There is renal enlargement with loss of corticomedullary differentiation and a hypodense area along the inferior pole of the kidney (∗) concerning for perinephric abscess.

Given the POCUS findings, an abdominal CT with IV contrast was performed, which confirmed the diagnosis of EP with associated bilateral staghorn calculi. Urology was consulted and the patient was admitted for a prolonged course of IV broad-spectrum antibiotics.

2.4. Case 4

A 39-year-old woman patient with history of nephrolithiasis presented with 2 weeks of right upper quadrant abdominal pain radiating to the right flank, associated with subjective fevers, nausea, and vomiting. Physical examination revealed diffuse lower abdominal tenderness and costovertebral tenderness bilaterally. Laboratory tests showed evidence of acute kidney injury (AKI) with a creatinine of 2.3 mg/dL and BUN 45.3 mg/dL. Urinalysis was consistent with a urinary tract infection due to significant pyuria. Given the AKI, renal POCUS was used to rule out hydronephrosis as a cause of renal impairment. Renal POCUS revealed altered echogenicity of the renal parenchyma bilaterally, with multiple hyperechoic foci within the right kidney associated with reverberation artifact and a hypoechoic area along the inferior pole of the kidney concerning for EP with associated perinephric abscess (Fig 5).

Figure 6.

Long-axis view of the right kidney with hyperechoic band-like areas in the renal parenchyma (→) with reverberation artifact suggestive of air.

Because of the ultrasound findings, an abdominal CT with IV contrast was requested, which confirmed the diagnosis of EP with associated perinephric abscess and bilateral staghorn calculi. Urology was consulted for management of the perinephric abscess and the patient was treated with a prolonged course of broad-spectrum antibiotics until time of discharge from the hospital.

2.5. Case 5

A 37-year-old woman with history of diabetes mellitus type 2 presented with right upper quadrant abdominal pain radiating to the right flank and subjective fevers for 2 weeks. Triage vital signs were notable for tachycardia to 102 beats per minute, and oral temperature of 38.8 °C. Examination was notable for right-sided costovertebral tenderness and primarily right upper quadrant tenderness. Laboratory tests showed elevated inflammatory markers with leukocytosis of 33,870 cells/mm³, normal renal function and a urinalysis revealing large amounts of pyuria. Because of the radiation of pain to the right flank, a renal POCUS was performed to evaluate for ureteral colic as a potential cause of the patient’s right-sided flank pain. POCUS revealed hyperechoic areas with reverberation artifact suggestive of gas throughout the renal parenchyma, concerning for EP (Fig 6).

Figure 4.

Longitudinal view of the left kidney demonstrating “dirty” shadowing (→) from air within the renal parenchyma and “clean” shadowing (∗) from staghorn calculi within the renal pelvis.

An abdominal CT with IV contrast confirmed the diagnosis of EP and the patient was hospitalized and treated with a prolonged course of IV antibiotics.

3. Discussion

EP is a necrotizing infection characterized by the presence of gas in the renal tissue. The most frequently isolated microbial agent is E coli, accounting for approximately 70% of cases.^3,4 However, other bacteria such as Klebsiella pneumoniae, Proteus mirabilis, group D Streptococcus, and coagulase-negative Staphylococcus can also cause EP.⁵ EP is an uncommon condition but carries a significant risk of mortality. Reported mortality rates vary depending on factors such as disease severity, comorbidities, and treatment approaches. Studies have shown mortality rates ranging from as high as 78% to as low as 12.5% in a recent meta-analysis of 1145 patients.¹ EP tends to occur more frequently in women and individuals with diabetes.^3,4

Patients with EP typically present with symptoms associated with an upper urinary tract infection, such as fever, dysuria, flank pain, nausea, and vomiting. The most common physical examination finding is costovertebral tenderness, and in some cases, crepitus may be detected.¹ Severe cases can rapidly progress to septic shock and lead to death, underscoring the importance of early diagnosis. Initial case reports identified pneumaturia as an almost pathognomonic sign of EP.^6,7 There are no symptoms or signs that can clinically differentiate between uncomplicated pyelonephritis and EP. In cases of EP, the most frequent symptoms include fever and flank pain.¹ CT is the imaging modality of choice due to its higher sensitivity for identifying air within the renal parenchyma compared with other imaging modalities, and its ability to also assess for signs of obstructive pyelonephritis.^3,5 Nevertheless, its use is limited because of cost, radiation exposure, and limited availability, particularly in low-resource settings. Abdominal radiography can also be used to identify air in the renal region, but has an overall low diagnostic accuracy.^1,3 Renal POCUS can alternatively be used to identify air within the renal parenchyma; however, a recent metanalysis reported only an accuracy of 68% for ultrasound diagnosis of EP.¹ Nevertheless, similar to CT, ultrasound can also be used to assess for signs of obstructive pyelonephritis, a complication that, similar to EP, is difficult to differentiate from severe, acute pyelonephritis based on presenting symptoms alone.

Air within the renal parenchymal is visualized as hyperechoic foci, typically mobile, that project reverberation artifacts, creating shadows of varying echogenicity, known as “dirty” acoustic shadowing.⁸ Acoustic shadowing is a form ultrasound artifact resulting from a lack of signal deep to the imaged tissue interface due reflection or absorption of all, or nearly all the transmitted sound waves.^9,10 Air and renal stones are both strong ultrasound wave reflectors, thus it is important to be able to distinguish shadowing from a tissue-air interface versus a renal stone, since both can be found on renal POCUS. “Clean” shadowing refers to the shadow that results when ultrasound waves encounter a highly reflective surface such as a calcified renal stone.⁸ “Clean” shadows are of homogeneous echogenicity, arising from a hyperechoic, typically well-demarcated, and nonmobile object, unlike the “dirty” shadows created by air.¹¹ Figure 4 demonstrates the presence of both air resulting in “dirty” shadowing and a renal calculus resulting in “clean” shadowing. Patients with EP can also have blurring of the renal parenchymal due to overlying “dirty” shadowing, renal enlargement, and loss of corticomedullary differentiation as seen in Figure 1.

The treatment of EP consists of aggressive resuscitation, broad-spectrum antibiotics, and percutaneous nephrostomy if renal abscess and/or concomitant obstructive pyelonephritis is present.^2,3 In severe cases, or settings where percutaneous nephrostomy is not available, nephrectomy can be considered, but this is a highly morbid procedure.¹² The majority of the patients discussed were poorly controlled diabetics presenting with symptoms consistent with severe acute pyelonephritis. POCUS was performed to assess for signs obstructive uropathy, given most of the patients presented in acute renal failure and/or with pain suggestive of ureteral colic. POCUS findings suggestive of EP prompted the clinical team to order CT imaging, which confirmed in all cases the presence of air within the renal parenchyma.

We recognize that the accuracy of POCUS for diagnosing EP is operator-dependent. In our case series, all the renal POCUS were performed by emergency physicians undergoing an emergency ultrasound fellowship in Lima, Peru. Although previous case reports have documented the use of POCUS to diagnose EP, this is the first case series demonstrating the potential role for using POCUS to risk stratify patients presenting with severe acute pyelonephritis, thereby expediting the need for additional imaging and/or surgical consultation.

Timely diagnosis of EP is vital given its high mortality, complications, and potential long-term effects. Our experience highlights the utility of renal POCUS for identifying signs of complicated pyelonephritis, including EP and obstructive pyelonephritis. Renal POCUS can therefore be used as screening tool to risk stratify patients presenting with severe acute pyelonephritis, thereby optimizing CT usage and decreasing risks of delayed diagnosis of complications from acute pyelonephritis.

Funding and Support

By JACEP Open policy, all authors are required to disclose any and all commercial, financial, and other relationships in any way related to the subject of this article as per ICMJE conflict of interest guidelines (see www.icmje.org). The authors have stated that no such relationships exist.

Conflict of Interest

All authors have affirmed they have no conflicts of interest to declare.