A  55 year-old man with poorly controlled diabetes and bilateral renal transplants presents to the Emergency Department with a 6 day history of :

  • Right upper quadrant pain  and severe back pain
  • Fever
  • Nausea and vomiting
  • lethargy and confusion

On examination:

  • Temperature: 37.5
  • Heart rate: 95/min
  • Systolic BP of 97mmHg
  • Saturating 95% on room air
  • Tenderness  in the RIF and less so in the RUQ
  • Distended but not peritonitic abdomen.

Initial blood tests demonstrate:

  • WCC 23.5× 10⁹/L
  • CRP 482 mg/L
  • glucose 27.1 mmol/l
  • total bilirubin 80 umol/l with elevated LFT
  • Amylase 122U/l
  • Lactate 4.5 mmol/L
  • Acute kidney injury : Creatinine 518umol/l from a baseline of 97 umol/l

The working diagnosis  is cholecystitis +/- cholangitis . A Point of Care Ultrasound is requested to look at his gallbladder. The gall bladder appears normal, the CBD is not dilated, the liver has normal echogenicity.  The area of maximum tenderness is in the RIF  over the right transplant kidney. This is his right transplant kidney POCUS ultrasound.

This is an emphysematous kidney. Notice the coarse echogenic foci associated with “dirty shadowing”. This is gas within the renal parenchyma. This ultrasound appearance should immediately raise concern for emphysematous pyelonephritis (EPN).

The left transplant kidney looks normal

A CT scan showed extensive amount of gas in the renal parenchyma confirming the diagnosis of emphysematous kidney. CT remains the gold standard for confirming the diagnosis and determining disease severity.

A predominant growth of  E coli was found in his urine  and  E coli in his blood cultures. He was started on tazosin and went emergently to theatre for a right renal transplant nephrectomy,  and recommenced haemodylsis in ICU.

Discussion

Ultrasound features of a normal kidney are

  • Hyperechoic renal capsule
  • Isoechoic or hypoechoic parcenchyma (cortex and meduclla) compared with the normal liver or spleen
  • The cortex and medulla usually have the same echogenicity. The medulla may be slightly darker
  • Hyperechoic renal sinus

Normal Kidney

What is Emphysematous Pyelonephritis?

Emphysematous pyelonephritis is a necrotising bacterial infection of the kidney characterised by gas production within the renal parenchyma, collecting system, or surrounding tissues.

It is a true urological emergency with a high risk of septic shock and death if diagnosis is delayed.

Fortunately, early recognition and advances in drainage techniques have substantially improved outcomes over the past two decades.

Who is at Risk?

More than 90% of patients have diabetes mellitus.

Other important risk factors include:

  • Urinary tract obstruction
  • Renal calculi
  • Neurogenic bladder
  • Immunosuppression
  • Chronic kidney disease

The most common organisms are:

  • Escherichia coli
  • Klebsiella pneumoniae
  • Proteus mirabilis

Ultrasound Features

Emergency physicians should look for:

✅ Enlarged kidney

✅ Hyperechoic gas within the collecting system or renal parenchyma

✅ Dirty shadowing

✅ Reverberation (ring-down/comet-tail) artefacts

✅ Hydronephrosis if obstruction is present

Gas can occasionally obscure much of the kidney, making the anatomy difficult to appreciate.

Ultrasound Pearls
Stone or Gas?

Renal stones produce:

  • Bright echogenic focus
  • Clean acoustic shadow

Gas produces:

  • Bright echogenic foci
  • Dirty shadow
  • Reverberation artefact
  • Comet-tail artefact

Recognising this distinction is one of the key learning points in renal ultrasound.

Differential Diagnosis

Consider:

  • Emphysematous pyelitis (gas confined to the collecting system)
  • Obstructing infected calculus
  • Xanthogranulomatous pyelonephritis
  • Renal abscess

CT helps differentiate these conditions.

Emergency Department Management

Treatment begins immediately:

  • Aggressive fluid resuscitation
  • Broad-spectrum intravenous antibiotics
  • Blood and urine cultures
  • Early urology consultation
  • Glycaemic control
  • CT imaging to determine disease extent

Many patients can now be managed successfully with:

  • Percutaneous drainage
  • Relief of urinary obstruction
  • Antibiotic therapy

Nephrectomy is generally reserved for patients who fail conservative management or have extensive renal destruction.

Practical POCUS Approach

In any septic patient with flank pain:

  • Scan both kidneys.
  • Look for hydronephrosis.
  • Assess renal size.
  • Look carefully for hyperechoic gas with dirty shadowing.
  • If gas is identified, treat as emphysematous pyelonephritis until proven otherwise.
  • Arrange urgent CT and involve Urology early.

Clinical Pearls 

Diabetes + septic shock + flank pain = always scan the kidneys.

Gas produces dirty shadowing. Stones produce clean shadowing.

A normal renal ultrasound does not exclude uncomplicated pyelonephritis, but abnormal gas within the kidney is a red flag that requires immediate action.

Key Learning Points

  • Emphysematous pyelonephritis is a life-threatening necrotising infection.
  • Diabetes and urinary obstruction are the major risk factors.
  • POCUS can rapidly identify gas within the kidney at the bedside.
  • CT is essential for confirming the diagnosis and staging disease.
  • Early antibiotics, resuscitation, drainage, and multidisciplinary management have markedly improved survival compared with historical treatment strategies.

References:

  1. Spectrum of gas within the kidney: Emphysematous pyelonephritis and emphysematous pyelitis
    Author links open overlay panelG.V.EvanoffM.D.C.S.ThompsonM.D.R.FoleyM.D.E.J.WeinmanM.D
    The American journal of medicine Volume 83, Issue 1, July 1987, Pages 149-154
  2. Emphysematous Pyelonephritis in a Transplant Kidney
    M. Salehipour, J. Roozbeh, A. R. Rasekhi, M. A. Afrasiabi, H. Rezaee, K. Izadpanah, and S. A. Malek-Hosseini Int J Organ Transplant Med. 2010; 1(1): 49–51.
  3.  Radiopaedia
  4. Medscape : Emphysematous pyelonephritis