Gas in the renal parenchyma of a diabetic, septic patient — CT separates parenchymal destruction (emphysematous pyelonephritis) from gas limited to the collecting system (emphysematous pyelitis), because the first may need nephrectomy.
Orient first
- Almost always diabetic or obstructed; E. coli and Klebsiella ferment glucose into gas.
- CT classifications (e.g. Huang–Tseng) grade gas in the collecting system, parenchyma, perinephric and pararenal spaces — verify the scheme the urologists use.
- Emphysematous pyelitis (gas only in the pelvicalyceal system) has a much better prognosis.
Acquire the study
- Unenhanced CT KUB then contrast-enhanced nephrographic phase if renal function allows; lung window for gas.
The manoeuvre
- Lung window: where is the gas — collecting system only, parenchyma (streaky, mottled), perinephric space, pararenal space or beyond?
- Nephrographic phase: size of the non-enhancing destroyed parenchyma; fluid collections.
- Look for obstruction: stone, dilated ureter; state the level.
- Contralateral kidney and bladder (emphysematous cystitis).
What confirms it
- Gas within the renal parenchyma (not only the collecting system) with parenchymal destruction in a septic patient.
What licenses you to exclude it
- Gas confined to the collecting system without parenchymal gas = emphysematous pyelitis; reflux of gas after instrumentation must be excluded from the history.
The classic misread
- Gas after recent ureteric stenting or nephrostomy mistaken for infection.