Pyelonephritis

Case Discussion

• Findings: There is a right obstructing renal stone with upstream hydronephrosis and perinephric stranding, thick and enhancing urothelium, fluid/stranding surrounding the proximal ureter.

• Objectives:

  • To identify signs that indicate pyelonephritis in the appropriate clinical setting

  • To assess for common complications of pyelonephritis

  • To identify enhancement patterns of the kidney

• Protocolling:

  • Contrast enhanced CT to assess for complicated disease/complications

• Key Points:

  • CT signs of pyelonephritis:

    • Enlargement of kidneys

    • Perinephric stranding/fluid

    • Altered enhancement - most commonly striated nephrogram

    • Urothelial thickening/enhancement

    • Can be normal, pyelonephritis is a clinical diagnosis!

  • Pyelonephritis is a clinical diagnosis that does not routinely require imaging. CT considered if:

    • History of obstruction, stones, or urologic surgery

    • Persistent symptoms after 2-3 days on antibiotics

    • Diabetic, elderly, immunocompromised

  • Complications of pyelonephritis:

    • Abscess

    • Pyonephrosis - obstructed collecting system with clinical signs of infection

      • On CT will see obstructed/dilated collecting system and possibly higher density fluid within renal pelvis or other signs of infection

    • Renal vein thrombosis

    • Emphysematous pyelonephritis - gas within the parenchyma, a surgical emergency (note that gas within the collecting system, emphysematous pyelitis, is generally treated medically)

• Enhancement patterns of the kidney (suggest “Wolin EA, Hartman DS, Olson JR. Nephrographic and Pyelographic Analysis of CT Urography: Differential Diagnosis. AJR 2013;200:1197-1203 ”)

  • Absent nephrogram (lack of functioning kidney):

    • No blood in = Acute arterial occlusion

    • No blood out = Complete renal vein occlusion (uncommon, most renal vein thrombi are incomplete)

    • No urine out = Complete long standing obstruction (uncommon, usually some residual function)

    • Non-functioning kidney (congenital or acquired).

  • Unilateral delayed nephrogram/pyelogram (slow urine out):

    • Slow urine out = Partially obstructed collecting system (stone, tumor, clot, compression)

    • Slow blood in = renal artery stenosis, large subcapsular fluid collection

    • Slow blood out = partial renal vein occlusion

    • Unilateral poor renal function = i.e. acute pyelo

  • Bilateral persistent nephrogram/delayed pyelogram

    • Systemic hypotension (look for other shock complex findings)

    • Bilateral intrarenal obstruction (crystal disease, protein, tumor lysis syndrome)

    • Much less common: bilateral renal artery stenosis, partial renal vein thrombosis, bilateral collecting system obstruction

  • Rim nephrogram (only retained subcapsular enhancement)

    • Acute renal artery occlusion/global infarction - most specific sign for vascular compromise. Usually seen days to weeks after infarct

  • Reverse rim nephrogram (enhancement centrally, no cortical enhancement)

    • Acute cortical necrosis (rare form of acute renal failure).

  • Striated nephrogram (alternating low and normal enhancing bands, radially arranged)

    • Unilateral: acute pyelonephritis, ureteric obstruction, renal vein thrombosis

    • Bilateral: acute pyelonephritis, tubular obstruction, hypotension, polycystic kidney disease.

  • Spotted nephrogram (segmental areas of non-enhancement due to tiny infarctions)

    • Intrarenal vasculitis

    • Embolic disease

    • Pyelonephritis (though, classically a striated nephrogram)