CASE OF BILATERAL PYELONEPHRITIS

Case : Bilateral Pyelonephritis.

Discussion on : 

1. Appearance of Pyelonephritis on USG and CECT.

2. Acute Pyelonephritis Vs Renal Infarct.

Patient is a 35year old diabetic female, who presented with fever with chills, right upper quadrant and loin pain, with Urine examination showing plenty of pus cells, RBCs and Bacteria.

Clinical diagnosis was of acute right pyelonephritis.

USG showed only hypoechoic mildy enlarged kidneys bilaterally. No urolithiasis / hydroureteronephrosis was seen.

CECT was done, which showed areas with relative lack of enhancement in lower and posterior interpolar region of right kidney and upper pole of left kidney. Only  minimal perinephric fat stranding was seen. Minimal pararenal fascial thickening was seen on right side only.

No urolithiasis / hydroureteronephrosis was seen in CT.

Click on the images to view full size.

ACUTE PYELONEPHRITIS Vs SEGMENTAL RENAL INFARCTS

In segmental renal arterial infarcts, the peripheral most cortical supply gets derived from the capsular arteries, which shows an enhancing rim around the infarct - which is called as the Cortical Rim Sign. This is not seen in Pyelonephritis, in which case the entire thickness is involved. Rim sign is however only seen in approximately 50% cases of renal infarcts.

ACUTE PYELONEPHRITIS

Image courtesy of Dr Frank Gaillard From the case Acute pyelonephritis

Cortical Rim Sign : SEGMENTAL RENAL INFARCTS

Image courtesy of Dr Frank Gaillard, Radiopaedia.org. From the case Renal infarction

CECT findings of Acute Pyelonephritis (abnormal in 65-90%):

1. Perinephric fat stranding and pararenal fascial thickening.
2. Hypoattenuating (80-90 HU) wedge-shaped area of cortex extending from papilla to renal capsule
3. During nephrographic phase(= lobar segments of hypoperfusion +edema)
4. Striated nephrogram
5. Poor corticomedullary differentiation
6. Dense parenchymal staining on scan delayed 3-6 hr in area of earlier diminished enhancement(= functioning renal parenchyma)
7. Soft-tissue filling defect in collecting system (=papillary necrosis, inflammatory debris, blood clot) calyceal effacement.

CT is best for detecting presence of calculi, level of obstruction and complications.

 

 US findings in Acute Pyelonephritis (abnormal in <50%):

1. Swollen kidney of decreased echogenicity – (kidney becomes more globular, with increased AP dimension. The parenchymal thickness sometimes appears to be increased because of parenchymal edema.)
2. Loss of central sinus complex (Sometimes described as effacement of renal sinus fat)
3. Wedge-shaped hypo to isoechoic zones, rarely hyperechoic (due to hemorrhage)
4. Thickened sonolucent corticomedullary bands
5. Blurred corticomedullary junctions
6. Localized increase in size + echogenicity of perinephric fat ± fat within renal sinus
7. Localized perinephric exudate
8. Thickening of wall of renal pelvis
9. Focally decreased blood flow on power Doppler.

Ultrasound is often the first imaging study, and it is difficult to diagnose Pyelonephritis by USG.

Differentiating between Ascending and Hematogenous infection may not be possible, however these points might be of some help -

            'In acute bacterial nephritis, the alternating bands of hypo- and hyperattenuation, which correspond to differential enhancement of infected and noninfected parenchyma, are sharply defined. Over time, the differential enhancement becomes less distinct and ultimately will either completely normalize or evolve to scar, as evidenced by loss of parenchymal volume. When round, peripheral hypoattenuation renal lesions are seen in the clinical setting of pyelonephritis, hematogenous seeding should be considered'. [3]



References : 

• [1] Dahnert, Radiology Review Manual 7th Edition.
• [2] Radiopedia.org article on Cortical Rim Sign.
• [3]  Pyelonephritis: Radiologic-Pathologic Review, Radiographics.