- CHRONIC PYELONEPHRITIS
- Angell Memorial Animal Hospital
- Chronic pyelonephritis is a difficult morphologic diagnosis for young pathologists (who always want to call them multifocal cortical infarcts), but with a bit of practice and using your noodle, all the signs are here that point to an ascending urinary tract infection rather than a vascular event. The depressions in the cortex (actually loss of cortical nephrons) should initially bring up the thought cortical infarcts (maybe this animal had left-sided vegetative valvular endocarditis and threw some emboli which lodged in the arcuate arteries resulting in cortical infarcts.) That's a distinct possibility. But look closer at the picture. First examine the shape of the renal pelvis - it's wider than normal and assymetric - there has been damage in this area and tissue loss, and you won't get that with a cortical infarct. Then look at the medulla - the areas of hemorrhage are interesting, but it is the areas of white streaking between them that are even more important here - those are areas of fibrosis and probably aggregates of lymphocytes and plasma cells in the interstitium). If the lesion was a bit more acute, they might be distended tubules full of neutrophils and necrotic debris, but I think we are way past that in this case Why is the capsule indented then? Pyelonephritis spreads from the pelvis up through to the capsule through the collecting ducts, which run all the way to the capsular surface. These ducts fill with bacterial and inflammation, and eventually rupture, spilling inflammatory cells, debris and urine into the adjacent endothelium. When you lose a nephron (usually when the basement membrane is breached), it's gone forever, and is replaced with scar tissue. Lose a big area of nephrons, you get lots scar tissue which will eventually...you got it - CONTRACT. This contraction, and the cleanup of the lost nephrons, looks like a cortical infarct, but is really a classic gross sign of pyelonephritis.