Chronic pyelonephritis

With chronic pyelonephritis, pyelo- means pelvis, and -neph- refers to the kidney, so in this case it’s the renal pelvis, which is the funnel-like structure of the kidney that drains urine into the ureter, and -itis means inflammation.

This inflammation is usually caused by bacterial infection of the kidney, which is called acute pyelonephritis. When somebody has recurrent episodes of acute pyelonephritis, the kidney becomes visibly scarred, at which point it’s referred to as chronic pyelonephritis.

Now a urinary tract infection, or UTI, is any infection of the urinary tract, which includes the upper portion of the tract—the kidneys and the ureters, and the lower portion of the tract—the bladder and urethra.

So acute and chronic pyelonephritis are types of upper urinary tract infection. Now, an episode of acute pyelonephritis often clears up without much complication.

Certain people, though, are predisposed to having recurring bouts of acute pyelonephritis, which eventually leads to chronic pyelonephritis and permanent scarring of the renal tissue.

The most common risk factor for recurrent acute pyelonephritis and therefore chronic pyelonephritis, is vesicoureteral reflux, or VUR, which is where urine is allowed to move backward up the urinary tract, which can happen if the vesicoureteral orifice fails.

The vesicoureteral orifice is the one-way valve that allows urine to flow from each ureter into the bladder, but not in the reverse direction.

VUR can be the result of a primary congenital defect or it can be caused by bladder outlet obstruction, which increases pressure in the bladder and distorts the valve.

That being said, chronic obstruction is its own independent risk factor for chronic pyelonephritis. Obstructions in the urinary tract causes urinary stasis, meaning it tends to cause urine to stand still, which makes it easier for bacteria to adhere to and colonize the tissue, making lower UTIs more likely and therefore upper UTIs more likely.

Bilateral obstruction increases pressure in the bladder and therefore affects both kidneys, causes include congenital malformations, like posterior urethral valve, which obstructs the flow of urine through the urethra, as well as benign prostatic hyperplasia in men, which is an enlarged prostate, and cervical carcinoma in women, both of which can compress the urethra shut.

Obstructions could be unilateral as well, which would be a little higher and affect only one kidney, as in the case of urinary calculi or kidney stones.

Although one episode of inflammation might not lead to any permanent damage, recurrent episodes of inflammation eventually leads the renal interstitium to undergo fibrosis and scarring, and the tubules atrophy, and these changes are generally found on the upper and lower poles of the kidney.

Also, on imaging studies like for example, computed tomographic urogram or CTU, the renal calyces become “blunted” or flattened.

On histology, some tubules might be dilated and full of glassy-appearing proteinaceous material, called colloid, that forms as a result of the chronic inflammation.

This material ends up being shaped like the tubules and therefore forms “colloid casts”. Since this material happens to look a little like thyroid tissue, this process is sometimes referred to as thyroidization of the kidney.

Those colloid casts can then get peed out and show up in the urine. A rare type of chronic pyelonephritis is called xanthogranulomatous pyelonephritis, or just XGP, which happens when an infected kidney stone causes chronic obstruction.

The combination of infection and the increased pressure creates granulomatous tissue, which is full of foamy or fat-laden macrophages and can easily be confused for a kidney tumor on imaging.

Treatment for chronic pyelonephritis involves correcting the underlying cause of recurrent infection, which could be surgery to correct congenital structural causes or to remove obstruction like kidney stones.

In some cases, though, dialysis or nephrectomy, which is removal of some or all of a kidney, might be needed. Alright, as a quick recap, chronic pyelonephritis is where the kidney develops permanent scarring as a result of recurring bouts of acute pyelonephritis, which is bacterial infection and inflammation of the kidney.

The two major predisposing factors to developing chronic pyelonephritis are vesicoureteral reflux and chronic obstruction.

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