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.