Acute pyelonephritis

With acute 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. So acute pyelonephritis describes an inflamed kidney that develops relatively quickly, usually as a result of a bacterial infection.

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 pyelonephritis is a type of upper urinary tract infection.

Acute pyelonephritis is most often caused by ascending infection, meaning bacteria start by colonizing the urethra and bladder, which would be a lower urinary tract infections, and make their way up the ureters and kidney, therefore upper UTI shares a lot of the same risk factors as lower UTI, things like female sex, sexual intercourse, indwelling catheters, diabetes mellitus, and urinary tract obstruction.

One major factor that increases the risk of an upper UTI from a lower UTI spreading upward 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.

As kind of a double-whammy, obstruction also leads to urinary stasis, where urine stands still, which makes it easier for bacteria to adhere and colonize the urinary tract.

So, for ascending infections that cause acute pyelonephritis, the most common organisms are E coli, Proteus species, and Enterobacter species, all of which are commonly found in the bowel flora.

Now, it’s also possible that kidneys get infected via hematogenous infection, or spread through the bloodstream, although this is a lot less common.

Usually pyelonephritis from hematogenous spread is a consequence of septicemia or bacteremia—which is bacteria in the blood, as well as infective endocarditis, an infection of the inner layer of the heart. In these situations, the most common organisms are Staphylococcus species and again E. coli.