Incompetence of the vesicoureteral orifice, resulting in vesicoureteral reflux (VUR), is an important cause of ascending infection. The reflux allows bacteria to ascend the ureter into the pelvis. VUR is present in 20% to 40% of young children with UTI, usually as a consequence of a congenital defect that results in incompetence of the ureterovesical valve. VUR also can be acquired in individuals with a flaccid bladder resulting from spinal cord injury or with bladder dysfunction secondary to diabetes. VUR results in residual urine after voiding in the urinary tract, which favors bacterial growth. Furthermore, VUR affords a ready mechanism by which the infected bladder urine can be propelled up to the renal pelvis and further into the renal parenchyma through open ducts at the tips of the papillae (intrarenal reflux). Additional risk factors for UTI include preexisting renal conditions with renal scarring and intraparenchymal obstruction and also immunosuppressive therapy and immunodeficiency.
Although hematogenous spreadis far less common than ascending infection, acute pyelonephritis may result from seeding of the kidneys by bacteria in the course of septicemia or infective endocarditis.
Oneorbothkidneysmaybeinvolved.Theaffectedkidneymay benormalinsizeorenlarged.Characteristically,discrete,yellowish, raised abscesses are grossly apparent on the renal surface (
Fig. 14.13 ).They may be widely scattered or limitedtooneregionofthekidney,ortheymaycoalescetoform asinglelargeareaofsuppuration.
Thecharacteristichistologicfeatureofacutepyelonephritis is liquefactive necrosis and abscess formation within the renal parenchyma.Intheearlystages,pusformation(suppuration)is limited to the tubularlumina, but later abscessesruptureinto the interstitial tissue. Large masses of intratubular neutrophils frequentlyextendwithininvolvednephronsinto thecollecting ducts,givingrisetocharacteristicwhitebloodcellcastsinthe urine.Typically,theglomeruliarenotaffected.
When obstruction is prominent, the pus does not drain and may fill the renal pelvis, calyces, and ureter, producing pyonephrosis.
Asecond(rare)formofpyelonephritisispapillary necrosis, which has three predisposing conditions: diabetes, urinary tractobstruction,andsicklecellanemia.Thislesionismarkedby ischemicandsuppurativenecrosisofthetipsoftherenalpyramids(renalpapillae).Thepathognomonicgrossfeatureissharply definedgray-whitetoyellownecrosisoftheapicaltwo-thirdsof
Fig.14.13 Acutepyelonephritis.Thecorticalsurfaceisstuddedwithfocal paleabscesses,morenumerousintheupperpoleandmiddleregionofthe kidney;thelowerpoleisrelativelyunaffected.Betweentheabscesses,there isdarkcongestionoftherenalsurface.
thepyramids.Onepapillaorseveralorallpapillaemaybeaffected.Microscopically,thepapillarytipsshowcoagulativenecrosissurroundedbyaneutrophilicinfiltrate.
Involvementoftheprostatecangiverisetoacuteorchronicprostatitis,particularlyinoldermaleswithbenignprostatichyperplasia.