MORPHOLOGYBilateralhydronephrosis(orunilateralhydronephrosiswhentheotherkidneyisalreadydamagedorabsent)leadstorenalfailure,andtheonsetofuremiatendstoabortthenaturalcourseofthelesion.Withsubtotalorintermittentobstruction,thekidneymaybemassivelyenlarged(lengthsintherangeof20cm),andtheorganmayconsistalmostentirelyofthegreatlydistendedpelvicalycealsystem.Therenalparenchymaitselfiscompressedandatrophied,withobliterationofthepapillaeandflatteningof thepyramids(Fig.14.23 ).Ontheotherhand,whenobstructionissuddenandcomplete,glomerularfiltrationiscompromisedrelativelyearly,andasaconsequence,renalfunctionmayceasewhiledilationisstillcomparativelyslight.Dependingontheleveloftheobstruction,oneorbothuretersalsomaybedilated (hydroureter).
Onmicroscopicexamination,theearlylesionsshowtubulardilation,followedbyatrophyandfibrousreplacementofthetubularepitheliumwithrelativesparingoftheglomeruli.Eventually,inseverecasestheglomerulialsobecomeatrophicanddisappear,convertingtheentirekidneyintoathinshelloffibroustissue.Withsuddenandcompleteobstruction,theremaybecoagulativenecrosisoftherenalpapillae,similartothechangesofpapillarynecrosis.Inuncomplicatedcases,theaccompanyinginflammatoryreactionisminimal.Superimposedpyelonephritis,however,iscommon.
Bilateral complete obstruction produces anuria. When the obstruction is distal to the bladder, the dominant symptoms are those of bladder distention. Paradoxically, incomplete bilateral obstruction causes polyuria rather than oliguria as a result of defects in tubular concentrating mechanisms, and this may obscure the true nature of the lesion. Unilateral hydronephrosis may remain completely silent for long periods unless the other kidney also is dysfunctional. Often the enlarged kidney is discovered on routine physical examination. Sometimes the underlying cause of the hydronephrosis, such as renal calculi or an obstructing tumor, produces symptoms that uncover the hydronephrosis. Removal of obstruction usually permits full return of function within a few weeks; however, with time the changes become irreversible.
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Fig. 14.23 Hydronephrosisofthekidney,withmarkeddilationofthepelvisandcalycesandthinningofrenalparenchyma.
Congenital and developmental abnormalities of the kidney and urinary tract represent the most common cause of end-stage renal disease in individuals younger than 21 years of age and account for 40% to 50% of pediatric renal failure worldwide. The abnormalities can affect the kidney (e.g., dysplasia, agenesis, and hypoplasia), collecting system (e.g., duplicated collecting system, hydronephrosis, and megaureter), bladder (e.g., ureterocele and vesicoureteral reflux), or urethra (e.g., posterior urethral valves). They may present as an isolated feature or as part of a syndrome in association with extrarenal manifestations. Most result from sporadic developmental defects of unknown cause. Others are caused by germline mutations in genes that impact the development of the kidney and especially the ureteric bud. The spectrum of clinical presentations is broad depending on the nature and severity of the underlying abnormalities. We restrict the discussion to a few forms affecting the kidney.