Protein losing kidney disease: What to do (Proceedings)
Etiology and pathophysiology
The disease is a result of immune complexes accumulating within the glomerulus. This can occur either because pre-formed immune-complexes are deposited in the glomerulus (trapping) or because immune-complexes are actually formed in the glomerulus. In the latter situation circulating antibodies are attracted to the glomerulus because of antigen located in the glomerulus (planted). The immune-complexes draw in inflammatory cells, which then sustain an immune response that can eventually lead to destruction of the glomerulus. It is seen not infrequently in dogs, it is rare in cats.The destruction of the glomerulus is a complex event. With antigen-antibody complexes being present, a variety of inflammatory reactions occur including complement activation, homing and migration of white blood cells, and activation of the coagulation system. Platelet activation is also considered of primary importance as the platelets release substances that further increase inflammation and cause the development of "scar tissue" through hyalin formation and sclerosis. Initially the damage manifests as abnormal protein loss because the barrier function of the glomerular capillary is lost (size and charge dependent barrier). Smaller proteins are lost preferentially, albumin being the one that is of most importance. The sieve never becomes leaky enough to allow large proteins such as globulins to go across. Once damage is extensive enough the glomerulus becomes non-functional and then tubular function is lost. Once glomerular injury is widespread enough with a sufficient number of nephrons being damaged, azotemia can occur as tubular function is lost. It is important to remember that significant glomerular injury can be present without azotemia being noted.
Proteinuria is of course the hallmark of glomerular involvement. When protein is found in the urine it must first be decided if it is a clinically significant amount. The same dipstick reaction could indicate significantly different amounts of protein loss depending upon how concentrated the urine is. The definitive way to determine if protein loss is significant is with the urine protein to urine creatinine ratio. With this ratio it is possible to get an objective number with regard to the degree of proteinuria, so that it is a good way to follow up on the case. This no longer holds true however when azotemia occurs as then GFR is significantly decreased and less protein will be lost (fewer places for it to filter out). A complete urinalysis is of course needed to rule out other sources of "non-glomerular" protein such as through inflammation or blood contamination. The UP:UC only holds true if the sediment is "benign", that is few to none WBC and few RBCs are present. In some cases an infection or inflammatory lesion located downstream from the glomerulus could still be the cause. At times, a urine culture may be indicated to rule out cystitis, especially if the urine specific gravity is low. An electrophoresis should be informative in these cases as with inflammation serum "weeps" into the urine in a pattern very similar to serum whereas with glomerular injury smaller proteins will be preferentially found.
In theory proteinuria can occur in ways other than glomerular injury and urogenital inflammation. Small amounts of protein can be lost with tubular dysfunction (Fanconi syndrome) though this usually is not of great significance. With abnormally large amounts of certain proteins in the blood stream some glomerular protein leakage can occur (so called pre-glomerular or glomerular overload proteinuria). A good example is with Bence-Jones proteinuria as can occur with multiple myeloma (will see a monoclonal spike in the urine protein as well). It will also be seen with hemoglobinuria secondary to massive hemolysis or myoglobin with massive muscle breakdown.
Definitive diagnosis is with renal biopsy. This allows differentiation between amyloidosis and glomerulonephritis. A variety of morphologic patterns may be seen with glomerular pathology (membranous, membranoproliferative, etc.), although to date it does not appear that in veterinary medicine the differentiation is of prognostic or therapeutic importance.