Page 793 - Small Animal Clinical Nutrition 5th Edition

Introduction to Canine Urolithiasis 823

ty. Blood clots are radiolucent and may be mistaken for radiolu- 1981; Rothuizen and van den Ingh, 1980; Barrett et al, 1976;
VetBooks.ir cent uroliths. Radiolucent uroliths may be readily distinguished Marretta et al, 1981; Center et al, 1985).
from blood clots when evaluated by two-dimensional, gray-
Urine Chemistry
scale ultrasonography. Uroliths are usually in the dependent
portion of the bladder lumen, produce sharply marginated Detection of the underlying causes of specific types of urolithi-
shadows containing few echoes and are associated with acoustic asis is often linked to evaluation of the biochemical composi-
shadowing. Blood clots may be located anywhere in the blad- tion of urine. For best results, at least one and preferably two
der lumen, typically have an irregular outline and indistinct consecutive 24-hour urine samples should be collected because
margins and are not associated with acoustic shadowing. determination of fractional excretion of many metabolites in
Uroliths that are radiodense on survey radiographs may “spot” urine samples does not accurately reflect 24-hour
appear to be radiolucent when evaluated by positive-contrast metabolite excretion (Table 38-6).
radiography. This finding is related to the fact that many Water consumption and hydration status must be considered
uroliths are more radiodense than body tissue, but less radio- when interpreting laboratory results. Decreased water con-
dense than the contrast material. A diagnosis of radiolucent sumption and dehydration are associated with several alter-
uroliths should be based on their radiodensity compared with ations, including decreased renal clearance of metabolites and
soft tissues, and not their radiodensity compared with positive- increased urine specific gravity and urine solute concentrations
contrast medium. (Taburu et al, 1993). Caution must be used in interpreting 24-
A urolith may be larger than that depicted by its radiodensi- hour excretion of solutes in the diagnosis and therapy of uro-
ty if only a portion of it contains radiodense minerals.This phe- lithiasis if hospitalized animals consume less water than in the
nomenon is most likely to occur with rapidly growing struvite home environment.
uroliths that contain large quantities of matrix. Urine concentrations of potentially lithogenic metabolites
are also influenced by the amount and composition of food
Hematology and Serum Chemistry consumed, and whether urine was collected during conditions
Hemograms of dogs with uroliths are usually normal unless of fasting or food consumption (Lulich et al, 1991, 1991a).
there is concomitant generalized infection of the kidneys or Aldosterone secretion increases following food deprivation.
prostate gland associated with leukocytosis. Microcytosis, ane- Increased aldosterone secretion promotes renal tubular sodium
mia, target cells and leukocytosis have occasionally been associ- reabsorption and potassium excretion. As a consequence, plas-
ated with portal vascular anomalies in dogs with and without ma potassium concentration decreases, urinary potassium ex-
urate uroliths (Cornelius et al, 1975; Ewing et al, 1974; cretion increases and urinary sodium and chloride excretion
Griffiths et al, 1981; Rothuizen and van den Ingh, 1980). decrease (Lulich et al, 1991a). Urinary calcium, magnesium and
Serum chemistry values are usually normal in patients with uric acid excretions are reduced during fasting. However, uri-
infection-induced magnesium ammonium phosphate, cys- nary excretion of phosphorus, oxalate and citrate are apparent-
tine and silica uroliths unless obstruction of urine outflow or ly unaffected by fasting (Lulich et al, 1991a). In dogs, urinary
generalized renal infection leads to changes characteristic of ammonia, titratable acid and hydrogen ion excretion decrease
renal failure. Although most patients with calcium oxalate and urinary pH values increase when food is withheld (Lulich
and calcium phosphate uroliths are normocalcemic, some are et al, 1991a; Lemieux and Plante, 1968). Therefore, values for
hypercalcemic. 24-hour urinary solute excretion may differ when measured fol-
Calcium phosphate and sterile struvite uroliths may be asso- lowing food consumption vs. values obtained when food is
ciated with distal renal tubular acidosis characterized by hyper- withheld.
chloremic (normal anion gap) metabolic acidosis, urinary pH Consumption of food stimulates gastric secretion of hydro-
values consistently greater than approximately 6 and chloric acid. As a result, concentrations of chloride decrease and
hypokalemia. bicarbonate increase in venous blood draining the stomach.
A variety of biochemical alterations may exist in patients Total serum concentration of carbon dioxide increases. The
with urate urolithiasis.The following changes may be observed resulting metabolic alkalosis is commonly called the postpran-
in patients with urate uroliths due to congenital or acquired dial alkaline tide. Urinary pH will increase unless acidifying
hepatic disorders (Rothuizen and van den Ingh, 1980; Barrett substances are contained in the food. In a study of healthy bea-
et al, 1976; Marretta et al, 1981): 1) decreased urea nitrogen gles, eating was associated with increased urinary excretion of
concentrations, 2) decreased total protein and albumin concen- hydrogen ions, ammonia, sodium, potassium, calcium, magne-
trations, 3) abnormal bile acid concentrations, 4) increased con- sium and uric acid (Lulich et al, 1991a).
centrations of total bilirubin and fasting blood ammonia and 5) Laboratory results may be markedly affected by changes in
increased serum alanine aminotransferase and serum alkaline foods fed in a home environment vs. different foods fed in a
phosphatase enzyme activities. Dogs with portal vascular hospital environment. For example, urinary excretion of poten-
anomalies typically have reduced hepatic functional mass and tially lithogenic metabolites while animals consume foods fed
altered portal blood flow evidenced by abnormally elevated bile in the hospital may be different from those excreted by animals
acid concentrations, prolonged sulfobromophthalein retention eating at home. To determine the influence of home-fed foods
times and abnormal ammonia tolerance tests (Griffiths et al, on laboratory test results, consider asking clients to bring