Nephroliths in a cat: What is your diagnosis

Nephroliths in a cat: What is your diagnosis

Jul 01, 2002

Figure 1: Survey radiograph of the lateral aspect of the abdomen of an 8-year-old domestic shorthair cat with persistent gross hematuria.
An 8-year-old spayed female domestic shorthair cat was referred to the University of Minnesota Veterinary Teaching Hospital for evaluation of bilateral nephroliths associated with persistent gross hematuria.

According to the owner, there was no evidence of dysuria or pollakiuria. Physical examination revealed no abnormalities. Results of a CBC and serum biochemical profile were normal (urea nitrogen concentration = 29 mg/dl, creatinine concentration = 1.5 mg/dl, calcium concentration = 9.9 mg/dl, and phosphorus concentration = 3.7 mg/dl). Analysis and aerobic culture of a voided urine sample revealed the following results:

  • Color = Red
  • Turbidity = Cloudy
  • Specific gravity = 1.037
  • pH = 6.5
  • Glucose = Negative
  • Acetone = Negative
  • Bilirubin = Negative
  • Occult blood = 3+
  • Protein = 3+
  • RBC = TNTC/hpf
  • WBC = 3-5 /hpf
  • Casts = 0
  • Epithelial cells = Occasional
  • Bacteria = None
  • Crystals = 0
  • Culture = sterile

Survey abdominal radiography revealed bilateral nephroliths; their radiodensity was comparable to bone (Figure 1). Intravenous urography revealed no evidence of obstruction to urine flow.

Mineral composition In your opinion, what is the mineral composition of the nephroliths? You may find the following information to be helpful in formulating your answer.

Table 1: Mineral composition of feline upper tract uroliths
The primary mineral in approximately 69 percent of the nephroliths submitted to the Minnesota Urolith Center is composed of calcium oxalate (Table 1). Magnesium ammonium phosphate, calcium phosphate, purines, xanthine and cystine occur much less frequently.

Figure 2: Nephroliths composed primarily of clotted blood and mineralized with calcium phosphate.
Approximately 9 percent of the nephroliths are composed on noncrystalline matrix. On the basis of this information, and the fact that the radiodensity of the nephroliths is comparable to bone, you may have narrowed your selection to calcium oxalate and possibly calcium phosphate.

Please examine the radiograph again and note the shape and location of the nephroliths. Their linear shape and distribution suggest that they are located in the diverticula of the renal pelves. Next, examine Figure 2. Note that the appearance of the nephroliths removed from one of the kidneys is similar to dried blood clots. Microscopic evaluation of stained sections of the nephroliths confirm that they are primarily composed of red cells and proteinaceous material (75 percent) and mineral salts containing calcium (25 percent; Figure 3). Evaluation of the nephroliths by polarizing light microscopy and infrared spectrophotometry reveal that their mineral component is calcium phosphate.

Figure 3: Photomicrograph of a section of a nephrolith described in Table 1. Notice the red blood cells surrounded by proteinaceous matrix. The purple stained amorphous material is calcium phosphate. (Hematoxylin and eosin stain; original magnification = 450X).
We have observed uroliths composed of blood clots partially mineralized with calcium phosphate on numerous occasions. In our experience, they are most common in the kidneys of cats, although on occasion they have been detected in the ureters, urinary bladder and urethra.

Less commonly they have been observed in dogs (Figure 4). Probable risk factors for formation and retention of blood clots in the urinary tract include:

  • marked hemorrhage
  • reduction in the rate of urine flow.
  • formation of highly concentrated urine.
  • formation of acid urine (proteins tend to precipitate in an acid environment).

Mechanism of precipitation We speculate that the mechanism of precipitation of calcium phosphate in the blood clots is similar to dystrophic mineralization that may occur in devitalized tissue anywhere in the body. In our experience, affected cats and dogs were not hypercalcemic or hyperphosphatemic.

Figure 4: Cross section of the left kidney surgically removed from a 5-year-old female Poodle with a history of persistent gross hematuria caused by a hamartoma. Note the blood clots in the renal pelvis. These nephroliths were partially mineralized with calcium phosphate.
If blood clots contain calcium phosphate as a result of dystrophic mineralization, minimizing or controlling the underlying cause of hematuria would be of obvious therapeutic benefit.

If hematuria persists, therapeutic strategies designed to reduce urine concentration and increase the flow of urine through the excretory pathway may reduce the concentration of factors that promote formation of clots. Consumption of diets designed to minimize urinary excretion of excess calcium, phosphorus and acid catabolites might reduce mineralization of blood clots.

Despite persistent idiopathic renal hematuria, the nephroliths in the cat described at the beginning of this Diagnote remained inactive (did not increase in number or size, or cause outflow obstruction, or result in bacterial UTI) during a three-year period of evaluation.

Serial evaluation of CBC's during this period revealed no evidence of anemia.

To obtain further information about quantitative analysis of uroliths from the Minnesota Urolith Center, FAX your request to (612) 624-0751.