Ultrasonography best to assess character, consistency of renal parenchyma
Q. Renomegaly is often seen in older cats. What are the differential considerations?
A. At the 19th Veterinary Forum of the American College of Veterinary Internal Medicine (ACVIM) in Denver, Colorado, Dr. Alice M. Wolf, diplomate of ACVIM and ABVP, from Texas A&M University College of Veterinary Medicine presented an excellent review of renomegaly in cats.
During physical examination, both kidneys in cats are usually palpable. However, the ability to critically evaluate the kidneys is clearly dependent on the cat's body weight and temperament. If asymmetry of the kidneys is palpated, the veterinarian should determine whether it is because of reduced size of one kidney or an enlargement of the other kidney. If both kidneys are changed in size symmetrically, it may be more difficult to assess them as being abnormal.
In addition to size, kidneys should be palpated for being rough surfaced or smooth surfaced. If pain is indicated during renal palpation, this should also be noted. Care should be taken not to over-interpret apparent renal pain because existing epaxial muscle and spinal pain may also be present.
Causes of renomegaly
Renal dysgenesis is a congenital condition in which one kidney fails to properly develop and the remaining kidney will undergo compensatory hypertrophy to maintain normal renal function.
Renal dysgenesis is usually detected during physical examination early in the cat's life and may lead one to investigate an apparently large kidney. Any disease that causes significant damage to one kidney can contribute to compensatory hypertrophy of the opposite kidney. The hypertrophied kidney usually maintains normal renal function.
Primary renal tumors (usually adenocarcinomas) are often unilateral and usually produce an enlarged, irregular-shaped renal mass on abdominal palpation. A polar portion of the remaining normal kidney may be palpable in association with the renal mass that is a normal pole on one end of the kidney and a mass effect on the opposite end. Unilateral renal tumors do not cause renal failure.
Polycystic kidney disease of Persian, Himalayan and crossbred cats usually causes bilateral renal enlargement. One should remember that all Persian and Himalayan cats (especially male cats) have cortical cysts in their kidneys until proven otherwise.
Cortical cysts are an autosomal dominant trait in these breeds. Cortical cysts can be detected by ultrasound examination as early as a few months of age. The cortical cysts may occur throughout the renal parenchyma and are present from birth. Cortical cysts may continuously enlarge throughout the life of the cat. Renal surfaces are generally smooth and occasionally soft spots (superficial cysts) can be detected by abdominal palpation.
Bacterial pyelonephritis, especially in the early acute stage, may cause bilateral swollen and painful kidneys. Ethylene glycol intoxication will often cause swollen and painful kidneys. With ethylene glycol intoxication, urine output is often markedly reduced or absent.
Other disorders that may cause differential kidney size include perinephric pseudocyst, obstructive hydronephrosis and neoplasia (most notably lymphoma).
Feline infectious peritonitis (FIP) usually produces bilateral renomegaly, but unilateral involvement occurs in some cats. Trauma similarly may affect one or both kidneys.
The signalment (age, breed, gender) will help prioritize the differential list. A complete history will help rule in or out exposure to toxicants, trauma (past or present), and other urologic injury or disease (e.g. calcium oxalate urolithiasis or previous diagnoses of neoplasia).
Physical examination should be thorough to evaluate for the presence of systemic illness or other organ/system abnormalities in addition to the detection of renomegaly. Laboratory evaluation should include a complete blood count, serum chemistry profile and urinalysis.
The cat's urine should ideally be collected by cystocentesis or directly from the renal pelvis so that bacterial culture can be performed immediately on the same urine sample should bacteria or inflammatory urine sediment suggest UTI.
The new IndicatoRx device (IDEXX) is excellent for obtaining immediate bacterial urine culture results results are obtained overnight and done in-hospital. The FeLV antigen and FIV antibody tests should also be performed.
Survey abdominal radiographs will often be helpful to more accurately assessing the kidneys for true enlargement than is abdominal palpation or ultrasonography. If the renal shadow(s) is 2.5 times or greater the length of the 2nd lumbar vertebral body, the kidney is larger than normal. The entire urinary system should be carefully examined for the presence of radiopaque calculi.
Ultrasonography is the best tool to assess the character and consistency of the renal parenchyma and to evaluate the urinary system for the presence of radiolucent calculi, ureteral dilatation and cystic calculi and masses. In addition, an excretory urogram or double contrast cystogram have not been replaced by ultrasonography.
Ultrasound-guided fine needle aspiration (FNA) of the renal cortex, renal pelvis or a perinephric pseudocyst for cytologic examination may be a useful tool for such disorders as renal lymphoma and pyelonephritis. FNA is low risk and can be performed without sedation in most cats. If the kidney is readily palpable, ultrasound guidance may not be needed in order to perform FNA.
Needle biopsy may be required for diagnosis in some cats with renomegaly and this is best performed under anesthesia with ultrasound guidance or with a keyhole technique. If nephrectomy is being considered, nuclear scintigraphy can be used to determine the amount of renal function being carried by each individual kidney.