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 Collegeof Veterinary Internal Medicine (ACVIM) in Denver, Colorado, Dr. Alice M.Wolf, diplomate of ACVIM and ABVP, from Texas A&M University Collegeof 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 dependenton the cat's body weight and temperament. If asymmetry of the kidneys ispalpated, the veterinarian should determine whether it is because of reducedsize of one kidney or an enlargement of the other kidney. If both kidneysare changed in size symmetrically, it may be more difficult to assess themas being abnormal.

In addition to size, kidneys should be palpated for being rough surfacedor smooth surfaced. If pain is indicated during renal palpation, this shouldalso be noted. Care should be taken not to over-interpret apparent renalpain because existing epaxial muscle and spinal pain may also be present.

Causes of renomegaly

Renal dysgenesis is a congenital condition in which one kidney failsto properly develop and the remaining kidney will undergo compensatory hypertrophyto maintain normal renal function.

Renal dysgenesis is usually detected during physical examination earlyin the cat's life and may lead one to investigate an apparently large kidney.Any disease that causes significant damage to one kidney can contributeto compensatory hypertrophy of the opposite kidney. The hypertrophied kidneyusually maintains normal renal function.

Primary renal tumors (usually adenocarcinomas) are often unilateral andusually produce an enlarged, irregular-shaped renal mass on abdominal palpation.A polar portion of the remaining normal kidney may be palpable in associationwith the renal mass ­ that is a normal pole on one end of the kidneyand a mass effect on the opposite end. Unilateral renal tumors do not causerenal failure.

Polycystic kidney disease of Persian, Himalayan and crossbred cats usuallycauses bilateral renal enlargement. One should remember that all Persianand Himalayan cats (especially male cats) have cortical cysts in their kidneysuntil proven otherwise.

Cortical cysts are an autosomal dominant trait in these breeds. Corticalcysts can be detected by ultrasound examination as early as a few monthsof age. The cortical cysts may occur throughout the renal parenchyma andare present from birth. Cortical cysts may continuously enlarge throughoutthe life of the cat. Renal surfaces are generally smooth and occasionallysoft spots (superficial cysts) can be detected by abdominal palpation.

Bacterial pyelonephritis, especially in the early acute stage, may causebilateral swollen and painful kidneys. Ethylene glycol intoxication willoften 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 perinephricpseudocyst, 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 affectone or both kidneys.

Diagnostic evaluation

The signalment (age, breed, gender) will help prioritize the differentiallist. A complete history will help rule in or out exposure to toxicants,trauma (past or present), and other urologic injury or disease (e.g. calciumoxalate urolithiasis or previous diagnoses of neoplasia).

Physical examination should be thorough to evaluate for the presenceof systemic illness or other organ/system abnormalities in addition to thedetection of renomegaly. Laboratory evaluation should include a completeblood count, serum chemistry profile and urinalysis.

The cat's urine should ideally be collected by cystocentesis or directlyfrom the renal pelvis so that bacterial culture can be performed immediatelyon the same urine sample should bacteria or inflammatory urine sedimentsuggest UTI.

The new IndicatoRx device (IDEXX) is excellent for obtaining immediatebacterial urine culture results ­ results are obtained overnight anddone in-hospital. The FeLV antigen and FIV antibody tests should also beperformed.

Survey abdominal radiographs will often be helpful to more accuratelyassessing the kidneys for true enlargement than is abdominal palpation orultrasonography. If the renal shadow(s) is 2.5 times or greater the lengthof the 2nd lumbar vertebral body, the kidney is larger than normal. Theentire urinary system should be carefully examined for the presence of radiopaquecalculi.

Ultrasonography is the best tool to assess the character and consistencyof the renal parenchyma and to evaluate the urinary system for the presenceof radiolucent calculi, ureteral dilatation and cystic calculi and masses.In addition, an excretory urogram or double contrast cystogram have notbeen replaced by ultrasonography.

Ultrasound-guided fine needle aspiration (FNA) of the renal cortex, renalpelvis or a perinephric pseudocyst for cytologic examination may be a usefultool for such disorders as renal lymphoma and pyelonephritis. FNA is lowrisk and can be performed without sedation in most cats. If the kidney isreadily palpable, ultrasound guidance may not be needed in order to performFNA.

Needle biopsy may be required for diagnosis in some cats with renomegalyand this is best performed under anesthesia with ultrasound guidance orwith a keyhole technique. If nephrectomy is being considered, nuclear scintigraphycan be used to determine the amount of renal function being carried by eachindividual kidney.