Diseases of the feline kidney (Proceedings)
POLYCYSTIC RENAL DISEASE
Polycystic kidney disease was first described in adult male and female long-haired, Persian-type cats in the late 1960's. In 1996, the disorder was shown to be inherited as an autosomal dominant trait in a family of Persian cats. Both male and female cats were affected. In affected × unaffected crosses, 42% of offspring were affected and 58% were unaffected. In affected × affected crosses, 73% of progeny were affected and 27% were unaffected. These results were consistent with autosomal dominant transmission. Recently, the prevalence of autosomal dominant polycystic kidney disease (ADPKD) in Persian cats has been studied in the United Kingdom, Australia and the United States. The prevalence of ADPKD ranged from 35 to 57% of cats examined depending on whether the cats studied consisted of long-haired cats referred for any reason or those referred specifically for ADPKD evaluation.
Many young Persian cats with ADPKD are asymptomatic, and renomegaly is recognized as an incidental finding on physical examination with confirmation of renal cysts by abdominal ultrasonography. The kidneys of cats with ADPKD become progressively enlarged and irregular over time as the cysts increase in number and size. Renal failure usually does not develop until 7 or 8 years of age. Some affected cats appear outwardly normal for many years, and clinical signs of renal failure can seem to the owner to appear relatively abruptly. Clinical signs in cats with ADPKD and chronic renal failure include polyuria, polydipsia, anorexia, weight loss, poor hair coat and lethargy. On physical examination, enlarged irregular kidneys can be palpated. Dehydration, pallor of mucous membranes, and emaciation also may be observed on physical examination of affected cats with chronic renal failure. Laboratory findings in ADPKD cats in renal failure include azotemia, hyperphosphatemia, isosthenuria, nonregenerative anemia, and metabolic acidosis. Systemic blood pressure is normal or mildly increased in cats with ADPKD that are not in renal failure at the time of evaluation, and the role of the renin-angiotensin-aldosterone system in blood pressure regulation in affected cats is unclear.A skilled ultrasonographer can detect cysts during renal ultrasound examination in affected cats as young as 6 to 8 weeks of age. In one study, renal ultrasonography had a sensitivity of 75% when performed at = 4 months of age and a sensitivity of 91% when performed at = 9 months of age. The cysts are smooth, round, anechoic and characterized by distal acoustic enhancement. Cyst size and number vary greatly among affected cats, and cysts increase in number and size over time. Currently, ultrasonography is the diagnostic test of choice for identifying ADPKD in Persian cats, and the ability to detect cysts before sexual maturity provides an opportunity to eliminate this disorder from the breed by careful genetic counseling.
On pathologic examination, multiple cysts of varying size are found in the renal cortex and medulla of the kidney. Lymphoplasmacytic interstitial inflammation and fibrosis also may be observed. Ultrastructural examination and lectin staining of affected kidneys suggests that the cysts arise from both proximal and distal segments of the nephron. Some affected cats also have hepatobiliary hyperplasia and fibrosis. Hepatic cysts occasionally have been observed, and rarely pancreatic cysts have been detected.
Treatment of ADPKD in affected Persian cats is limited to control of hypertension if present and medical management for chronic renal failure. Enalapril or amlodipine can be used for blood pressure control in affected hypertensive cats.
Spontaneous systemic amyloidosis is uncommon in the domestic cat, but occurs as a familial disease in the Abyssinian, Siamese and Oriental Shorthair breeds. Affected Abyssinian cats usually are presented between 1 and 5 years of age, and amyloid deposits first appear in the kidneys of affected Abyssinian cats between 9 and 24 months of age. In some of these cats, amyloid deposition is rapid and severe and renal failure develops within one year of diagnosis. In others, amyloid deposition in the kidney is mild, and affected cats may live to an advanced age without detection of their amyloid deposits. Such mildly affected cats likely transmit and maintain the disorder in the affected breeds. The mode of transmission of amyloidosis in Abyssinian cats has been difficult to establish because no ante-mortem gold standard of diagnosis exists.
The amyloid deposits in affected Abyssinian cats are composed of amyloid A protein, an amino terminal degradation product of the acute phase reactant serum amyloid A protein. In one study of Siamese cats with familial amyloidosis and severe hepatic involvement, 2 amino acid substitutions were found when comparing the amino acid sequence of their amyloid A protein to that reported for Abyssinian cats with familial amyloidosis. These sequence differences potentially could explain the different tissue tropisms for the amyloid deposits in these 2 breeds (i.e. renal in Abyssinian cats vs hepatic in Siamese cats).
Abyssinian cats with familial amyloidosis usually are presented for poor hair coat, weight loss, polydipsia, polyuria, lethargy, and anorexia. Physical examination findings include dehydration, pallor of mucous membranes, gingivitis, and kidneys that are small, firm, and irregular on abdominal palpation. Laboratory evaluation usually reveals evidence of chronic renal failure including azotemia, hyperphosphatemia, metabolic acidosis, nonregenerative anemia, and isosthenuria. Proteinuria is a variable finding and reflects the severity of glomerular involvement. In Siamese and Oriental shorthair cats with amyloidosis, large amyloid deposits in the liver may lead to hepatic rupture and spontaneous hemoabdomen necessitating emergency abdominal exploration to control hemorrhage.
The diagnosis of amyloidosis requires proper pathologic evaluation (e.g. Congo red staining) of an adequate biopsy specimen. A wedge of kidney containing both cortical and medullary tissue obtained at laparotomy is more likely to yield a definitive diagnosis than a percutaneous needle biopsy specimen because of the prominent medullary distribution of renal amyloid deposits in the Abyssinian cat. Despite a conscientious effort to obtain an adequate renal biopsy sample, results can be negative in affected cats if no glomerular amyloid deposits are present and only small deposits are present in the renal medullary interstitium.
Treatment of amyloidosis is limited to symptomatic therapy of chronic renal failure. Underlying inflammatory disease is uncommonly detected, but any concomitant infections should be treated appropriately.