Overview

• Capsulogenic renal cyst, capsular cyst, pararenal pseudocyst, capsular hydronephrosis, perirenal cyst, and perirenal pseudocyst are terms used to describe renomegaly caused by accumulation of fluid between the kidney and its surrounding capsule. One or both kidneys are affected.
• The tissue adjacent to the fluid accumulation is not lined with secretory epithelium; and thus the name “pseudo”-cyst is appropriate.

Signalment

• Primarily older male cats (>8 years).
• When detected in young cats, the disease is usually unilateral.
• Rare in dogs; the difference in prevalence between species may be related to the prominent network of subcapsular veins that characterize feline kidneys.

Signs

• May be asymptomatic
• Non-painful, enlarged abdomen is common
• Signs of concomitant renal failure in some patients

Causes & Risk Factors

• Cause of perirenal accumulation of fluid is not completely understood.
• Accumulation of pseudocyst fluid is a dynamic, not a static, process.
• Cytologic and biochemical evaluation of pseudocyst fluid may aid understanding of pathophysiologic mechanisms.
• Fluid with characteristics of a transudate may accumulate because of high capillary hydrostatic pressure or lymphatic obstruction. Some cats have light microscopic evidence of renal fibrosis. However, it is not known whether progressive renal parenchymal contraction occludes lymphatics and blood vessels, promoting transudation of fluid.
• Perirenal accumulation of transudate can also result from rupture of renal cysts.
• Accumulation of perirenal urine may indicate disruption of the renal pelvis or the proximal ureter.
• Accumulation of blood in pseudocysts can result from external trauma, surgery, neoplastic erosion of blood vessels, rupture of aneurysms, coagulopathies, or paracentesis.

Differential Diagnosis

• Causes of renomegaly include renal neoplasia, hydronephrosis, polycystic kidney disease (common), feline infectious peritonitis, and mycotic or bacterial nephritis (less common).

• Ascites and enlargement of other abdominal organs can cause non-painful distension of the abdomen.

CBC/Biochemistry/Urinalysis

• Results unremarkable until patient develops renal insufficiency.
• Azotemia and inappropriately low urinary specific gravity (<1.035) indicate concomitant chronic kidney disease (CKD).

Other Laboratory Tests

N/A

Imaging

• Renomegaly is commonly detected by abdominal radiography.
• Excretory urography and ultrasonography can be used to determine whether the underlying renal parenchyma is normal or abnormal. Small kidneys beneath an abnormally wide fluid-filled intracapsular space is a common finding.

Diagnostic Procedures

Cytologic examination of pseudocyst fluid may provide evidence of the underlying disease process resulting in fluid accumulation (e.g., transudation, hemorrhage, lymphatic obstruction, inflammation, urine, etc.) or secondary complications (e.g., infection). Creatinine concentrations that are higher in pseudocyst fluid compared to serum are consistent with urinary tract rupture.

Drug(s)

Consider appropriate antimicrobic (i.e., lipid-soluble antibiotic chosen on the basis of antimicrobial susceptibility) if pseudocysts become infected.