Rare Kidney Fusion Anomalies in Cats: What Your Radiographs Might Be Missing

Imaging Features of Renal Ectopia and Fusion in 13 Cats

Powell T. Slinkard, Augstina Anson, Swan Specchi, Maria Frau Tascon, Laura Muñiz-Moris, Dominique Penninck, Nele Eley Ondreka

Background
Renal fusion anomalies such as crossed fused renal ectopia (CFRE) are rare in veterinary medicine but well-documented in human populations. These congenital malformations may remain asymptomatic or be associated with urinary tract complications. Despite occasional feline case reports, a structured description of renal fusion types in cats and their imaging characteristics has been lacking. This study aimed to systematically document the imaging findings associated with renal fusion anomalies in cats, using a classification adapted from human medicine.

Methods
This retrospective multicenter case series analyzed 13 cats with renal fusion anomalies identified between 2005 and 2021 across six veterinary institutions. Data from radiography, ultrasonography, and computed tomography (CT) were reviewed. Fusion types were classified based on the McDonald system, adjusted for anatomical terminology used in veterinary contexts. Clinical presentations, imaging characteristics, and concurrent congenital abnormalities were recorded.

Results
Of the 13 cats (median age: 9 years), 11 cases were classified as CFRE and two as other renal fusion anomalies. Fusion types included five lump kidneys, four disc kidneys, one horseshoe kidney, one L-shaped kidney, one caudal ectopia, and one pelvic (pancake) kidney. Most cases involved left-to-right fusion (7/13) and the majority were males (9/13). Ultrasonography successfully identified and classified fusion types in 12/13 cases, commonly revealing reduced corticomedullary distinction, increased cortical echogenicity, or pyelectasia. CT, available in three cases, provided detailed information on ureteral and vascular anatomy. Two cats presented with additional congenital anomalies: a peritoneal-pericardial diaphragmatic hernia and azygous continuation of the caudal vena cava.

Limitations
The retrospective design and incomplete clinical data limited interpretation of the functional consequences of these anomalies. Imaging availability varied, and ultrasonography did not assess ureteral insertion or renal vasculature in most cases. The rarity of these conditions also restricts broader generalizability.

Conclusions
Crossed fused renal ectopia and related fusion anomalies, while rare, can be identified in cats using standard imaging modalities. Most cases are incidental findings, with unclear clinical significance. Further research is needed to determine their impact on renal function and long-term health outcomes. This case series expands the veterinary literature and reinforces the utility of ultrasonography and CT in diagnosing renal fusion disorders.

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