9-week-old, intact female, Domestic Shorthair

• There is partial fusion of the caudal aspect of the renal cortices by a wide isthmus.
• The fused cortices display shared vascularity on color Doppler.
• Two separate renal medullas and renal pelves are present. The renal medullas may be bilaterally thickened.
• No renal pelvic dilation or ureteral dilation is present.
• There is no evidence of nephrolithiasis and the perinephric fat is normal.
• The spleen is moderately to severely enlarged, with slightly rounded margins. The spleen measures 1.1 cm in thickness. Several small ill-defined hypoechoic nodules are scattered throughout the splenic parenchyma, but do not deform the splenic capsule.
• A scant volume of anechoic peritoneal effusion is also present, considered physiologic in a juvenile patient.
• No other abnormalities are detected

1. Congenital renal fusion anomaly (horseshoe kidney).
2. Moderate to severe splenomegaly. Extramedullary hematopoiesis is prioritized, given the reportedly severe anemia. Other etiologies (e.g. splenitis) cannot be excluded.

Renal fusion and positional anomalies occur secondary to disruption of normal renal embryologic migration. Renal fusion anomalies have rarely been described in veterinary medicine, with crossed fused renal ectopia (CFRE) being the most common congenital abnormality reported. CFRE involves fusion of the kidneys on one side of the abdomen, with migration of the left kidney into the right abdomen being the most common.

Six different subtypes have been classified in human medicine: inferior crossed fusion; sigmoid (or S-shaped) kidney; lump kidney; disc kidney; L-shaped kidney; superior crossed fusion. Both inferior crossed fusion and L-shaped kidneys have been described in cats, with the former being the most frequently described type of CFRE in humans. The most common type of renal anomaly in humans overall, however, is termed “horseshoe kidney.” This anomaly is characterized by the kidneys being fused to one another across midline (versus fusion localized to one side of the abdomen). Typically, the fusion occurs at the caudal poles of the kidneys, with an isthmus of renal parenchyma or fibrous tissue connecting one kidney to the other.

To the authors’ knowledge, this type of renal anomaly has not been previously reported in cats. Renal fusion anomalies are frequently incidental in humans, although associated pathologies have been documented (e.g. nephrolithiasis, ureteropelvic obstruction, infection). An increased incidence of neoplasia and higher susceptibility to trauma has also been described in conjunction with horseshoe kidneys. There is only 1 case of a cat with CFRE having signs of renal dysfunction. In all other reported cases, CFRE was an incidental diagnosis in cats. In this case, both kidneys displayed orthotopic positioning, therefore classifying the cat’s renal fusion anomaly as a horseshoe kidney was deemed most appropriate. Given the very small size of the patient (1.1 kg), the possibility that the left kidney was mildly ectopic (with slight migration to the right of midline) cannot be entirely excluded. This finding was considered incidental, as the patient’s anemia was regenerative and no azotemia was present on serum biochemistry. It was communicated to the owner that although this renal anomaly was not currently related to the patient’s clinical signs, the long term prognosis of being born with a horseshoe kidney is unknown in cats.

After receiving 2 packed red blood cell transfusions, the kitten’s hematocrit stabilized at 23% and her appetite and demeanor markedly improved. The owner declined further diagnostics to determine the cause of the severe regenerative anemia. The kitten was discharged into the owner’s care on antibiotics (doxycycline) to treat for presumptive infectious anemia. Recheck examination was recommended within the week, however the patient was lost to follow up.

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