Perinephric pseudocyst in a two-month-old female cat

Background: Perinephric pseudocyst is defined as an accumulation of fluid in fibrous sacs surrounding one or both kidneys, and may be located in the subcapsular or extracapsular region. Histologically, it is characterized by the absence of an epithelial lining to the cyst wall and the term pseudocyst is used. This condition have been reported sporadically in cats, and is associated with chronic kidney disease, mainly in older animals. The diagnosis is based on imaging tests and the prognosis is related to the severity of renal dysfunction. The aim of this report is to describe an unusual presentation of perinephric pseudocyst in a very young female cat. Case: A mixed-breed 2-month-old female cat was evaluated for presenting abdominal distension and anatomical deformities of the limbs. Renomegaly on the left side was noted during abdominal palpation. On presentation, the serum creatinine was 134 μmol/L. Urinalysis showed mild proteinuria, presence of squamous and transitional cells, and rare bacteria. The urine specific gravity was 1.044 and urine protein to creatinine ratio was 0.23. Abdominal ultrasonography revealed a large anechoic subcapsular cyst on the left kidney, in adittion to dilatation of renal pelvis and calyces. The excretory urography showed left renomegaly, and the nephrogram and pyelogram were not clearly observed in this kidney, indicating unilateral deficiency in renal filtration. The cat was referred to unilateral left nephrectomy but died shortly before surgery. At necropsy, marked left kidney enlargment was observed (6.5 x 4.5 x 1.5 cm), with a cystic aspect, occupying almost the entire abdominal cavity. The cystic content was translucent and was compressing the renal parenchyma, resulting in hydronephrosis. The capsule forming the cyst wall was composed of connective tissue, characterizing the perinephric pseudocyst. Discussion: Reports show cases of perinephric pseudocyst affecting mostly older animals, above eight years old. In one retrospective study of 26 cases, 73% were male and the mean age was eleven years, ranging from four to 18 years old. Another study of 13 cases showed a mean age of 16 years. In this report, unexpectedly, the patient was too young compared to these other cases, but the clinical presentation was similiar. Being a young animal, in the absence of ureterolithiasis or history of trauma, it is suggested a congenital origin for the disease. Loss of renal function and azotemia may occur in about 90% of the cases by compression of the renal parenchyma by the pseudocyst or due to associated interstitial fibrosis. In this case, the serum creatinine concentration was above the reference value proposed for pediatric patients therefore, the cat may be considered azotemic. Beyond that, the excretory urography indicated no proper excretory function on the left kidney. It is proposed that the renal function was being compensated by the contralateral kidney. The clinical and imaging findings of this case are compatible with perinephric pseudocyst, which was confirmed by necropsy and histopathological examination. This condition is not commonly reported in cats of pediatric age, althought it must be considered as a differencial diagnosis to cats presenting abdominal distension and renomegaly, even young ones. Imaging tests were important in this case, not only to direct the diagnosis, but also contributing in evaluating the renal excretory function