Supernumerary Kidney with Ipsilateral Cryptorchidism in a Cat

Introduction

A kitten presented with an abdominal mass that was confirmed and noted to be complex and cystic on ultrasonographic examination. Normal right and left kidneys were also identified. At surgery, the mass was separated from a normal right kidney, removed and was unexpectedly identified on examination by the pathologist as a hydronephrotic kidney with an attached, normal caliber ureter. Follow-up ultrasound identified subsequent hydronephrosis of the cranial right kidney that was also removed. There was ipsilateral cryptorchidism and apparent ureteral hypoplasia. To the authors’ knowledge, this is the first reported case of supernumerary kidney in a cat.

Case Report

An 8 wk old male domestic longhair was presented after the owner noted swelling of the abdomen. The kitten behaved normally with no other apparent symptoms. Physical examination revealed a well-developed, well-nourished, 0.8 kg kitten, cryptorchid on the right. A firm mass, approximately 2 cm in diameter, was palpable in the mid abdomen. Discomfort was noted on palpation. The remainder of the physical examination was normal. The body condition was 5 out of 9.

Laboratory results were unremarkable, including a hematocrit of 32% (reference range, 29–48%), total protein of 5.8 g/dL (reference range, 5.2–8.8 g/dL), alanine aminotransferase of 43 U/L (reference range, 10–100 U/L), blood urea nitrogen of 25 mg/dL (reference range, 14–36 mg/dL), creatinine of 0.2 mg/dL (reference range, 0.6–2.4 mg/dL), and glucose of 103 mg/dL (reference range, 64–170 mg/dL). Urinalysis was normal with neither blood nor protein.

An abdominal ultrasound to better define the character of the mass revealed a complex, septated, cystic mass measuring 3 cm × 2.5 cm (Figure 1A). The mass appeared ventrolateral, somewhat caudal to, and possibly related to, the right kidney. The right kidney appeared normal except for mild fullness of the pelvis (Figure 1A). A normally appearing left kidney was identified (Figure 1B). There was no hepatosplenomegaly or lymphadenopathy, the bladder appeared normal, and the location of the cryptorchid testis was not identified. All ultrasounds were performed by one of the authors (D.P.) who had completed multiple continuing education courses and laboratories in abdominal ultrasonography and had been performing and interpreting studies for over 10 yr.

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At that point in the examination, the origin of the mass was uncertain. The causes of abdominal masses in 8 wk old kittens included a duplication cyst, testicular tumor/torsion, teratoma, hydronephrotic kidney, polycystic kidney, intussusception, and bladder obstruction. A supernumerary kidney was not suspected as it had never been described before in a cat; therefore, further renal workup was not considered, and surgical excision appeared to be the best diagnostic and therapeutic option.

Surgical removal of the mass was elected and performed without complication. At surgery, a retroperitoneal mass was found on the right side. The mass was surrounded by a capsule that adhered to the surrounding fascia and right renal capsule, which was carefully separated from the latter. Three attachments were noted to the mass. One attachment with vasculature was noted running caudodorsally into the inguinal region. A second attachment with vasculature was noted medially with what appeared to be a possible ureter running caudally to the bladder, within the fascia (Figure 2). The third attachment was fibrous in character and connected to the right kidney capsule proper. The three attachments were separated and ligated appropriately. Care was taken to avoid damage to the right kidney and ureter. The cryptorchid testis was not located, suggesting that the mass could be an abnormal cryptorchid testis. Following removal of the attachment of the mass to the right renal capsule, a small tear in the renal capsule was made. No break in the parenchyma of the right kidney was identified confirming that the mass was separated from the right kidney per se. The right renal capsule was sutured closed. The kitten recovered from surgery uneventfully.

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The mass and attachments were submitted for pathologic examination. The mass measured 3 cm × 2 cm, was a horseshoe in shape, and well delineated. The mass was identified as a kidney, had marked dilatation of the renal pelvis, and compression and loss of adjacent medullary and cortical parenchyma. Microscopic examination confirmed well-differentiated cortical and medullary cells, a palisade of urethral cells consistent with pelvis, and a ureteral tube arising from the pelvic space. The caliber of the submitted ureter was normal. The fact that the structure was found adjacent to, but separate from, the right kidney and an ultrasound identified a normal left kidney defined this mass as a hydronephrotic supernumerary right kidney. Although the findings suggested ureteral pelvic junction obstruction, a specific reason for the obstruction was not identified on ultrasound, intraoperatively, or on pathologic examination.

Postoperative ultrasound 1 wk later confirmed the presence of a normal left kidney. Although there continued to be mild fullness of the right renal pelvis, there was no further ureteral or pelvic dilation. The physical examination remained unremarkable.

A follow-up ultrasound was performed 1 mo postoperatively to search for the cryptorchid testis. The left kidney remained normal. The cryptorchid testis was not identified; however, over the interim 4 wk, the right cranial kidney that previously showed fullness but no obstruction had developed overt signs of obstruction. There was moderate to severe hydronephrosis with pelvic dilation accompanied by cortical and medullary compression (Figure 3). There was no ureteral dilation. Subsequently, right nephrectomy and identification and removal of an intra-abdominal cryptorchid testis were performed by an affiliated board certified veterinary surgeon (Figure 4). The cryptorchid testis was located in the pelvis, dorsal to the distended bladder. The kitten had an uneventful recovery following the second surgery.

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Pathologic examination confirmed a kidney with marked hydronephrosis with only a narrow zone of cortical medullary tissue. An epididymis, spermatic cord, and normally functioning immature testis were confirmed. Regarding the site of obstruction, the ureteropelvic junction, per se, was described as dilated. The attached ureter was diffusely narrowed externally and the lumen was difficult to identify throughout its course, suggesting a congenital hypoplastic ureter.

At the 1 yr follow-up examination, the BUN was 23 mg/dL and creatinine was 1.5 mg/dL. Ultrasound of the left kidney showed minimal pelvicaliceal dilation with normal cortex. In view of the normal renal values, the dilatation was felt to be most consistent with enhanced flow through the single kidney.

Discussion

This kitten presented with an abdominal mass and was confirmed to have a right caudal supernumerary kidney with ipsilateral cryptorchidism and an apparent hypoplastic ureter of the right cranial kidney. Supernumerary kidney is a third or, rarely, a fourth kidney without parenchymal attachment to the main kidneys.¹ This finding is rare in humans, with about 100 cases reported.² The third kidney is most often caudal to the main kidney and its separate ureter often inserts into the bladder although ectopic insertion into the ureter or even vagina.¹ Embryological data suggests that two separate ureteral stalks enter two nephrogenic entities; however, that theory is somewhat controversial.¹ Associated anomalies such as cryptorchidism may be present.¹ Most cases in humans are diagnosed incidentally at surgery, as was true in this kitten. Supernumerary kidney must be distinguished from either renal duplication or duplex systems wherein a single continuous parenchyma gives rise to two collecting systems and two ureters. Duplicated kidneys are the most common congenital abnormality of the urinary tract in infants with an incidence of approximately 1% of live births.³ However, duplicated kidneys and supernumerary kidneys are considered rare in dogs.^4,5 A single case of ureteral duplication has been reported in the cat.⁶ To the authors’ knowledge, the current case is the first report of a supernumerary kidney in a cat.

An abdominal mass presenting at 8 wk of age is unusual and is most often due to congenital anomalies such as duplication cysts, tumors such as teratomas or embryonal tumors, or polycystic kidney disease. In an older cat, the more common causes of an enlarged kidney include renal masses (both benign and malignant) and hydronephrosis due to obstruction at either the ureteropelvic or ureterovesicular junction or within the ureter due to uroliths, ureteral polyps, strictures, ureteroceles, neoplasia, accidental ligation of a ureter during an ovariohysterectomy, or, more unusually, related to retroperitoneal fibrosis.^7,8 Other masses such as nonrenal tumors or urinoma could be considered.⁹ The ultrasound identified the mass in this case, and the complexity suggested a tumor, possibly of testicular origin. This is because the cryptorchid testis was not identified and a supernumerary kidney was not considered because supernumerary kidney had not been previously reported in a cat.

The presentation of the caudal right kidney with hydronephrosis, due to presumably congenital obstruction, is unusual. Although the obstruction might have manifested shortly after birth, the degree of parenchymal destruction supports significant obstruction in utero. No overt cause for the obstruction of the caudal right kidney was identified, and why it presented with hydronephrosis before the cranial right kidney with a hypoplastic ureter is unclear. Ureteral pelvic obstruction without obvious cause may be due to simple in-folding of the mucosa. More often, the ureteropelvic juncture is normally patent and the ureter is characterized by a decrease in musculature, an increase in collagen, and distorted peristalsis.³ Other causes of ureteral pelvic obstruction include stricture, prior ischemia, and, rarely, polyps. Ureteral obstruction may be extrinsic due to compression from fibrous bands, aberrant vessels, or a circumcaval course of the ureter itself.¹⁰ In light of the negative pathologic examination findings for either intrinsic or extrinsic anatomic obstruction, either undetected mucosal in-folding or occult structural abnormalities of the ureter accompanied by abnormal peristalsis would seem to be the most likely explanation for the ureteropelvic obstruction in this kitten. In contrast to this kitten, duplex kidneys in children characteristically have an abnormal upper pole, and the upper pole ureter inserts lower into the bladder, which may be obstructed leading to ureteral vesicular junction obstruction.³

The subsequently manifested obstruction of the cranial right kidney is of additional interest. Pathologic observation that the cranial ureter was hypoplastic leads to a hypothesis that with growth and increasing activity, the kitten’s renal demands, such as increasing renal blood flow and glomerular filtration rate, surpassed the flow reserve of the hypoplastic ureter, leading to secondary hydronephrosis. If the caudal right kidney had residual function, removing that kidney might also have led to an increasing load on the cranial right kidney. However, the extensive hydronephrosis and medullary cortical thinning speaks against the caudal kidney having significant function under which conditions its removal would not be anticipated to affect the dynamics of the right cranial kidney. If a supernumerary kidney had been in the original differential diagnosis, and certainly if it is in the future, individual renal function tests of the three renal moieties using radionuclear imaging techniques or even MRI urography could be helpful. Significant function in an obstructed kidney would speak against nephrectomy. Additional studies to explore such circumstances would include excretory urography and even retrograde ureteral visualization to better define the site of obstruction. None of those modalities were available for this case.

Indications for nephrectomy in cats include severe renal or ureteral trauma, renal or perirenal abscess, end state hydronephrosis, and primary renal tumors.¹¹ The caudal right kidney was removed in this case because nonrenal tumor was suspected. Had the mass been identified as a supernumerary kidney, then kidney imaging and function tests could have guided treatment. Unfortunately, advanced imaging modalities were not fiscally an option with respect to the management of the right cranial kidney once it had become hydronephrotic. The rapid progression and extent of hydronephrosis of the cranial right kidney prompted nephrectomy by the affiliated board certified veterinary surgeon. In other situations, there are numerous options to treat an obstructed kidney including ureteral reconstruction, diversion, and stenting.¹² In any event, the extent of ureteral hypoplasia in this case would have made it a challenging surgical case. Hypoplastic ureters have infrequently been reported in human and canine neonates, but to the authors’ knowledge, not in cats.^13,14

Conclusion

This case is unique in that, to the authors’ knowledge, this is the first report of a supernumerary kidney in a cat. A third or supernumerary kidney has been documented in humans and dogs, but not previously in the cat. Postoperative pathologic characterization of the mass and the intraoperative observation of the separation of the mass from the parenchyma of the right cranial kidney were paramount in diagnosing the presence three kidneys in this kitten. The caudal right kidney apparently obstructed in utero at the ureteropelvic junction without overt cause, whereas subsequent hydronephrosis of the cranial right kidney appeared to be due to ureteral hypoplasia. The complex embryologic development of the nephros increases the probability of multiple anomalies. In this case, the concordant occurrence of ipsilateral supernumerary kidney, ureteral hypoplasia, and cryptorchid testis underscores a common embryologic abnormality of ureteral bud/metanephric blastema formation.¹ Supernumerary kidney, either with or without associated malformations, should be considered in the differential diagnoses of abdominal masses in cats.