A Horseshoe Kidney With Solid Mass: A Case Report

Abstract

Sonography can be used to detect horseshoe kidneys. Sonographic findings for horseshoe kidney commonly include an isthmus connection occurring anterior to the aorta and just inferior to the inferior mesenteric artery. This anomaly can occur with or without complications. Each kidney is associated with its separate collecting system, leading to increased risk for complications. This case study describes the importance of identifying a horseshoe kidney, isthmus, and blood supply to ensure prompt intervention if needed. A case report of a horseshoe kidney with a solid mass, in the isthmus, is provided.

Horseshoe kidneys (HSKs) are the most common congenital renal fusion abnormality, affecting 1 in 500 people and a 2:1 ratio of men to women.^1,2 HSK occurs due to the congenital fusion of the lower poles of the right and left kidneys, with a parenchymal or fibrous isthmus connection located inferiorly. ¹ Superior pole fusion can occur but is considered rare. ² HSKs are prone to a variety of pathologies and must be fully evaluated to exclude any pathological conditions. In the event a pathology is visualized by sonography, full interrogation of the finding can lead to prompt intervention. This case report reviews a rare HSK finding found with sonography, which led to prompt treatment.

Case Study

A 56-year-old man was referred for a complete abdominal sonogram due to a chief complaint of epigastric pain. A review of the patient’s medical history showed gastroesophageal reflux disease, for which he had a prior intervention, and a family history of prostate cancer. The patient experienced mild pain with palpation of the right lower quadrant during evaluation prior to completing the sonographic study.

The sonographic evaluation was performed using a Philips IU22 ultrasound system (Philips North America Corporation, Andover, MA) using a 5-1 MHz curved-array transducer. Sonographic evaluation revealed an empty left and right renal fossa. The right kidney was located to the right of midline in the right lower quadrant, while the left kidney was located to the left of midline in the left lower quadrant. A connection between the two kidneys was found to be anterior to the aortic bifurcation (Figure 1). A horseshoe kidney was suspected. An evaluation of the isthmus parenchyma revealed a complex solid mass measuring 3.72 cm in the largest plane (Figures 2 and 3). This area was seen with increased peripheral vascularity (Figure 4). All other abdominal structures appeared unremarkable. A computed tomogram (CT) scan was recommended to confirm horseshoe kidney and provide information regarding the isthmus mass.

Figure 1.

Transverse midline sonographic image showing horseshoe kidney with isthmus anterior to aortic bifurcation.

Figure 2.

Transverse sonographic image of isthmus demonstrating a questionable mass.

Figure 3.

Sagittal sonographic image of isthmus demonstrating a complex mass.

Figure 4.

Color Doppler of solid mass seen within isthmus of horseshoe kidney.

A CT of the abdomen and pelvis was performed with and without intravenous contrast. The CT showed a renal migration anomaly with the kidneys fused in the midline, consistent with the sonographic findings of HSK. An arterially enhancing mass was demonstrated within the isthmus, concerning for renal neoplasm. There was no extension of the mass outside of the margins of the kidney and no evidence of metastatic adenopathy. The vascular supply to the kidney was aberrant. The right and left portions of the horseshoe kidney were fed directly from aortic branches. The isthmus and mass were supplied by a branch of the left common iliac artery and inferior mesenteric artery, which coursed tightly over the mass and isthmus (Figure 5). The CT demonstrated two separate ureters arising from both the right and left kidney. The duplicated ureters fused with one another just distal to the renal pelvis to form one ureter one each side, which descended into the bladder.

Figure 5.

Coronal computed tomography image showing multiple large vessels feeding mass: (1) branch off inferior mesenteric artery and (2) branch off left common iliac artery.

The patient was referred for a surgical consult where an isthmus mass resection was determined to be the best option. Six days after the CT findings, an open left isthmus mass removal was performed using a Hitachi Hi Vision 900 (Hitachi Healthcare Americas, Twinsburg, OH) endocavity 8-4 probe as guidance. Two surgeons were present due to the complex anatomy and bleeding. Sonographic evaluation demonstrated increased peripheral blood flow and blood flow within the mass (Figure 6). Upon resection, significant venous oozing began, which resulted in the removal of the left side of the horseshoe kidney in addition to the isthmus mass. The patient lost approximately 6 L of blood and was transferred to the critical care unit. The pathology report classified the isthmus mass as clear cell renal cell carcinoma while the left portion of the kidney that was removed was negative for any neoplasm.

Figure 6.

Intraoperative color Doppler image showing increased peripheral vascularity in the isthmus mass.

An immediate postsurgical CT demonstrated postoperative changes, including left kidney moiety resection, simple fluid collection measuring 1.9 cm, and a normal enhancing right kidney moiety (Figure 7). There was no evidence of urinoma. Patient was discharged six days postoperatively with instructions to return within five days for staple removal.

Figure 7.

Coronal computed tomography images showing postoperative changes (left) and preoperative mass (right).

Discussion

HSK is the most common urinary tract congenital fusion anomaly. ¹ The lower poles of both renal masses are fused by functioning parenchymal or fibrous isthmus tissue.^1–3 Less commonly, the superior poles may be fused, creating an inverted HSK. ² This abnormality is often incidentally discovered in adults during routine intravenous pyelography, sonography, or CT but can be seen in the fetus as early as the first trimester of pregnancy. ²

Two theories exist describing the origin of HSK. During the metanephric stage when the kidneys are still located in the pelvis, one theory proposes that the lower poles come into contact with one another, fuse, and form an HSK. When the fused kidneys migrate to their adult position, the inferior mesenteric artery (IMA) prevents ascension, thus explaining the lower location of HSK on sonography. The second theory involves a teratogenic event that causes abnormal migration of nephrogenic cells, forming a midline isthmus with functional parenchymal tissue.^2,4

The typical position of an HSK is inferior to the IMA, posterior against the lumbar spine, and inferior near the aortic bifurcation. ⁴ This anatomical position predisposes HSK to abdominal trauma due to a lack of rib protection. In addition, there is a wide variety of renal arterial origins, with the renal arteries originating off the abdominal aorta, common iliac arteries, or inferior mesenteric artery.^2,4 HSKs are commonly supplied by aberrant arteries entering directly into the kidney and multiple accessory renal arteries. One study identified that 63% of HSK cases had more than three renal arteries. ⁶

HSK typically does not produce symptoms but can be associated with other vascular, central nervous system, skeletal, chromosomal, and genitourinary abnormalities.^2,3 Pathological conditions such as ureter stricture and aberrant arterial supply are also commonly associated with HSK.^1,2 Large staghorn calculi are also commonly found due to stasis caused by calyx orientation and impaired drainage. ¹ Transitional cell carcinoma accounts for 20% of HSK masses. Wilms tumor, oncocytomas, angiomyolipomas may also be seen. ⁷

Sonography can be used to detect HSK, often visualizing the connecting isthmus anterior to the spine. Careful attention must be made to not mistaken a retroperitoneal mass for HSK. To evaluate the arterial supply, digital subtraction angiography may be used. ⁶ CT or magnetic resonance imaging (MRI) may be used to evaluate arterial supplies in preoperative planning to reduce the likelihood of unexpected hemorrhage. Contrast-enhanced ultrasound (CEUS) may also be used to evaluate renal masses. ⁸

The incidence of renal cell carcinoma within horseshoe kidneys is rarely reported and even rarer for the mass to be involved in the parenchyma of the isthmus. Worldwide incidence of renal cell carcinoma in horseshoe kidneys totals around 150 reported cases. Of these reported cases, nearly 50% involved renal cell carcinoma clear cell type, with Wilms tumor and renal pelvic tumors equally accounting for the other 50% of malignancies.^3,9 Of all the published cases, only two were found with bilateral malignancies, and fewer than 10 reported cases involving the isthmus. ¹⁰

The most common types of renal cell carcinoma are clear cell, papillary, chromophobe, oncocytoma, and collecting duct. Clear cell carcinoma arises from the epithelial cells that line the proximal convoluted tubules of the nephron within the collecting system. This form of renal cell carcinoma is considered a renal cortical tumor and is also known as common renal cell carcinoma.^3,5,9

Clear cell type renal cell carcinoma, CCRCC, is also known as common renal cell carcinoma and nonpapillary renal cell carcinoma. This type of cancer is found to typically affect men over the age of 55, with a 2:1 incidence rate of males to females. Three main established risk factors for CCRCC include smoking, obesity, and hypertension. It should be noted that CCRCC develops in most cases of patients with Von-Hippel Lindau disease. Other risk factors include a family history of renal cancer; having undergone treatment for chronic kidney disease, including long-term dialysis; and exposure to harmful substances such as herbicides. The highest incidence of CCRCC occurs in North America and northern Europe, with the lowest incidences found in Africa and Asia.^3,9

Clear cell renal cell carcinoma arises from the epithelial cells that line the proximal convoluted tubules within the collecting system of the kidney. It is a type of renal cortical tumor. Clear cell renal carcinoma has been found to have a high tendency to spread via the lymphatic system, as well as extending into the inferior vena cava (IVC), increasing the importance of sonographic recognition for early detection. ¹¹ The signs and symptoms associated with renal cell carcinoma can be helpful to solidify suspicions in incidental findings. However, in the early stages of the cancer, it is unlikely to have major symptoms affecting daily activities. Symptoms indicative of late-stage kidney cancer include pain in the back or side, unexplained weight loss, loss of appetite, fatigue, intermittent fever, and blood in the urine. ¹¹

Treatments for renal cell carcinoma vary widely based on the stage and degree of the cancer. More than 50% of patients diagnosed during the early stages of renal cell carcinoma are cured, but prognosis in the later stages is very poor. A review of the literature revealed that outcomes can be predicted based on tumor size, histological tumor necrosis, sarcomatoid component, cystic architecture, nuclear grade, multifocality, and surgical margin status. ¹² Surgical resection is the most common treatment used, especially if the cancer is localized. Another rising option is the thermal ablation of minimally invasive carcinomas. In 95% of cases where the tumor measures 3 cm or less, the renal cell carcinoma tumors can be completely ablated. ¹³ The major advantage to the thermal ablation procedure is the minimal damage done to the remaining renal tissues, as long-term renal function is directly associated with the number of functioning nephrons.

Conclusion

HSKs are commonly associated with other congenital, benign, or malignant findings. Complete interrogation of the kidneys, isthmus, and blood supply is essential in the diagnosis of HSK and its associated pathologies. In the case of clear cell renal cell carcinoma, as with most malignancies, earliest detection is key to ensure the best prognosis possible. In the case presented, an HSK could have made an isthmus mass difficult or impossible to locate. The early incidental detection of CCRCC was treated with surgical resection of the isthmus tumor and ultimately the left portion of the horseshoe kidney, due to a complicated aberrant blood supply confirmed by intraoperative sonographic guidance.