A 13 year old domestic short haired feline presented to Alamo Feline Health Center as a second opinion for vomiting and a decreased appetite. The patient had shown clinical signs for approximately three days at the time of presentation. History revealed decreased thirst and appetite, vomiting four times in a 12 hour period two days prior, and lethargy. The vomitus initially contained grass and then white foam. According to the owner, ingestion of insecticide/poison or a foreign body was unlikely. Physical examination revealed significantly enlarged kidneys upon abdominal palpation. All other physical examination findings were clinically insignificant.

[enlarge]

Figure 1. Ultrasound view of the mass in the renal pelvis of the left kidney.

A urinalysis, urine culture, complete blood count, and serum chemistry were previously performed. Urinalysis revealed tenacious, stringy, amber colored urine with severe proteinuria (2000 mg/dl), severe hematuria (4+), and hyposthenuria (USG = 0.019; N=1.015-1.060). All other urinalysis findings were within normal limits. Urine culture was negative at 72 hours. The CBC revealed no significant abnormal findings. The blood chemistry panel revealed a severely elevated creatine (>13.6 mg/dL; N=0.3-2.1 mg/dL), a severely elevated blood urea nitrogen (>130 mg/dL; N=10-30 mg/dL), and hyperphosphatemia (8.1 mg/dL; N=3.4-5.5 mg/dL).

[enlarge]

Figure 2. (a). Right lateral view of the abdomen showing lack of contrast material 25 minutes after administration. (b). Ventrodorsal view of the abdomen showing lack of contrast material into the renal pelvis 25 minutes after administration.

An ultrasound study of the kidneys revealed severe enlargement (Right=5.63 cm; Left=5.26 cm; N=3.8-4.2 cm longitudinal), a left dilated renal pelvis, and a single, well-defined, circular mass in the left renal pelvis. The mass (approximately 2.2 cm in diameter) appeared to have mixed echogenicity and smooth, distinct margins (Figure 1). An excretory urogram was performed with 4 ml of MD-76 Ra given intravenously. After 25 minutes, there was no filtration of contrast into the renal pelvis of either kidney (Figure 2(a) and (b)). Due to the lack of filtration by the kidneys, the patient was euthanized.

[enlarge]

Figure 3. Gross view of the mass in the renal pelvis of the left kidney.

A necropsy was performed. The mass in the left kidney was cloudy white, gelatinous, and easily detachable from the kidney (Figure 3). The mass and both kidneys were submitted for histopathological assessment. The tissue samples collected were fixed in 10% neutral buffered formalin, sectioned into cassettes, dehydrated in alcohols, cleared in xylene, infiltrated and then blocked in paraffin. Three sections of the tissue block were floated onto glass slides. These were routinely stained with Hematoxylin and Eosin using an autostainer, and permanently mounted with coverslipping film.

[enlarge]

Figure 4. Gross visualization of the myxoid material.

Histopathology revealed the mass to be composed of small, monomorphic, wispy spindled cells suspended in an abundant, loose, myxomatous material. No mitotic figures were observed in the sample. The left kidney histologically appeared to have an obstructive nephritis presumed to be due to the mass itself and secondarily to the release of the myxoid substance. The right kidney also contained the myxoid substance which was most likely due to retrograde flow (Figure 4-6). The right renal collecting tubules were widely dilated, supporting the theory of outflow obstruction.

[enlarge]

Figure 5. Attachment between the mass and the renal pelvis.

Myxomas are rare tumors of mesenchymal origin [1-3], but the exact histogenesis is still debated [3, 4]. There have been no reported cases of a myxoma in a feline. In humans, myxomas are rare and are mainly found in the heart and skin [1]. But, they can also be found in the skin, soft tissues, bone, sinonasal cavity, juxta-articular space, maxillary antrum, eye, heart, ovary, kidney, and the viscera [4]. They were originally described in 1863 by Virchow as a tumor that mirrored the structure of the umbilical cord [5, 6] and then by Stout in 1948 as a true neoplasm composed of stellate cells set in a loose myxoid stroma that contain delicate reticulin fibers, as a way to establish the basic histologic criteria [3, 6]. Stout nullified Virchow’s analogy to an umbilical cord, but agreed that a myxoma was a single entity [6]. He also stated that myxomas do not metastasize [6]. According to a recent case report in the human literature, 11 cases of kidney myxomas have been reported, and only one of these developed in the renal sinus [1]. Renal myxoma development is considered extremely rare and according to some only three cases have been convincingly diagnosed as renal myxomas [2, 4]. A literature review found the other cases exhibited features of sarcoma, myxolipoma, or a fibroepithelial polyp [7]. In humans the disease is so rare it is not included in a recent review on tumors of the urinary system and male genital organs [4].

[enlarge]

Figure 6. The myxoid material observed in the kidney.

According to Bolat [3], differential diagnoses must include other soft-tissue lesions that exhibit prominent secondary myxoid features, including, but not limited to, perineurioma, myxoid neurofibroma, myxoid leiomyoma, myxolipoma, low grade fibromyxoid sarcoma, the myxoid varients of malignant fibrous histiocytoma, liposarcoma, leiomyosarcoma, rhabdomyosarcoma, and extraskeletal chondrosarcoma. All of the differentials listed above have a gelatinous consistency, but they all have exclusive morphologic, immunophenotypic, ultrastructural, and genotypic features [3, 4]. Nishimoto stressed the importance of recognizing the existence of renal myxomas to avoid confusion with malignant neoplasms with secondary myxoid features.

Cytologically, a renal myxoma is described as a benign fibroblastic tumor with cellular components lacking pleomorphism. It also differs from fibroblasts because it lacks the ability to produce mature collagen [5]. Nucleoli and mitotic figures are not present [2]. Upon sectioning, these tumors can be easily nucleated from the surrounding parenchyma due to the formation of loose attachments. Grossly, they are gelatinous in nature due to the accumulation of glycosaminoglycans, which is produced in excessive amounts. It has also been shown that the cells have lost their capacity to polymerize collagen [4].

In conclusion, there is limited information regarding myxoma formation in either human or veterinary species. There are many differential diagnoses to consider when diagnosing this disease, but key cytological factors help to differentiate between it and other soft-tissue masses.

This research received no grant from any funding agency in the public, commercial or not-for-profit sectors.

The authors do not have any potential conflict of interest to declare.