History
A 16-year-old approximately 450-kg (990-lb) Arabian stallion was examined because of an enlarged right side of the scrotum. The enlargement had been gradual and was not associated with a known traumatic or infectious event. Caretakers of the horse had been aware of the enlargement for several years. The horse was a retired breeding stud, last used for breeding approximately 10 years earlier.
On physical examination, the stallion was in good general health, had vital signs within reference limits, and had a body condition score of 5 (on a scale of 1 to 9). Palpation of the scrotum revealed that the enlarged right side had an obvious fluid wave and a firm mass at the region of the head of the epididymis, whereas no abnormalities were detected in the left side (Figure 1). Regional lymph nodes were not examined.
Clinical image of a 16-year-old approximately 450-kg (990-lb) Arabian stallion examined because of long-term enlargement of the right side of the scrotum.
Citation: Journal of the American Veterinary Medical Association 256, 11; 10.2460/javma.256.11.1225
Clinical image of a 16-year-old approximately 450-kg (990-lb) Arabian stallion examined because of long-term enlargement of the right side of the scrotum.
Citation: Journal of the American Veterinary Medical Association 256, 11; 10.2460/javma.256.11.1225
Clinical image of a 16-year-old approximately 450-kg (990-lb) Arabian stallion examined because of long-term enlargement of the right side of the scrotum.
Citation: Journal of the American Veterinary Medical Association 256, 11; 10.2460/javma.256.11.1225
Ultrasonography of the right side of the scrotum revealed an increased volume of anechoic fluid around the right testicle, suggestive of a hydrocele, and a round, well-circumscribed, heteroechoic mass (approx 5.5 × 5.0 × 5.0 cm) near the head of the epididymis (Figure 2); however, the testicle and spermatic cord appeared clinically normal. Ultrasonography of the left side of the scrotum revealed no abnormalities in the associated structures.
Transscrotal ultrasonographic image of the head of the right epididymis of the stallion in Figure 1 showing a heteroechoic mass that is 54.6 × 50.0 mm, as measured with digital calipers of the ultrasound machine.
Citation: Journal of the American Veterinary Medical Association 256, 11; 10.2460/javma.256.11.1225
Transscrotal ultrasonographic image of the head of the right epididymis of the stallion in Figure 1 showing a heteroechoic mass that is 54.6 × 50.0 mm, as measured with digital calipers of the ultrasound machine.
Citation: Journal of the American Veterinary Medical Association 256, 11; 10.2460/javma.256.11.1225
Transscrotal ultrasonographic image of the head of the right epididymis of the stallion in Figure 1 showing a heteroechoic mass that is 54.6 × 50.0 mm, as measured with digital calipers of the ultrasound machine.
Citation: Journal of the American Veterinary Medical Association 256, 11; 10.2460/javma.256.11.1225
Question
What are 4 differential diagnoses for the epididymal mass? Please turn the page.
Answer
Four differential diagnoses for the epididymal mass in the stallion of the present report are neoplasia, epididymal cyst, granuloma, and epididymitis.
Results
The owner declined further diagnostic procedures. For unrelated reasons, the stallion was euthanized (sodium pentobarbitone, 120 mL, IV [left jugular vein]) days after scrotal ultrasonography was performed. Although a full necropsy was not performed, postmortem evaluation of the epididymal mass was conducted. The tunica vaginalis of the right testicle was incised, and approximately 50 mL of clear, serous fluid was present. A firm, well-demarcated, round (approx 5.6-cm-diameter), tan-to-white mass that had a smooth surface was attached to the head of the epididymis (Figure 3). The mass was sectioned longitudinally, and half of the mass was placed in neutral-buffered 10% formalin solution for histologic analysis. The tissues were routinely processed, embedded in paraffin wax, sliced at a thickness of 5 μm, then stained with H&E. In addition, consecutive sections were prepared for Masson trichrome and phosphotungstic acid-hematoxylin histochemical staining and immunohistochemical (IHC) labeling with a monoclonal mouse anti-human smooth muscle actin antibodya and monoclonal mouse anti-human desmin antibody.b
Postmortem image of the isolated right testicle (dagger), epididymis (double dagger), and epididymal mass (asterisk) of the stallion in the previous figures. Bar = 3 cm.
Citation: Journal of the American Veterinary Medical Association 256, 11; 10.2460/javma.256.11.1225
Postmortem image of the isolated right testicle (dagger), epididymis (double dagger), and epididymal mass (asterisk) of the stallion in the previous figures. Bar = 3 cm.
Citation: Journal of the American Veterinary Medical Association 256, 11; 10.2460/javma.256.11.1225
Postmortem image of the isolated right testicle (dagger), epididymis (double dagger), and epididymal mass (asterisk) of the stallion in the previous figures. Bar = 3 cm.
Citation: Journal of the American Veterinary Medical Association 256, 11; 10.2460/javma.256.11.1225
Microscopic examination revealed an unencapsulated, well-circumscribed, densely cellular tumor comprised of elongated spindle cells that replaced and expanded the lamina propria (Figure 4). The tumor was noninfiltrative, and the adjacent epididymal duct remained unaffected, with occasional spermatozoa observed within the lumen. Less than 2-fold anisocytosis and anisokaryosis were observed, and the neoplastic cells showed no evidence of multinucleation, mitotic activity, or nuclear atypia. Calcification, necrosis, and inflammation were absent. Results of Masson trichrome staining indicated that moderate amounts of collagen supported the neoplastic cells, and results of phosphotungstic acid-hematoxylin staining indicated that cross-striations were not observed in neoplastic cells. Results of IHC staining (not shown) were positive in tissue sections with neoplastic cells, which indicated cytoplasmic expression of both antibodies. External and internal control tissues of vascular smooth muscle cells had strong immunolabeling, providing controls for IHC staining results. On the basis of these results, epididymal leiomyoma was diagnosed.
Photomicrographs of the epididymal mass of the stallion in the previous figures. A—A tissue section of the unencapsulated, noninfiltrative, densely cellular tumor comprised of elongated spindle cells. H&E stain; bar = 200 μm. B—A tissue section in which the epididymal duct (arrows) adjacent to the tumor (asterisk) is unaffected by the tumor. H&E stain; bar = 1,000 μm.
Citation: Journal of the American Veterinary Medical Association 256, 11; 10.2460/javma.256.11.1225
Photomicrographs of the epididymal mass of the stallion in the previous figures. A—A tissue section of the unencapsulated, noninfiltrative, densely cellular tumor comprised of elongated spindle cells. H&E stain; bar = 200 μm. B—A tissue section in which the epididymal duct (arrows) adjacent to the tumor (asterisk) is unaffected by the tumor. H&E stain; bar = 1,000 μm.
Citation: Journal of the American Veterinary Medical Association 256, 11; 10.2460/javma.256.11.1225
Photomicrographs of the epididymal mass of the stallion in the previous figures. A—A tissue section of the unencapsulated, noninfiltrative, densely cellular tumor comprised of elongated spindle cells. H&E stain; bar = 200 μm. B—A tissue section in which the epididymal duct (arrows) adjacent to the tumor (asterisk) is unaffected by the tumor. H&E stain; bar = 1,000 μm.
Citation: Journal of the American Veterinary Medical Association 256, 11; 10.2460/javma.256.11.1225
Discussion
Epididymal neoplasms are rare in domestic animals, and to our knowledge, the present report is the first to detail a leiomyoma in the epididymis of a horse. Differential diagnoses for epididymal masses in horses include epididymitis, epididymal cysts, granulomas, and neoplasia.1–4 Cystic structures of the epididymis in horses have been reported and are thought to arise from remnant mesonephric or paramesonephric tissue.1 Sperm granulomas may also affect the head of the epididymis1,2 and are soft, associated with signs of pain, and often induced by inflammation associated with extratubular spermatozoa or, in rare cases, migrating strongyle larvae.5 Bacterial epididymitis has been diagnosed in horses and appears to have a predilection for the tail of the epididymis, where a firm swelling may result.4 In addition, epididymal nodules in horses may be caused by adenomyosis, a nonneoplastic proliferation of the epithelial lining of the epididymis that may occasionally be associated with reduced fertility.2 Other causes of intrascrotal enlargement, such as hydroceles, hernias, hematomas, or varicoceles, may be difficult to differentiate from masses that originate from the epididymis by palpation alone; thus, further diagnostic investigation would be needed.
Leiomyomas in domestic animals are typically benign, expansile tumors derived from any smooth muscle origin. Other tumors of mesenchymal cell origin may have similar histologic features and therefore will require IHC staining or electron microscopy for differentiation.6 Fibroma, fibrosarcoma, dermatofibroma, schwannoma, malignant nerve sheath tumor, and various sarcomas can be misdiagnosed as tumors of smooth muscle origin on the basis of histologic appearance alone. In the horse of the present report, positive results for IHC staining for desmin and smooth muscle actin confirmed a neoplasm of smooth muscle origin. Smooth muscle tumors have traditionally been categorized as either benign (leiomyoma) or malignant (leiomyosarcoma). Generally, differentiation between leiomyoma and leiomyosarcoma can be made on gross and light microscopy findings.5 It should be noted, however, that myofibroblast tumors or reactive myofibroblasts present within a tumor may also express muscle-specific actin and α-smooth muscle actin; therefore, IHC staining results must be interpreted in conjunction with histologic features. Leiomyomas are discrete, noninvasive tumors that microscopically appear as densely packed spindle cells, which are morphologically similar in appearance to smooth muscle cells. Leiomyomas do not typically undergo necrosis, have few mitotic figures, and may be difficult to differentiate from normal smooth muscle. In contrast, leiomyosarcomas are frequently invasive and have neoplastic cells that may be homogenous or variably pleomorphic, with frequent mitoses and multinucleation. Smooth muscle is found in abundance in the epididymis, and neoplasia may originate from the thick layer that surrounds the highly coiled epididymal duct or from the small amounts of smooth muscle in the serosal layer.
In horses, leiomyoma has been reported in the esophagus, stomach, small and large intestines, rectum, omentum, urinary bladder, testicles, testicular tunica, and skin.6,7 Further, in older females of domestic species, leiomyomas are among the most common tumors of the reproductive system.6 Leiomyomas are rarely reported in the male genital tract of domestic animals, although there are single case reports of leiomyoma in the tunica albuginea of a horse,7 tunica vaginalis of a dog,8 and testicles of a ram.9 Perhaps an explanation for the decreased incidence of paratesticular neoplasia in domestic animals is surgical castration, which is commonly performed in these species.
Leiomyomas of the epididymis have been reported in humans, and in a previous study10 of 341 epididymal tumors, 28 (8%) of these neoplasms were leiomyomas. Investigators have reported that epididymal leiomyomas in humans are slow growing, smooth, well demarcated, and generally nonpainful and that ultrasonography could be used to differentiate extratesticular from intratesticular masses with 95% to 100% sensitivity in humans.11
Outcome
The stallion was euthanized for unrelated reasons shortly after the ultrasonographic examination of the present report. Findings for this stallion highlighted the importance of considering neoplasia as a cause of scrotal hydrocele in horses and for including leiomyoma as a differential diagnosis for paratesticular masses in horses. Epididymal tumors are infrequently described in veterinary medicine, and the available literature is primarily based on reports in humans. It should be noted, however, that the gross, microscopic, and IHC features of epidydimal leiomyoma in the horse of the present report were similar to those described in humans, in whom most are benign masses with low recurrence once surgically excised.11 Therefore, unilateral castration of affected horses should be considered, and we suspect that it would be unlikely for a stallion's reproductive capabilities to be substantially affected, unless involvement was bilateral.
Although leiomyomas are typically benign, the biological behavior of epididymal leiomyoma in stallions is unknown. We recommend that veterinarians treating similarly affected horses in the future include investigation with diagnostic imaging, clinicopathologic assessments (eg, hematologic and serum biochemical analyses, urinalysis, and semen analysis), and histologic evaluations to exclude associated systemic or metastatic disease and to assess any effect on reproductive capabilities.
Acknowledgments
The authors thank QML Vetnostics, Murarrie, Australia, for its assistance in performing IHC assessments of the tissues described in the present report.
Footnotes
Clone 1A4, Dako, Glostrup, Denmark.
Clone NCL-L-DES-DERII, Leica Biosystems, Mount Waverly, VIC, Australia.
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