Epibulbar melanoma in a foal

Abstract

A case of epibulbar melanoma in a 6-month-old, gelded, chestnut Hanoverian foal is reported. The location and clinical appearance upon initial presentation led to the tentative diagnosis of staphyloma or a congenital mass of unknown origin. An attempt was made to surgically excise the mass under general anesthesia, but due to its infiltrative nature and intraoperative appearance, most, but not all was removed without compromising the integrity of the globe. Histopathological evaluation revealed a multinodular to packeted, poorly demarcated, unencapsulated, infiltrative exophytic melanocytic neoplasm composed of bundles and nests of plump spindloid to polygonal heavily pigmented epithelioid neoplastic cells interspersed with pigment-laden macrophages within a fine fibrovascular stroma. Upon examination after enucleation, neoplastic cells were found to infiltrate into the lateral cornea, sclera and the choroid. This is a unique case of an epibulbar melanoma with choroidal invasion in a foal. Based on the sudden onset and rapid growth as well as the histological evidence of invasion, well-differentiated features, heavy pigmentation, and no apparent mitoses, this neoplasm was considered to be a low-grade malignant melanoma. At 14 months after excision there is no evidence of recurrence.

INTRODUCTION

A 6-month-old Hanoverian cross gelding was presented to the North Carolina State University College of Veterinary Medicine (NCSU-CVM) Equine Ophthalmology Service for evaluation of an epibulbar mass located at the dorsal limbus of his OS. Nine weeks prior to presentation, the foal incurred a laceration above the left eyelid that was repaired surgically by the referring veterinarian. There was no ocular injury or mass observed at that time. The foal was hospitalized 3 weeks later for surgical removal of a sequestrum of the left distal third metatarsal bone. During this hospitalization, a mass involving the OS was discovered. No treatment was recommended and the mass rapidly increased in size in the 6 weeks prior to referral to the NCSU-CVM.

Initial clinical findings

On the initial examination, the foal appeared comfortable and no blepharospasm was observed in either eye. Focal swelling of the left upper eyelid was observed upon initial examination. Normal results were found with a Schirmer tear test (22 mm/min OD and 25 mm/min OS) and for IOP (23 mmHg OD and 26 mmHg OS by applanation tonometry; Tono-Pen XL, Mentor, Norwell, MA). Direct and consensual pupillary light reflex (PLR), palpebral reflex, menace response, and dazzle reflex were normal for OU. No abnormalities were seen in the OD on slit-lamp biomicroscopy and indirect ophthalmoscopy examination. In the OS, a large, firm, solid, subconjunctival, black mass was present at the dorsal limbus. The mass extended 6 mm into the cornea, while the most posterior extension of the mass onto the sclera could not be visualized. The medial-to-lateral corneal dimension was approximately 15 mm, and the scleral dimension was 35 mm. The mass was raised 5–7 mm above the corneal and 30–35 mm above the dorsal scleral surfaces (Fig. 1a,b). Diffuse corneal epithelial and superficial stromal pigment deposition was present extending along the dorsal limbus from 9 to 3 o’clock, most pronounced at the dorsal limbus and along the leading edge of the corneal mass and accompanied by a halo of corneal edema (Fig. 2). Slit-lamp biomicroscopy, and indirect ophthalmoscopy revealed no further abnormalities. Vestibuloocular reflex and retropulsion were normal OU. The physical examination, with the exception of the left hind limb (i.e. healing sequestrum), was within normal limits.

Figure 1.

(a,b) Darkly pigmented, subconjunctival mass approximately 35 mm wide (mediolateral), extending posteriorly well beyond the limbus, and reaching a height of 30–35 mm above the adjacent scleral surface. Invasion of the mass into the corneal stroma resulted in an increase in elevation compared to the surrounding cornea. The conjunctiva remained transparent and was freely moveable above the mass.

Figure 2.

Darkly pigmented mass approximately 15 mm wide at the dorsal limbus (yellow arrow), extending 6 mm into the cornea within the stroma, causing this area of the cornea to be elevated 5–7 mm higher than the adjacent corneal tissue. Diffuse pigmentation can be seen extending into the cornea from along the leading edge of the mass (red arrow). A distinct cresent-shaped arc of corneal edema delineates the edge of the mass within the cornea.

Differential considerations

Differential considerations for the subconjunctival, episcleral mass included staphyloma,^1,2 orbital fat prolapse,^3–5 parasitic granuloma or cyst,^6,7 corneal or scleral rupture with iris prolapse,⁸ ocular neoplasia (hemangioma, angiosarcoma, melanocytoma or malignant melanoma, mastocytoma, lymphosarcoma, and medulloepithelioma),^9–21 or corneal or conjunctival dermoids.^8,22–26

Laboratory and ancillary diagnostic tests

Transcorneal ultrasonography (Biosound ESAOTE Megas ES, S.p.A., Genoa, Italy, 10 MHz probe) of the OS and mass revealed a homogeneous mass of mixed echogenicity that compressed the dorsal iridocorneal angle and invaded the scleral wall adjacent to the posterior aspect of the pars plana. Complete blood count and serum chemistry results were within normal limits for a foal. A fine needle aspirate of the left mandibular lymph node revealed no signs of neoplasia or inflammation. Thoracic radiographs were normal and did not show signs of pulmonary metastatic disease.

Initial treatment and rationale

Surgery was recommended to obtain a biopsy, potentially remove the mass, and to reconstruct the globe. Perioperative antibiotic treatment consisted of gentamicin sulfate solution (Phoenix Scientific Inc., St. Joseph, MO, GENTOZEN^®, 6.6 mg/kg IV, q24 h) and penicillin G potassium (Roerig, Division of Pfizer Inc., New York, NY, buffered PFIZERPEN^®, 20 000 IU/kg IV, q6 h) over a 24-h period. With the foal under general anesthesia and in right lateral recumbency, the OS was prepared for surgery. An incision was made in the conjunctiva with a #64 microsurgical blade, exposing the entire external surface of the mass (Fig. 3). An attempt was made to sharply dissect and completely excise the mass from the sclera; however, its friability, large size, and degree of invasion into the sclera only permitted debulking. The intraoperative appearance of the mass was suggestive of neoplasia rather than a congenital defect. In order to await the histopathology results before deciding on a course of treatment, the remaining portion of the mass was covered with bulbar conjunctiva. Excised tissues were placed in 10% neutral buffered formalin and submitted for histological sectioning and routine H&E staining. Postoperative treatment consisted of topical broad spectrum antibiotics (neomycin and polymyxin B sulfates and gramicidin ophthalmic solution, Bausch & Lomb Inc., Tampa, FL, 0.2 mL through a subpalpebral lavage catheter (SPL) q6 h), and atropine sulfate (1%, Bausch & Lomb Inc., 0.2 mL through the SPL q12 h), flunixin meglumine anti-inflammatory medication (Schering-Plough Animal Health Corp., Union, NJ, Banamine^®, 1 mg/kg IV, q12 h), and ranitidine, a gastroprotectant (Apotex Inc., Toronto, Ontario, Canada, 6.6 mg/kg PO, q8 h) for a period of 10 days.

Figure 3.

Intraoperative photograph of the exposed epibulbar melanoma OS. The conjunctiva has been incised and retracted, exposing the darkly pigmented mass.

Progress report

One day after surgery, the foal was comfortable and the menace response remained intact. Direct PLR OS, and consensual PLR OD to OS could not be elicited due to pharmacological mydriasis resulting from the topical treatment with atropine. The direct OD and OS to OD consensual PLR remained intact. The multifocal epithelial pigmentation and associated stromal edema extending from the dorsal limbus appeared more advanced than prior to surgery. Over the course of the next 3 days, the mass continued to grow in size, appearing to extend further into the cornea and causing elevation of the surgical site.

Clinical diagnosis and confirmation

Histopathologic evaluation of the surgical biopsy specimens confirmed a diagnosis of melanoma. Heavily pigmented neoplastic cells were arranged in packets, bundles and nests of spindle to polygonal cells within fine fibrovascular stroma. The neoplastic cells had variably distinct cell borders, and moderate to abundant eosinophilic cytoplasm with numerous brown to black punctate granules (melanin) that often obscured the nuclei. Evaluation of bleached sections (potassium permanganate to remove pigment) revealed a multinodular to packeted, poorly demarcated, unencapsulated, infiltrative exophytic melanocytic neoplasm comprised of bundles and nests of plump spindloid to polygonal heavily pigmented epithelioid neoplastic cells separated by pigment-laden macrophages and fine fibrovascular stroma Neoplastic cells had moderate anisokaryosis, anisocytosis and cellular and nuclear pleomorphism. The nuclear morphology ranged from variably sized, fusiform to round, densely basophilic to vesiculated with rare cytoplasmic invaginations. No mitoses were seen in more than 50 400× fields examined by two pathologists. In consideration of the diagnosis of melanoma and the aggressive clinical nature of the mass both prior to and following surgery, the decision was made to proceed with enucleation. This was performed via a routine transpalpebral approach²⁷ under general anesthesia 8 days after the initial surgical procedure. Histopathologic evaluation of the enucleated globe identified marked corneal, scleral, and choroidal infiltration by a pigmented epibulbar melanoma (Figs. 4–6d), confirming the initial diagnosis. Multifocal areas of tumor necrosis were often surrounded by mild to moderate lymphocytic infiltration. Based on the sudden onset and rapid clinical growth as well as the histological evidence of invasion, well-differentiated features, heavy pigmentation, and no apparent mitoses, this neoplasm was considered to be a low-grade malignant melanoma. At 14 months after excision there is no evidence of recurrence.

Figure 4.

Gross section of the globe. Note that the globe was enucleated after initial debulking of the mass. Therefore, the mass only extends a few millimeters above the sclera (yellow arrow). The darkly pigmented mass can be seen extending deep within the sclera (red arrow), and appears to reach the choroid. The epibulbar melanoma in this case report was large in size and extended into both the cornea (green arrow) and sclera.

Figure 6.

(a) A photomicrograph of the unbleached H&E section: The heavily pigmented limbal melanoma is infiltrating into the cornea (green arrow), well beyond the limbus (red arrow). (Bar = 500 µm). (b) A photomicrograph of the limbal biopsy: The paraffin section is heavily bleached (potassium permanganate to remove pigment) and stained with H&E. The neoplastic cells are arranged in large aggregates within a fine fibrovascular stroma. In this section, the neoplastic cells are polygonal with abundant cytoplasm and vesiculated nuclei. The neoplastic cells exhibit mild anisocytosis, and moderate anisokaryosis. (Bar = 50 µm). (c) A photomicrograph of bleached and H&E stained section: Neoplastic melanocytes infiltrating the perivascular areas within the limbus. The neoplastic cells have spindloid (arrow head) to polygonal (arrow) morphology. (Bar = 0.25 mm). (d) A photomicrograph of bleached (potassium permanganate to remove pigment) and H&E stained section: Neoplastic melanocytes can be seen infiltrating the perivascular areas within the choroid. The neoplastic cells have spindloid to polygonal morphology. (Bar = 0.25 mm).

DISCUSSION

The primary differential diagnoses for the pigmented epibulbar mass in the case presented in this report were a congenital defect (i.e. staphyloma) or a neoplastic mass. The young age of the foal and the limbal location of the mass were suggestive of a staphyloma, or a focal scleral defect with subsequent external protrusion of the underlying uveal tract, while the solid appearance on ultrasonographic evaluation and the progressive growth suggested a neoplastic growth. Surgical exploration identified an invasive, solid exophytic growth, and surgical biopsies indicated a melanoma. Rapid proliferation of the mass following surgery necessitated enucleation, with subsequent histopathology identifying the limbal sclera as the origin of the mass, refining the diagnosis to epibulbar melanoma.

Limbal or epibulbar melanomas are most commonly reported in dogs^28–34 and are generally less aggressive than intraocular melanomas in that species. A bimodal age distribution with distinct peaks at 3–4 and 7–10 years of age has been reported.²⁸ Limited reports of limbal melanomas in cats exist^31,35–38 with the youngest affected cat being 1 year of age. In horses, melanomas occur most commonly on the skin of older gray horses,^39,40 particularly involving the ventral aspect of the tail base, peri-anal region, lips, and eyelids.⁴¹ Ocular, conjunctival, retrobulbar, and epibulbar melanomas are very rare in horses^{12,20,21,42–47} with the youngest reported case being a 20-month-old horse.⁴² A temporally located, progressively enlarging epibulbar melanocytoma has been reported in a 7-year-old Argentinian horse.¹² The mass, covered by heavily pigmented conjunctiva, was incompletely excised and was without evidence of recurrence 2 years postoperatively. There was no reference to the mitotic activity of the mass, and its origin could not be determined due to the advanced stage of the tumor.

Histologically, canine limbal melanomas have been described as a mixture of small, lightly pigmented spindle-shaped cells and large round to polyhedral, heavily pigmented ‘plump’ (epithelioid) cells evident in various proportions, with the latter cell type being most prominent.^28,32–34 A uniformly low-index of mitotic activity (< 1 mitotic figure per 10 HPF’s) was seen in all cases in one study.²⁸ The authors of that study reported tumor necrosis, indicating vascular obstruction in 4 of 22 cases (18%). Diters et al.³⁴ similarly reported tumor necrosis in 3 of 12 cases (25%) and increased mitotic rate (8 per 10 HPF’s) in 1 of 12 (8%) cases of limbal melanoma. Therefore, it should not be assumed that canine limbal melanomas are inherently benign.²⁸ Feline melanomas most consistently involve the iris often spreading into the ciliary body and iridocorneal angle⁴⁸ but a series of atypical primary ocular melanoma have also been described.⁴⁹ Duncan and Peiffer⁴⁸ categorized the neoplasms morphologically into one of 3 groups: (1) spindle, (2) plump spindle and (3) pleomorphic. The majority (26 of 38, 68%) were made up of plump spindle cells, were elongated to polygonal in shape, and had oval to round (vesicular) nuclei. Cases were categorized as fatal or nonfatal⁴⁸ and tumor location was consistently associated with one of the groups. Histologically, atypical ocular melanomas in cats were made up of primarily round, heavily pigmented cells lacking anaplastic nuclear features.⁴⁹ These tumors are darkly pigmented and consist mainly of heavily pigmented round cells, features that are shared with canine ocular melananocytomas. Canine round cell melanocytomas coexist with pigmented spindle cells, whereas the feline atypical tumors lack a spindle cell population. Feline atypical melanomas also tend to originate multifocally, while canine tumors tend to consist of a solitary mass.⁴⁹ Based solely on this morphology, it may be predicted that atypical melanomas are less aggressive than diffuse iris melanomas. This is, however, contradicted by the extensive involvement in most of the globes examined in that series of cases.⁴⁹ Previous reports of ocular melanoma in horses described similar histological findings to that presented in this case report, and consisted of plump, heavily pigmented cells and polyhedral cells with round nuclei and prominent nucleoli.^{20,21,43,44,47,50} Increased mitotic activity was identified in 2 of the reported cases^20,21 and areas of necrosis, indicating rapid growth, were identified in the case reported by Moore et al.²⁰ The histological findings in this report are similar to those reported in cases of benign dermal melanomas in horses.^40,51 However, based on the sudden onset and rapid clinical growth as well as the histological evidence of invasion, well-differentiated features, heavy pigmentation, and low mitotic rate, this neoplasm was considered to be a low-grade malignant melanoma.

Immunohistochemistry (IHC) using antibodies against S-100, melan-A, Ki-67, and proliferating cell nuclear antigen (PCNA)⁴⁰ may have provided additional information regarding the characterization of the melanoma described in this case report. However, prolonged bleaching of the heavily pigmented neoplastic tissue led to loss of antigenicity against all the above antibodies tested.

Ultraviolet (UV) radiation exposure, chemical exposure, trauma or chronic irritation, coexistence of cutaneous and ocular melanomas, and genetic variables have been examined as factors in the development of melanomas in humans and animals.^28,52–54 In a review of records from 1680 research colony Beagles housed outdoors in a sunny, high altitude (1500 m) environment at Colorado State University, only 19 proliferative or neoplastic lesions developed in 14 of the dogs, none of which were melanomas.⁵⁴ The absence of data supporting its involvement in the development of ocular melanomas, coupled with the age and epibulbar location of the melanoma in this foal, makes UV radiation a less likely factor in this case. Single or persistent traumatic events are often incriminated in the development of cutaneous malignant melanomas (CMM) and nail apparatus melanomas (NAM) in humans, suggesting a possible link between the head trauma sustained to the left side of the head prior to initial presentation in this case. Conclusive evidence linking the development of melanoma directly to isolated, or persistent traumatic incidents in cases of CMM and NAM is lacking; however, repeated mechanical injury to a clinically apparent tumor should be considered a potential risk factor for CMM development in humans.^55,56 Some authors conclude that simple trauma probably does not cause melanoma, but because it is less likely to be ignored by the patient, may be the reason for detection of the tumor.⁵³ In our patient, the mass was not detected until well after the initial traumatic incident. However, it cannot be ruled out that the epibulbar melanoma was present prior to the injury, and that the physical trauma contributed to its accelerated growth.

Therapy for limbal melanomas in dogs and cats includes photocoagulation,³¹ partial (lamellar) or full-thickness resection with or without adjunctive strontium-90βplesiotherapy, grafting procedures (porcine small intestinal submucosa, synthetic, and third eyelid), cryotherapy,^{29,32,34,35,38,57–60} periodic surveillance,^38,57 or enucleation.³⁴ The large size, invasive nature, and rapid progression of the mass, and the likelihood of excessive choroidal damage associated with adjunctive therapeutic modalities suggested a poor prognosis for maintenance of vision, or of the globe following surgery.

Congenital tumors are both rare and sporadic in horses,⁶¹ and congenital ocular melanomas have not been previously reported in horses. Only 5 cases of congenital ocular melanoma have been documented in humans,⁶² and one case of intraocular melanoma in a 7-day-old Charolais-cross calf has been reported.⁶³ Given the age of our patient, and the size and relatively obscure location of the epibulbar melanoma, it may represent a congenital defect.

This is the first report of an epibulbar melanoma in a foal. It also documents putative extensive choroidal invasion, which has not been previously reported. Although the mass had grown to considerable size, it was not compromising vision or ocular movements, making early detection difficult. Epibulbar melanoma should be considered a differential diagnosis for any pigmented ocular masses in horses, regardless of age.

Figure 5.

Subgross H&E stained image corresponding with the gross image in Fig. 4. The dark areas indicate where the melanoma is infiltrating the limbus (red arrow), the cornea (green arrow), and the sclera (yellow arrow). (Bar = 2.5 mm).