Diagnostic Ophthalmology

History and clinical signs

A 10-year-old Welsh pony cross gelding was examined at the ophthalmology service at the Western College of Veterinary Medicine for evaluation of a change of color of the right bulbar conjunctiva from black to pink (Figure 1). The menace response, palpebral, oculocephalic, and direct and consensual pupillary light reflexes were present bilaterally. Schirmer tear test (Schirmer Tear Test Strips; Alcon Canada, Mississauga, Ontario) values were 25 and > 35 mm/min in the right and left eyes, respectively. The intraocular pressures were estimated with a rebound tonometer (Tonovet, Tiolat, Helinski, Finland) and were 26 mmHg bilaterally. On direct examination, there was an irregularly shaped area of depigmentation with a mottled surface affecting the lateral bulbar conjunctiva. Following application of 0.5% tropicamide (Mydriacyl; Alcon Canada, Mississauga, Ontario), biomicroscopic examination (Osram 64222; Carl Zeiss Canada, Don Mills, Ontario) revealed no further abnormalities bilaterally. Indirect ophthalmoscopic (Heine Omega 200; Heine Instruments Canada, Kitchener, Ontario) examinations were completed bilaterally and no further abnormalities were detected.

Figure 1.

Photograph of the right eye of a 10-year-old Welsh pony cross gelding. There was an irregularly shaped area of depigmentation on the bulbar conjunctiva.

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Discussion

Differential diagnoses for the depigmented lesion included neoplasia (squamous cell carcinoma (SCC), papilloma, adenoma, lymphoma, mast cell tumor) or an inflammatory lesion (parasitic, granuloma, immune-mediated disease, foreign body reaction). An excisional biopsy was submitted for histopathology. Light microscopy revealed an undulating basement membrane of the epithelium with packets of epithelial cells invading into the substantia propria consistent with a diagnosis of SCC (Figure 2).

Figure 2.

A light microscopic image of the depigmented lesion. There is an increase in epithelial layers, undulating epithelial basement membrane, and multiple nests of epithelial cells in the substantia propria below the basement membrane of the epithelium. There are also scattered lymphocytes and plasma cells within the substantia propria. Image collected at 5×, Bar = 0.2 mm.

Squamous cell carcinoma is the most common neoplasm of ocular tissue in horses, affecting the eyelids, conjunctiva, limbus, cornea, and nictitans (1,2). An increased prevalence has been reported in Haflingers (3,4), Appaloosas, Paint horses, Quarter horses, draft horses, Belgians, and Percherons (4). Prevalence increases with age (2), with horses between 8 and 13 years most affected (5). A predisposition for ocular and adnexal SCCs has been reported in geldings (5). Horses with a lighter coat color are overrepresented, likely due to a lack of photoprotective pigment in the skin and conjunctiva (4).

Although the pathogenesis has not been fully elucidated, it is likely that several factors play a role, including prolonged exposure to sunlight and genetics (2–4). Ultraviolet radiation causes mutations in the tumor suppressor gene p53, a gene that codes for a protein involved in cell cycle and tumor suppression (6). This protein is overexpressed in SCC of multiple species, including horses (6). Damage-specific DNA binding protein 2 (DDB2) is another gene important for the repair of ultraviolet-induced mutations as it is involved in the excision and repair of ultraviolet damaged DNA (3,4). Bellone et al (4) identified a mutation in this gene that was strongly associated with SCC of the limbus and third eyelid in the Haflinger and Belgian, and to a lesser extent in the Percheron (3,4). Genetic testing is available for screening Belgians and Halflingers for the DDB2 mutation (https://vgl.ucdavis.edu/services/HaflingerSCC.php). Other factors such as papillomavirus (7), changes in estrogen and androgen levels in the blood (5), and parasitic irritation of tissues (2) have been suggested to play a role in the pathogenesis.

Early limbal SCC may appear clinically as an area of conjunctival depigmentation or hyperemia, which can progress to become ulcerated, raised, or multilobulated (2). It will often begin at the lateral limbus and extend into the adjacent cornea and conjunctiva (8). Affected eyes may have concomitant conjunctivitis and ocular discharge (9).

Ocular SCCs can be locally invasive, exhibit metastatic potential, and are prone to recurrence (10). Local invasion can involve the sclera, intraocular structures, orbit, nasal cavity, guttural pouch, and sinuses (8). Metastases are reported in 6% to 18% (5,8,11) of cases and most commonly involve the regional lymph nodes, salivary glands, or thorax (11). The recurrence rate following treatment has been reported to be between 11.1% to 66.7% depending on completeness of surgical excision and adjunctive therapy (5,8). Recurrences are more common in eyelid SCC than the limbal form of the tumor (12), and third eyelid and eyelid tumors tend to spread and metastasize more frequently than limbal SCC, possibly due to the sclera and cornea acting as barriers to the penetration of cells (13).

Surgical excision is the most common treatment option for ocular SCC and can be adequate when a wide margin is excised (13). Cryotherapy, radiofrequency hyperthermia, photo-dynamic therapy, immunotherapy, chemotherapy with cisplatin, mitomycin C, radiotherapy, and beta-irradiation have all been described as adjunctive therapies (1,2,5,8,10,13–16).

Beta-irradiation is applied to the neoplastic tissue using a strontium probe (Figure 4) and is an effective treatment of ocular SCC with a local control rate of 64% to 100% with range of follow-up varying between studies of 1 to 3 years after treatment (14–16). Beta-irradiation has been reported to be efficacious in cases of superficial SCC of the cornea and limbus after keratectomy (2,10). It is a safer adjunctive therapy for corneal and limbal lesions due to its limited range of penetration into soft tissue (~ 2 mm) (13).

Figure 4.

Strontium-90 beta-irradiation was applied to the surgical site under standing sedation.

In this case, the leading edge of the biopsy had neoplastic cells extending to the tissue margin. Upon re-examination 3 months later, the right eye revealed a focal area of conjunctival thickening and cobblestone texture of 1 mm in diameter (Figure 3). A resection including a peripheral superficial keratectomy was completed to allow a 1 cm margin around the abnormal tissue. Beta-irradiation was applied at a dose of 20 000 rads using a strontium probe (Figure 4) positioned directly over the surgical site and repositioned as needed to expose the entire surgical site. Application of topical antibiotic therapy (BNP ointment; Vétoquinol, Lavaltrie, Quebec) was initiated postoperatively, every 6 hours for 10 days. Administration of phenylbutazone (Phenylbutazone tablets; Dominion Veterinary Laboratories, Winnipeg, Manitoba) was initiated, 1 g, per os for 3 to 5 days. The excised tissue was submitted for histopathology, which revealed no evidence of neoplastic cells. The owner was instructed to monitor for recurrence and a recheck was recommended in 6 months, or earlier if there were concerns.

Figure 3.

Photograph of the right eye of a 10-year-old Welsh Pony cross gelding after the excisional biopsy had healed. There was a focal area of conjunctival thickening and cobblestone texture of 1 mm in diameter at the junction of the pigmented and non-pigmented conjunctiva.