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for intraocular tumours. Those who wish to preserve the eye and confirm diagnosis should be referred to an ophthalmologist for diagnostics and potential sector iridectomy or other therapies. Histopathological evaluation with special staining helps confirm diagnosis, but does not consistently define malignant potential in horses. Histological appearance of equine melanocytic tumours is highly variable and should not be used in isolation to predict their clinical behaviour. This adds to the confusion of how to treat these lesions and what to tell clients about prognosis.
Clinical behaviour and treatment
Treatment for periocular melanoma frequently involves surgical excision with or without adjunctive therapy such as cryotherapy, brachytherapy or photodynamic therapy (Giuliano et al. 2005). Surgical excision is generally curative for eyelid melanomas in horses, and there are few reports of specific treatments for equine adnexal melanomas; therefore, the success rates of various treatments are unknown. Oral cimetidine has been used to shrink nonocular dermal melanomas in horses, but there have been no published reports on the use of oral cimetidine for the treatment of adnexal melanomas (Goetz et al. 1990). There are no known studies on the efficacy of intralesional chemotherapy or immunotherapy for equine adnexal melanomas although the author has had success with intralesional cisplatin. Epibulbar or limbal melanomas are rarely reported in the
horse. Limbal melanomas may be pigmented, fan-shaped, or nodular, and most commonly occur on the dorsal limbus (Fig 3). These are predominantly benign growths with the potential for local infiltration (Wang and Kern 2015). Most limbal melanomas can be treated with surgical excision and cryotherapy. A malignant epibulbar melanoma was described in a 6-month-old Hanoverian cross gelding with a rapidly growing pigmented epibulbar mass (McMullen et al.
2008). The lesion continued to grow rapidly despite debulking for histopathological evaluation, so enucleation was performed. No recurrence was reported 14 months post- operatively. In contrast to epibulbar melanomas, conjunctival melanomas in humans and domestic animals appear to have a more aggressive clinical nature. Conjunctival melanomas may be variably pigmented and have the potential for metastasis. Unfortunately, there have been few reports of conjunctival melanomas in horses, but uncomplicated cases seem to respond well to surgical excision and cryotherapy (Moore et al. 2000; Wang and Kern 2015). Recurrence and malignancy has been reported, such as in the case of a 16-year-old grey Arabian mare with multiple recurrences of a conjunctival melanoma following two surgical excisions and one treatment with cryotherapy. Exenteration was elected and no recurrence was reported 5 years post-operatively (Moore et al. 2000). Melanomas of the anterior uvea are also rare. They are
typically nodular, pigmented masses that are benign in nature, but with the potential for rapid local growth. Nodular intraocular neoplasms may cause secondary keratitis, uveitis, cataract and glaucoma. Distortion of the pupil may occur and focal corneal oedema is commonly seen if the mass contacts the cornea (Fig 4). Tumours in horses with lightly pigmented irises may appear pink and fleshy. In the case of rapidly growing tumours, or those involving ciliary body, sector iridectomy, enucleation, or exenteration should be considered. There has been high success with these procedures and metastasis has not been reported (Gilger 2016b). Small, uncomplicated tumours may be monitored. Local laser therapy may also be considered to shrink melanocytic neoplasms; however, uveitis and corneal oedema are common adverse effects (Gilger 2016b). Collectively, there are few reports of ocular melanomas in
horses and the effectiveness of specific therapies, so prognosis is difficult to determine. There is a need for a
Fig 3: Perilimbal, heavily pigmented mass suspicious for conjunctival or epibulbar melanoma. Histopathology was inconclusive in this case and surgical excision and concurrent cryotherapy resolved the lesion in this case. Any limbal mass should be biopsied in an attempt to confirm diagnosis.
Fig 4: Intraocular melanoma associated with the corpora nigra. This mass was associated with cornea oedema due to tumour touch on the endothelial surface. Additionally, this mass was associated with intermittent haemorrhage and uveitis within the eye. Enucleation was curative in this horse and no evidence of melanoma elsewhere in the horse was found.
© 2017 EVJ Ltd
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