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EQUINE VETERINARY EDUCATION


Equine vet. Educ. (2019) 31 (8) 410-412 doi: 10.1111/eve.12847


Clinical Commentary Melanocytic ocular and periocular tumours of the horse


K. Myrna†* and C. Sheridan‡ †Small Animal Medicine and Surgery, University of Georgia, Athens; and ‡Department of Ophthalmology, Louisiana State University, School of Veterinary Medicine, Baton Rouge, USA. *Corresponding author email: kmyrna@uga.edu


Keywords: horse; melanoma; iris; eyelid; melanocytoma; cancer


Introduction Reports of ocular melanocytic neoplasms are rare in horses. However, many ocular melanocytic tumours are likely unreported since these lesions often progress slowly or are benign in nature. Therefore, there are limited case reports in the literature, and the biologic behaviour of tissue-specific melanoma as well as the relative effectiveness of specific therapies is not well established. Strauss et al. (2019) present the first published case of a primary corneal melanoma in this issue, reminding us of the need for larger, multi-institutional studies to characterise these tumours in the horse. Ocular melanocytic neoplasms comprise about 9% of all


equine ocular neoplasms and have been described in adnexal, epibulbar, intraocular and orbital tissues (Dugan 1992). The most commonly reported ocular melanocytic neoplasms in horses involve the periocular skin (Figs 1 and 2). They are more frequently found in older grey or white-coated horses (Fleury et al. 2000). Ultraviolet light exposure, chemical exposure, trauma, chronic irritation, concurrent dermal melanomas and genetics have been considered in the pathogenesis of these tumours in humans and animals. No predisposing factors have been identified for the development of corneal and scleral melanocytic proliferation or neoplasia (Gilger 2016a). Intraocular melanocytic neoplasms occur less commonly than periocular tumours and are also more frequently reported in older grey horses. In a retrospective study of 52 horses with intraocular melanocytic neoplasms, 83% had grey coat colour and 67% had concurrent cutaneous melanoma at the time of diagnosis (LaBelle et al. 2012).


Clinical examination


Periocular melanocytic neoplasms are typically pigmented, partially alopecic, and slowly progressive in nature (Giuliano 2010). Although some masses and adjacent tissues may be heavily pigmented, the tissue may be nonpigmented as in the corneal tumour identified by Strauss et al. (2019). For this reason, biopsy is indicated for definitive diagnosis in all cases. Most neoplasms can be identified with direct illumination, but slit lamp biomicroscopy and ocular ultrasound are valuable for determining extent of tissue involvement. In particular, ocular ultrasound or other methods of advanced imaging are helpful in determining the magnitude of tissue involvement; especially with iridociliary tumours. Horses should also be examined for concurrent nonocular cutaneous melanocytic tumours.


Diagnostics


Exfoliative cytology may aid in the diagnosis of eyelid, conjunctival, third eyelid, corneal and scleral tumours, while tissue histopathology remains the gold standard (Dugan 1992; Wang and Kern 2015). Fine needle aspiration of uveal masses is usually nondiagnostic due to limited sample cellularity and may be contraindicated as the uvea is likely to bleed resulting in substantial intraocular sequelae (Singh and Biscotti 2012; Wang and Kern 2015). Unfortunately, biopsy of intraocular tissues is extremely difficult, and for the general practitioner, enucleation is the only method of direct biopsy


Fig 1: Eyelid melanoma in the medial canthus of a horse. This sessile mass was treated locally with injectable cisplatin resulting in clinical resolution of the neoplasia.


© 2017 EVJ Ltd


Fig 2: Eyelid melanoma only visible by flipping the lid to examine the conjunctival surface. This neoplasia was not involving the overlying conjunctiva and resolved with local excision rather than full thickness wedge excision. The area was treated with cryotherapy as well and there is no recurrence 5 years post-operatively.

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