Anterior uveal melanoma
The uvea is the part of the eye that is made up of the pigmented iris, the ciliary body, the choroid and pars plana. These tumors usually arise from the anterior surface of the iris and extension into the ciliary body and choroid.
These melanocytic tumors are a neoplastic proliferation of the uveal melanocytes and involve the iris, ciliary body, and, rarely, the choroid. They are usually benign in nature, nodular in appearance, and primarily contain large, heavily pigmented round-polygonal melanocytes and often less than 5% spindle shaped germinal cells.
Clinical signs in affected dogs include a visible irregularly-pigmented, iris with possible anterior uveitis, scleritis or episcleritis. Hyphema may be observed in large masses where secondary vascular infiltration has led to intraocular hemorrhage.
On ophthalmic examination, these tumors are characteristically flat and diffuse (unlike intraocular melanomas, which are raised masses) and typically affect only one eye. They need to be distinguished from iris melanosis which is a benign pigmentation change to the iris.
Affected dogs may present with bradycardia due to stimulation of the oculocardiac reflex associated with elevated intraocular pressure. A significant number of these melanomas express COX-1 and COX-2 enzymes, suggesting a palliative benefit of using COX-inhibitors in the management of early cases.
Diagnosis is based on clinical history, opthalmic examination and histopathological examination of biopsied tissue samples. Unfortunately, the cytological appearance or pattern of growth does not correlate with biologic behavior.
Uveal and choroidal melanomas are rarely metastatic but can invade surrounding tissue, particularly the choroid, leading to secondary diseases such as glaucoma. In more rare cases, distant metastasis to the brain or lungs has been reported.
In many cases, glaucoma is inevitable and usually poorly responsive to antiglaucoma therapy, necessitating anterior chamber shunt placement, evisceration, or both, or enucleation when the eye becomes blind
Under specialist veterinary care, more advanced surgical interventions may include cryotherapy.
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