Anterior uveal melanoma

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Visible appearance of a uveal melanoma in a dog - mass located in upper quadrant of iris[1]

Anterior uveal melanoma are the most common intraocular neoplasia of the canine iris.

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.

A breed predisposition has been reported in the Boxer and a hereditary link observed in the Golden Retriever and Labrador Retriever[2].

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[3][4].

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[5].

Affected dogs may present with bradycardia due to stimulation of the oculocardiac reflex associated with elevated intraocular pressure[6]. 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[7].

Diagnosis is based on clinical history, opthalmic examination and histopathological examination of biopsied tissue samples[8]. Unfortunately, the cytological appearance or pattern of growth does not correlate with biologic behavior[9].

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[10] 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[11].

References

  1. Vetnext
  2. Donaldson D et al (2006) Canine limbal melanoma: 30 cases (1992-2004). Part 1. Signalment, clinical and histological features and pedigree analysis. Vet Ophthalmol 9(2):115-119
  3. Wilcock B et al (2002) Histologic Classification of Ocular and Otic Tumors of Domestic Animals. Washington: Armed Forces Inst of Pathol, pp:22–25
  4. Grahn B et al (2004) Veterinary Ophthalmology Essentials. Philadelphia: Butterworth and Heinemann. pp:235–236
  5. Riis R (2002) Small Animal Ophthalmology Secrets. Ocular melanosis in cairn terriers. Philadelphia: Hanley and Belfus, pp:105–109
  6. Steinmetz A et al (2012) Oculocardiac reflex in a dog caused by a choroidal melanoma with orbital extension. J Am Anim Hosp Assoc 48(1):66-70
  7. Pires I et al (2010) COX-1 and COX-2 expression in canine cutaneous, oral and ocular melanocytic tumours. J Comp Pathol 143(2-3):142-149
  8. Peiffer RL et al (1990) The pathogenesis and significance of pre-iridal fibrovascular membrane in domestic animals. Vet Pathol 27(1):41-45
  9. Martens AL (2007) Unusual presentation of an anterior uveal melanocytoma in a 3-year-old poodle. Can Vet J 48(7):748-750
  10. Galán A et al (2009) Presumed primary uveal melanoma with brain extension in a dog. J Small Anim Pract 50(6):306-310
  11. Featherstone HJ et al (2009) Efficacy of lamellar resection, cryotherapy, and adjunctive grafting for the treatment of canine limbal melanoma. Vet Ophthalmol 12(1):65-72