A change in color of the eye is rapidly noted by most owners. Pigmented lesions of the eye vary greatly in their appearance and their clinical significance.
Limbal melanomas originate from the rim of melanocytes normally found within the junction of the cornea and sclera. These tumors are histologically benign but progressively enlarge with intraocular extension, invasion of the iridocorneal angle and secondary glaucoma. There appears to be a breed pre-disposition for large breeds, with Golden retrievers and Labrador retrievers among the most commonly affected breeds. When these tumors affect dogs less than 5 years of age, they tend to progress more rapidly with early treatment indicated.
Fortunately these tumors are amenable to diode laser photocoagulation. Partial surgical excision is performed with treatment of the remaining affected limbal tissue with the laser delivered through the operating microscope. These photographs illustrate the appearance of a typical limbal melanoma before and 6 weeks after treatment with the diode laser. Although some corneal opacity remains, there has been no progression of the tumor for over three years.
Eyelid melanomas are also typically benign. When less than 25% of the eyelid margin is involved, surgical excision is the treatment of choice. However, when the lesions are larger or multiple, laser treatment is of value.
This series of photographs illustrates multiple eyelid melanomas in a five year old Vizsla dog. During the initial three weeks postoperatively, there was necrosis at the site of laser treatment. With time, sloughing of the treated area resulted in a smooth, functional eyelid margin. Using conventional surgical excision, a grafting procedure would have been necessary.
Conjunctival melanomas have biological behavior that is often very different from other ocular melanomas. These tumors more closely resemble melanomas associated with other mucous membranes (i.e. oral mucosa melanomas) and may be highly malignant. Aggressive surgical excision, including enucleation if necessary, is indicated.
Uveal melanomas represent the most common primary intraocular tumor. The iris is most often affected, with the ciliary body and/or choroid affected secondarily by extension. These tumors appear as a densely pigmented, localized mass within the iris. In dogs, uveal melanomas are generally benign with a very low incidence of metastasis. In many cases, the lesin is not noted or referred until the tumor has extended into the iridocorneal angle and/or posteriorly into the ciliary body. In these cases, there are no options for treatment and, ultimately glaucoma occurs necessitating enucleation.
Earlier iris melanomas in dogs can be successfully treated using diode laser photocoagulation with excellent results and preservation of vision. These photos illustrate a small iris melanoma in a 3 year old Labrador retriever. The lesion was treated transcorneally using a diode laser. One year postoperatively, the treated area exhibits iris atrophy and a small iris defect with no remaining tumor.
In cats the prognosis is very different. Feline iris melanomas are diffuse within the iris stroma with aggressive growth and frequent metastasis. Thus, in cats, enucleation is recommended once this diagnosis is made.
Iris cysts represent a developmental abnormality of the pigmented epithelium on the posterior iris surface. A portion of this bilayered epithelium fills with aqueous humor and may separate from the iris, forming a spherical mass floating within the anterior chamber. Cysts may be multiple and may interfere with vision by obstructing the pupil however their primary importance is in the differential diagnosis from an iris melanoma. If necessary, cysts my be ruptured using the diode laser.
Veterinary Vision acknowledges the support of IRIDEX, Inc. in providing the equipment and technical support which has made possible the development of many of the techniques described in this article.