Uveal Melanoma and Malanosis

March 7, 2020

Definition: Increased pigmentation within the uvea can be caused by neoplastic (melanoma) or non-neoplastic disease processes (melanosis).

Melanoma is a type of tumor arising from the pigmented cells within the iris, ciliary body, or choroid.  In dogs, these tumors are usually benign but expansion can lead to secondary glaucoma; mortality approximates 10%.  In cats, tumors have metastatic potential and are more likely to spread to other areas of the body, with mortality rates as high as 30%.

Melanosis is a non-cancerous proliferation of pigmented cells with a variety of possible causes and ocular effects but a favorable prognosis for general health and well-being. Elderly Cairn Terriers suffer from a genetic condition characterized by a benign proliferation of pigmented cells that while invariably leading to glaucoma do not influence the rest of the body. Labs, goldens, and Shepherds have benign pigmented tumors that arise from the limbus that likewise remain localized.

Clinical Signs: In most cases, the first sign noted is a change in iris color.  Additionally, change in the shape of the iris or a raised mass of the iris may be seen.  Later in the disease process, the changes may produce a high pressure within the eye (glaucoma), causing redness, cloudiness, squinting or loss of vision and eventually eye enlargement.

Examination: A thorough examination of the eye is needed in order to determine if the pigmented areas are flat or raised, if intraocular pressure (IOP) is affected and if there are pigmented cells floating in the anterior chamber.  The pupil may be dilated to evaluate symmetry of iris dilation.  The periphery of the iris (iridocorneal angle) may be evaluated by gonioscopy for involvement.  In some cases where melanoma is strongly suspected, evaluation of the lungs and abdominal organs with radiographs and ultrasound is recommended to look for cancer spread.  The definitive diagnosis is made by examining the tissue under a microscope, but this is not often pursued initially because obtaining a tissue sample is an invasive procedure and interpretation of small samples may be problematic.

Monitoring: For melanosis, examinations at regular and initially frequent intervals are often recommended.  Photographs may be taken to document the appearance over time.

Differentiation of melanosis from melanoma may depend upon growth rate, extravasation of tumor cells, and elevated intraocular pressure.

Treatment: No treatment is usually not required for melanosis, only regular monitoring.

Treatment of melanoma depends on stage of the disease and species.  Small, focal lesions may be observed, excised, or ablated with laser.  Larger or diffuse lesions may necessitate removal of the eye, especially if glaucoma has developed.  Pre-operative thoracic radiographs and ocular ultrasound may be recommended to confirm extent of the tumor and when the eye is removed histopathology is performed to determine tumor type and behavior.

Prognosis: The prognosis for melanosis is excellent, although occasional benign processes may lead to localized complications including glaucoma. Melanomas have the potential to spread to other areas of the body.  In dogs metastatic disease usually manifests within a few months; cats may demonstrate long latencies of several mos or years.  Early removal of the eye is preferable but does not always prevent spread of tumor cells.