UVEAL MELANOMA AND MELANOSIS
Increased pigmentation within the iris can be caused by melanoma or melanosis.
Melanoma is a type of tumor, arising from the pigmented cells within the eye. In dogs, these tumors are usually benign but expansion can lead to secondary glaucoma. In cats, tumors are more likely to spread to other areas of the body.
Melanosis is a non-cancerous tissue pigmentation, similar to a skin freckle.
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.
A thorough examination of the eye is needed in order to determine if the pigmented areas are flat or raised, 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 eye (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 done initially because obtaining a tissue sample is an invasive procedure.
For melanosis, examinations at regular and initially frequent intervals are often recommended. Photographs may be taken to document the appearance over time.
To differentiate melanosis from melanoma, we look for progressive changes in pigmentation, raised areas of pigmentation and elevated intraocular pressure.
No treatment is required for melanosis, only regular monitoring.
Treatment of melanoma depends on stage of the disease and species. Small, focal lesions can be destroyed with laser. Larger or diffuse lesions may necessitate removal of the eye, especially if glaucoma has developed. When the eye is removed, microscopic exam is performed to determine tumor type and likelihood of spread.
The prognosis for melanosis is excellent. There is a change in the appearance of the iris, but there are no risks to systemic health with this condition.
Melanomas of the iris have been described to spread to other areas of the body. This is especially true for cats, although early removal of the eye does not always prevent spread of tumor cells. In dogs, most intraocular pigmented tumors do not spread beyond the eye.