It is difficult to differentiate a benign freckle from a potentially malignant melanoma in the iris of a cat. Iris melanosis in cats is generally a benign change that should be monitored closely, while uveal melanoma is a malignant tumor, usually of the iris, that can be detrimental to the eye (causing inflammation and glaucoma) and has the potential to metastasize (spread to other parts of the body). The difficulty with these diseases is determining at which point the benign melanosis changes to the malignant melanoma.
There are several characteristics that veterinary ophthalmologists use to try and differentiate the two conditions; however, the only way to definitively diagnose a lesion is via biopsy (usually obtained by removal of the eye). Some patients are candidates for sampling of the mass prior to enucleation (either with an aspirate or biopsy), although this creates a significant amount of inflammation within the eye, and is often not worth the risk.
Characteristics of intraocular pigmented masses that often raise concern for a more malignant process include:
- Rate of progression (quick progression is more concerning)
- Whether pigment extends into the iridocorneal (drainage) angle
- A velvety appearance to the pigment
- Development of dyscoria (abnormal pupil shape)
- Presence of free-floating pigmented cells within the anterior chamber
When the drainage angle becomes clogged, it can lead to fluid backup in the eye and cause increased pressures, known as glaucoma. This is painful and may feel similar to a migraine. Historically, cats that already have glaucoma due to an intraocular melanoma are at higher risk of having metastatic disease, and may have a shorter overall survival time.
Other diagnostics to ensure the mass has not spread to other parts of the body include generalized bloodwork (complete blood count and chemistry panel), aspirates of the submandibular lymph node (that drains the eye), thoracic radiographs (chest X-rays), and abdominal ultrasound.
The difficulty is determining at what time (if at all) to remove the eye. If the eye is blind and painful due to glaucoma, the decision is fairly straightforward. However, if the eye is still visual, there is risk of removing a visual eye that may only have a benign lesion. If the mass has changed to a malignant tumor and spread elsewhere in the body, prognosis is generally poor and removing the eye would no longer be beneficial in preventing systemic disease.