Cats have more eye colors than most other domestic species. Your cat’s eye color is hereditary, and depends on how much melanin is distributed to their iris, which is the colored tissue around your cat’s pupil that helps control the pupil size. This structure can be invaded by certain cancers, with diffuse iris melanoma the most common. Our team at Veterinary Vision Center would like you to know more about this concerning disease, and how your cat’s eyes can be affected.
What is iris melanoma in cats?
Melanocytes are cells that produce melanin. If these melanocytes proliferate in a disorganized and uncontrolled manner, a cancerous tumor called a melanoma results. Melanomas affecting the iris are the most common primary intraocular tumors found in cats, and can metastasize to other organs, leading to a poor prognosis. These tumors seem to have no breed or gender predilection, but cats affected average 9.4 years of age. Iris melanoma has a highly variable progression rate. Benign pigmentation on the iris, known as melanosis, can remain static for the cat’s lifetime, or can progress to advanced melanoma in a relatively short time.
When the cancerous cells invade the iridal layers, the disease is considered diffuse iris melanoma (DIM). As the disease progresses, the melanoma’s invasion can result in changes in pupil shape and motility. The lesions can also affect other eye structures, including ciliary body, choroid, and iridocorneal angle. If the iridocorneal angle becomes affected, secondary glaucoma may develop. DIM has a high metastatic potential and can affect many organs, including the liver, lungs, kidneys, spleen, bone, and lymph nodes. When metastasis occurs, the cat’s prognosis for survival is poor.
How is diffuse iris melanoma in cats diagnosed?
DIM begins as focal or multifocal hyperpigmented areas confined to the iris surface. These lesions are known as iris freckles, or nevi, which can be round, irregular, or linear in shape, and are typically light brown initially, turning dark brown as they progress. Since a biopsy of these lesions would be too damaging to the eye, a clinical diagnosis of iris melanosis is suspected when the nevi appear. Once observed, these lesions should be monitored closely, because if they are allowed to metastasize, your cat will likely not survive. The lesions can be photographed to help determine if they are changing over time. Other recommended monitoring methods include gonioscopy to monitor the cat’s iridocorneal angle, and tonometry to monitor their intraocular pressure. Glaucoma development has been linked to higher metastatic rates.
How is diffuse iris melanoma in cats treated?
The only way to treat DIM is to enucleate the cat’s eye. The struggle is deciding when the eye should be removed, since some cats go their entire life without the melanosis progressing to DIM. Enucleation may be elected when the pigmented lesions progress, or when glaucoma develops. Early nucleation has been reported as paramount to preventing premature death because of metastasis.
What are potential rule-outs for diffuse iris melanoma in cats?
Since definitive diagnosis requires histopathological examination that cannot be performed because the eye damage would be too severe, other diagnoses should be considered. These include:
- Uveal cyst — These are fluid filled, circular structures that are brown to yellowish in color. They occur spontaneously, and may be free floating, or attached to the iris. Uveal cysts are usually benign, and require treatment only if they impede vision. Uveal cysts are rare in cats, and evaluation by an ophthalmologist using a slit lamp can distinguish between a uveal cyst and a melanoma.
- Intraocular sarcoma — These lesions are the second most common primary ocular tumor in cats. They occur after a cat has suffered an eye injury, and are caused by the subsequent chronic inflammation. Intraocular sarcomas are locally invasive and have metastatic potential. Enucleation is the recommended treatment.
- Uveitis — Inflammation inside the eye can cause pigmentary changes, but this form of uveitis is not common in cats, and other signs, such as squinting and tearing, would likely be observed.
What if my cat has diffuse iris melanoma and needs an enucleation?
Enucleation means surgical eye removal. Once removed, your cat’s eye will be sent for evaluation by a pathologist, to determine the DIM disease progression. You can expect your cat to have some swelling and tenderness that may worsen during the first 24 hours. A small amount of blood-stained fluid from the wound is normal, but you should contact our Veterinary Vision Center team, or your referring veterinarian, if you see more than a few drips. Your cat will need antibiotics and pain medication for several days following surgery, and may also need a special collar to prevent them from rubbing the surgical site. DIM typically affects only one eye, and most cats acclimate well to enucleation of one eye. Your cat should be back to normal in about five days.
No one wants their cat’s eye removed, but the procedure could save your cat’s life if they are affected by DIM. If your cat is diagnosed with this disease, our veterinary professionals will work with you to determine how to manage your cat. If you are concerned your cat is affected by DIM or another eye condition, do not hesitate to contact our team at Veterinary Vision Center to schedule an appointment.