Iris Melanosis in Cats: Causes, Symptoms, and Treatment

Urgency: Prompt assessment recommended

Home Remedies: Monitoring only

Vaccine Available: None

Treatment Options: None needed, unless progression to malignant uveal melanoma

May be Linked to: Uveal melanoma

Diagnosis: Examination by a veterinary ophthalmologist, with possible biopsy of affected tissue

Requires Ongoing Medication: No medication is required

Common Symptoms: The appearance of dark pigment (brown or black) on the iris of the cat's eye

Other Names: Iris freckle, iris nevus

Diagnosed in Cats: Commonly

Home Treatment: Home treatment is not possible, but home monitoring of any changes to the pigmentation in the eye is very important.

Iris melanosis is a type of iris hyperpigmentation, which is an increase in dark pigmentation on the iris. The iris is the colorful part of the eye and varies between cats—from yellow or amber to hazel, green, or blue. In iris melanosis, dark spots or patches of dark brown pigment appear on the iris. They can usually be seen quite clearly without exam tools.

These patches of pigment can be likened to freckles on human skin and they are typically harmless. However, in some cases, iris melanosis can transform into uveal melanoma, a type of malignant tumor. So it is important to monitor this disease closely despite its predominantly benign nature.

Causes of Iris Melanosis

The cause of iris melanosis is unknown. Simply, the pigmented cells (melanocytes) in the iris start to multiply and produce more pigment than normal, creating a brown or black lesion that looks abnormal. A small patch of pigment on the iris may be called an “iris freckle” and a larger patch of pigment on the iris may be called an “iris nevus” (plural: iris nevi).

Symptoms of Iris Melanosis in Cats

Iris melanosis is common and is most often seen in middle-aged cats. All cats can be affected—there is no breed predisposition.

The signs of this condition are usually obvious to an owner. There are patches of dark brown or black pigmentation in one or both of a cat’s eyes so that the iris looks different from normal. It’s usually seen just in one eye, but both eyes can be affected.

Complications of Iris Melanosis in Cats

In some cases, iris melanosis can transform over time into uveal melanoma, a type of malignant tumor that can spread to the lymph nodes, lungs, spleen, kidneys, bone, and other parts of the body. This metastatic disease then leads to premature death. The malignancy of uveal melanoma is the reason why it is so important to carefully monitor iris melanosis. This is similar to the idea that humans need to monitor freckles on their skin in case they develop into malignant melanoma. Uveal melanoma is also known as Feline Diffuse Iris Melanoma (FDIM).

A similar but unrelated condition that involves new pigment in the eyes is uveal cysts. Uveal cysts are small, brown or yellow, fluid-filled, circular structures that may be attached to the iris or floating in the front of the eye. These can be confused with iris melanosis.

Diagnosis of Iris Melanosis

If your veterinarian suspects that your cat may have iris melanosis, they will examine them carefully and take a history of when you first noticed any changes. They will then make a detailed examination of the eye with an ophthalmoscope or by using a slit lamp. Your veterinarian may perform this examination or they may take a quick look and refer you to a specialist. An ophthalmic veterinarian can perform a much more detailed, close-up eye examination.

They will search for signs that the pigmented areas were caused by benign iris melanosis, with the hope of ruling out uveal melanoma. The warning signs that such a uveal melanoma transformation may be happening include:

• Changes to the surface of the iris—e.g., smooth and unblemished to rough and irregular
• Iris thickening or growth
• Spreading of the pigment into other parts of the eye or into a larger part of the iris
• Changes in shape of the pupil so that it looks uneven (dyscoria)
• Secondary uveitis (inflammation of the iris)

They may also measure the pressure of the eye (intraocular pressure or IOP) or examine the eye using an ultrasound scanner. To make a definitive diagnosis, a specialist veterinary ophthalmologist may be needed to biopsy the pigmented tissue in the iris and then conduct a histopathology (microscopic tissue examination).

Treatment of Iris Melanosis

Iris melanosis does not require treatment because it is a benign condition that does not affect your cat’s vision. However, close monitoring of iris melanosis is important. Talk to your veterinarian about physical rechecks every 3–4 months to record any changes in the pigmentation of the iris. Photographs and measurements of objective criteria (such as the size of the pigmented area, the IOP, and the iridocorneal angle) may be taken and stored with the clinical records for future comparisons.

If there are concerns that areas of pigment may be progressing toward uveal melanoma, treatment options include laser therapy (photocoagulation) for early cases or possibly early enucleation (surgical removal of the entire eye). Early removal of the eye aims to maximize survival times in cases of iris melanoma. But it can be difficult to decide to remove a functional eye due only to a suspected risk of a serious problem when, in fact, the cat may just have a benign problem.

Cat Care Tips

Iris melanosis doesn’t require treatment but close monitoring is important, so here are our top tips.

• Regular checks: All cats should have regular veterinary exams, which can help discover any potential problems early.
• Take photos: If your cat has iris melanosis, take regular pictures or take notes so that any changes can be easily and quickly seen.
• Be vigilant for changes: Iris melanosis is benign and relatively slow to change. If the conditions seem to be progressing rapidly or causing other changes, such as vision impairment, speak to your veterinarian promptly.