Eye Tumors: Primary Intraocular Tumors

What are primary intraocular tumors?

Primary intraocular tumors are tumors that form in the eye and arise from structures within the eye. These types of tumors are uncommon to rare.

In dogs and cats, primary intraocular tumors can develop in the iris and ciliary body. The iris is the thin, circular structure in the eye that gives the eye its color and controls the size of the pupil. The ciliary body is part of the wall of the eye that makes the fluid that fills the eye. Tumors that develop from the ciliary body are called adenomas (if benign) or adenocarcinomas (if malignant).

In dogs and cats, the most common intraocular tumor is melanoma. More information about this tumor can be found in the handouts “Eye Tumors: Melanoma in Dogs” and “Eye Tumors: Melanoma in Cats”.

In dogs, the second-most common eye tumors are ciliary body adenomas and adenocarcinomas. Other primary intraocular tumors occur but are extremely rare. One rare tumor is a type of spindle cell tumor in dogs known as uveal schwannomas of blue-eyed dogs. These tumors typically arise in the iris and ciliary body. Although they predominantly occur in blue-eyed dogs, they have also been reported in dogs with brown eyes.

In cats, the second-most common primary ocular tumor is feline post-traumatic ocular sarcoma (FPTOS). FPTOS occurs because of injury to the eye and the subsequent chronic inflammation, and it only occurs in cats. Cats can also develop tumors from the ciliary body and iris (iridociliary adenomas and adenocarcinomas), but these are rare.

Primary intraocular tumors only affect one eye. If both eyes are affected, the tumors are likely secondary, meaning they have metastasized (spread) from a cancer located elsewhere in the body.

What causes these types of tumors?

The reason a pet develops these types of tumors, or any cancer, is not straightforward. Few cancers have a single known cause. Most seem to be caused by a complex mix of risk factors, some environmental and some genetic or hereditary. Many eye tumors are attributed to ultraviolet rays/sunlight. With uveal schwannomas of blue-eyed dogs, there may be a genetic cause (a gene mutation) as well. In cats, feline post-traumatic ocular sarcoma is caused by trauma to the eye. The average delay between the trauma and tumor formation is 6-7 years. It also occurs when there is a history of severe eye disease.

What are the signs of these types of tumors?

Primary intraocular tumors can cause glaucoma (increased pressure in the eye), which can cause the eyeball to enlarge protrude, preventing the eyelids from closing. When this happens, the cornea will develop ulcers (sores on the cornea). Both glaucoma and corneal ulcers are painful and may cause your pet to rub or scratch the eye, which can cause further irritation and lead to eye infections.

The sclera (white of the eye) and conjunctiva (membrane that covers the eye and lines the inside of the eyelids) may become reddened and inflamed, conditions termed scleritis and conjunctivitis, respectively. The front part of the eye may become cloudy, making it difficult to see the iris or pupil, a condition called uveitis. All these changes can lead to vision impairment or loss (blindness). Vision impairment or loss can be hard to detect in your pet, as pets tend to function well with one eye.

How are these types of tumors diagnosed?

If your veterinarian suspects a tumor, they may recommend an ultrasound of the eye. An ultrasound determines if a tumor is present, where it is located, and how extensive it is, including whether it has grown through the eye and into the tissues behind and around the eye. This procedure is typically done by referral to a veterinary ophthalmologist (a veterinarian that specializes in eyes).

"If your veterinarian suspects a tumor, they may recommend an ultrasound of the eye."

If your pet has lost vision in the eye or is in pain, primary intraocular tumors are often diagnosed by surgically removing the eye (enucleation) and examining it under the microscope. This examination of the tissue cells, called histopathology, is performed by a veterinary pathologist. Histopathology helps make a diagnosis and can also indicate how the tumor is likely to behave (probability of local recurrence or spread to other areas).

Some intraocular tumors can be diagnosed through fine-needle aspiration (FNA). Fine-needle aspiration involves suctioning a sample of cells directly from the tumor, placing them on a slide, and examining the cells under a microscope. However, to establish a definite diagnosis, enucleation is often necessary.

How do these types of tumors typically progress?

Most intraocular tumors tend to grow within the eye, usually leading to the signs noted above (e.g., uveitis, glaucoma, corneal ulcers). The risk for metastasis is low.

In contrast, uveal schwannomas of blue-eyed dogs are usually locally invasive, causing the signs noted above and also extending beyond the eye into the surrounding tissues, including the bone and nervous tissue, which is very painful. This local invasion can make it difficult to surgically remove the eye and the affected tissues.

Post-traumatic sarcomas in cats also tend to be locally invasive, can involve the optic nerve, and can spread to the brain.

What is the treatment for these types of tumors?

The treatment of any intraocular tumor is based on several factors, including the type of tumor, the extent of damage to the eye, whether there is metastasis, and the pet’s overall health.

The initial treatment may be medical, with topical anti-inflammatories (eye drops) and systemic anti-inflammatories (oral or injectable) administered to help reduce uveitis so the inside of the eye can be better examined. Medical treatment often reduces the pain and helps your pet feel better, but if a mass is found, the best treatment is to surgically remove the eye (enucleation). Enucleation is always recommended when uveitis, glaucoma, or ocular discomfort cannot be managed with medications.

For tumors that have extended outside the eye and into the surrounding tissues, removal of the eye may be recommended, as well as removal of the surrounding tissues. The enucleated eye and locally removed tissues should be submitted for histopathology to determine the type of tumor and whether any further treatment is recommended (e.g., chemotherapy or radiation).

What is the prognosis?

Intraocular tumors are uncommon, but will be considered in pets with unexplained uveitis, glaucoma, or an obvious ocular mass. The prognosis depends on the type of tumor and how early it is found and treated. Although tumors of the eye often require enucleation, most have low metastatic potential, so the overall prognosis is good. In the case of post-traumatic ocular sarcomas in cats, any eye that has become blind from trauma should be considered for early enucleation because of the high rate of malignancy.