What are primary intraocular tumors?
Primary intraocular tumors are tumors that specifically form and arise from structures within the eye. These types of tumors are uncommon to rare.
In dogs, 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, the second most common eye tumor are ciliary body adenomas and adenocarcinomas (melanomas of the eye are the most common and more information can be found in the handout "Eye Tumors - Melanoma in Dogs"). Other primary intraocular tumors occur, but are extremely rare. One of these is a rare 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.
The second most common primary ocular tumor in cats is feline post-traumatic ocular sarcoma (FPTOS). FPTOS occurs as a result of injury to the eye and the subsequent chronic inflammation. 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 from a cancer located elsewhere in the body.
What causes these types of tumors?
The reason why a pet may develop these types of tumors, or any cancer, is not straightforward. Very 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.
Ultraviolet rays/sunlight has been attributed to causing many forms of eye tumors. 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, as its name suggests, 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?
These types of tumors can cause glaucoma (increased pressure in the eye) which can cause the eyeball to enlarge and protrude and prevent 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, causing scleritis and conjunctivitis. The front part of the eye may become cloudy, making it difficult to see the iris or pupil. This is 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, an ultrasound of the eye may be recommended. 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. If your pet’s eye appears to protrude, your veterinarian may recommend a referral to a veterinary ophthalmologist.
"If your veterinarian suspects a tumor, an ultrasound of the eye may be recommended."
If your pet has lost vision in the eye or is in pain, these types of tumors are often diagnosed by surgically removing the eye (enucleation) and examining it under the microscope. This examination of the tissue cells, performed by a veterinary pathologist, is called histopathology. Histopathology is not only helpful to make a diagnosis but can 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, in order 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, etc.). The risk for metastasis (spread) is low.
In contrast, uveal schwannomas of blue-eyed dogs tend to be locally invasive, and not only cause the signs above, but they tend to extend beyond the eye into the surrounding tissues, including the bone and nervous tissue, which is very painful. This local invasion can make the surgical removal of the eye and the affected tissues difficult.
Post-traumatic sarcomas in cats also tend to be locally invasive, can involve the optic nerve, and spread to the brain.
What are the treatments for these types of tumors?
The treatment of any intraocular tumors 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 (eye drops) and systemic (oral or injectable) anti-inflammatories to help reduce the 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 the uveitis, glaucoma, or ocular discomfort cannot be managed with medications.
"The treatment of any intraocular tumors 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."
For tumors that have extended outside the eye and into the surrounding tissues, removal of the eye, as well as the surrounding tissues may be recommended. The enucleated eye and locally removed tissues should be submitted for histopathology to determine the type of tumor and whether any further treatment (e.g., chemotherapy or radiation) is recommended.
Is there anything else I should know?
Intraocular tumors are uncommon, but will be considered in pets with uveitis, glaucoma, or an ocular mass. The prognosis depends on the type of tumor and how early it is found and treated. Although tumors of the eye often necessitate enucleation, most have low metastatic potential, so the overall prognosis is good. In the case of post-traumatic ocular sarcomas in cats, because of the high rate of malignancy, any eye that has become blind from trauma should be considered for early enucleation.