In this VETgirl online veterinary continuing education blog, Dr. Shelby Reinstein, DVM, DACVO reviews how to treat vision loss in dogs and cats. She also reviews how to perform appropriate examination of the lens and retina in our veterinary patients.
By Dr. Shelby Reinstein, DVM, DACVO
Vision loss in dogs and cats can be one of the most challenging types of cases. A complete neuro-ophthalmic exam should be completed, and basic eye tests performed to rule out common causes. However, special attention should always be paid to the lens and retina as these two structures often undergo diseases that lead to loss of vision.
The vast majority of lens pathology involves changes in the clarity. Nuclear sclerosis will appear as a round, opalescent appearance to the center of the lens. This change should not affect the ability to examine the retina to a significant degree (although it may appear hazy). Cataract formation is the most common change to the lens. Cataracts have a widely variable appearance based upon their location, stage, and underlying cause. Cataracts can form on the anterior or posterior lens capsule, the anterior or posterior lens cortex, or the nucleus. They can range in appearance from a dot or line, wedge-shaped, triangular, or can completely opacify the lens. As discussed above, the lens may also undergo changes in its position, becoming either anteriorally or posteriorally luxated. Lens subluxation can occur when only a portion of the lens zonules become broken, and the lens may tip or tilt and can cause asymmetric changes in the anterior chamber depth. Pigment or blood may be deposited on the anterior lens capsule following inflammation or trauma.
The most important aspect of fundic examination is to be able to recognize normal variations. The color and presence of the tapetum and retinal pigment varies with coat and eye color, as well as with age, and the appearance of the optic nerve varies with species and breed. For example, dogs with light hair coats or blue eyes will also lack pigment in their fundus, allowing visualization of the choroidal vessels. Young dogs and cats (<4 months of age) will have a blue tapetum, which matures to the more common green color, however tan colored dogs, such as Cocker Spaniels will often have an orange tapetum. The optic nerve of most dogs is highly myelinated (white) and often triangular in shape. The retinal blood vessels should pass onto the surface of the optic nerve head, often forming a C-shaped anastomosis. Cats, on the other hand, have poorly myelinated optic nerves that appear dark, small, and round. Retinal blood vessels appear to originate from the periphery of the nerve head. These normal variations can be misinterpreted as lesions or abnormalities to the untrained eye.
Fundic pathology can involve the retina, the tapetum, the choroid, and the optic nerve head. Basic processes of inflammation, degeneration, detachments and bleeding are the most common.
- Retinal and choroidal inflammation will result in a fuzzy appearance that often times produces a dull area to the tapetal reflection and an engorged or tortuous appearance to the retinal vessels. Often, multifocal circular or wedge-shaped lesions develop.
- Small focal retinal bullae, often seen with early hypertensive disease in cats, appear as diffuse grey lesions in the tapetum. Examination of these lesions at high magnification will demonstrate elevation of the blood vessels as they travel over these focal elevations.
- Retinal hemorrhages can vary in their appearance from small dots to larger splotches depending upon the underlying cause and severity.
- Retinal detachments are classified broadly as bullous or rhegmatogenous. A bullous retinal detachment appears as a fluid bubble under the retina causing elevation of the retina into the vitreous. The vessels will have an abnormal course and appear out of focus. A rhegmatogenous detachment occurs when the retina tears fully from its peripheral attachment and falls over the optic nerve, which appears as a hazy “curtain” of tissue that may billow in the vitreous.
- Retinal degeneration (such as in progressive retinal atrophy) is a thinning of this tissue which leads to tapetal hyperreflectivity and a decrease in the size and distribution of the retinal blood vessels.
- Inflammation is the primary abnormality seen in the optic nerve head (optic neuritis). The tissue will appear puffy or swollen and hyperemic, and the vessels may appear to “fall off” the edge of the optic nerve head. The surrounding retina may be hazy or slightly detached, and hemorrhages on or adjacent to the optic nerve head are not uncommon. Patients with inflammation of the optic nerve head are almost always significantly visually compromised with loss of PLRs.