Causes of Blindness
Veterinary Ophthalmology: Dedicated to the diagnosis and treatment of ocular disease in animals
Progressive Retinal Atrophy
Progressive Retinal Atrophy (PRA) describes a large group of inherited diseases that lead to blindness in dogs and cats. Vision loss typically occurs over months and the age of onset varies depending on breed. An ophthalmic exam may reveal dilated pupils and evidence of retinal degeneration including a decrease in retinal vessels and increased reflectivity of the tapetum (eye shine). When the ophthalmology exam provides initial evidence of the disease, the diagnosis can be confirmed with an electoretinogram (ERG) or genetic DNA testing. An ERG is a test to demonstrate a decrease in retinal function in response to flashing bright lights.
Unfortunately, there is no treatment for PRA but there is no pain associated with this disease.
In some cases, retinal degeneration is associated with the development of cloudiness that could be cataracts. Since the retina is the source of the blindness, cataract surgery is not typically recommended.
Sudden Acquired Retinal Degeneration Syndrome describes rapid and complete loss of vision that can occur over the course of days. While the specific cause of SARDS remains controversial, the syndrome appears to have an immune-mediated component.
Clinical signs include a sudden decrease in vision and dilated pupils. This is common in middle-aged, female dogs and in many cases has an association that we do not yet completely understand with Cushing’s disease or similar clinical signs (i.e. increased drinking, eating, urination, weight gain).
Ophthalmic exam may reveal a decrease in vision and absent light reflexes, but the structures of the eye including the lens, retina, and optic nerve frequently appear normal. An electroretinogram (ERG) is a test to demonstrate a decrease in retinal function in response to flashing bright lights. In SARDS patients, the ERG is most commonly a “flatline”, meaning there is no response to light from the retina. Much investigation is underway to identify a reliable treatment for SARDS and possibilities are on the horizon.
Glaucoma describes elevated pressure inside the eye that can result in blindness by damaging the optic nerve and retina. In a healthy eye, fluid called aqueous humour is constantly produced within the eye to nourish ocular structures, including the lens and the cornea. Intraocular pressure increases when the region that drains fluid from within the eye is narrowed (primary glaucoma) or blocked (secondary glaucoma), thereby preventing the drainage of aqueous. Primary glaucoma is an inherited condition and specific breeds including Beagles, Bassett hounds, Cocker spaniels, Siberian huskies, Labrador retrievers and Shar peis are predisposed. When secondary glaucoma occurs, inflammation or a loose (luxated) lens clogs the drain and blocks the outflow of fluid within the eye.
The onset of glaucoma can be sudden and severe, and clinical signs can include loss of vision, cloudiness, redness, and pain. Medical management of glaucoma includes topical eye drops to decrease the production of fluid within the eye or increase the outflow of fluid from the eye. In a specific subset of patients, surgical intervention is possible including placement of drainage shunts or laser destruction of the part of the eye that produces fluid. Unfortunately, the prognosis for glaucoma can be guarded and pressures can increase rapidly and severely. When vision has been lost and high intraocular pressures are causing pain, our treatment goal shifts to comfort. When maintaining vision is no longer possible, surgery to remove the eye (enucleation), place a prosthetic (evisceration), or perform a ciliary body ablation injection (chemical cycloablation) may be recommended depending on each specific pet.
Cataract describes an area of decreased transparency within the lens. These opacities, most commonly white in appearance, can be very small (incipient or incomplete) or mature (100% complete) and can obstruct vision to varying degrees. Causes of cataracts commonly include age-related changes, diabetes mellitus in dogs, inherited genes, and chronic inflammation. Surgery is the only effective treatment for removing cataracts to restore vision but unfortunately, not all patients are good candidates. A thorough ophthalmic exam is required to make this assessment.
Prior to performing cataract surgery and general anesthesia, a veterinary ophthalmologist will work with the patient and the family vet to make sure that pre-operative physical exam and blood work are normal. A veterinary ophthalmologist may perform an ultrasound and electroretinogram (ERG) to ensure that the eye is structurally healthy and verify that the retina is functioning well prior to removing the cataract in the lens. The cataract is most frequently removed using the same technique as in people (phacoemulsification), and an artificial lens may be placed depending on the health of the eye.
In some cases, animals are born without eyes (anophthalmia) or with very small eyes (microophthalmia) that do not function normally. The underlying cause can range from toxin or infection exposure while the eyes were forming in utero or can represent inherited defects in specific breeds of dogs.
When a patient presents with sudden blindness, the most important question to answer is whether the loss of vision originates in the eye or the central nervous system (brain). Once the absence of vision is confirmed using maze testing and failure to react to visual cues, a thorough ophthalmic exam will rule out structural abnormalities such as cataract or retinal detachment that would obstruct vision within the eye. If the retina appears normal on ophthalmic exam, an electroretinogram (ERG) can be performed to demonstrate decrease or absent retinal function. The ERG can be performed awake on most patients and helps rule out Central Nervous System disease without general anesthesia that is necessary for an MRI and CSF tap.
Ocular Trauma can be devastating and typically takes two forms, blunt or penetrating trauma. Blunt trauma often results from accidents with cars and can result in a blinding “blow-out” injury in which the back of the eye splits open. Penetrating trauma is most commonly cause by a stick or a cat claw and can lead to severe damage to the internal structures of the eye. The prognosis is particularly grave when the penetrating object ruptures the capsule of the lens and severe inflammation ensues. In some cases, surgery can be performed to stabilize the eye and maintain a comfortable globe, but saving vision is not always possible.
The retina lines the back of the eye and uses a complicated network of photoreceptors (rods and cones) and nerves to transmit images to the brain through the optic nerve. High blood pressure or severe inflammation within the eye (panuveitis) are examples of disease that lead to a retinal detachment. If the retinal detachment is large, significant vision loss occurs. If the underlying cause of the retinal detachment can be identified and treated, a retina can reattach and some vision may be spared. In some cases that meet specific criteria, retinal reattachment surgery may be possible with a specialized veterinary ophthalmologist.
The optic nerve is the bundle of nervous tissue that transmits images from the retina to the brain. Optic neuritis describes swelling or inflammation of the optic nerve that typically leads to a loss of vision and pupillary light reflexes. The blindness is often sudden and complete. Causes can include infectious diseases, cancer, and inflammatory diseases that stem from elsewhere in the body. Common infectious diseases in dogs are fungal and tick-borne disease. Inflammatory disease can be auto-immune such as granulomatous meningoencephalitis (GME). Cancerous processes can include a focal tumor in the nervous system or a more diffuse form such as lymphosarcoma. It is important to differentiate between these diseases in order to determine the best treatment, and spinal tap and MRI may be necessary for diagnosis.