Optic Neuritis in Dogs
Dr. Rhea Morgan
Optic neuritis is inflammation of the nerve that leads from the retina of the eye to the brain. Inflammation of this nerve interferes with normal function of the eye by preventing retinal information from reaching the brain. There is one optic nerve for each eye, and optic neuritis may involve one or both of the nerves. If both nerves are affected, then the animal is usually blind. The onset of blindness is usually sudden with optic neuritis. No signs may be detected if only one nerve is affected. Animals that are blind in one eye may act normal.
Inflammation of the optic nerves may be associated with inflammation of the retina, inflammation of the brain, or may involve only the nerves themselves. Causes of optic neuritis in the dog include viral infections (canine distemper), protozoal infections (toxoplasmosis, neosporosis), fungal infections (blastomycosis, cryptococcosis, histoplasmosis, coccidioidomycosis), and tick borne infections (Lyme disease, ehrlichiosis, Rocky Mountain spotted fever).
Immune diseases, head trauma, tumors, some forms of meningitis, and certain systemic diseases may also cause optic neuritis. In some cases of optic neuritis, the cause is never discovered and these are called "idiopathic" forms.
The optic nerve may be inflamed along its entire length from the retina to the brain, or only a portion of the nerve may be affected. When the beginning of the nerve is inflamed, this inflammation is visible by examining the retina. When the nerve behind the eye is inflamed, the retina may appear normal.
What to Watch For
Signs of blindness occur if both nerves are affected. They include bumping into objects, getting lost in the normal environment, reluctance to go outside or up/down stairs, fearful or timid behavior, possibly aggressive behavior, and inability to find or catch toys.
If the brain is affected, other neurologic signs may be noted.
If the animal has a systemic disease, other widespread signs may occur, such as lethargy, decreased appetite and weakness.
Optic neuritis can sometimes be difficult to diagnose. Tests may include the following:
A complete eye examination is indicated, including thorough assessment of neurologic and pupil reflexes, fluorescein staining of the cornea, Schirmer tear test, tonometry to measure pressure in the eye, and a detailed examination of the interior structures of the eye. Your veterinarian may refer your dog to a veterinary ophthalmologist for further evaluation using specialized instrumentation. Evidence of optic neuritis that may be visible upon examination of the retina include the following:
– Papilledema or swelling of the optic nerve from the accumulation of inflammatory fluid or from increased pressure behind the eye
– Redness or hemorrhage in the vicinity of the head (optic disk) of the optic nerve
– Inflammation or hemorrhage in the nearby retina
– Detachment of the retina near the optic nerve
A thorough physical examination is very helpful in detecting abnormalities in other organs, especially if a systemic illness is present.
A complete neurologic examination is required to detect deficits in other nerves, particularly those around the head.
Numerous laboratory tests may be submitted, such as a complete blood count, biochemistry profile, and tests for tick, fungal and protozoal diseases.
Chest and abdominal X-rays may also be recommended.
If brain disease is suspected, a CT scan or MRI may be helpful and a spinal tap may be performed.
In cases of acute blindness when the eye exam is normal, specialized electrodiagnostic testing may be required to identify whether the retina is the source of the blindness (via an electroretinogram) or whether the blindness may involve the nerve or other structures behind the eye (via a visual evoked potential test).
Treatment primarily involves the use of medications directed towards the underlying cause of the inflammation. In order for appropriate treatment to be instituted, the cause of the optic neuritis must be identified if at all possible.
Corticosteroids are often used when the optic neuritis is considered to be idiopathic or immune in origin, or if meningitis is involved.
Home Care and Prevention
When treating optic neuritis, be sure to follow precisely the medication instructions provided by your veterinarian. Repeated follow-up examinations are important to monitor response to therapy and to detect any worsening of the disease. Some dogs respond well to treatment and regain their vision, while others remain blind. Optic neuritis is considered a serious condition and may sometimes be life-threatening to the dog.
There are no preventative measures available for optic neuritis. Early intervention and treatment may prevent other neurologic signs from developing.