Optic Neuritis and Multiple Sclerosis
One of the common neuro-ophthalmologic manifestations of multiple sclerosis (MS) is unilateral vision loss due to optic neuritis (ON).
The classic clinical picture of MS is multiple neurologic symptoms disseminated in space and time. More specifically, over time, patients manifest episodic neurologic dysfunction due to inflammation in different regions of the central nervous system (CNS), which includes the brain and spinal cord.
An episode of optic neuritis typically begins with eye pain, especially with eye movement. Within a few days, patients will notice blurred vision in the affected eye. Often, this appears like a “thumb-print” or smudge that blurs the vision. Within a week, this may progress to darkening of part of the visual field. Reading can become difficult, especially if the central portion of the visual field is affected. In addition to blurred vision, most patients notice that colors appear less bright, or “washed-out” with the affected eye.
What causes Optic Neuritis?
The optic nerve is the cable that connects the eye to the brain. In optic neuritis, there is an inflammation of the covering of the optic nerve.
This covering is composed of a fatty substance known as myelin. When inflammation damages the myelin sheath, the optic nerve cannot properly transmit information from the eye to the brain, and vision becomes decreased in that eye. Optic neuritis most often occurs between the ages of 20 to 50 and is three times more frequent in women. In adults, it affects only one eye in most cases, but in children, it can affect both eyes simultaneously.
How is optic neuritis diagnosed?
The doctor will diagnose optic neuritis based on history and characteristic findings at the time of a careful examination. Almost all patients with optic neuritis will have what is called an afferent pupillary defect, where the pupil on the affected side will not constrict to the light stimulus as much as the fellow eye. Some patients with optic neuritis will have mild swelling of the optic nerve and surrounding retina that is visible to the physician during examination with the ophthalmoscope.
However, most patients with optic neuritis will not have observable abnormalities of the retina or optic nerve because the inflammation of the optic nerve has occurred behind the eye itself (retrobulbar). There is often a “blind-spot” either in the center of vision or diffusely throughout the entire field of vision. A computerized visual field test can quantify the extent of the field of vision that is reduced. Reduced color vision is a common finding and is detected by tests for color-blindness.
What conditions may mimic optic neuritis?
Uncommon symptoms and findings on examination should alert the doctor to search for other causes of visual loss. These other conditions include occlusion of the small arteries supplying blood to the eye, especially in older people with high blood pressure or diabetes; other types of inflammation; infections; tumors compressing the optic nerve; genetic diseases; and other disorders of the retina.
Important signs that may suggest one of these other causes include an unusual time course of visual loss (for example, progressive loss of vision lasting more than two weeks or failure to recover vision within 1 month), the absence of pain, or an eye exam that shows other abnormalities that are not typical for optic neuritis (such as severe swelling of the optic nerve or visible bleeding within the eye).
All patients with optic neuritis should have a brain MRI because the occurrence of optic neuritis is a risk factor for development of multiple sclerosis (MS). Even if there are no symptoms other than the loss of vision, the brain MRI may reveal additional areas of prior inflammation in the brain, separate from the optic nerve, that reflect demyelination at other sites. Experts believe that patients in whom these additional areas of demyelination are found should be treated with medications that reduce the chance of developing symptoms of clinically definite MS.
What is the prognosis for optic neuritis?
The visual loss caused by optic neuritis usually worsens for 7-10 days and then gradually begins to improve between 1-3 months. Fortunately, the prognosis for visual recovery from optic neuritis is very good. Over 90% will regain their vision back to 20/40 or better. On the other hand, there is usually some degree of permanent dysfunction of the optic nerve. For example, even after the recovery of vision to 20/40 or better, there may still be subtle changes in their vision (such as reduced brightness for colors, decreased contrast sensitivity, and reduced depth perception).
Most people with optic neuritis will eventually get diagnosed with MS. Some may be diagnosed immediately while others will be diagnosed years after the onset of optic neuritis. Although there is no cure for MS, there are many very effective treatment options available. As stated above, many neurological conditions do not have effective treatment. MS is one of the few neurological conditions that has gone through tremendous advancement in effective treatment, especially in last 10-20 years. Most people with recently diagnosed MS are doing very well with minimal, if any, disability with proper treatment. One of the ways to assess the progression of MS is by measuring the thickness of the optic nerve head, which is seen in the back of the eye. This is accomplished by a test called optical coherence tomography (OCT), which will be discussed in detail later. OCT is a very reliable and precise machine that can measure the thickness of the optic nerve. Numerous research has shown the progression of MS correlates with how much the optic nerve thins out. For example, more advanced MS gets, more thinning of the optic nerve. Optic nerve is very unique in the sense that it is the only part of the central nervous system we can actually see and measure the thickness directly. Therefore, I perform