Optic Neuritis (ON) is an inflammation, with accompanying demyelination, of the Optic Nerve (Cranial Nerve II) serving the retina of the eye. It is a variable condition and can present with any of the following symptoms: blurring of vision, loss of visual acuity, loss of some or all colour vision, complete or partial blindness and pain behind the eye.
Optic Neuritis is one of the most frequently presenting symptoms of multiple sclerosis, although there are other causes. Indeed, ON is the most common symptom at onset of MS and presents unilaterally (in one eye only) in 70% of cases. The majority of all cases of ON, MS-related or not, are caused by demyelination.
The minority (35%) of children presenting with ON go on to develop MS. Similarly, ON in adults aged over 50, for whom it is their first neurological symptom, is rarely associated with MS. Indeed, for this group, ON is often a misleading diagnosis and the cause is often ischemic optic neuropathyor some other condition.
Most typically, Optic Neuritis first affects people aged between 15 and 50 years of age. In this age group, studies indicate that more than 50% of patients will convert to Multiple Sclerosis within 15 years. However, the results of individual studies vary greatly, coming up with conversion rates from 33% to 71% and higher. As with MS, women are about twice as likely as men to present with ON and the prevalence in Caucasian peoples is higher than in other racial groups.
The main symptoms of Optic Neuritis are:
1. Loss of visual acuity (blurring of vision) which occurs in around 58% of ON cases. This can range from mild blurring of vision in 34% of cases, through moderate loss of acuity (12%), to severe or total loss of light perception (complete blindness) in 54% of cases.
2. Eye pain occurs in 53% to 88% of ON presentations. In one study examining the pain associated with ON, 12% of cases reported no associated pain, 21% patients reported that the pain was only noticeable with eye movement, in 16% the pain preceded a loss of visual acuity, in 22% it occurred as a general eye ache, in 16% it was reported as a headache centering on the affected eye and in 13% it was reported as a generalised dull headache [Nikoskelainen, E. 1975].
3. Dyschromatopsia (reduced colour vision) occurs in 100% of optic neuritis cases. Typically this is reported as a reduction in colour vividness, particularly reds. An Ishihara colour chart and/or Farnsworth-Munsell hue tests are often used to evaluate the degree of dyschromatopsia. If the inflammation is not visible on the macular (back of the retina), then dyschromatopsia is the most sensitive clinical diagnostic measure of optic neuritis.
4. Movement and sound phosphenes (visual flashing sensations brought about by side-to-side eye movement or sound) often occur with optic neuritis. They are most obvious in a dimly lit room. Obscuration of the visual field in bright light is another common symptom of optic neuritis and many people with ON, particularly those with a chronic condition, report that they see better in dimly lit rooms. It is likely that both of these symptoms are caused by fluctuating interference of the nerve transmissions along the visual pathways.
5. Uhthoff's symptom, the worsening of symptoms with heat or exhaustion, is present in about 58% of cases of Optic Neuritis.
Visually Evoked Potential (VEP, VER) tests, which detect the speed of nerve transmissions down the optic nerve, are used as a diagnostic test for optic neuritis.
The rate of onset of Optic Neuritis varies from a few hours to few days and occasionally greater. Typically sight is at its worst around one week after onset of symptoms.
Recovery from ON is usually very good, often complete and even in the worst cases patients seldom fail to recover some sight. In around 65-80% of patients, the sight recovers to 20/30 or better, although many report some enduring visual deficits.
Reoccurrence of Optic Neuritis in one or other eye is seen in about 33% of cases. Reoccurrence is more likely in people with Uhthoff's symptom(48%).
The inflammation associated with Optic Neuritis can often be observed with an opthalmoscope on the back of the eye (macular) provided that the inflammation is not further back along the optic nerve (retrobulbar optic neuritis).
Cortico-Steroids (oral prednisone and IV methylprednisolone) have been shown to significantly increase the rate of the recovery from optic neuritis. Continued use of steroids for around 3 years has been shown to delay the reoccurrence of ON and conversion to multiple sclerosis [Beck, R. W., 1995]. However, such studies have shown that after 4 years of use, they have no long term effect on either the reoccurrence of ON nor on conversion to MS. Indeed one study has shown oral prednisone treatments to actually increase the long-term probability of reoccurrence. It should also be remembered that cortico-steroids, particularly their continued use, have significant side effects.
COP-1 (Copaxone) and beta interferons have both been shown to reduce the probability and severity of reoccurrences of optic neuritis in addition to the other symptoms of multiple sclerosis.
