Optic Neuritis

Optic neuritis is the inflammation of the optic nerve that may cause a complete or partial loss of vision. When it is associated with a swollen optic disc, it is called papillitis or anterior optic neuritis. When the optic disc appears normal, the terms retrobulbar optic neuritis or retrobulbar neuritis are used. Acute optic neuritis is by far the most common type of optic neuritis that occurs throughout the world and is the most frequent cause of optic nerve dysfunction in young adults.

Demographics

The annual incidence of acute optic neuritis is estimated in population-based studies to be between 1 and 5 per 100,000. Optic neuritis typically affects young adults ranging from 18–45 years of age, with a mean age of 30–35 years. There is a strong female predominance.

Etiology

The most common etiology is multiple sclerosis (MS). Up to 50% of patients with MS will develop an episode of optic neuritis, and 20-30% of the time optic neuritis is the presenting sign of MS. The presence of demyelinating white matter lesions on brain MRI at the time of presentation of optic neuritis is the strongest predictor for developing clinically definite MS. Almost half of the patients with optic neuritis has white matter lesions consistent with multiple sclerosis. At five years follow-up, the overall risk of developing MS is 30%, with or without MRI lesions. Patients with a normal MRI still develop MS (16%), but at a lower rate compared to those patients with three or more MRI lesions (51%). From the other perspective, however, almost half (44%) of patients with any demyelinating lesions on MRI at presentation will not have developed MS ten years later.

Some other causes of optic neuritis include infection (e.g. Syphilis, Lyme disease, herpes zoster), autoimmune disorders (e.g. lupus), Inflammatory Bowel Disease, drug induced (e.g. chloramphenicol, Ethambutol) vasculitis and diabetes.

Symptoms

Major symptoms are sudden loss of vision (partial or complete), or sudden blurred or “foggy” vision, and pain on movement of the affected eye. Many patients with optic neuritis may lose some of their color vision in the affected eye (especially red), with colors appearing subtly washed out compared to the other eye. A study found that 92.2% of patients experienced pain, which actually preceded the visual loss in 39.5% of cases.

EXAMINATION

Opthalmological

On medical examination the head of the optic nerve can easily be visualised by an ophthalmoscope; however frequently there is no abnormal appearance of the nerve head in optic neuritis, though it may be swollen in some patients. In many cases, only one eye is affected and patients may not be aware of the loss of color vision until the doctor asks them to close or cover the healthy eye. A relative afferent pupillary defect is demonstrable with the swinging flashlight test in all unilateral cases of optic neuritis. When such a defect is not present, there is either a coexisting optic neuropathy in the fellow eye (e.g., from previous or concurrent asymptomatic optic neuritis) or the visual loss in the affected eye is not caused by optic neuritis or any other form of optic neuropathy About one third of patients with acute optic neuritis have some degree of disc swelling (Figure 1). The optic disc may be slightly or markedly blurred; however, the degree of disc swelling does not correlate with the severity of either visual acuity or visual field loss.

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Figure 1. Color photograph of a patient with acute anterior optic neuritis (papillitis).
Note swelling of the optic disc.

Disc or peripapillary hemorrhages and segmental disc swelling are less common in eyes with acute optic neuritis than in eyes with anterior ischemic optic neuropathy. The majority of patients with acute optic neuritis have a normal optic disc in the affected eye unless they have had a previous attack of acute or asymptomatic optic neuritis. Over approximately 4 to 6 weeks, the optic disc in an eye with acute optic neuritis may become or remain normal or become pale, even as the visual acuity and other parameters of vision improve. The pallor may be diffuse or located to a particular portion of the disc, most often the temporal region (Figure 2).

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Figure 2. Color photograph of a patient with an optic nerve atrophy.
Note pallor of the of the optic disc, especially temporal.

Visual Fields

Visual field loss can vary from mild to severe, may be diffuse or focal, and can involve the central or peripheral field. Indeed, almost any type of field defect can occur in an eye with optic neuritis, including central and cecocentral scotomas, altitudinal and arcuate defects, diffuse field loss, and even hemianopic defects.

MRI examination

Thin (2-3 mm) fat-suppressed T2-weighted images, through the optic nerves may show characteristic high-signal intensity foci in the minimally or non expanded nerve. These lesions frequently enhance following intravenous contrast administration, which is not seen in a healthy optic nerve.

The real contribution of imaging in the setting of optic neuritis is made by imaging the brain, not the optic nerves themselves. This is due to the fact that the most valuable predictor for the development of subsequent multiple sclerosis is the presence of white-matter abnormalities (Figure 3). From 27 to 70% (in various studies) of patients with isolated optic neuritis show abnormal MRI findings of the brain, as defined by 2 or more white-matter lesions on T2-weighted images.

mri
Figure 3. Magnetic resonance imaging shows multiple white-matter lesions

The role of cerebrospinal fluid (CSF) analysis in the evaluation of patients with acute optic neuritis is not clear. Although the presence of oligoclonal banding in the CSF is associated with the development of MS, the powerful predictive value of brain MRI for MS has reduced the role of lumbar puncture in the evaluation of patients with optic neuritis. Lumbar puncture can help define a very low-risk population for MS if both CSF and MRI are normal. CSF studies in patients with optic neuritis are mostly useful to detect another inflammatory or infectious disorder.

Treatment and prognosis

Much of our knowledge regarding this form of optic neuritis was obtained from the Optic Neuritis Treatment Trial (ONTT). The investigators in this trial enrolled 455 patients with acute unilateral optic neuritis and followed them for 15 years.

In the Optic Nerve Treatment Trial, the 5-year risk of developing multiple sclerosis was 16% in patients with normal brain MRI findings, 37% with 1-2 lesions, and 51% with 3 or more lesions. At 10 years, the only statistically significant difference was between no lesions (22% risk) and one or more lesions (56% risk).

Visual functions return to near normal within eight to ten weeks, but they may also advance to a complete and permanent state of visual loss. Therefore, systemic intravenous treatment with corticosteroids, which may quicken the healing of the optic nerve, is often recommended, but it does not have a significant effect on the visual acuity at one year, when compared against placebo. Intravenous corticosteroids have also been found to reduce the risk of developing MS in the following two years in those patients who have MRI lesions; but this effect disappears by the third year of follow up.

Paradoxically it has been demonstrated that oral administration of corticosteroids in this situation may lead to more recurrent attacks than in non-treated patients (though oral steroids are generally prescribed after the intravenous course, to wean the patient off the medication). This effect of corticosteroids seems to be limited to optic neuritis and has not been observed in other diseases treated with corticosteroids.

In a study using interferon beta-1a (Avonex) in patients with optic neuritis with 2 or more white-matter lesions on MRIs of the brain, a decreased risk of developing multiple sclerosis at 3 years was demonstrated.