Neuro-ophthalmology

The visual pathway

Lesions before the chiasm These will produce a field deficit in the ipsilateral eye. Field defects from damage to the optic nerve tend to be central, asymmetrical and unilateral. Visual acuity is often affected. Consider optic neuritis or optic atrophy.

Lesions at the chiasm These classically produce a bitemporal hemianopia as the fibers representing the nasal retina (temporal field) are compressed. The most common lesion is a pituitary tumor.

Lesions after the chiasm These produce homonymous field defects; a lesion in the right optic tract produces left visual field defect. Fibers in the optic tracts gradually rotate until the fibers reach the geniculate body, so lesions in the tract before geniculate body may produce incongruous defect.(the extent of field loss is not similar in both eyes)

Lesions in the optic radiation produce congruous homonymous field defect (the extent of field loss in both eyes is similar). Lesions of the optic tract and radiation are usually either vascular or neoplastic.

Optic neuritis Inflammation or demyelination of the optic nerve . Termed papillitis if the optic nerve head is affected and retrobulbar neuritis if the optic nerve is affected more posteriorly.

Causes : The most common cause is multiple sclerosis. Up to 50% of patients with MS will develop an episode of optic neuritis. Some other causes include : Viral_bacterial infection ( herpes zoster,TB). Other autoimmune disease like lupus.

Vasculitis of vessels nourishing the optic nerve. Drugs like chloramphenicol, ethambutol.

Epidemiology Affects young adults ranging from 18-45 years of age, with a mean age of 30-35. There is a strong female predominance. Prevalence is 115 / 100,000

Symptoms Optic neuritis usually affects one eye, although it may occur in both eyes simultaneously. Optic neuritis symptoms may include: Pain. Most people who develop optic neuritis experience eye pain that's worsened by eye movement. Pain associated with optic neuritis usually peaks within one week and then goes away within several days.

Visual loss. The extent of visual loss associated with optic neuritis varies. Some people experience severe difficulty seeing, while others might not notice any changes in their vision. Vision loss usually develops over the course of a day to two weeks and may be worsened by heat or exercise..

Loss of color vision. Optic neuritis often affects the perception of colors. The patient may notice that the colors of objects, particularly red ones, temporarily appear "washed out" or less vivid than normal.

Examination : Reduced visual acuity. Reduced color vision. Relative afferent pupillary defect. Central scotoma on field testing. a normal disc in retrobulbar neuritis. A swollen disc in papillitis.

Treatment An MRI scan will help to identify additional silent plaques of demyelination. One or more white matter lesions in the brain correlates with a 56% risk of developing clinical MS within 10 years. With no white matter lesions seen, the risk is 22%.

Corticosteroid given IV for few days may speed up visual recovery. After intravenous steroid therapy . Oral prednisone could be taken for about 2 weeks. Oral steroids usually follow an intravenous course of steroids, because using oral steroids alone to treat optic neuritis has been associated with an increased risk of recurrence .

interferon beta-1a and interferon beta-1b are used to prevent or delay the development of multiple sclerosis in people with optic neuritis who have two or more brain lesions evident on MRI scans.

Prognosis The prognosis following optic neuritis is generally good. Most people regain close to normal vision within six months after an episode of optic neuritis.

People with multiple sclerosis may experience recurrent attacks of optic neuritis sometime after they've recovered from the initial episode. Other people without any underlying conditions also may have recurrent optic neuritis. These people have a better prognosis for their vision in the long term than do people with MS.