Demyelination

Demyelination is a pathological process in which myelinated nerve fibers lose their insulating myelin sheath. Myelin, phagocytosed by microglia and macrophages, and subsequently by astrocytes, is replaced by fibrous tissue (plaques). Demyelination disrupts impulse conduction along the white matter pathways of the brain and spinal cord; peripheral nerves are not affected.

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Demyelinating diseases that cause eye pathology:

• isolated optic neuritis with no clinical manifestations of generalized demyelination, which often develops subsequently;
• multiple sclerosis is the most common;
• Devic's disease (neuromyelitis optica) is a rare disorder that can occur at any age. It is characterized by bilateral optic neuritis and subsequent development of transverse myelitis (demyelination of the spinal cord) within days or weeks;
• Schilder's disease is a very rare, inexorably progressive, generalized disease that begins before age 10 and is fatal within 1 to 2 years. Bilateral optic neuritis may develop without any prospect of improvement.

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Eye manifestations

• lesions of the visual pathway most often affect the optic nerves and lead to optic neuritis. Demyelination can sometimes occur in the chiasm area and very rarely in the optic tracts;
• Brainstem lesions can lead to internuclear ophthalmoplegia and gaze palsies, lesions of the oculomotor, trigeminal and facial nerves and nystagmus.

Association of optic neuritis with multiple sclerosis. Although some patients with optic neuritis do not have a clinically demonstrable associated systemic disease, there is a strong association between the neuritis and multiple sclerosis.

• Patients with optic neuritis and normal brain MRI results have a 16% chance of developing multiple sclerosis within 5 years.
• In the first episode of optic neuritis, approximately 50% of patients with no other symptoms of MS have evidence of demyelination on MRI. These patients are at high risk of developing symptoms of MS within 5 to 10 years.
• Signs of optic neuritis can be found in 70% of cases of established multiple sclerosis.
• In a patient with optic neuritis, the risk of developing multiple sclerosis increases in the winter season, with HLA-OK2 positivity and the UhlolT phenomenon (increased symptoms with increased body temperature, physical exertion or after a hot bath).

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Demyelinating optic neuritis

Presents as subacute monocular visual impairment; simultaneous involvement of both eyes is rare. Discomfort in or around the eye is common and is often aggravated by eye movement. Discomfort may precede or accompany visual disturbances and usually lasts for several days. Some patients experience frontal pain and eye tenderness.

Symptoms

• Visual acuity is usually between 6/18 and 6/60, and may occasionally be reduced to the point of no light perception.
• The optic disc is normal in most cases (retrobulbar neuritis); much less often - a picture of papillitis.
• In the fellow eye, pallor of the temporal part of the disc may be noted, indicating previous optic neuritis.
• Dyschromatopsia is often more severe than would be expected for the level of visual impairment.
• Other manifestations of optic nerve dysfunction as described previously.

Visual field defects

• The most common manifestation is diffuse loss of sensation within the central 30s, followed by altus and/or arcuate defects and central scotomas.
• Scotomas are often accompanied by general depression.
• Changes may also be present in the field of vision of the fellow eye, which does not have obvious clinical signs of the disease.

Course. Vision restoration usually begins in 2-3 weeks and continues for 6 months.

Prognosis. Approximately 75% of patients recover visual acuity to 6/9 or better; 85% recover to 6/12 or better, even if visual acuity is reduced during the acute stage to the point of loss of light perception. Despite recovery of visual acuity, other visual functions (color vision, contrast sensitivity, and light sensitivity) often remain impaired. An afferent pupillary defect may persist and optic nerve atrophy may develop, especially with recurrent disease.

Treatment

Indications

• For moderate vision loss, treatment is probably unnecessary.
• If visual acuity decreases within 1 month of the disease >6/12, treatment can speed up the recovery period by several weeks. This is important in acute bilateral lesions, which are rare, or in poor vision in the fellow eye.

Mode

Intravenous administration of sodium succinate methylprednisolone 1 g per day for 3 days, then oral prednisolone 1 mg/kg per
day for 11 days.

Efficiency

• Slowing down the development of further neurological manifestations characteristic of multiple sclerosis over 2 years.
• Acceleration of vision recovery in optic neuritis, but not to the original level.

Oral steroid monotherapy is contraindicated because it is of no benefit and doubles the recurrence rate of optic neuritis. Intramuscular interferon beta-la during the first episode of optic neuritis is useful in attenuating clinical manifestations of demyelination in patients at high risk of developing multiple sclerosis, as determined by the presence of subclinical brain changes on MRI.

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