Optic neuropathy

Optic neuropathy refers to damage to the optic nerve due to any cause.

Causes
Methyl alcohol consumption leads to a toxic optic neuropathy, with rapid deterioration in vision and irreversible blindness if not treated within 24-48 hours. Glaucoma is a special type of optic neuropathy, usually associated with high intraocular pressure (IOP).

Optic neuritis is inflammation of the optic nerve, which causes loss of vision usually due to the swelling and destruction of the myelin sheath covering the optic nerve. Some causes are viral-bacterial infections (e.g. herpes zoster), autoimmune disorders (e.g. lupus), multiple sclerosis and the inflammation of vessels (vasculitis) nourishing the optic nerve.

Signs and symptoms
Major symptoms are sudden or gradual loss of vision (partial or complete). There may or may not be pain on movement of the affected eye. Patients may lose some of their color vision in the affected eye, with colours appearing subtly washed out compared to the other eye.

On medical examination the head of the optic nerve can easily be visualised by an ophthalmoscope. A pale disc is characteristic of optic neuropathy. Primary and secondary optic neuropathy may have different morphological presentation. The nerve head in optic neuritis 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.

Pathophysiology
The primary change occurring in optic neuropathy is nerve cell damage. The optic nerve comprises axons that emerge from the retina of the eye and carry visual information to the primary visual nuclei, most of which is relayed to the occipital cortex of the brain to be processed into vision. Neurons from retina leave the eye at the optic disc, and go to the visual cortex via the optic nervce, optic chiasm, optic tract and optic radiations. Damage and death of these neurons leads to characteristic features of optic neuropathy.