Neurological Significance of the Disorders of Vision – Optic Neuritis, Papilledema and Optic Atrophy

Amaurosis reflects to blindness from any cause. Amblyopia refers to injury or loss of vision which is not due to an error of refiation or to other disease of the eye itself. Visual impairment can be secondary to disease process anywhere along the visual pathway from the cornea to visual cortex.

Disorders affecting the media of the eye are included under ophthalmological disorders. Diseases of retina may cause visual impairment due to involvement of the Peripheral neurones, the rods or cones. Conditions like retinities, chorioretinitis, vascular diseases, retinal detachment and hemorrhages cause patchy or even complete loss of vision depending on the extent of lesion. The optic nerve may be affected by degenerative disorders, infections, neoplasms, toxins, trauma or vascular insufficiency. Such disorders begin as a central scotoma or a partial field defect and proceed to complete visual loss in that eye.

A lesion in the chiasma produces a characteristic field defect depending on whether the lesion begins at the center of the chiasm or from the outer aspect. The former produces betemporal hemianopsia, eg, Optic nerve glioma. The latter produces binasal hemianopsia, usually due to extrinsic compression by tumor. Lesions in the Optic tract produce contralateral hemianopsia, with absent pupillary light reflex. However, if the pupillary light reflex is preserved then the lesion is either at the geniculate body or in the optical radiation. Involvement of the lower fibers of the optic radiation coursing in the temporal lobe causes contralateral superior quadrantic hemianopsia, and affection of the upper fibers in the parietal lobe causes contralateral inferior quadrantic hemianopsia. Lesions of one occipital lobe cause contralateral hemianopsia with intact pupillary light reflex. Involvement of the upper lip of the calcarine fissure causes inferior quadrantic hemianopsia, and that of the lower lip of the calcarine fissure causes superior quadrantic hemianopsia, and that of the lower lip of the clacarine fissure causes superior quandrantic hemianopsia. Incomplete lesions of the optic tract and radiation usually partial central (macular) vision. A lesion of the tip of one occipital lobe produces a central homosexual hemianopic scotoma since half the macular fibers from both eyes terminate there. Lesions of bot occipital poles as in bilateral posterior cerebral artery embolism cause bilateral central scotoma, and bilateral lesions of the entire calcarine cortex cause cortical blindness.

Optical neuritis : In this, there is unilateral or bilateral loss of vision. It may be due to demyelination of toxic damage to the optic nerve. Common causes include toxic agents such as methyl alcohol, nicotine, lead, quinine, ethambutol or demyelinating diseases such as multiple sclerosis and neuromyelitis optica (a combination of optic neuritis and transverse myelitis). The condition is associated with pain behind the eyeball. When the under cause cause is removed or treated, the vision typically recovers, but some cases may lead to inflammation of the optic disc (papillitis) and optic atrophy.

Papilledema : This is swelling of the optic disc as seen through the ophthalmoscope. The margins of the disc are indistinct. The physiological cup is filled. The medical margin of the disc shows more marked changes. The veins are congested.

Causes : Increased intracranial tension, malignant hypertension, disease of the retinal arteries such as giant cell arteritis, retinal vein thrombosis, cavernous sinus thrombosis, hematological disorders like acute leukemia and plycythemia vera, emphysema and pre-eclamptic toxemia. Visual loss consistants of concentric reduction of the field of vision and enlargement of the blind spot.

Optic atrophy The Ophthalmoscopic appearance of optic atrophy consists of pallor of the optic disc, with a punched out appearance, prominence of the lamina cribrosa and narrowing of the arteries. Optic atrophy may be "primary" or "secondary" and the fundoscopic findings are distinct. In primary optical atrophy, no other lesions is detectable on funduscopy. the lamina cribrosa is less distinct .. The term "consonic optical atrophy" is used when the Optic atrophy results from retinal diseases.

Causes : All causes of retrobulbar neuritis may lead to optic atrophy as well. Other rarer causes include pressure on the optic nerve or optic chiasm, occlusion of the central retinal artery and injury to the optic nerve. Primary optical atrophy occurs in association with hereditary cerebellar ataxias, Oras an isolated abnormality. Optic atrophy leads to gross dimunition of vision, leading to total blindness.