Peripheral Vision and the Visual Pathway
Vision is a very complex task that begins in the eye and ends in visual cortex located in the back end of the brain. For visual information to get to the back of the head it must take a long and complex course. If there is an obstruction along the way, a scotoma results. A scotoma is a missing piece of the visual puzzle. Based on the size, shape and location of the missing vision we can predict the location of abnormal brain function.
Normal peripheral vision consists of a 150 degree view for an individual eye and 180 degrees for both eyes working together. Peripheral vision can be roughly estimated by counting fingers at the edge of the visual field (confrontations) or more precisely with a small beam of light at random spots in the visual field (automated visual field testing. Different structures in the eye can influence peripheral vision testing.
Cataracts or a corneal condition can dim vision and only allow patients to see brighter test points. Retinal defects produce a scotoma opposite their location. This means a superior retinal scar will cause a loss of vision in the inferior portion of one's vision (visual field). Optic nerve problems work the same way. Glaucoma damaging the inferior part of the optic nerve will cause superior visual field loss.
Visual information begins its pathway into the brain via the optic nerve. Glaucoma is the most common defect of the optic nerve. The most common tumor of the optic nerve is a glioma. Glaucoma causes a partial scotoma in one eye while gliomas often cause complete vision loss in one eye. The optic nerve separates its nasal and temporal visual information before reaching the optic chiasm.
A lesion before the optic chiasm can produce a junctional scotoma. For a lesion on the right before the chiasm. There is a central scotoma on the right and a temporal lesion on the left visual field. This is due to a structure called Wilbrand's knee. In Wilbrand's the optic nerve fibers from the nasal retina of the opposite (left) eye meet the central optic nerve fibers of the other (right) eye before the chiasm.
The optic chiasm is where temporal visual field or nasal retinal fibers of the two eyes cross. A tumor in this area creates a bitemporal scotoma. If more inferior than superior visual information is missing the tumor is more likely to be a craniopharyngioma. If superior vision is affected more a pituitary tumor is more likely to be present. Once we reach the chiasm, obstructions in the visual pathway cause vision loss in both eyes.
Past the optic chiasm are the optic tracts. A stroke or tumor is the most likely cause of vision loss in these areas. Scotomas in this area have a unique pattern. Vision loss is incongruous between the eyes or uneven. The shape of the area of vision loss is different between the eyes. Inferior scotomas indicate the area of damage is more likely to be in the parietal lobe. Superior scotomas are more often due to temporal lobe damage.
Visual information ends its journey in the optic radiations. Lesions are usually a combination of occipital with parietal and/or temporal lobe damage. The more congrous (same size) the scotomas are the further into the occipital lobe they are. It is very rare to have central vision loss due to a stroke. This is because the occipital cortex devoted to the macula (center of vision) has two blood supplies (middle and posterior cerebral arteries).
Summary
Normal peripheral vision is 150 degree view in one eye and 180 for both eyes.
Retina and optic nerve damage decrease peripheral vision of one eye.
Strokes and brain tumors can cause peripheral vision loss in both eyes.
The pattern of peripheral vision loss can locate brain damage from a stroke or tumor
Central vision is preserved during a stroke by a dual blood supply
Normal peripheral vision consists of a 150 degree view for an individual eye and 180 degrees for both eyes working together. Peripheral vision can be roughly estimated by counting fingers at the edge of the visual field (confrontations) or more precisely with a small beam of light at random spots in the visual field (automated visual field testing. Different structures in the eye can influence peripheral vision testing.
Cataracts or a corneal condition can dim vision and only allow patients to see brighter test points. Retinal defects produce a scotoma opposite their location. This means a superior retinal scar will cause a loss of vision in the inferior portion of one's vision (visual field). Optic nerve problems work the same way. Glaucoma damaging the inferior part of the optic nerve will cause superior visual field loss.
Visual information begins its pathway into the brain via the optic nerve. Glaucoma is the most common defect of the optic nerve. The most common tumor of the optic nerve is a glioma. Glaucoma causes a partial scotoma in one eye while gliomas often cause complete vision loss in one eye. The optic nerve separates its nasal and temporal visual information before reaching the optic chiasm.
A lesion before the optic chiasm can produce a junctional scotoma. For a lesion on the right before the chiasm. There is a central scotoma on the right and a temporal lesion on the left visual field. This is due to a structure called Wilbrand's knee. In Wilbrand's the optic nerve fibers from the nasal retina of the opposite (left) eye meet the central optic nerve fibers of the other (right) eye before the chiasm.
The optic chiasm is where temporal visual field or nasal retinal fibers of the two eyes cross. A tumor in this area creates a bitemporal scotoma. If more inferior than superior visual information is missing the tumor is more likely to be a craniopharyngioma. If superior vision is affected more a pituitary tumor is more likely to be present. Once we reach the chiasm, obstructions in the visual pathway cause vision loss in both eyes.
Past the optic chiasm are the optic tracts. A stroke or tumor is the most likely cause of vision loss in these areas. Scotomas in this area have a unique pattern. Vision loss is incongruous between the eyes or uneven. The shape of the area of vision loss is different between the eyes. Inferior scotomas indicate the area of damage is more likely to be in the parietal lobe. Superior scotomas are more often due to temporal lobe damage.
Visual information ends its journey in the optic radiations. Lesions are usually a combination of occipital with parietal and/or temporal lobe damage. The more congrous (same size) the scotomas are the further into the occipital lobe they are. It is very rare to have central vision loss due to a stroke. This is because the occipital cortex devoted to the macula (center of vision) has two blood supplies (middle and posterior cerebral arteries).
Summary
Normal peripheral vision is 150 degree view in one eye and 180 for both eyes.
Retina and optic nerve damage decrease peripheral vision of one eye.
Strokes and brain tumors can cause peripheral vision loss in both eyes.
The pattern of peripheral vision loss can locate brain damage from a stroke or tumor
Central vision is preserved during a stroke by a dual blood supply
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