The span of one’s space around him that falls within the vision of the subject at one given time is called field of vision. Visual field span is in all 4 directions but to a different extent in each direction. Its temporal extent is 90 degrees, inferior extent is 70 degrees, nasal extent is 60 degrees and superior extent is 50 degrees. Any spots of blindness or impaired vision whether located in the center (due to defect or disease in optic disc or nerve) or periphery (due to defect in visual pathways from optic chiasm to proximally onward to visual cortex) of field of vision are called visual field defects. A visual field defect can be the reflection of a host of underlying conditions same as the tip of an iceberg. Here we will describe how to interpret various meanings of different types of visual field defects.
1. Arrangement of Visual Pathways:
I. Image Formation:
Inverted image is formed onto the retina after passing through lens.
Macula is a part of the retina that is responsible for central vision. Fibers from macula are located in the temporal/outer part of optic nerve when it exits the retina, while these fibers migrate to the central part of the optic nerve by the time it reaches optic chiasm.
III. The Crossing Over of Visual Fibers:
When the Optic nerve reaches the optic chiasm, fibers from its inner/nasal side cross over to the opposite side, while fibers from outer/temporal side continue on the same/ipsilateral side so that a combination nasal fibers from opposite side and temporal fibers from ipsilateral side combine o make optic tract. This arrangement results in information from same side of visual field passing onto same optic tract then geniculate body then optic radiation and lastly to the visual cortex. This arrangement causes left visual hemifield to project in right side of visual pathway beyond optic chiasm and vise versa is true for right hemifield.
2. Lesions at The Level of Retina:
Visual field defects with boundaries in horizontal plane are observed when there is retinal detachment or when any artery or vein other than central retinal artery or vein is occluded.
I. Retinal Detachment:
Retinal detachment is sudden in onset and quick in progression. It begins with visible floaters or blinking lights following trauma or any of the predisposing factors such as diabetes mellitus and hypertension. Retinal detachment can be seen on slit lamp in the form of a red or orange colored detached slip at the peripheral retina.
II. Retinal Artery Occlusion:
Sudden and complete visual loss in one eye results from occlusion of central retinal artery. If only one of the 4 arteries gets blocked then there will be loss of vision in only one quadrant. On examination there will be poor blood supply to retina with invisible artery and pale look to the retina.
III. Retinal Vein Occlusion:
Retinal vein occlusion has almost similar clinical expression as that of retinal artery occlusion however on examination retina gives a different look. There are scattered hemorrhages in the retina with cotton wool spots. If the retinal vein blockage is incomplete then hemorrhages will be few and far between.
IV. Age Related Macular Degeneration (AMD):
AMD progresses gradually in elderly people initially involving only Macula and hence central field of vision leading to involvement of periphery very late in the age.
There are various drugs that have adverse effect on retina leading to visual field defects. Examples include chloroquine and vigabatrin.