Visual field
The visual field is the "spatial array of visual sensations available to observation in introspectionist psychological experiments". Or simply, visual field can be defined as the entire area that can be seen when an eye is fixed straight at a point.
The equivalent concept for optical instruments and image sensors is the field of view.
In optometry, ophthalmology, and neurology, a visual field test is used to determine whether the visual field is affected by diseases that cause local scotoma or a more extensive loss of vision or a reduction in sensitivity.
Normal limits
The normal human visual field extends to approximately 60 degrees nasally from the vertical meridian in each eye, to 107 degrees temporally from the vertical meridian, and approximately 70 degrees above and 80 below the horizontal meridian.The binocular visual field is the superimposition of the two monocular fields. In the binocular field, the area left of the vertical meridian is referred to as the left visual field ; a corresponding definition holds for the right visual field. The four areas delimited by the vertical and horizontal meridian are referred to as upper/lower left/right quadrants. In the European Union, the minimum field requirement for driving is 50 degrees to either side of the vertical meridian and 120 degrees horizontally in total, and 20 degrees above and below the horizontal meridian. The macula corresponds to the central 17 degrees diameter of the visual field; the fovea to the central 5.2 degrees, and the foveola to 1-1.2 degrees diameter.
The nose is situated in the field of view of both eyes, but due to later processing carried out in the brain, it is not noticed during normal visual tasks.
Measuring the visual field
The visual field is measured by perimetry. This may be kinetic, where spots of light are shown on the white interior of a half sphere and slowly moved inwards until the observer sees them, or static, where the light spots are flashed at varying intensities at fixed locations in the sphere until detected by the subject. Commonly used perimeters are the automated Humphrey Field Analyzer, the Heidelberg Edge Perimeter, or the Oculus.Another method is to use a campimeter, a small device with a flat screen designed to measure the central visual field.
Light spot patterns testing the central 24 degrees or 30 degrees of the visual field, are most commonly used. Most perimeters are also capable of testing up to 80 or 90 degrees.
Another method is for the practitioner to hold up 1, 2 or 5 fingers in the four quadrants and center of a patient's visual field. If the patient is able to report the number of fingers properly as compared with the visual field of the practitioner, the normal result is recorded as "full to finger counting". The blind spot can also be assessed via holding a small object between the practitioner and the patient. By comparing when the object disappears for the practitioner, a subject's blind spot can be identified. There are many variants of this type of exam.
Visual field loss
Visual field loss may occur due to many disease or disorders of the eye, optic nerve, or brain. For the eye, e.g., Glaucoma causes peripheral field defects. Macular degeneration and other diseases affecting the macula cause central field defects. Lesions of the visual pathway cause characteristic forms of visual disturbances, including homonymous hemianopsia, quadrantanopsia, and scotomata.The main classification of visual field defects is into
- Lesions to the eye's retina
- Lesions of the optic nerve
- Lesions in the chiasm
- Lesions after the chiasm
- Altitudinal field defects, loss of vision above or below the horizontal meridian – associated with ocular abnormalities
- Central scotoma, loss of central vision
- Peripheral field loss including tunnel vision
- Generalized depression of the entire field of vision
Visual field defects in glaucoma
Following are the common glaucomatous field defects:
- Generalized depression: Generalized depression is seen in early stages of glaucoma and many other conditions. Mild constriction of the central and peripheral visual field due to isopter contraction comes under generalized depression. If all the isopters show similar depression to the same point, it is called a contraction of the visual field. Relative paracentral scotomas are the areas where smaller and dimmer targets are not detected by the patient. Larger and brighter targets can be seen. Small paracentral depressions, mainly superonasal, occur in normal-tension glaucoma. The generalized depression of the entire field may occur in cataract also.
- Baring of blind spot: Baring of the blind spot means the exclusion of the blind spot from the central field due to inward curve of the outer boundary of the 30° central field. It is only an early, non-specific visual field change, without much diagnostic value in glaucoma.
- Small wing-shaped Paracentral scotoma: Small wing-shaped Paracentral scotoma within Bjerrum’s area is the earliest clinically significant field defect seen in glaucoma. It may also be associated with nasal steps. Scotoma may be seen above or below the blind spot.
- Siedel’s sickle-shaped scotoma: Paracentral scotoma joins with the blind spot to form Siedel’s sickle-shaped scotoma.
- Arcuate or Bjerrum’s scotoma: This kind of scotoma is formed at later stages of glaucoma by extension of Seidel’s scotoma in an area either above or below the fixation point to reach the horizontal line. Peripheral breakthrough may occur due to damage of nerve fibers.
- Ring or Double arcuate scotoma: Two arcuate scotomas join together to form a Ring or Double arcuate scotoma. This defect is seen in advanced stages of glaucoma.
- Roenne’s central nasal step: It is created when two arcuate scotomas run in different arcs to form a right angled defect. This is also seen in advanced stages of glaucoma.
- Peripheral field defects: Peripheral field defects may occur in early or late stages of glaucoma. Roenne’s peripheral nasal steps occur due to contraction of peripheral isopter.
- Tubular vision: Since macular fibers are the most resistant to glaucomatous damage, central vision remains unaffected until the end stages of glaucoma. It results in Tubular vision, or Tunnel vision, by the loss of peripheral vision with retention of central vision, resulting in a constricted circular tunnel-like field of vision. Retinitis pigmentosa, is another disease that causes tubular vision.
- Temporal island of vision: It is also seen in end stages of glaucoma. The temporal islands lie outside of the central 24 to 30° visual field, so it may not be visible with standard central field measurements done in glaucoma.
Field defects in macular degeneration (AMD)
Field defects in visual pathway lesions
The visual pathway consists of structures that carry visual information from the retina to the brain. Lesions in the pathway cause a variety of visual field defects. The type of field defect can help localize where the lesion is located.- A lesions in the optic nerve of one eye causes partial or complete loss of vision in the same eye, with an intact field of vision in other eye.
- A lesion at the proximal part of optic nerve of one eye cause, central field defect in one eye and temporal half-field defect in the other eye.
- A lesion in the center of the optic chiasma causes bitemporal hemianopia
- A lesion to the optic tract, or involving the complete optic radiation causes homonimous hemianopia
- When part of the optic radiation in the parietal lobe is affected, the resulting field defect may be inferior quadrant hemianopia
Neurological Causes of Visual Field defects
- Cerebral palsy: Field loss due to cerebral palsy may be hemianopic affecting even upper or lower half, central scotoma, peripheral scotoma, or islands of vision.
- Epilepsy: Although field defects are rarely associated with epilepsy, defects may occur after temporal lobe surgery for epilepsy.
- Periventricular leukomalacia : Bilateral inferior visual field defects may occur due to damage of the optic radiations
Other causes of field defects
- Generalized depression is seen in cataract
- Central visual field loss is seen in Optic neuritis, Leber hereditary optic neuropathy, Macular hole, Cone dystrophy, Branch retinal artery occlusion etc.
- Peripheral visual field loss including tunnel vision is seen in Retinal detachment, Retinitis pigmentosa, Branch retinal vein occlusion etc.
- Hemianopia and quadrantanopia are seen in patients with stroke, traumatic brain injury, tumor etc.
- Altitudinal visual field defect is seen anterior ischemic optic neuropathy, compressive neuropathy, Branch retinal artety occlusion, Branch retinal vein occlusion, Coloboma, Papilloedema, lesions in visual cortex etc.
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