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Perimetry is the exact, quantitative assessment of the extent of space visible in a patient’s periphery when he directs his eyes straight in front of him. Kinetic perimetry involves a moving target, while static threshold perimetry uses fixed points of light of varying intensity. During eye tests, either a trained perimetrist or an automated system presents the stimuli in different parts of the patient’s peripheral field to determine the existence and size of defects in the field. Physicians use the information from perimetry to facilitate the early diagnosis and provide long-term tracking of eye diseases or brain lesions. Identifiable patterns of visual field loss can help to localize the lesion or disease process to the part of the eye or brain that is affected.
A natural blind spot in the visual field occurs slightly to the nasal side of the center due to the presence of the optic nerve. Diseases that affect the optic nerve, such as multiple sclerosis with associated optic neuritis, typically expand the size of the physiologic blind spot. Optic nerve swelling secondary to severe hypertension, brain masses, or increased pressure in the skull will also widen the blind spot. Optic nerve tumors, such as gliomas or meningiomas, and impingement on the optic nerve by masses, aneurysms, or traumatic injuries may also cause the appearance of this defect.
Ophthalmologists regularly incorporate perimetry into the assessment and care regimen for glaucoma. Glaucoma is a condition associated with increased eye pressure, progressive optic nerve damage, and constriction of the visual fields. The amount of visual field loss is linked to the severity of the optic nerve damage. Typical visual field defects that occur with glaucoma include nasal defects, called nasal steps, and arcuate defects, which extend circumferentially from the nasal mid line to the physiologic blind spot. This pattern of visual field loss coincides with the pattern of nerve fiber layer damage to the retina.
Visual field defects due to lesions in the brain typically affect the same side of the field in both eyes. For example, a stroke that occurs in the left side of the brain will create a defect in the right field of both eyes. This pattern of visual field loss occurs due to the crossing of nerve fibers in the optic chiasma, with all of the fibers from the same field of vision traveling in the opposite portion of the brain. These defects are called hemianopias. Congruity, the extent to which the field from each eye matches the field from the opposite eye, reflects the part of the brain affected, with more congruous defects indicating a more posterior location of the brain lesion.