What’s Perimetry?

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Perimetry is the exact quantitative assessment of the extent of visible space in a patient’s periphery when he directs his eyes in front of him. Kinetic perimetry involves a moving target, while static threshold perimetry uses fixed points of light of varying intensity. During eye testing, a trained perimetrist or automated system presents stimuli to different parts of the patient’s peripheral field to determine the existence and size of defects in the field. Doctors use perimetry information to aid in early diagnosis and provide long-term monitoring of eye disease or brain injury. Identifiable patterns of visual field loss can help locate the lesion or disease process to the affected part of the eye or brain.

A natural blind spot in the field of vision occurs slightly on the nasal side of center due to the presence of the optic nerve. Diseases affecting the optic nerve, such as multiple sclerosis with associated optic neuritis, typically expand the size of the physiological blind spot. Swelling of the optic nerve secondary to severe hypertension, brain masses, or increased pressure in the skull will also enlarge the blind spot. Optic nerve tumors, such as gliomas or meningiomas, and impingement on the optic nerve by masses, aneurysms, or traumatic injuries can also cause the appearance of this defect.

Ophthalmologists routinely incorporate perimetry into their glaucoma evaluation and treatment regimen. Glaucoma is a condition associated with increased pressure in the eye, progressive damage to the optic nerve, and constriction of the visual fields. The amount of visual field loss is related to the severity of the damage to the optic nerve. Typical visual field defects that occur with glaucoma include nasal defects, called nasal passages, and arcuate defects, which extend circumferentially from the nasal midline 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 crossing of nerve fibers in the optic chiasm, with all fibers of the same visual field traveling to the opposite side of the brain. These defects are called hemianopias. Congruity, the extent to which the field of each eye matches the field of the opposing eye, reflects the part of the brain affected, with more congruous defects indicating a more posterior location of the brain lesion.

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