Perimetry testing, often called a visual field test, is a non-invasive eye examination. It measures peripheral vision and sensitivity across various regions, providing a detailed map of the entire area a person can see while looking straight ahead.
Purpose of Perimetry Testing
Perimetry testing identifies and quantifies vision loss. It helps determine the extent to which various health conditions impact the visual system. It also detects specific patterns of vision loss that can indicate eye diseases or neurological issues.
The examination also plays a role in monitoring the progression of diagnosed conditions over time. By comparing current visual field maps with previous ones, healthcare providers can assess if a disease is stable, worsening, or responding to treatment. This ongoing assessment helps guide treatment adjustments and manage conditions that affect vision.
Understanding the Testing Process
During a perimetry test, a patient sits in front of a bowl-shaped instrument called a perimeter, resting their chin and forehead on supports. One eye is covered, and the patient focuses on a central target point within the dome.
Small lights or objects appear in different areas of the peripheral vision, and the patient presses a button or otherwise signals each time they see a flash or object. A computer system records the patient’s responses, mapping out areas of visible and non-visible light, which creates a comprehensive record of their visual field.
Different types of perimetry tests include:
- Static perimetry: Stationary lights of varying brightness are presented at fixed locations.
- Kinetic perimetry: Involves a moving light stimulus, typically moved from the periphery towards the center of vision until it is detected.
- Amsler grid: A simpler test, often a printed grid with a central dot, used to detect central vision defects by observing distortions or missing lines.
- Confrontation visual field tests: Basic screenings where the examiner moves their hand or fingers into the patient’s peripheral vision, asking them to report when they see it.
Interpreting Visual Field Results
The results of a perimetry test are presented as visual field maps or charts, representing the sensitivity of a patient’s vision across their field of view. These printouts often include a grayscale map, where darker areas indicate reduced sensitivity or vision loss, and lighter areas represent normal vision.
Numerical plots on the printout show the threshold sensitivity in decibels (dB) for each tested point, with higher numbers indicating better sensitivity. Healthcare providers also analyze global indices such as Mean Deviation (MD), which represents the average difference between the patient’s overall visual field sensitivity and that of a normal, age-matched population. Pattern Deviation plots help identify localized areas of vision loss by correcting for any general reduction in sensitivity, such as that caused by cataracts. These patterns, including blind spots or areas of reduced sensitivity, help medical professionals diagnose conditions, monitor disease progression, and make informed decisions about treatment strategies.
Perimetry Testing and Specific Eye Conditions
Perimetry testing is valuable in diagnosing and managing several specific eye and neurological conditions. In glaucoma, a progressive eye disease, perimetry helps detect characteristic patterns of nerve damage, such as arcuate scotomas or nasal steps, which are indicative of optic nerve fiber loss. Regular perimetry tests enable monitoring of glaucoma progression and evaluation of treatment effectiveness in preventing further vision loss.
Following a stroke, visual field defects, such as homonymous hemianopia (loss of vision in the same half of the visual field in both eyes), are common. Perimetry maps these defects, helping to identify the location of brain injury affecting the visual pathways. While the Amsler grid is commonly used for macular degeneration to assess central vision loss, standard automated perimetry can also detect reductions in overall visual field indices in early and intermediate stages of the disease. For multiple sclerosis, perimetry can identify visual field abnormalities, including optic neuritis-related defects and homonymous visual field defects, providing objective data on visual pathway involvement. This detailed mapping helps track disease activity and the impact on vision in these diverse conditions.