A visual field test measures the full scope of your vision, encompassing both central and peripheral (side) vision. It aims to detect any areas of dysfunction or “blind spots.” Understanding these results is important for monitoring eye health and identifying potential changes over time.
What a Visual Field Test Measures
During a visual field test, you sit in front of a bowl-shaped instrument called a perimeter. You focus on a central target without moving your eyes. Small lights appear briefly, and you press a button each time you see one. This process maps your entire area of vision.
The test assesses sensitivity to light across central and peripheral vision. Each eye is usually tested separately to map areas of strong or weak vision. The machine tracks which lights you did not see, creating a detailed map.
Key Elements of Your Visual Field Report
Your visual field report contains several components. Reliability indices indicate the accuracy of the test results. These include fixation losses (responding to a light in your blind spot), false positives (pressing the button when no light appeared), and false negatives (missing a light brighter than one previously seen). High percentages in these indices suggest an unreliable test.
The grayscale map visually represents your sensitivity across the visual field. Darker areas on this map indicate less sensitivity or potential blind spots, while lighter areas show better sensitivity. While intuitive, the grayscale map should be used as a starting point and not the sole basis for interpretation, as it can sometimes be misleading.
Numerical displays, often in decibels (dB), show the measured light sensitivity at each tested point. Higher decibel values signify better light sensitivity at that location. Conversely, lower or negative decibel values indicate reduced sensitivity.
The total deviation plot compares your vision to that of age-matched individuals without vision problems. Negative values on this plot indicate areas where your sensitivity is lower than expected for your age group. Solid black squares on the probability plot associated with total deviation suggest a statistically significant abnormality compared to lighter shades.
The pattern deviation plot refines this by highlighting localized vision loss, factoring out any generalized reduction in sensitivity that might be caused by conditions like cataracts or overall aging. This plot is particularly useful for identifying specific patterns of vision loss. It corrects for diffuse dimming across the entire field, allowing for a clearer view of focal defects.
Global indices provide overall summaries of the visual field. Mean Deviation (MD) represents the average overall loss of vision across the entire field, with more negative values indicating greater loss. Pattern Standard Deviation (PSD) quantifies the irregularity or localized nature of vision loss, with higher values suggesting more focal defects. The Visual Field Index (VFI) expresses the visual field status as a percentage compared to a normal age-adjusted field, with 100% being normal.
Interpreting Common Visual Field Patterns
Different patterns of visual field loss offer clues about the underlying cause. Generalized depression involves an overall reduction in light sensitivity across the visual field, manifesting as uniform dimming. This may be associated with conditions like cataracts or early stages of some eye diseases.
Localized defects appear as specific areas of reduced sensitivity. Examples include a nasal step (a horizontal boundary of vision loss on the nasal side) or an arcuate scotoma (an arc-shaped blind spot). These patterns often correspond to damage to specific nerve fiber bundles.
Hemianopia refers to vision loss in half of the visual field. This can affect the same side in both eyes (homonymous hemianopia) or different sides (heteronymous hemianopia). Quadrant loss is a less extensive form where vision is lost in only a quarter of the visual field. These broader patterns are often linked to issues within the brain’s visual pathways.
Understanding the Implications of Your Results
Visual field test results are interpreted by eye care professionals in conjunction with other eye examinations, your medical history, and other diagnostic tests. Your eye care provider analyzes the patterns and indices on your report to understand how your vision is affected.
Abnormal results may lead to further investigations to determine the cause of the vision loss. Your doctor may recommend monitoring your condition over time or developing a management plan. It is important to discuss your specific results with your eye care professional for personalized understanding and guidance on any next steps.