The visual field encompasses the entire area an eye can see, including both the sharp central vision and the broader peripheral vision, when the gaze is fixed on a single point. Visual field testing is a diagnostic tool used to map this full extent of vision, measuring sensitivity across the field to detect blind spots, or scotomas, and areas of reduced sight. It is a fundamental examination for assessing the function of the entire visual pathway, from the eye to the brain.
Clinical Reasons for Visual Field Testing
Specific patterns of vision loss revealed by this testing provide information about the location and type of underlying medical conditions. The most common reason for a visual field test is to monitor conditions that damage the optic nerve, such as glaucoma, which typically causes a progressive loss of side vision.
Visual field testing also plays a significant role in diagnosing and tracking neurological disorders because the visual pathway extends into the brain. Conditions like stroke, multiple sclerosis, and brain tumors, especially those near the pituitary gland, can cause distinct types of field loss. For instance, a tumor pressing on the optic chiasm can produce a bitemporal hemianopia, a specific loss of vision in the outer half of both visual fields. The precise nature of the defect helps pinpoint where the neurological problem lies.
Manual Confrontation Testing
Manual confrontation testing is a quick, qualitative screening method often performed as part of a routine eye or neurological examination. The test compares the patient’s visual field to that of the examiner, who is presumed to have normal sight.
The patient covers one eye and fixes their gaze on the examiner’s corresponding open eye, while the examiner closes their own opposite eye. The examiner then moves a target, such as a finger or a small object, from the periphery toward the center in all four quadrants of the visual field. The patient signals the moment they first see the object, and the examiner checks if this point aligns with their own field of vision. This simple technique is effective for identifying large defects in the peripheral field, but it lacks the sensitivity and quantitative detail of automated methods.
Automated Perimetry Procedure
Automated perimetry is the standard method for detailed and precise mapping of the visual field. The patient sits in front of a large, dome-shaped instrument, known as a perimeter. They are instructed to focus on a central fixation light and to press a response button whenever they see a brief flash of light within the dome.
The machine uses static threshold perimetry, where stationary light stimuli are presented at fixed points. The computer algorithm systematically varies the intensity of the light at each point to find the dimmest light the patient can perceive, called the threshold sensitivity. Modern testing strategies, like the Swedish Interactive Threshold Algorithm (SITA), have significantly reduced the test time, making the procedure more patient-friendly.
During the test, the machine constantly monitors the patient’s eye movement to ensure the gaze remains fixed. If the patient moves their eye, the test may register a fixation loss, which compromises accuracy. The reliability of the test also depends on minimizing false positive errors (pressing the button without a stimulus) and false negative errors (failing to see a bright light previously seen when dimmer).
Interpreting Visual Field Test Results
The results from automated perimetry are presented in a detailed printout containing several plots and numerical indices for interpretation. The primary component is the numerical plot, which displays the measured sensitivity at each tested location in decibels (dB); a higher decibel value indicates better vision.
This numerical data is often translated into a grayscale map, providing a simple visual representation of the field. Areas with normal sensitivity appear lighter, while areas of reduced sensitivity or blind spots are represented by darker shades. While the grayscale map is helpful for patient education, clinical interpretation relies more on the total deviation and pattern deviation plots, which compare the patient’s results to an age-matched normal population.
Key summary indices quantify the overall extent of the damage. The Mean Deviation (MD) represents the average difference between the patient’s field and the normal field; negative values indicate generalized vision loss. The Pattern Standard Deviation (PSD) measures the irregularity of the field, reflecting localized loss, where higher values suggest a more focal defect. Specific patterns of loss, such as arcuate scotomas or a nasal step, suggest damage from glaucoma, while loss respecting the vertical midline points toward a neurological problem.