What is Automated Static Threshold Perimetry?
Automated Static Threshold Perimetry, often simply called a visual field test or perimetry, is a sophisticated diagnostic procedure used in ophthalmology to map the entire area of vision, including central and peripheral (side) vision. It's a crucial tool for detecting and monitoring conditions that affect the visual pathway, most notably glaucoma, but also optic neuropathies, retinal diseases, and neurological conditions that impact vision.
Table of Contents
Definition of Automated Static Threshold Perimetry
Automated static threshold perimetry is a modern, computer-assisted technique used to assess the visual field — the entire area that can be seen when the eye is focused on a single point.
✅ Automated: The test is performed by a computer-controlled machine (like the Humphrey Field Analyzer, which is very common). This automation ensures standardization of stimulus presentation and recording of responses, leading to more reproducible results compared to older manual methods.
✅ Static: Unlike kinetic perimetry (which uses a moving light stimulus), static perimetry presents stationary light stimuli at specific, predetermined locations within the visual field.
✅ Threshold: The primary goal is to determine the "threshold" of vision at each tested point. The threshold is defined as the dimmest light intensity that a person can see 50% of the time at a particular location. This provides a quantitative measure of sensitivity across the visual field.
✅ Perimetry: Refers to the systematic measurement of the visual field.
Automated Static Threshold Perimetry is now considered the gold standard in clinical perimetry and plays an essential role in diagnosing and monitoring a variety of ocular and neurological diseases, particularly glaucoma.(alert-passed)
Clinical Applications of Automated Static Threshold Perimetry
Automated static threshold perimetry is a cornerstone in:
1. Glaucoma diagnosis and monitoring: Detects early functional loss, tracks progression, and correlates with optic nerve changes.
2. Neuro-ophthalmology: Helps identify and localize visual pathway lesions (e.g., strokes, tumors).
3. Retinal disease: Detects localized retinal sensitivity loss, such as in age-related macular degeneration.
4. Occupational screening: Assesses whether the visual field meets legal or professional standards.
It provides quantitative data, making it possible to detect subtle changes over time.
Types of Tests and Machines in Automated Static Threshold Perimetry
Among the most widely used and trusted machines are the Humphrey Field Analyzer (HFA), the Octopus Perimeter, and the Medmont Perimeter. Each of these devices offers its own set of test strategies, algorithms, and reporting formats, but they all share the common goal of providing an accurate, quantitative assessment of the visual field.
A. Humphrey Field Analyzer (HFA)
The Humphrey Field Analyzer, manufactured by Carl Zeiss Meditec, is considered the gold standard in clinical perimetry worldwide.
➧ It offers a range of test programs, including the 24-2, 30-2, and 10-2 protocols.
➧ The device uses advanced algorithms such as SITA Standard, SITA Fast, and SITA Faster, which reduce test time while preserving accuracy.
➧ Results are displayed in easy-to-read formats: grayscale maps, numeric threshold tables, total deviation and pattern deviation plots, and global indices like Mean Deviation (MD) and Pattern Standard Deviation (PSD).
HFA is especially known for its reliability and extensive normative database, making it ideal for diagnosing and monitoring glaucoma and other optic neuropathies.
B. Octopus Perimeter
The Octopus Perimeter, developed by Haag-Streit, offers both static and kinetic perimetry modes, providing flexibility for different clinical scenarios.
➧ Uses unique test patterns such as the G-program, which focuses on areas where glaucomatous damage is most likely to occur.
➧ Offers algorithms like TOP (Tendency Oriented Perimetry), which significantly shortens test duration by predicting thresholds based on surrounding points.
➧ Presents intuitive analysis tools, including progression analysis and cluster trend analysis to detect subtle field changes over time.
The Octopus is valued for its ability to combine static threshold testing with kinetic mapping in a single session.
C. Medmont Perimeter
The Medmont M700 Automated Perimeter is another respected instrument, commonly used in clinics and research settings.
➧ Supports test strategies like the 24-2, 30-2, and specialized macular programs.
➧ Noted for a user-friendly interface and detailed threshold mapping.
➧ Often chosen in academic and research institutions for its customizable test grids and detailed reporting.
Common Test Strategies
The choice of test strategy depends on the clinical question and the area of the visual field that needs to be assessed. The most commonly used protocols include:
🔼 24-2 and 30-2 Tests
➧ The 24-2 test covers the central 24° of the visual field, which includes most of the macula and paracentral region, using a grid of 54 points spaced 6° apart.
➧ The 30-2 test extends further to 30°, testing 76 points, and is useful when a broader view of the peripheral field is needed.
Both are widely used for diagnosing and monitoring glaucoma and other optic nerve diseases.
🔼 10-2 Test
➧ Focuses on the central 10°, testing 68 points with finer spacing (2° apart).
➧ Highly sensitive for detecting and monitoring damage close to fixation, which is critical in advanced glaucoma or macular diseases.
Specialized Protocols and Applications
Modern perimeters also offer:
➧ Neurological field tests, targeting hemianopias and quadrantanopias caused by brain lesions.
➧ Macular programs, useful in retinal conditions affecting central vision.
➧ Custom grids, allowing clinicians to tailor the test to specific clinical needs, such as monitoring localized retinal scotomas or optic nerve head drusen.
Additionally, some perimeters can perform binocular visual field testing, useful for assessing the functional impact on activities like driving.
How is Automated Static Threshold Perimetry Performed?
The test typically involves the following steps:
1. Patient Positioning: The patient sits in front of a large, bowl-shaped instrument called a perimeter. Their head is placed in a chin rest, and they are asked to fix their gaze on a central target light.
2. Eye Coverage: One eye is covered with a patch, as each eye is tested separately.
3. Corrective Lenses: Appropriate corrective lenses (glasses) for the testing distance are placed in front of the eye being tested to ensure the best possible vision.
4. Stimulus Presentation: Small, dim lights (stimuli) of varying intensities are briefly flashed at different locations within the bowl.
5. Patient Response: Whenever the patient sees a light, they press a button on a handheld remote control.
6. Threshold Determination: The machine uses algorithms (like SITA Standard or SITA Fast) to efficiently determine the threshold at each point. It does this by presenting stimuli of gradually increasing and decreasing intensities until it finds the dimmest light the patient can consistently see.
7. Reliability Monitoring: The machine continuously monitors the patient's fixation (ensuring they keep looking at the central target) and tracks false positives (pressing the button when no light was shown) and false negatives (not pressing the button when a clearly visible light was shown). These "reliability indices" are crucial for determining the validity of the test results.
8. Test Duration: The test takes several minutes per eye, and patients are encouraged to blink normally and can pause if they need a break.
What Does Automated Static Threshold Perimetry Measure?
Automated static threshold perimetry creates a detailed map of your "island of vision," which is a conceptual representation of your visual field. The "peak" of this island is the fovea (central vision), where sensitivity is highest, and it slopes downwards towards the periphery, where sensitivity decreases.
The test measures:
1. Differential Light Sensitivity
This is the primary measurement: the eye’s ability to detect a brief light stimulus against a constant, evenly illuminated background. Sensitivity is expressed in decibels (dB):
✔ Higher dB values mean greater sensitivity — the eye can detect dimmer stimuli.
✔ Lower dB values indicate decreased sensitivity, requiring brighter stimuli to be perceived.
By testing multiple points across the visual field, the machine creates a topographical sensitivity map.
2. Blind Spot
The physiological blind spot corresponds to the point on the retina where the optic nerve exits the eye (optic disc) — an area without photoreceptors. Automated perimetry maps this blind spot as part of the test, ensuring fixation accuracy and validating test reliability.
3. Visual Field Defects (Scotomas)
These are areas where sensitivity is reduced or absent compared to normal. Scotomas are classified as:
✔ Relative Scotomas: Areas where the retina still perceives light, but only at higher intensities than normal, indicating partial functional loss.
✔ Absolute Scotomas: Areas where the retina cannot detect even the brightest stimulus, representing complete functional loss.
Interpreting the Results (The Printout) of the Automated Static Threshold Perimetry
The results of automated static threshold perimetry are presented on a detailed printout, which includes several key plots and indices used to analyze and monitor the patient’s visual field.
🩺 Patient & Test Details
At the top of the printout, you’ll find basic information about the patient: name, date of birth, pupil size, any corrective lenses used, and the specific test pattern (e.g., 24-2 or 30-2). These patterns describe the extent and grid spacing of the area tested, usually the central 24° or 30° of the visual field.
Reliability Indices
These indices help assess whether the test results are trustworthy:
✔ Fixation Losses (FL): Indicates how often the patient's gaze drifted from the central target. High FL (e.g., >20%) suggests unreliable results.
✔ False Positives (FP): Indicates how often the patient pressed the button when no light was presented ("trigger happy"). High FP (e.g., >15-20%) can make the field look better than it is.
✔ False Negatives (FN): Indicates how often the patient missed a light that should have been easily seen (suggests inattention or fatigue). High FN (e.g., >15-20%) can make the field look worse than it is.
Numerical Plot (Threshold Values)
This table shows the measured sensitivity in decibels (dB) for each tested point. Higher numbers mean greater sensitivity (the patient can see dimmer lights); lower numbers indicate reduced sensitivity.
Grayscale Plot
A visual summary where lighter areas represent normal sensitivity and darker areas indicate reduced sensitivity or scotomas. While useful for quick overview, grayscale plots can exaggerate small variations.
Total Deviation Plot
This plot compares the patient’s sensitivity at each point to age-matched norms. It shows:
✔ Negative values: Areas where the patient’s sensitivity is lower than expected.
✔ Positive values (rare): Better-than-average sensitivity.
This plot reveals both localized defects and generalized depression (e.g., from cataract or small pupil).
Pattern Deviation Plot
A crucial plot, especially in glaucoma. It statistically adjusts for generalized depression (like that caused by cataract) to highlight true localized field defects. This helps distinguish diffuse media opacity effects from focal optic nerve damage.
Global Indices
These are summary statistics that describe the overall status of the visual field:
✔ Mean Deviation (MD): The average difference from normal across the field. Negative MD indicates overall sensitivity loss.
✔ Pattern Standard Deviation (PSD): Shows irregularity or patchiness in the field; higher PSD indicates more focal defects.
✔ Visual Field Index (VFI): Expressed as a percentage (0–100%), this emphasizes central vision and is designed to be less affected by cataract. Useful for tracking glaucoma progression.
✔ Glaucoma Hemifield Test (GHT): Compares upper and lower field regions that glaucoma typically affects asymmetrically. The test result can be: “Within Normal Limits,” “Borderline,” or “Outside Normal Limits.”
By carefully analyzing these plots and indices, eye care professionals can detect, classify, and monitor visual field changes, which is essential for diagnosing and managing various eye and neurological conditions.(alert-passed)
Advantages of Automated Static Threshold Perimetry
Automated static threshold perimetry offers significant benefits:
✔ Standardized and reproducible: Minimizes examiner bias.
✔ Detailed, quantitative mapping: Provides precise measurement at many test points.
✔ Sensitive to early changes: Useful for detecting subtle field defects before they become symptomatic.
✔ Relatively fast and user-friendly, especially with modern algorithms.
Limitations of Automated Static Threshold Perimetry
Despite its advantages, some limitations remain:
🗙 Test requires good patient cooperation, attention, and understanding.
🗙 Can be tiring; fatigue may affect reliability.
🗙 Less informative for kinetic defects or very peripheral fields beyond ~30°.
Automated static threshold perimetry has transformed the field of visual field testing, offering clinicians a highly sensitive, standardized, and quantitative method to detect and monitor functional visual loss. It remains an essential tool in ophthalmology, neurology, and visual science, improving diagnosis, guiding treatment decisions, and ultimately helping preserve patients’ vision.
