What Is Automated Perimetry?
Automated perimetry (also called computerized visual field testing) is the standard clinical method for quantitatively mapping the visual field. It presents light stimuli of varying brightness at predetermined locations across the visual field and records whether the patient can detect each stimulus. The result is a detailed visual field map that reveals the presence, pattern, and severity of field defects.
The most widely used instrument is the Humphrey Field Analyzer (HFA), which is found in the majority of optometry and ophthalmology practices. The CPOA is typically responsible for setting up and administering the test, which means your role directly affects the reliability of the results the doctor uses for diagnosis and treatment decisions.
Key Concepts in Automated Perimetry
Threshold vs. Suprathreshold Testing
- Threshold testing: Determines the minimum brightness (threshold) at which the patient can detect a stimulus at each test location. More detailed and time-consuming. Used for glaucoma monitoring and neurological assessment. Common programs: 24-2, 30-2 (the numbers refer to the angular extent of the field tested).
- Suprathreshold (screening) testing: Presents stimuli slightly brighter than expected normal threshold. The patient either sees the stimulus or not. Faster but less detailed. Used for initial screening.
Fixation
Reliable fixation is critical. The patient must keep their eye centered on the fixation target throughout the test. The HFA monitors fixation in two ways:
- Blind spot check (Heijl-Krakau method): Occasionally presents a bright stimulus in the patient's physiological blind spot. If the patient responds, they have moved their eye off fixation (fixation loss).
- Gaze tracking: Newer HFAs track the patient's eye position continuously using an infrared camera.
CPOA Role in Visual Field Testing
Before the Test
- Patient selection: Ensure the correct test program is selected (the doctor prescribes the test type and eye to test).
- Refraction setup: Insert the near addition lens in the trial lens holder if the patient needs one for the testing distance. Patients who wear bifocals or reading glasses for near should have the appropriate near correction. The instrument focuses stimuli at 33 cm, so near correction matters.
- Occlude the fellow eye with a patch or foam occluder -- completely cover the eye not being tested.
- Enter patient data: Age, name, test eye, and date in the HFA software.
- Position the patient: Chin in chin rest, forehead against bar. Align the patient's eye with the instrument's eye level mark. Ensure the trial lens (if used) is close to the eye without touching lashes.
- Instruct the patient fully before starting.
Patient Instructions (Essential)
Give thorough instructions before starting -- mid-test confusion wastes time and invalidates results:
- "You will see a blinking light inside the bowl. Keep looking at the center light (fixation target) at all times -- do not move your eyes to look for the flashing lights."
- "Whenever you see any flash of light, even if it seems faint or uncertain, press the button."
- "The lights will appear all over -- some bright, some very faint. There is no pattern; press whenever you see anything."
- "If you need to blink, blink normally. The test will pause briefly while you blink."
- "If you need a break, let me know and we can pause the test."
💡 Clinical Tip: Patients who do not understand the instructions will either press too rarely (missing stimuli they actually saw) or press constantly (false positives). Take extra time with first-time patients and elderly patients to ensure full comprehension before starting.
During the Test
- Monitor the patient's fixation and alertness through the instrument's monitor.
- Encourage the patient periodically if they seem fatigued: "You're doing great, keep going."
- If fixation is breaking frequently, stop and re-instruct the patient.
- Note unusual patient behavior (head tilting, frequent blinking, apparent fatigue) in the documentation.
After the Test
- Print the visual field result and review the reliability indices before giving them to the doctor.
- Flag tests with poor reliability for the doctor's attention.
- Remove the trial lens from the holder and replace it properly in the trial lens set.
Reliability Indices
The HFA printout contains three reliability indices:
- Fixation losses (FL): Percentage of catch trials where the patient responded when the stimulus was in the blind spot (indicating they were not fixating). Acceptable: less than 20%.
- False positives (FP): The patient pressed the button when no stimulus was presented. Acceptable: less than 15%. High FP rates make a field look better than it is.
- False negatives (FN): The patient did not respond to a bright stimulus in an area where they previously detected a dim one. Acceptable: less than 20%. High FN rates may indicate fatigue or poor attention, making the field look worse than it is.
⚠️ Common Mistake: Failing to insert the correct near addition lens for the patient's age and refraction. Without the near add, older patients will have a blurred image inside the bowl, producing artifactually poor central thresholds that mimic early glaucomatous damage. Always check the near correction before starting.
Key Takeaways
- Automated perimetry (Humphrey Field Analyzer) is the standard for quantitative visual field assessment.
- The CPOA sets up the test, inserts the correct near add lens, patches the fellow eye, enters patient data, positions the patient, and gives thorough instructions.
- Clear patient instructions before starting are essential for reliable results.
- Reliability indices (fixation losses, false positives, false negatives) determine whether the test is valid.
- High fixation losses indicate poor fixation; high false positives make the field look better than reality; high false negatives suggest fatigue.
- The near add lens is required for the correct focal distance (33 cm) -- omitting it causes artifactual central depression.