What is threshold testing in automated perimetry?

Threshold testing determines the dimmest light stimulus a patient can detect at each test location. The instrument uses a bracketing strategy, alternating between brighter and dimmer stimuli until it finds the boundary between what the patient can and cannot see. Results are recorded in decibels (dB), with higher values indicating better sensitivity.

What do fixation losses, false positives, and false negatives mean?

Fixation losses indicate the patient looked away from the central target during the test. False positives mean the patient pressed the button when no stimulus was presented, suggesting they were guessing. False negatives mean the patient failed to respond to a bright stimulus at a location where they previously demonstrated good sensitivity, suggesting inattention or fatigue.

Why is the 24-2 test the most commonly used protocol for glaucoma?

The 24-2 protocol tests the central 24 degrees of the visual field, which covers the area most commonly affected by glaucomatous damage. Points are spaced 6 degrees apart, providing a good balance between spatial resolution and test duration. It captures characteristic glaucomatous patterns like arcuate defects and nasal steps while keeping the test short enough for reliable patient performance.

Automated Perimetry: Humphrey Visual Field Testing

What Is Automated Perimetry?

Automated perimetry uses a computerized instrument to systematically map the visual field by presenting light stimuli of varying brightness at defined locations. The Humphrey Field Analyzer (HFA) is the most widely used device in clinical ophthalmology and the standard instrument referenced on the COA exam.

Unlike confrontation testing, automated perimetry provides quantitative, reproducible data that can be tracked over time to monitor disease progression, particularly in glaucoma.

How the Test Works

The patient sits with their chin in a rest and one eye occluded. They fixate on a central target inside a uniform white bowl. The instrument presents small points of light at predetermined locations across the visual field. Each stimulus varies in brightness (intensity).

Threshold Testing

Threshold testing determines the dimmest light the patient can detect at each test location. The instrument uses a bracketing strategy:

It presents a stimulus at a predicted intensity
If the patient responds, it presents a dimmer stimulus next
If the patient does not respond, it presents a brighter stimulus
This back-and-forth continues until the threshold (the boundary between "seen" and "not seen") is determined

The result is a map showing the sensitivity (in decibels, dB) at each test point. Higher dB values mean better sensitivity; lower values indicate reduced sensitivity or scotoma.

Common Test Protocols

24-2: Tests the central 24 degrees of the visual field with points spaced 6 degrees apart. The most common protocol for glaucoma.
30-2: Tests the central 30 degrees. Provides slightly wider coverage than 24-2.
10-2: Tests only the central 10 degrees with tightly spaced points. Used for advanced glaucoma, macular disease, or neuro-ophthalmic conditions affecting central vision.

Reliability Indices

The HFA monitors patient performance throughout the test. Three key reliability indices tell you whether the results can be trusted:

Fixation Losses (FL)

The instrument periodically presents a stimulus in the patient's known blind spot (where the optic nerve head is). If the patient responds to this stimulus, they must have been looking away from the fixation target. High fixation losses (generally over 20%) suggest the patient was not fixating properly.

False Positives (FP)

The instrument occasionally pauses stimulus presentation. If the patient presses the button when no light was shown, that counts as a false positive. High false positive rates (over 15%) suggest the patient is "trigger happy," pressing the button based on timing rather than actually seeing stimuli. This artificially elevates sensitivity values.

False Negatives (FN)

The instrument occasionally presents a very bright stimulus in a location where the patient previously demonstrated good sensitivity. If the patient fails to respond, it counts as a false negative. High false negative rates (over 33%) suggest inattention, fatigue, or fluctuating responses. Note that high false negatives can also occur in patients with genuine advanced field loss.

An unreliable visual field test is essentially uninterpretable. Before analyzing the field pattern, always check the reliability indices first. If fixation losses, false positives, or false negatives exceed acceptable thresholds, the test should be repeated.

The Role of the Ophthalmic Assistant

The assistant plays a critical role in obtaining quality visual field results:

Proper patient instruction: Explain the test clearly before starting. Tell the patient to look straight ahead at the fixation light, press the button whenever they see any flash of light (even dim ones), and not to search for lights.
Correct setup: Enter the proper near correction (trial lens), ensure proper eye occlusion, and position the patient comfortably
Monitor fixation: Watch the video monitor during the test and pause to reposition if the patient drifts
Encourage the patient: Remind them to blink normally and take brief rests if needed

Forgetting to place the correct near correction lens for the patient's age and test distance. Without the appropriate add, a presbyopic patient may not see dim stimuli near fixation due to blur rather than genuine field loss, producing falsely depressed central sensitivity values.

Key Takeaways

Automated perimetry quantitatively maps visual field sensitivity using threshold testing, where the dimmest detectable light is found at each location
The Humphrey Field Analyzer is the clinical standard, with 24-2 being the most common glaucoma protocol
Reliability indices (fixation losses, false positives, false negatives) must be checked before interpreting any field
The ophthalmic assistant's role in patient instruction, setup, and monitoring directly affects test quality
Serial testing over time is essential for detecting glaucoma progression