What is the Humphrey 24-2 visual field test?

The Humphrey 24-2 is an automated threshold perimetry test that measures visual field sensitivity at 54 points within the central 24° of the visual field (using a 6° grid pattern, with the '2' indicating the 2° offset from the horizontal and vertical meridians). It is the standard test for glaucoma detection and monitoring because glaucomatous field loss typically begins in the central 24°.

What are fixation losses in a visual field test?

Fixation losses are recorded when the patient presses the response button for a stimulus presented at their known blind spot location. Since the blind spot has no photoreceptors, no stimulus should be seen there. Pressing the button indicates the patient's eye has moved away from central fixation, placing a different retinal area at the blind spot location. Fixation loss rates above 20% make the field result unreliable.

Why should patients respond even when unsure if they saw a light?

Automated threshold perimetry deliberately presents stimuli near the patient's detection threshold, meaning many stimuli will be at the edge of visibility. Patients who only respond when certain will systematically miss these near-threshold stimuli, producing artificially depressed sensitivity values that can be misinterpreted as field loss. Encouraging responses for uncertain stimuli reduces false negatives and produces more accurate threshold measurements.

Automated Perimetry: CPO Exam Guide

Why Automated Perimetry Matters

Automated perimetry is the standard method for quantitative visual field assessment. Unlike confrontation testing, automated perimetry maps the sensitivity of the entire visual field at hundreds of specific test points with objective, reproducible measurements. It is essential for detecting and monitoring glaucoma, neurological field defects, and other conditions affecting the visual pathway. As a CPO, you will routinely set up and instruct patients for this test.

The Humphrey Visual Field Analyzer

The Humphrey Field Analyzer (HFA) is the most widely used automated perimeter. It presents light stimuli of varying intensity at specific locations within a defined test area (usually the central 24° or 30°) against a uniform background. The patient presses a response button each time they see a stimulus. The instrument determines the threshold sensitivity at each test location.

Setting Up the Test

Select the appropriate test: 24-2 (tests 54 points in central 24°, standard for glaucoma) or 30-2 (76 points in central 30°) are most common
Enter patient demographics and refraction data; the instrument may require a correction lens for near add
Seat the patient comfortably with the appropriate eye aligned to the bowl
Patch the non-tested eye
Position the patient so they are centered and comfortable, not straining forward
Adjust the chin rest to center the pupil in the instrument's view

Before starting, explain to the patient that the lights will vary in brightness and that some will be very faint — they may not be sure they saw one. Encourage them to press the button even when they are uncertain. Patients who respond only when very confident tend to produce depressed results that overestimate field loss.

Patient Instructions

Proper patient instruction is the most important factor in obtaining a reliable visual field. Key points to communicate:

"Keep your eye still and focused on the central fixation light at all times — even when you see a light in your side vision"
"Press the button each time you see any flash of light, even if it seems very dim or you are not completely sure"
"It is okay to blink normally, but try to do so right after you press the button"
"If you need to rest, tell me and I will pause the test"

Fixation loss is the most common cause of an unreliable visual field. The HFA monitors fixation by occasionally presenting a stimulus in the patient's known blind spot (the Heijl-Krakau method). If the patient presses the button for a blind spot stimulus, they are not maintaining central fixation. Fixation loss rate over 20% makes the field unreliable.

Reliability Indices

Every Humphrey printout includes three reliability metrics that must be evaluated before interpreting field results:

Index	What It Measures	Acceptable Threshold
Fixation losses	Responses to blind spot stimuli (indicates eye movement)	Less than 20%
False positives	Responses when no stimulus was presented (trigger-happy patient)	Less than 15%
False negatives	Missed stimuli at previously seen locations (inattentive or fatigued)	Less than 15%

Interpreting a field result without checking the reliability indices first. A field with 40% fixation losses and 30% false positives cannot be reliably interpreted for clinical decision-making. The test should be repeated with better patient instruction before the physician makes treatment decisions based on the result.

Key Takeaways

Automated perimetry maps visual field sensitivity at specific test points with quantitative results
The 24-2 test is standard for glaucoma monitoring
Patient instruction is the single most important factor in reliable results
Check reliability indices first: fixation losses under 20%, false positives and negatives under 15%
Encourage patients to respond even for dim, uncertain stimuli