Visual Field Testing: COA Guide to Automated Perimetry

Q: What is the purpose of visual field testing in ophthalmic practice?

Visual field testing (perimetry) maps a patient's entire visual field — including peripheral (side) vision — to detect areas of reduced sensitivity or complete loss (scotomas). It is essential for diagnosing and monitoring glaucoma (the most common indication), identifying neurological lesions (optic nerve, optic chiasm, visual cortex), detecting retinal diseases, assessing driving eligibility, and documenting disability. In glaucomatous optic neuropathy, visual field loss typically appears before patients notice any symptoms.

Q: What are the three reliability indices on a Humphrey visual field printout?

The three reliability indices are: (1) Fixation Losses (FL) — the percentage of trials where the patient presses the button when a stimulus is presented in the blind spot (indicates patient was not fixating properly). A reliable test has FL <20%. (2) False Positives (FP) — percentage of responses when no stimulus was presented (trigger-happy patient; overestimates sensitivity). Reliable tests have FP <15%. (3) False Negatives (FN) — percentage of non-responses to a bright stimulus in areas that already responded to a dimmer one (suggests fatigue or inattention). Reliable tests have FN <15%.

Q: What is the difference between SITA Standard and SITA Fast on the Humphrey perimeter?

Both are versions of the Swedish Interactive Thresholding Algorithm (SITA) used on the Humphrey Field Analyzer. SITA Standard is more accurate, takes approximately 6–8 minutes per eye, and is recommended for glaucoma diagnosis and monitoring. SITA Fast is approximately twice as fast (~3–4 minutes per eye) with slightly more variability; used for screening or when patient fatigue is a concern. Both are superior to older Full Threshold algorithms in terms of patient comfort and test time, with comparable sensitivity.

Q: What visual field pattern is characteristic of glaucoma?

Glaucomatous visual field loss follows the arcuate nerve fiber bundles. Classic patterns include: (1) Arcuate (Bjerrum) scotoma — an arc-shaped defect connecting the blind spot to nasal periphery, following the superior or inferior arcuate nerve fiber layer. (2) Nasal step — an abrupt step in sensitivity at the horizontal nasal meridian, respecting the horizontal raphe. (3) Altitudinal defect — loss in the superior or inferior half of the visual field (more severe glaucoma). (4) Paracentral scotoma — small defects near fixation in early glaucoma. These patterns are reproducible on repeated testing, which distinguishes true glaucoma damage from artifacts.

Q: How should ophthalmic assistants prepare a patient for visual field testing?

Key patient preparation steps include: (1) Explain the test clearly — "You will see flashing lights of different brightness. Press the button every time you see a light, no matter how faint." (2) Emphasize fixation — staring at the central fixation target throughout the test, not searching for the lights. (3) Occlude one eye at a time with an occluder — testing is monocular. (4) Correct the patient's refractive error with the trial lens — uncorrected refractive error, especially in presbyopic patients, causes spurious scotomas (rim artifacts). (5) Warn about the duration — SITA Standard takes 6–8 minutes per eye, and patients should be encouraged to take breaks if needed by releasing the button.

Humphrey VFPerimetryGlaucoma Testing

Visual field testing is one of the most critical diagnostic procedures in ophthalmic practice, and the ophthalmic assistant plays a central role in ensuring that tests are performed correctly and that results are reliable. The primary instrument used in most ophthalmic offices is the Humphrey Field Analyzer (HFA), which performs automated static perimetry. Understanding the equipment, the different testing algorithms, how to properly instruct patients, and how to identify when a test is unreliable are all tested on the COA exam.

Perimetry maps a patient's "hill of vision" — the three-dimensional island of visual sensitivity surrounded by non-seeing space. The apex of this island represents the point of maximum sensitivity (the fovea), while the slopes represent decreasing sensitivity toward the periphery. A scotoma is an area of depressed or absent vision within the visual field. The clinical value of visual field testing lies in identifying where scotomas are located and whether their pattern correlates with a specific disease process such as glaucoma, optic neuritis, or neurological damage.

This guide covers everything you need to know about visual field testing for the COA exam: the Humphrey perimeter testing algorithms, patient instructions, reliability indices and how to interpret them, the most common artifacts that mimic disease, and the characteristic visual field patterns associated with glaucoma and neurological conditions.

Humphrey Field Analyzer: Testing Algorithms

The Humphrey Field Analyzer (Carl Zeiss Meditec) is the gold standard automated perimeter in most clinical settings. It uses a white dome background with a Goldmann size III stimulus (0.43° diameter) presented at varying intensities to map threshold sensitivity at each test location.

Strategy	Duration/Eye	Best For	Notes
Full Threshold	12–15 min	Research; rarely used clinically today	Most accurate; staircase algorithm; causes fatigue
SITA Standard	6–8 min	Glaucoma diagnosis and monitoring (standard of care)	Bayesian algorithm; high accuracy; less fatigue
SITA Fast	3–4 min	Screening; elderly/fatigued patients	Slightly more variable; good for screening
SITA Faster	~2 min	Monitoring established glaucoma	Newest algorithm; comparable to SITA Fast
Frequency Doubling (FDT)	~1 min	Mass glaucoma screening	Different instrument; tests magnocellular pathway
Esterman Binocular	~5 min	Driving eligibility assessment	Binocular suprathreshold; 120-point grid

Test Patterns (Grid Programs)

24-2

54 points within 24° of fixation, plus 4 nasal points to 27°. Most commonly used for glaucoma. Adequate for most glaucoma monitoring.

30-2

76 points within 30° of fixation. Slightly wider than 24-2; used when peripheral involvement suspected or for neurological cases.

10-2

68 points within 10° of fixation (central field only). Used for advanced glaucoma monitoring when only central island remains.

Patient Preparation and Instructions

Proper patient preparation is the ophthalmic assistant's primary responsibility in visual field testing. Poor preparation leads to unreliable results, wasted time, and potentially incorrect clinical decisions.

Set Up the Trial Lens Correctly

Use the patient's distance correction (sphere and cylinder) in the trial lens holder. For patients over 40, add the age-appropriate near correction (reading add) based on the testing distance. Incorrect or missing lens correction causes rim artifacts — false scotomas at the edges of the lens aperture.

Position the Patient Carefully

The chin rest positions the eye level with the bowl center. Brow arch should be above the instrument aperture — overhanging brows/lids cause lid artifacts. Adjust the chin rest and head tilt to minimize lid droop. The center of the pupil should align with the calibration mark.

Occlude the Fellow Eye

Use a clean eye patch or occluder on the non-tested eye. Never rely on the patient to keep one eye closed — this leads to inconsistent occlusion and potential binocular summation. Ensure the occluder does not press on the eyelid of the tested eye.

Give Clear Instructions

Standardized instruction: "Keep your eye on the yellow fixation light in the center. You will see small flashing lights of different sizes and brightness. Press the button whenever you see a light — even if it's very faint or in the corner of your vision. Try not to search for the lights; just keep staring at the center target." Demonstrate with a practice trial.

Instruction: Keep your eye on the yellow fixation light in the center. You will see small flashing lights of different sizes and brightness. Press the button whenever you see a light — even if it\'s very faint or in the corner of your vision. Try not to search for the lights; just keep staring at the center target.

Run the Practice Test

Always run the brief practice test before the actual test. This allows the patient to understand the task and calibrates their response pattern. Patients who struggle with the practice may need additional instruction.

Monitor During the Test

Watch the fixation monitoring display throughout. The HFA uses the Heijl-Krakau method (presents stimuli at the blind spot) to measure fixation losses in real time. If the patient is losing fixation excessively, gently remind them via the intercom to keep staring at the center target.

Reliability Indices: Reading the Printout

The three reliability indices on a Humphrey VF printout tell you whether the test results are trustworthy. An unreliable test should be repeated — acting on an unreliable result can lead to incorrect diagnoses or missed disease.

Fixation Losses (FL) — < 20% — Patient not fixating on central target — looking around the bowl — Unpredictable results; may miss scotomas or create false ones — blue
False Positives (FP) — < 15% — Trigger-happy patient — pressing button even when no light appeared — Visual field appears falsely better than actual sensitivity (artefactual improvement) — red
False Negatives (FN) — < 15% — Patient failed to respond to bright stimulus in area that previously responded — May indicate fatigue, inattention, or true deep scotoma — amber

Global Indices on the Printout

Mean Deviation (MD)

Average deviation of all test points from normal age-matched values. Negative MD indicates overall depression. MD of −2dB to −6dB = mild loss; −6 to −12dB = moderate; >−12dB = severe. Early glaucoma typically shows MD between −1 and −6dB.

Pattern Standard Deviation (PSD)

Measures irregularity (spatial variability) of the visual field. A high PSD indicates a localized scotoma pattern (typical of glaucoma). Low PSD with low MD = diffuse depression (media opacity, pupil miosis). PSD is more specific to glaucoma than MD.

Common Visual Field Artifacts

Artifacts are false visual field defects produced by testing conditions rather than true pathology. Recognizing artifacts is essential — confusing an artifact for disease can result in unnecessary workup or treatment.

Artifact Type	Appearance on VF	Cause	Fix
Lid artifact	Superior depression, often broad and straight-edged	Ptosis or heavy brow blocking superior field	Tape lid up; retest with proper lid position
Rim artifact	Ring-shaped peripheral depression	Trial lens frame edge obscuring peripheral stimulus	Adjust lens position; use rimless trial lens
Lens correction artifact	Scotoma near the blind spot or central area	Incorrect trial lens power — usually uncorrected add for presbyopia	Verify and correct the trial lens before retesting
Fatigue effect	Diffuse depression in second half of test; high FN	Patient tires during long test and stops responding	Break between eyes; use SITA Fast for fatigable patients
Media opacity (cataract, cornea)	Diffuse depression across all locations (low MD, low PSD)	Cloudy media reduces light transmission globally	Note cataract grade; results may not be valid for glaucoma monitoring
Pupil miosis	Diffuse depression	Small pupil reduces retinal illumination	Dilate if pupil <2.5mm; note pupil size on report

Glaucoma Visual Field Patterns

Glaucomatous visual field defects follow the anatomy of the retinal nerve fiber layer (RNFL), which runs in arcuate bundles from the peripheral retina to the optic disc. These bundles respect the horizontal raphe (a boundary between superior and inferior fiber bundles at the macula).

Early to Moderate

An arc-shaped defect that begins at or near the blind spot, curves around fixation (5–25° from center), and ends at the nasal horizontal midline. May be superior, inferior, or both (double arcuate). The most classic glaucoma field pattern.

Pattern: Arcuate Scotoma (Bjerrum Scotoma)

Tips: Often the first defect detected, May start as a nasal step or paracentral scotoma before becoming arcuate, Does not cross the horizontal midline

Early

An abrupt difference in sensitivity above vs. below the horizontal midline on the nasal side of the visual field. The step corresponds to a difference in damage between the superior and inferior arcuate nerve fiber bundles at the horizontal raphe.

Pattern: Nasal Step

Tips: Highly specific for glaucoma, Early sign — look carefully in ocular hypertension, Confirmed only if reproducible on repeat testing

Advanced

Loss of the entire superior or inferior half of the visual field, respecting the horizontal midline. In glaucoma, represents severe damage to one arcuate bundle. Can also result from AION (anterior ischemic optic neuropathy), BRVO, or branch retinal artery occlusion.

Pattern: Altitudinal Defect

Tips: If from glaucoma, often associated with large C/D ratio, AION: sudden onset; glaucoma: gradual, Must respect horizontal midline exactly in glaucoma

Early

Small, dense defects within the central 10° of the visual field near but not touching fixation. May be detected earlier than arcuate defects in some patients, especially with superior pressure peaks. Important for monitoring with 10-2 testing in advanced disease.

Pattern: Paracentral Scotoma

Tips: Not always related to glaucoma — check for other causes, In advanced glaucoma: monitor central field preservation, 10-2 program preferred for detecting paracentral loss

Neurological Visual Field Patterns (Non-Glaucoma)

Lesion Location	Visual Field Defect	Example Cause
Optic nerve (unilateral)	Any monocular VF defect; central scotoma common	Optic neuritis, glaucoma, AION
Optic chiasm	Bitemporal hemianopia (temporal loss in both eyes)	Pituitary adenoma, craniopharyngioma
Optic tract	Homonymous hemianopia (same-side loss in both eyes)	Stroke, tumor, trauma
Temporal lobe	Superior homonymous quadrantanopia ("pie in the sky")	Temporal lobe stroke or tumor
Parietal lobe	Inferior homonymous quadrantanopia ("pie on the floor")	Parietal lobe stroke or tumor
Occipital cortex	Congruous homonymous hemianopia with macular sparing	Posterior cerebral artery stroke

Esterman Binocular Field Testing

The Esterman binocular visual field is a specialized suprathreshold test that maps the combined visual field of both eyes open. It uses a 120-point grid extending from 70° temporal to 60° nasal in the horizontal meridian. Unlike threshold tests, it uses a fixed suprathreshold stimulus — the patient either sees it or doesn't.

Clinical Uses of Esterman Testing

Driving eligibility assessment (most common use in the US)
DMV/licensing authority requirements vary by state
Social Security disability determination
Legal visual impairment documentation

Legal Blindness Visual Field Definition

Legal blindness is defined as visual acuity of 20/200 or worse in the better eye with best correction, OR visual field of 20° or less in the better eye (regardless of acuity).

On the Esterman binocular field, a score of <50% (fewer than 60 of 120 points seen) generally indicates significant field restriction. However, specific criteria vary by jurisdiction and purpose.

Practice COA Visual Field Questions

Test your knowledge on perimetry, reliability indices, glaucoma patterns, and all COA exam domains with adaptive practice questions.

Frequently Asked Questions

What is the purpose of visual field testing in ophthalmic practice?

Visual field testing (perimetry) maps a patient's entire visual field — including peripheral (side) vision — to detect areas of reduced sensitivity or complete loss (scotomas). It is essential for diagnosing and monitoring glaucoma (the most common indication), identifying neurological lesions (optic nerve, optic chiasm, visual cortex), detecting retinal diseases, assessing driving eligibility, and documenting disability. In glaucomatous optic neuropathy, visual field loss typically appears before patients notice any symptoms.

What are the three reliability indices on a Humphrey visual field printout?

The three reliability indices are: (1) Fixation Losses (FL) — the percentage of trials where the patient presses the button when a stimulus is presented in the blind spot (indicates patient was not fixating properly). A reliable test has FL <20%. (2) False Positives (FP) — percentage of responses when no stimulus was presented (trigger-happy patient; overestimates sensitivity). Reliable tests have FP <15%. (3) False Negatives (FN) — percentage of non-responses to a bright stimulus in areas that already responded to a dimmer one (suggests fatigue or inattention). Reliable tests have FN <15%.

What is the difference between SITA Standard and SITA Fast on the Humphrey perimeter?

Both are versions of the Swedish Interactive Thresholding Algorithm (SITA) used on the Humphrey Field Analyzer. SITA Standard is more accurate, takes approximately 6–8 minutes per eye, and is recommended for glaucoma diagnosis and monitoring. SITA Fast is approximately twice as fast (~3–4 minutes per eye) with slightly more variability; used for screening or when patient fatigue is a concern. Both are superior to older Full Threshold algorithms in terms of patient comfort and test time, with comparable sensitivity.

What visual field pattern is characteristic of glaucoma?

Glaucomatous visual field loss follows the arcuate nerve fiber bundles. Classic patterns include: (1) Arcuate (Bjerrum) scotoma — an arc-shaped defect connecting the blind spot to nasal periphery, following the superior or inferior arcuate nerve fiber layer. (2) Nasal step — an abrupt step in sensitivity at the horizontal nasal meridian, respecting the horizontal raphe. (3) Altitudinal defect — loss in the superior or inferior half of the visual field (more severe glaucoma). (4) Paracentral scotoma — small defects near fixation in early glaucoma. These patterns are reproducible on repeated testing, which distinguishes true glaucoma damage from artifacts.

How should ophthalmic assistants prepare a patient for visual field testing?

Key patient preparation steps include: (1) Explain the test clearly — "You will see flashing lights of different brightness. Press the button every time you see a light, no matter how faint." (2) Emphasize fixation — staring at the central fixation target throughout the test, not searching for the lights. (3) Occlude one eye at a time with an occluder — testing is monocular. (4) Correct the patient's refractive error with the trial lens — uncorrected refractive error, especially in presbyopic patients, causes spurious scotomas (rim artifacts). (5) Warn about the duration — SITA Standard takes 6–8 minutes per eye, and patients should be encouraged to take breaks if needed by releasing the button.

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