6.2 Automated perimetry / visual fields

Automated Perimetry and Visual Fields

Perimetry maps the visual field — the entire area a patient can see while holding fixation straight ahead — to detect and monitor damage from glaucoma, retinal disease, and neurologic lesions. The COT sets up the test, positions and coaches the patient, and screens the printout for reliability before the physician interprets it. Good coaching (steady fixation, blinking between stimuli, pressing promptly) is often the difference between a usable field and a repeat test.

Static vs Kinetic Perimetry

Automated static perimetry, performed on instruments such as the Humphrey Field Analyzer, presents stationary lights of varying brightness at fixed retinal locations while the patient fixates centrally and presses a button each time a light is seen.

• Threshold testing finds the dimmest light detectable at each point. It is precise and is used for diagnosis and follow-up.
• Suprathreshold testing presents lights brighter than expected normal sensitivity. It is faster and gives a pass/fail result, so it is used for screening.

Common Humphrey test patterns:

• 24-2 — 54 points within the central 24°; the standard field for glaucoma.
• 30-2 — extends to 30° with more points, sampling farther into the periphery.
• 10-2 — a dense grid within the central 10°, used for macular disease, advanced glaucoma, and defects that threaten fixation.

Goldmann kinetic perimetry is a manual technique in which the perimetrist moves a target of fixed size and brightness from a non-seeing area inward until the patient first reports seeing it, plotting isopters (contour lines of equal sensitivity). It remains valuable for very advanced field loss, complex neurologic cases, and patients who cannot manage the sustained attention of automated testing.

Reliability Indices

Before trusting a field, check the reliability catch trials:

• Fixation losses — a stimulus is projected into the physiologic blind spot; a button press there means the eye was not fixating. High losses signal an unreliable test.
• False positives — the patient presses when no stimulus is shown ('trigger-happy'). This produces abnormally high sensitivities and a white 'clover-leaf' printout.
• False negatives — the patient misses a bright stimulus in an area already shown to detect dimmer lights, suggesting inattention, fatigue, or genuine disease.

As a rule of thumb each index should remain below roughly 20–33%; higher values prompt a repeat.

Global Indices and Defect Patterns

Key summary indices printed on the report:

• MD (Mean Deviation) — the overall departure from an age-matched normal field. It becomes more negative with diffuse loss such as cataract or advanced glaucoma.
• PSD (Pattern Standard Deviation) — the degree of localized irregularity; it rises with the focal defects typical of glaucoma.
• VFI (Visual Field Index) — the field expressed as a percentage of normal (100% = perfect), used to chart progression over time.
• GHT (Glaucoma Hemifield Test) — compares mirror-image zones of the upper and lower field and flags results such as 'outside normal limits.'

Pattern recognition ties the field to the disease:

• Glaucomatous defects follow the arc of the retinal nerve fiber layer — arcuate (Bjerrum) scotomas, nasal steps, and paracentral defects. Because nerve fibers do not cross the horizontal raphe, these defects respect the horizontal midline.
• Neurologic defects respect the vertical midline. A bitemporal hemianopia points to chiasmal compression (for example a pituitary tumor), while a homonymous hemianopia (the same-side half missing in both eyes) points to a retrochiasmal lesion. Whether a defect is homonymous or bitemporal helps localize the lesion along the visual pathway.

Patient Setup and Test Logistics

Proper setup prevents artifactual defects. Correct any refractive error with the appropriate trial lens placed close to the eye, and add the patient's near correction for the bowl distance in patients over about 40 so blur does not depress sensitivity. Occlude the non-tested eye with a patch, center the pupil, and confirm the pupil is at least about 3 mm — a very small or undilated pupil and a drooping upper lid can create false peripheral loss, so tape or lift a ptotic lid when needed. Enter the correct age and pupil size so the software selects the right normative comparison.

Coach the patient before starting: fixate the central light, press the button whenever a light is seen anywhere even if faint, do not chase the lights, and rest by holding the button down if a break is needed. A brief practice run reduces the learning effect, which commonly makes a first-ever field look worse than reality, so the physician often discounts an unreliable first test. Standard threshold programs run several minutes per eye, so fatigue is real — encourage blinking and reassure the patient that missing some lights is completely normal. For gross binocular screening, such as a driving-vision assessment, the Esterman test may be used. Keep the physiologic blind spot on the printout in mind as a built-in landmark of proper alignment.

Quick Reference: Field Indices