Glaucoma Visual Field Test: Board Guide
Master the glaucoma visual field test for NBEO and OKAP: defects, reliability indices, GHT and the classic meridian trap explained.
Which visual field pattern is relatively common in normal-tension glaucoma?
Glaucoma Visual Field Test in 30 Seconds
Standard automated perimetry measures functional vision loss caused by glaucomatous optic neuropathy. It quantifies scotomas that correlate with retinal nerve fiber loss patterns. Boards test this heavily because interpretation drives diagnosis and progression calls on NBEO and OKAP.
What the Test Measures
Standard automated perimetry measures differential light sensitivity, the dimmest stimulus a patient detects at each tested point. The 24-2 program is the standard glaucoma test because it efficiently samples the central 24 degrees, including the arcuate nerve fiber bundle region where early glaucomatous defects typically occur. A 10-2 program is used when central or paracentral glaucoma is suspected, particularly in normal-tension glaucoma or advanced disease where macular function is at risk. The printout gives a grayscale map, a total deviation plot comparing raw sensitivity to age-matched norms, a pattern deviation plot that adjusts for overall depression, and global indices including mean deviation, pattern standard deviation, and the Visual Field Index. The VFI is a percentage estimate of remaining visual function that is less affected by cataract than mean deviation and is commonly used to monitor progression.
Classic Glaucomatous Field Defects
- Nasal step: defect respecting the horizontal meridian in the nasal field, reflecting the anatomic split of superior and inferior retinal nerve fiber bundles at the horizontal raphe.
- Arcuate (Bjerrum) scotoma: arching defect from the blind spot toward the nasal raphe, following the arcuate nerve fiber bundle course.
- Seidel scotoma: an early sickle-shaped extension of the blind spot that often precedes a complete arcuate scotoma. Frequently tested as an early glaucomatous sign.
- Paracentral scotoma: small dense defect near fixation, often an early sign of localized nerve fiber damage.
- Advanced loss: dense arcuate defects that coalesce into hemifield loss and eventually tunnel vision as damage becomes diffuse.
4 Facts Exams Always Ask
- Structural change on OCT, especially retinal nerve fiber layer and ganglion cell thinning, often precedes measurable visual field loss by months to years.
- A nasal step respects the horizontal meridian because it follows retinal nerve fiber bundle anatomy, unlike chiasmal or retrochiasmal lesions, which respect the vertical meridian due to fiber decussation at the chiasm.
- High false-positive responses can make fields appear deceptively normal, whereas excessive fixation losses or false negatives reduce confidence in interpretation. High false-positive rates are generally considered the most misleading reliability error because they can make visual fields appear artificially normal.
- Inferior optic nerve damage produces superior visual field loss, while superior nerve fiber loss produces inferior field defects, because retinal images are inverted.
The Classic Trap
The most common exam trap is mixing up glaucomatous field defects with neurologic ones. Glaucomatous defects respect the horizontal meridian because retinal nerve fiber bundles split into superior and inferior arcades at the horizontal raphe, so damage to one bundle spares the other side. Chiasmal and retrochiasmal lesions respect the vertical meridian instead, because nasal and temporal fibers separate at the chiasm and project to opposite hemispheres. A defect stopping at the vertical midline should push the differential toward neurologic disease, not glaucoma.
Clinical Pearl
Pattern deviation is more useful than total deviation for catching early glaucoma. Total deviation reflects raw sensitivity loss, which can be dragged down by generalized depression from cataract, media opacity, or a small pupil. Pattern deviation statistically adjusts for generalized depression, isolating the focal loss that is the real signature of early nerve fiber bundle damage. Pattern deviation becomes less informative in advanced disease, where diffuse loss predominates. The Glaucoma Hemifield Test compares corresponding points above and below the horizontal meridian, and an "outside normal limits" result strongly supports glaucomatous damage.
Checkpoint 1
Why may standard 24-2 perimetry become less informative in advanced glaucoma near fixation?
Checkpoint 2
Which statement about retinal nerve fiber layer loss in glaucoma is most accurate?
A 58-year-old patient with an IOP of 24 mmHg and a cup-to-disc ratio of 0.7 undergoes 24-2 perimetry. The printout shows a defect in the inferior nasal field that stops sharply at the horizontal meridian. Fixation losses are 2 percent, false positives 4 percent and false negatives 8 percent.
Which visual field defect is classically considered an early glaucomatous change?
Sources & Reviewer Info
Diagram created with AI image generation, clinically reviewed for accuracy.
References
- American Academy of Ophthalmology. Basic and Clinical Science Course, Section 10: Glaucoma.
- Shields' Textbook of Glaucoma.
- American Academy of Ophthalmology. EyeWiki: Visual Field Testing in Glaucoma.
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