Differentiating Glaucomatous from Neurologic Field Loss

The Meridian Rule: Your First Filter

Glaucomatous field defects follow the anatomy of the retinal nerve fiber bundles and respect the horizontal meridian. Chiasmal and retrochiasmal (neurologic) defects respect the vertical meridian instead, a consequence of hemidecussation at the chiasm and the hemispheric organization of the visual pathway beyond it. Orientation is the single most useful discriminator between the two: a defect that stops sharply at the vertical midline points away from glaucoma.

Specific Red Flags

• Altitudinal defect with a pale, non-cupped disc. This pattern suggests anterior ischemic optic neuropathy (AION), not glaucoma. AION also classically produces an altitudinal defect that respects the horizontal meridian, but pallor without the cupping expected in glaucoma is the tell.
• Cecocentral scotoma that crosses the horizontal meridian and merges with the blind spot: typical of toxic, nutritional, and hereditary optic neuropathies (e.g., Leber hereditary optic neuropathy) affecting the papillomacular bundle. Glaucoma spares fixation until very late in the disease and does not produce this pattern.
• Field defect respecting the vertical midline in a suspected NTG workup should prompt neuroimaging to exclude a compressive intracranial lesion mimicking normal-tension glaucoma. A "normal" IOP paired with a vertically organized defect does not fit true glaucomatous anatomy.