Visual Field Defects & Structure-Function Correlation

The Arcuate Pattern & Why It Respects the Horizontal Meridian

Retinal nerve fiber bundles arc above and below the horizontal raphe without crossing it. Glaucomatous scotomas therefore respect the horizontal meridian, the opposite of neurologic (chiasmal or retrochiasmal) defects, which respect the vertical meridian instead. This single fact is the most useful discriminator between glaucomatous and neurologic field loss (see below). An arcuate scotoma follows this curved course from the disc to the horizontal raphe. Because of retinal inversion, damage to the inferior neuroretinal rim produces a corresponding superior arcuate defect, and vice versa.

A nasal step is a related but distinct pattern: a defect confined to the nasal field that still respects the horizontal meridian, arising from asymmetric damage to the superior and inferior arcuate bundles. A temporal wedge defect can occur too, but from a different source: loss of the nasal radial fibers, which enter the disc along a straight course rather than the curved arcuate path.

How Defects Progress

Early changes include baring of the blind spot (enlargement of the physiologic blind spot) and the Seidel scotoma, a comma-shaped extension from the blind spot curving toward fixation and one of the earliest recognizable arcuate-type defects. As arcuate defects enlarge in both hemifields and merge, they form a ring scotoma encircling fixation. In end-stage disease only a small central island remains, along with a residual temporal crescent: the pattern described clinically as tunnel vision. The papillomacular bundle runs directly from macula to disc rather than arcing, and stays relatively spared through this whole progression. That is why central fixation outlasts the peripheral field.

A paracentral scotoma, within the central 10° of fixation, is a distinct early pattern associated with normal-tension glaucoma. Because it falls entirely within that central 10°, it can threaten fixation early even while the rest of the field looks relatively intact.

Discriminating Glaucomatous From Non-Glaucomatous Loss