Post-Stroke Homonymous Hemianopia: Peli Prisms & Peripheral Visual Field Expansion

When the left occipital lobe suffers an infarction, the left visual cortex ceases to register signals from the right hemifield of both eyes. The visual field drops from normal 180° binocular span to less than 90°:

Early optical attempts used full-field prisms across the entire spectacle lens. However, by Prentice's rule, displacing central visual gaze by $10\Delta$ creates constant central diplopia (double vision) and peripheral scotomas, causing severe confusion and nausea.

Dr. Eli Peli discovered that visual field expansion can be achieved without central double vision by mounting high-powered Fresnel or rigid prism segments ($40\Delta$ to $57\Delta$) strictly in the upper and lower periphery of the lens, positioned above and below the normal pupillary axis:

$$\text{Prism Location: } \ge 5\ \text{mm above and below primary line of sight}$$

With Base pointing toward the blind hemifield:

• Central vision remains completely clear and single through the unprismatic center aperture.
• When walking, moving obstacles in the blind field (e.g., a person walking into the patient's right side) are refracted into the upper and lower seeing peripheral retina.
• The patient notices peripheral movement (focal awareness), triggering a rapid reflex head turn to identify the obstacle with their clear central vision.

Using standard Goldmann perimetry or Humphrey visual field testing, patients fitted with a $40\Delta$ Peli lens design demonstrate an immediate functional visual field expansion of $20^\circ$ to $28^\circ$ into their blind quadrant. This dramatic improvement restores confidence in pedestrian navigation, preventing catastrophic falls and pedestrian collisions.