Oblique Multi-periscopic Prism for Field Expansion in Homonymous Hemianopia Driving

Abstract

Purpose : Peripheral prisms have been used for field expansion in homonymous hemianopia (HH). We recently developed multi-periscopic prism (MPP) using half-penta prisms, which provided higher prism power (45°) with wider eye scanning (15°) and better image quality. For a useful field expansion through a car windshield, we further develop oblique peripheral MPP that provides a vertical shift as well as a horizontal shift.

Methods : Five half-penta prisms that provide 45° angular shift through a double internal reflection were arranged in a cascade to enable 45° wide field expansion into the blind side. Since the peripheral prisms are located 20° above and below the horizontal midline, we designed the oblique MPP to achieve 18° vertical shift toward the horizontal midline by rotating the half-penta prisms (20° relative to the vertical meridian). This reduces the lateral shift to 42°. To minimize the obscuration scotoma, the half-penta prisms were also tilted (pitch) 20° to match the line of sight. A 3D printed module was developed to mount the prisms at the required position and orientation in front of the carrier lens (Fig. 1). Field expansion through the oblique MPP was verified using optical simulations (LightTools and KeyShot) and also measured (Goldmann perimetry) for a patient with right HH.

Results : Monocular perimetry demonstrated about 36° field expansion with almost 17° vertical shift. The oblique MPP provided the desired field expansion to the blind side through the car windshield (Fig 2). The slight deviation from the optical simulation results may be attributed to the prototyping process and perimetry measurement errors.

Conclusions : The oblique peripheral MPP provides vertically shifted field expansion toward the horizontal midline. The expanded field covers the view through the car windshield, necessary for HH driving. The current configuration of the oblique peripheral MPP may be improved to reach even wider field expansion.

This is a 2021 ARVO Annual Meeting abstract.

 

Fig 1: Upper oblique peripheral MPP prototype for right HH fitted over the left eye; The yellow line is passing through the lens optical center. The rightmost half-penta prism supports eye scanning to the right.

 

Fig 2: Monocular Perimetry of a right HH patient wearing an upper oblique peripheral MPP. The red dotted rectangle represents the field of view through the windshield.