Community-based trial of a peripheral prism visual field expansion device for hemianopia

Abstract

Objective: To determine the functional utility for general mobility of peripheral prism glasses, a novel visual field expansion device for hemianopia, in a large-scale, community-based, multicenter study with long-term follow-up.

Methods: Forty-three participants with homonymous hemianopia were fitted with temporary press-on Fresnel peripheral prism segments of 40 prism diopters. Follow-up questionnaires evaluating functional benefits for mobility were administered in the office at week 6. Participants who continued wearing the prisms were interviewed again by telephone after a median of 12 months. Primary outcome measures included clinical success (a clinical decision to continue wear) and 5-point ratings of prism helpfulness for obstacle avoidance when walking.

Results: Thirty-two participants (74%) continued prism wear at week 6, and 20 (47%) were still wearing the prisms after 12 months (median time, 8 hours per day). These participants rated the prism glasses as very helpful for obstacle avoidance and reported significant benefits for obstacle avoidance in a variety of mobility situations. Success rates varied among clinic groups (27%-81%), with higher rates at the clinics that fitted more patients.

Conclusion: Our results demonstrate the functional utility of peripheral prism glasses as a general mobility aid for patients with hemianopia.

Figure 1

Press-on™ 40Δ Fresnel peripheral prism segments placed base out on the left spectacle lens of a left hemianope (11mm inter-prism separation). The patient has an uninterrupted binocular view through the central prism-free area of the lens. The prism segments provide field expansion in the upper and lower peripheral fields (Figure 2). For bifocal users, a small aperture was cut from the lower segment to enable short duration reading. Due to the angle from which the photo was taken, the top of the lower prism segment appears closer to the pupil center than the 6mm below at which it was fitted.

Figure 2

Binocular visual field (Goldmann V4e) of a left hemianopic patient (a) without peripheral prisms and (b) with 40Δ peripheral prisms fitted at 11mm inter-prism separation showing about 20° horizontal field expansion in the upper and lower peripheral fields. Dashed line represents the extent of the normal binocular visual field.

Figure 3

Participant flow through the study.

Figure 4

Spectacles with permanent 40Δ Fresnel prism segments developed by Chadwick Optical, Inc., shown for a patient with right hemianopia. A small bifocal segment (outlined for the purposes of illustration) was placed below the lower prism segment for patients who needed a reading correction; the bifocal segment for the fellow eye was set at the same height.

Figure 5

Percentage of participants fitted with prisms continuing prism wear throughout the study. a) All participants fitted with prisms. b) Participants grouped by final status. X - time point at which each participant who continued long-term prism wear was last interviewed.

Figure 6

Prism fitting positions at week 6; participants grouped by final status. There were no differences in prism fitting positions or inter-prism separations for participants who discontinued and continued prism wear (p > 0.2). The thick horizontal line within the box represents the median of the distribution. The vertical extent of the box represents the interquartile range (IQR). The vertical lines extending from the ends of the box represent 1.5x IQR. Open circles represent data points that are outliers (1.5x to 3x IQR) and asterisks represent extreme (far) outliers (> 3x IQR)

Figure 7

Questionnaire responses at week 6; participants grouped by final status. (a) Reported daily wearing times. (b) Ratings of vision comfort. (c) Ratings of how helpful the prisms were for detecting obstacles in time to avoid them when walking. Participants who continued long-term prism wear reported longer daily wearing times, higher ratings of vision comfort and prism-helpfulness for obstacle avoidance than participants who discontinued wear at week 6. Rating scale: 1 not comfortable/helpful at all; 5 very comfortable/helpful.