Amplitude of accommodation for different head positions and different directions of eye gaze.

One of the theories of the mechanism of presbyopia is the Hess-Gullstrand theory. This theory predicts that amplitude of accommodation should increase in downgaze because of movement of the lens under the influence of gravity, and that this should be more marked for older than for younger subjects. We assessed this theory by measuring the effect of direction of eye gaze and head posture on amplitude of accommodation for two age groups. Farpoints, nearpoints, and amplitude of accommodation were determined for two subject groups, a young group aged 18 to 25 years and an older group aged 35 to 45 years. Small but significant shifts of nearpoints toward the eye were observed when head position or eye gaze was shifted from above to below the horizontal, for the younger observers only (the maximum mean difference between conditions was 1.1 D, compared with a mean accommodation level of 9.8 D for this young group). Previous studies by others found changes in the same direction for eye gaze and head position, respectively, but our changes were much smaller. We do not believe that particular care is needed in selection of head position and eye gaze during clinical measurements of the amplitude of accommodation. Because the shift of the nearpoint was noted only for the younger group, this study does not support the Hess-Gullstrand theory of presbyopia.