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A Glasser, AS Vilupuru, S Kasthurirangan; Dynamics of Accommodative Fatigue in Rhesus Monkeys and Humans . Invest. Ophthalmol. Vis. Sci. 2002;43(13):2014.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: To study changes in accommodative dynamics following repetitive, fatiguing accommodative tasks in rhesus monkeys and humans. Prior studies of accommodative fatigue in humans measured changes in tonus and adaptability of the accommodative system. Repeated central stimulations of accommodation in rhesus monkey resulted in complete loss of ciliary body movement. No prior studies have systematically explored how accommodative dynamics change with repeated accommodative events. The present study was undertaken to better understand the nature and possible sites of fatigue using dynamic analyses. Methods: In one young rhesus monkey, maximal centrally stimulated accommodative responses of 10 D were measured dynamically with infrared photorefraction in the left eye. Repeated accommodative events were induced using three paradigms: 4sec. on 4sec. off (4x4), 4x1.5 and 2x1 for at least 15 min. in each case. In humans, a PowerRefractor (PlusOptix) was used to dynamically measure repeated binocular accommodative responses to step changes in target position. Responses to two fatiguing paradigms (5x5-5D, 2x2-6D) were recorded from the right eye of one subject (25 years). In each paradigm, 20 accommodative responses at equal time intervals were analyzed. Results: In both monkey and human, peak accommodative amplitude did not decrease significantly during any of the experimental trials. In the monkey, accommodative state was maintained for the duration of each accommodative stimulus with the 4x4 and 2x1 paradigms, but not for the 4x1.5 paradigm. The 4x1.5 paradigm also resulted in a progressive decrease of 28% and 24% in the peak velocity of accommodation and disaccommodation respectively. The 2x1 paradigm produced a 21% decrease in disaccommodative peak velocity. No changes in any of the dynamic metrics were observed with the 4x4 paradigm. In the human, the 2x2 and 5x5 paradigms produced a 56% decrease and a 12% decrease in the disaccommodative peak velocities respectively. Conclusion: Accommodative fatigue is experimental paradigm and measurement criteria dependent and is not necessarily expressed as a decreased amplitude. Dynamic measures such as peak velocity provide a good indication of accommodative fatigue in the absence of a decline in amplitude. Accommodative fatigue in the anesthetized rhesus monkey may be of neuromuscular rather than neurological origin, implying that the accommodative plant may fatigue. In the awake, behaving human, neuromuscular fatigue and fatigue in the accommodative/vergence control system could have augmented the decrease in peak velocity of disaccommodation in the 2x2 paradigm.
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