Development of visual acuity in experimentally strabismic monkeys

1. The purpose of this study was to document the time course for the development of strabismic amblyopia in infant monkeys, and to evaluate the factors involved in the production of amblyopia. Twenty-one monkeys were raised with experimentally produced strabismus; 14 had esotropia surgically induced and 7 had esotropia induced by injection of Botulinum A neurotoxin into the lateral rectus muscle. The ages of induction of esotropia ranged from 1 to 15 weeks. 2. Amblyopia, defined as a difference in grating acuity between the eyes of greater than one octave (factor of two), developed in 67% of these monkeys. 3. The time course of amblyopia development was different on average for the two groups of esotropes. In the surgical group, the most common time course for amblyopia development showed a period of continued normal, parallel acuity development in both eyes for a period of weeks following surgery before amblyopia began to appear in the deviated eye. In the neurotoxin group, amblyopia generally appeared soon after the induction of esotropia. 4. The development of amblyopia was most clearly associated with the pattern of fixation. Animals that adopted a unilateral fixation pattern were more likely to develop amblyopia. A multiple regression analysis revealed that age of esotropia onset, refractive error, and alternation percentage together accounted for 39% of the variance in the extent of the acuity difference between the eyes for the entire group of monkeys. 5. A separate multiple regression analysis was performed to analyze the variance in the extent of amblyopia for the group of 14 amblyopic monkeys. The analysis revealed that the size of the esotropic deviation and the refractive error of the deviated eye accounted for a significant proportion of the variance.