Esotropia associated with early presbyopia caused by inappropriate muscle length adaptation.

Background: The purpose of this study is to investigate the occurrence of esotropia accompanying early presbyopia. The two primary long-term mechanisms for maintenance of ocular alignment are vergence adaptation (neurologic) and muscle length adaptation (anatomic). Both mechanisms depend upon disparity-driven motor fusion for proper operation. A possible cause for an esotropic shift in early presbyopic adults with insufficient or absent disparity-driven motor fusion is inappropriate muscle length adaptation (medial rectus muscle shortening) occurring in response to increased convergence tonus accompanying increased accommodative effort.

Methods: Of 617 patients, age 10 and older who underwent surgery for esotropia during the period of 1980 to 1996, the age when the deviation occurred or worsened could be determined with confidence in 140. A plot was made of the number of these patients versus the age of onset of the deviation. This was compared with a similar plot of patients operated for exotropia.

Results: A statistically significant increase (P = 0.017) in the incidence of an esotropic shift in the age range from 30 to 50 years was found when compared with the incidence of an exotropic shift.

Conclusion: If the postulated mechanism is correct, full correction of any hyperopia as well as prompt prescription of a reading add (or conversion to monovision correction) may help prevent further progression of small esodeviations accompanying early presbyopia.