A 6-year-old patient has a 10∆ esophoria at distance and a right 40∆ esotropia at near. Optimum refractive correction of +3.00DS for eye reduces the deviation at near to 20∆ esotropia. There are no signs that this has a sinister neurological cause. What is the most likely cause of this anomaly? What is a reasonable course of action?
The near deviation is reduced by 20∆ with +3.00Ds. This means that by reducing the accommodative response by 3DS the deviation is reduced by 20∆. The sum 20/3 (AC/A-accommodative convergence/accommodative stimulus) gives an AC/A ratio of just under 7. The normal AC/A ratio has a range of between 3 and 4. This means that the patient has a convergence excess esotropia. A further +3.00DS will reduce the near deviation to around zero but if prescribed in a single vision lens the distance vision would be very blurred. A reasonable course of action would be to prescribe a bifocal with a +3 add in the near portion. The best type is an executive style where all of the lower half of the lens (the segment) carries the near addition. The segment has an obvious flat top and this along with its large size make it the best choice for children. Spectacles should be worn all waking hours. There is likely to be amblyopia in the right eye. This could improve just through spectacle wear but is unlikely to reach normal levels without occlusion therapy. It may be possible when the child is older (10-12 years) to gradually reduce the power of the addition, over a period of several months, as the child learns to accommodate without converging.