The Gradient AC/A Ratio: What's Really Normal?

The two most commonly used methods for determining the AC/A ratio are the Gradient Method and the Clinical Method. Though both methods are simple, practical, and often used interchangeably, they are really quite different. The Gradient AC/A measures the amount of convergence generated by a diopter of accommodative effort, while the Clinical Method detects the presence of a distance-near disparity of 10 or more prism diopters of esotropia. The purpose of this prospective study was to compare the Gradient and Clinical AC/A in 69 consecutive patients presenting with acquired nonparalytic esotropia. In addition, two methods of calculating the Gradient AC/A were compared: "minus" AC/A in which accommodation (and therefore accommodative convergence) is stimulated, and "plus" AC/A in which accommodation (and accommodative convergence) is relaxed. Mean Gradient AC/A for patients with a normal Clinical AC/A was 2:1, below the range traditionally thought of as normal. Mean Gradient AC/A for patients with a high Clinical AC/A was 5:1. Forty-four percent of those with a high Clinical AC/A had a normal or low Gradient AC/A, suggestive of nonaccommodative convergence excess. Mean Gradient AC/A with plus lenses was statistically identical to the AC/A with the minus lens method.