What does the accommodative convergence to accommodation (AC/A) ratio represent, and what is a simple method to estimate it?

What does the accommodative convergence to accommodation (AC/A) ratio represent, and what is a simple method to estimate it?

• A. AC/A is the total vergence response required for stable fusion at all viewing distances.
• B. AC/A is the amount of accommodative convergence per diopter of accommodation; estimated by retinoscopy.
• C. AC/A is the amount of accommodative convergence per diopter of accommodation; it can be estimated by comparing near and distance phorias with lenses or using a gradient method.
• D. AC/A measures the speed of accommodative changes during focusing.

Answer: (C)

AC/A ratio tells you how much accommodative convergence the eyes produce for each diopter of accommodation. In other words, it’s the amount of vergence (prism diopters of convergence) that occurs per one diopter of focusing effort. Clinically, a higher AC/A means more convergence is triggered when you accommodate, which can affect near alignment and stability of fusion. A simple way to estimate it is to see how a person’s phoria changes as you change their accommodative demand. Two common approaches work: - Compare near and distance phorias while adjusting accommodation with lenses (the heterophoria method). Measure the phoria at a distance, then at a near target, using lenses to alter the amount of accommodation. The difference in phoria (in prism diopters) divided by the known change in accommodation (diopters) gives the AC/A. - Use a gradient (or lens-difference) method. Keep the target at a fixed distance and incrementally change the accommodative demand with small lenses (for example ±1 diopter) while measuring the phoria. The ratio of the phoria change to the change in accommodation provides the AC/A. Retinoscopy isn’t typically used to estimate AC/A, so it’s not part of the standard estimation methods.

AC/A ratio tells you how much accommodative convergence the eyes produce for each diopter of accommodation. In other words, it’s the amount of vergence (prism diopters of convergence) that occurs per one diopter of focusing effort. Clinically, a higher AC/A means more convergence is triggered when you accommodate, which can affect near alignment and stability of fusion.

A simple way to estimate it is to see how a person’s phoria changes as you change their accommodative demand. Two common approaches work:

• Compare near and distance phorias while adjusting accommodation with lenses (the heterophoria method). Measure the phoria at a distance, then at a near target, using lenses to alter the amount of accommodation. The difference in phoria (in prism diopters) divided by the known change in accommodation (diopters) gives the AC/A.

• Use a gradient (or lens-difference) method. Keep the target at a fixed distance and incrementally change the accommodative demand with small lenses (for example ±1 diopter) while measuring the phoria. The ratio of the phoria change to the change in accommodation provides the AC/A.

Retinoscopy isn’t typically used to estimate AC/A, so it’s not part of the standard estimation methods.