The accommodative convergence/accommodation (AC/A) ratio is defined as the amount of convergence measured in prism diopters per unit (diopter) change in accommodation. There are 2 methods of clinical measurement (see also BCSC Section 3, Clinical Optics): the gradient method and the heterophoria method.
The gradient method arrives at the AC/A ratio by dividing the change in deviation in prism diopters by the change in lens power. An accommodative target must be used, and the working distance (typically at 1/3 m or 6 m) is held constant. Plus or minus lenses (eg, +1, +2, +3, –1, –2, –3) are used to vary the accommodative requirement. This method measures the stimulus AC/A ratio, which is not necessarily identical to the response AC/A ratio. The latter can be determined only with the use of an optometer that records the change in accommodation actually produced.
The heterophoria method employs the distance–near relationship, measuring the distance and near deviations. A similar alignment is normally present for distance and near fixation. If the patient is more exotropic or less esotropic at near, too little convergence, or a low AC/A ratio, is present; if the patient is more esotropic or less exotropic at near, a high AC/A ratio is present. In accommodative esotropia, an increase of esotropia of 10Δ or more from distance to near fixation is considered to represent a high AC/A ratio.
An abnormally high AC/A ratio can be managed optically, pharmacologically, or surgically. For example, plus lens spectacles for hyperopia reduce accommodation and therefore reduce accommodative convergence. This principle is the mainstay of the medical management of esotropia. Bifocals reduce or eliminate the need to accommodate for near fixation. This optical management may be used for excess convergence at near—that is, an esodeviation greater at near. Underplussed or overminused spectacles create the need for greater-than-normal accommodation. This excess accommodation creates more accommodative convergence and is occasionally used to reduce an exodeviation.
Long-acting cholinesterase inhibitors (eg, echothiophate iodide) can be used to decrease accommodative convergence. These drugs act directly on the ciliary body, facilitating transmission at the myoneural junction. They reduce the central demand for accommodative innervation and thus reduce the amount of convergence induced by accommodation.