To determine the near point of convergence, the patient fixates on an object in the midsagittal plane, and the object, positioned initially at 40 cm, is moved toward the patient until 1 eye loses fixation and turns out. The distance from the object to the patient is then measured, giving the near point of convergence, which is normally 8–10 cm or less. The eye that maintains fixation is considered the dominant eye. This test does not distinguish between fusional convergence and accommodative convergence.
Accommodative convergence/accommodation ratio
The accommodative convergence/accommodation (AC/A) ratio is defined as the amount of convergence (in prism diopters) per unit change in accommodation (in diopters). There are 2 methods of clinical measurement (see also BCSC Section 3, Clinical Optics):
The gradient method derives the AC/A ratio by stimulating a change in accommodation using lenses, and dividing the resulting change in deviation (in prism diopters) by the change in lens power. An accommodative target must be used, and the working distance (typically 33 cm or 6 m) is held constant. Plus or minus lenses (eg, +1.00, +2.00, +3.00, −1.00, −2.00, −3.00) are used to vary the accommodative requirement (plus lenses at distance can be used only with uncorrected hyperopia). This method measures the stimulus AC/A ratio, which may differ from the response AC/A ratio. The latter can be determined only by simultaneously measuring the refractive state of the eyes to quantify the change in accommodation actually produced.
In the heterophoria method, the measurements of the distance and near deviations are used, along with the interpupillary distance, to calculate the AC/A ratio. Comparing distance and near deviations gives a rough clinical estimate. In accommodative esotropia, a near deviation exceeding distance deviation by 10Δ or more is considered to represent a high AC/A ratio. Note, however, that in intermittent exotropia, the near deviation may be smaller than the distance deviation despite a normal AC/A ratio, because of tenacious proximal fusion (see Chapter 9).
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.