accommodative-convergence by jubairiya firoz

1. ACCOMMODATIVE-CONVERGENCE
Accommodation and convergence are inter-related and they develop together so that a single
clear image is appreciated. The ratio accommodative-convergence (AC) over accommodation
(A) indicates the relationship between the amount of convergence produced by a stimulus to
accommodate and the amount of accommodation which produces that convergence. To see
something clearly and singly at 1 meter, 1 meter angle of convergence between the two eyes and
1 dioptre of accommodation are exerted. This is known as 1 : 1 relationship. Clinically, however,
it is easier to compare one person with another by measuring convergence in prism dioptres
(P.D.). Six prism dioptres (P.D.) = 1 meter angle when the interpupillary distance (I.P.D.) is 60
mm. Therefore, to see clear as a single image at one meter, 6 P.D. of convergence and one
dioptre of accommodation are being used. Two-thirds of this convergence that takes place (i.e. 4
Prism Dioptre) being accommodative the AC/A ratio becomes 4 : 1.
There are 2 methods of clinical measurement:
1. Gradient Method
2. Heterophoria Method.
1. 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 (e.g. +1, +2, +3, –1, –2, –3) are used to vary the accommodative
requirement.
AC/A= (∆L-∆O) / D
∆ L = deviation in prism dioptre when lenses are put; ∆ O = Original deviation in prism dioptre; D = power of lens used in
dioptre.

2. 2. 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.
AC/A= IPD+ (∆n-∆d)Xd
IPD – distance IPD (cm), ∆n- near deviation, ∆d – distance deviation, D – distance (m)
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.
SN Binocular Vision anomaly Ac/A Deviation
1 Convergence Excess High ∆n > ∆d
2 Convergence Insufficiency Low ∆n > ∆d
3 Divergence Excess High ∆d > ∆n
4 Divergence Insufficiency Low ∆d > ∆n