AC/A RATIO

AC/A RATIO Measurement of the convergence induced

by accommodation, per diopter of accommodation.

To determine the change in accommodative convergence that occurs when the patient accommodates or relaxes accommodates a given amount.

Unit = ∆∕D

Normal range = ( 4 – 6) ∆/D

CONTINUE……..Abnormalities of the AC/A ratio are very

important causes of strabismus.

A high AC/A ratio may cause excessive convergence & produce a convergent squint during accommodation on a near object.

A low AC/A ratio may cause a divergent squint when the patient looks at a near object.

2 TYPES 1. Stimulus AC/A Ratio Ratio of accommodative convergence to stimulus to accommodation.

2. Response AC/A Ratio Ratio of accommodative convergence to response to accommodation.

Usually stimulus AC/A > Response AC/A

METHODS OF MEASUREMENT OF AC/A RATIO Heterophoria method

Gradient method

Fixation disparity method

Graphical method

HETEROPHORIC METHOD Simple method, consists of comparing the

measurement of the latent deviation of eyes

Using prisms & alternate cover test at a point of distance fixation ( 6m ) with refractive correction.

At a point of near fixation ( 33cm ) with refractive correction.

IPD should be measured.

+ve sign for esodeviation, -ve sign for exodeviation.

AC/A ratio is calculated from this following formula:-

AC/A = IPD + (∆n - ∆d/D)

GRADIENT METHOD The change in the stimulus to

accommodation is produced by means of ophthalmic lenses not by a change in viewing distance.

Concave lenses placed before the eyes increase the requirement for accommodation & convex lenses relax accommodation.

-1D lenses produce an equivalent of 1D of accommodation, where as +1D lenses relax accommodation by 1D.

CONTINUE……. Original deviation is found at near while

the patient wears optical correction & then with additional lens.

Ac/A ratio is calculated from this following formula:-

AC/A = ∆L - ∆O/D

FIXATION DISPARITY METHOD Used by Ogle & co-workers.

Indirect method, more reliable due to test under binocular condition.

Consists of 2 sets of data

First changes in fixation disparity induced by force convergence using prism

Second change in fixation disparity induced by altering the accommodative stimulus with lens

From these 2 sets of data they determined stimuli for convergence & accommodation that gave same fixation disparity.

Because of its complexity, this test is not performed in routine clinical practice.

GRAPHICAL METHOD Convergence (in prism diopter) is

plotted along the x- axis.

Stimulus to accommodation is indicated along the y-axis.

Slope of graph represents AC/A ratio.

USES OF AC/A RATIODiagnosis of convergence excess

type of esodeviation.

Divergence excess exodeviation

Divergence insufficiency

Convergence insufficiency

MANAGEMENT OF HIGH AC/A RATIO Apply in those patient who have chance

of obtaining some fusion when eyes are straight or within 10PD in distance with optical correction.

Try to keep eyes straight for distance upto age of 8 years.

If eso deviation is less then 10PD in distance- has chance of developing fusion with full optical correction.

If eso deviation is greater than 10PD in distance has no chance of developing fusion unless the deviation is reduced to under 10PD.

PRESCRIBING ON BASIS OF AC/A Prescribing Prism

Prescribing Visual Training

Alteration of Sph. Portion of Correction if AC/A sufficiently large additional plus for

convergence excess

CONVERGENCE EXCESS:n>d high AC/A ratioTreatment

Near add (overcorrection of plus power or under correction of minus power most effective) e.g.

Base-out prism (alternative method)

Base-in training(not so effective)

CONVERGENCE INSUFFICIENCYn> dLow AC/A ratioTreatment

Base-out training (most effective)“Pencil push-up” training

DIVERGENCE EXCESSn< dhigh AC/A ratioTreatment

overcorrection of minus power or undercorrection of plus power

in mild case base-in prism for full time wear

DIVERGENCE INSUFFICIENCYn>d low AC/a ratioTreatment

Base-out prism

THANK YOU…….