This document discusses the AC/A ratio, which is the ratio of accommodative convergence to accommodation. It defines the AC/A ratio and notes the normal range is 3-5 prism diopters per diopter of accommodation. Abnormal AC/A ratios can cause strabismus. The document outlines methods to measure the AC/A ratio clinically and discusses its uses in diagnosing different types of strabismus and their management approaches.

1. AC/A RATIO
Presented by
Dr Anupama Manoharan
First year – D.O
Stanley Medical College
Chennai

2. INTRODUCTION
The Concept of AC/A Ratio
Designed by Fry who later with Haines introduced the abbreviation AC/A.
Definition
The ratio of the amount of accommodative convergence measured (in prism dioptre) to the
number of dioptres of accommodation which causes the convergence, other factor causing
convergence remaining constant. (Proposed by Fry)
The relationship between accommodation convergence and accommodation is constant
throughout the life in emmetropes.
The normal value of ac/a ratio is 3-5 prism dioptre for one dioptre of accommodation.
Unit expressed in ∆/D.

3. SIGNIFICANCE OF AC/A RATIO
• Has effect on near phoria.
• 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 look at a near
object.

4. TYPES
1. Stimulus AC/A ratio
Amount of accommodative convergence produced by a change of 1D in the
stimulus to accommodation.
Change in convergence is related to change in stimulus.
2. Response AC/A ratio
Amount of accommodative convergence produce by an actual change of 1D of
accommodation.

5. RESPONSE VS STIMULUS
• Usually Stimulus AC/A > Response AC/A.
• In practice - Stimulus AC/A is used.
• Difficult to measure - Response AC/A.
 Not appropriate to measure in older age (accommodation gradually decreased)
 Useful in younger Patient with binocular vision problems
• Alpern et. al (1959) found that response will be 10% less than the stimulus . Thus when we
measure accommodation response directly, we find for a +2.50D (40cms) stimulus there will be
a lag of accommodation of about +0.25 to +0.75 D.

6. METHODS OF MEASUREMENT OF AC/A RATIO
• Heterophoria method
• Gradient method
• Fixation disparity method
• Graphical method

7. TWO WAYS TO DETERMINE AC/A RATIO
• Clinical calculation
• Gradient method
 A simple comparison of deviation in distance and near fixation is commonly
used in clinical practice to estimate the AC/A ratio.

8. HETEROPHORIC METHOD
• Simple method , consists of comparing the measurement of the latent deviation of eye.
• Using prisms & alternate cover test at a point of distance fixation (6m) with full refractive correction
• At a point of near fixation (33 cm ) with refractive correction.
• Along with target like Snellen’s test type letter (to ensure a steady accommodation both at distance
and near ) or Maddox tangent scale.
• IPD should be measured.
• +ve sign for esodeviation, -ve sign of exodeviation.
• AC/A ratio is calculated from this following formula.
• AC/A = IPD +(N-D’/D).

9. CALCULATED AC/A RATIO
AC/A = IPD +[N - D¹] / D
Where,
IPD = interpupillary distance in centimetres
N = near fixation distance in meters
D¹= near phoria (eso is plus and exo is minus)
D = far phoria (eso is plus and exo is minus)

10. CALCULATION METHOD - CONTINUED
Ocular deviation AC/A
Distance = near IPD(cm)
More eso at near > IPD(cm)
More exo at near < IPD(cm)

11. GRADIENT METHOD
• The change in the stimulus to accommodation is produced by means of ophthalmic lenses and not
by a change in viewing distance.
• Concave lenses placed before the eyes increase the requirement for accommodation for the same
distance both for producing clear binocular single vision.
• Convex lenses relax accommodation.
• -1 D lenses produce an equivalent of 1D of accommodation,
• +1D lenses relax accommodation by 1D
• Estimation of AC/A : The difference between the deviation produced by placing spherical lenses
and the original deviation in prism dioptres, with a division of the difference by the power of the
lenses used in the determination.
AC/A=∆ⁱ−∆⁰/ D

12. FIXATION DISPARITY METHOD
• Used by Ogle & co-worker.
• Indirect method.
• More reliable due to test under binocular condition
• Consists of 2 set of data.
 First change in fixation disparity induced by force convergence using prism.
Second change in fixation disparity induced by altering the accommodative
stimulus with lens.

13. FIXATION DISPARITY - CONTINUED
• From these 2 sets of data they determined stimuli for convergence &
accommodation that gave same fixation disparity.
• Because of its complexity , the test is not performed in routine clinical
practice.

15. GRAPHICAL METHOD
• Convergence (in prism diopeter ) is plotted along the x axis.
• Stimulus to accommodation is indicated along the y – axis.
• Slope of graph represents AC/A ratio.

16. GRAPHICAL METHOD

17. USES OF AC/A RATIO
• Diagnosis of convergence excess type of esodeviation
• Divergence excess esodeviation
• Divergence insufficiency
• Convergence insufficiency

18. MANAGEMENT OF HIGH AC/A RATIO – USING OPTICAL
• Apply in those patient who have chance of obtaining some fusion when the
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 than 10PD in distance has chance of developing
fusion with full optical correction
• If exodeviation is greater than 10PD in distance has no chance of
developing fusion unless the deviation is reduced to under 10PD

19. MANAGEMENT OF HIGH AC/A RATIO –
PHARMACOLOGICALLY
• Mitotic
• 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.

20. CONVERGENCE EXCESS
• N>D’
• High AC/A ratio
Treatment
 Near add (over correction of plus power or under correction of minus power most effective )
 Base – out prism ( alternative method )
 Base in training

21. DIVERGENCE EXCESS
• N<D
• High AC/A ratio
Treatment
 Over correction of minus power or under correction of plus power
 In mild case base in prism for full time wear.

22. DIVERGENCE INSUFFICIENCY
• N>D
• Low AC/A ratio
Treatment
Base out prism

23. Thank you