AC/A

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1. AC/A Ratio

2. 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

3. Continue…….. A bnormalities 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.

4. 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

5. Methods of measurement of ac/a ratio Heterophoria method Gradient method Fixation disparity method Graphical method

6. 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 .

7. AC/A ratio is calculated from this following formula:- AC/A = IPD + (∆n - ∆d/D)

8. 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.

9. 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

10. 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

11. 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.

12. 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.

13. Uses of AC/A ratio Diagnosis of convergence excess type of esodeviation . Divergence excess exodeviation Divergence insufficiency Convergence insufficiency

14. 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.

15. 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.

16. 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

17. CONVERGENCE EXCESS:  n >  d high AC/A ratio Treatment 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)

18. CONVERGENCE INSUFFICIENCY  n >  d Low AC/A ratio Treatment Base-out training (most effective) “Pencil push-up” training

19. DIVERGENCE EXCESS  n <  d high AC/A ratio Treatment overcorrection of minus power or undercorrection of plus power in mild case base-in prism for full time wear

20. DIVERGENCE INSUFFICIENCY  n >  d low AC/a ratio Treatment Base-out prism

21. THANK YOU…….