1. The AC/A
Ratio
Catherine L. Heyman,O.D
Assistant Professor

2. AC/A Ratio
 Definition
 Determination
 Clinical significance

3. AC/A Ratio
 Definition: The measurement of the
convergence induced by accommodation per
diopter of accommodation
 Purpose: To determine the change in
accommodative convergence that occurs
when the patient accommodates or relaxes
accommodates a given amount

4. AC/A Ratio
 Why?

5. AC/A Ratio
 Importance
 Significant in
 Diagnosis
 Treatment

6. AC/A Ratio
 Distance phoria dependant on
 Tonic vergence
 Near phoria dependant on
 AC/A ratio

7. AC/A Ratio
 Two ways to determine AC/A
 Calculation
 Gradient

8. AC/A Ratio
 Calculated AC/A
 AC/A = IPD (cm) + N (m) (D’-D)
IPD = interpupillary distance in centimeters
N = near fixation distance in meters
D’ = near phoria (eso is plus and exo is minus)
D = far phoria (eso is plus and exo is minus)

9. AC/A Ratio
 Calculation method continued
 Example: IPD = 60 mm, the patient is 2Δ
exophoric at distance and 10Δ exophoric at near
(40 cm)
AC/A = 6 + 0.4(-10 - (-2))
= 6 + 0.4(-10 + 2)
= 6 + 0.4(-8) = 6 + (-3.2)
= 2.8

10. AC/A Ratio
 Example: IPD = 56 mm, the patient is 3Δ
exophoric at distance and 7Δ esophoric at
near (40 cm)
AC/A = 5.6 + 0.4(7 -(-3))
= 5.6 + 0.4(10)
= 5.6 + 4
= 9.6

11. AC/A Ratio
 Note
 Be sure to use the correct signs for Eso and Exo
 Be sure to use correct units

12. AC/A Ratio
 Gradient AC/A
 Phoria is measured a second time using a -1.00/
+1.00 lens
 The change in phoria with the additional minus
or plus is the AC/A ratio

13. AC/A Ratio
 Gradient continued
 Example: If the near phoria is 2Δ eso through the
subjective finding and with -1.00 it is 7Δ eso
 AC/A ratio is 5/1

14. AC/A Ratio
 Gradient continued
 Example: If the near phoria is 4Δ exo through
the subjective finding and with +1.00 it is 10Δ
exo
 AC/A ratio is 6/1

15. AC/A Ratio
 Clinically both methods are used
 Calculation
 Only requires a cover test and simple math
 Gradient
 Quick and easy
 Gives Dr immediate feedback

16. AC/A Ratio
 Differences between the two methods
 Proximal vergence
 Lag of accommodation

17. AC/A Ratio
 Difference between the two methods:
 Calculated AC/A includes the effect of proximal
vergence
 Gradient AC/A measures the near phoria twice at
a fixed distance, proximal vergence is held
constant and thus does not alter the final result

18. AC/A Ratio
 Response vs Stimulus
 Alpern et.al (1959) found that the accommodative
response will be about 10% less than the stimulus.
Thus when we measure accommodative response
directly we find for a +2.50 D (40 cm) stimulus there
will be a lag of accommodation of about +0.25 to
+0.75D
 Clinically we measure the stimulus AC/A

19. AC/A Ratio
 Lag of accommodation
 Generally +0.25 to +0.50 D
 This under accommodation can significantly effect
your findings

20. AC/A Ratio
 Controlling Accommodation
 Can be a source of measurement error
 Clinician should emphasize that clarity of the target is
essential
 A target with detail will help control accommodation

21. AC/A Ratio
 Expected AC/A Ratio values
 4/1with a standard deviation of +/- 2
 Between 2-6 normal
Morgan (1944)
 High > 6/1
 Low < 2/1

22. AC/A Ratio
Ocular Deviation AC/A
 Dist = Near  IPD (cm)
 More eso at near  >IPD (cm)
 More exo at near  <IPD (cm)

23. AC/A Ratio
 Plug and chug

24. AC/A Ratio
 Review
 Clinical Examples

25. AC/A Ratio
 AC/A is a measure of the amount of
accommodative induced convergence per
unit of accommodation
 The amount convergence is affected by
changes in accommodation

26. AC/A Ratio
 Two ways to measure the AC/A Ratio
 Calculation
 Gradient

27. AC/A Ratio
 Calculation method
 Example: IPD = 60 mm, the patient is 3Δ
exophoric at distance and 3Δ exophoric at near
(40 cm)
AC/A = 6 + 0.4(-3 - (-3))
= 6 + 0.4(-3+ 3)
= 6 + 0.4(0)
= 6/1

28. AC/A Ratio
 Expected AC/A Ratio values
 4/1with a standard deviation of +/- 2
 Between 2-6 normal
Morgan (1944)
 High > 7/1
 Low < 3/1

29. AC/A Ratio
Ocular Deviation AC/A
 Dist = Near  IPD (cm)
 More eso at near  >IPD (cm)
 More exo at near  <IPD (cm)

30. AC/A Ratio
 Effect of uncorrected refractive errors on
phorias
 Myopia
 Hyperopia
 Astigmatism
 Anisometropia

31. AC/A Ratio
 Clinical Examples

32. Case #1
 AC a 7 year old girl presents with complains of
headaches and eye strain with near work
 Unaided VA’s
OD 20/20
OS 20/20
At distance and near
 Cover Test: Ortho/5EP’
 IPD=53mm

33. Case #1
 AC/A=?
 High or low?

34. Case #1
AC/A- calculation
AC/A = 5.3 + 0.4(5 - 0)
= 5.3 + 0.4(5)
= 5.3 + 2
= 7.3/1

35. Case #1
 High AC/A
 More bang for your buck

36. Case #1
 Gradient
 Trial frame + 1.00 DS
 Re measure CT at near find?

37. Case #1
 Through +1.00 DS AC’s near phoria
measures 2XP’
 Treatment
 Rx +1.00 DS OU for near only
 RTC 1 month

38. Case #1
 F/U one month after SRx
 AC asymptomatic and loves her new glasses
 The OD (YOU) are the hero!!

39. Case #2
 TA a 8 year old boy presents with complaints of
headaches and eye strain with near work
 Unaided VA’s at distance
OD 20/50
OS 20/50
At near 20/20 OD,OS,OU
 Cover Test: 2XP/10XP’
 IPD=54mm

40. Case #2
 AC/A=?
 High or low?

41. Case #2
 AC/A- calculation
AC/A = 5.4 + 0.4(-10 - (-2))
= 5.4 + 0.4(-8)
= 5.4 + (-3.2)
= 2.2/1

42. Case #2
 To accurately determine the AC/A we need
to have the best refraction in place.
 Do we have that?

43. Case #2
 Unaided VA’s at distance
OD 20/50
OS 20/50

44. Case #2
 Examination findings:
 Refraction
 OD: -1.50 DS 20/20
 OS: -1.50 DS 20/20

45. Case #2
 Management
 Rx -1.50 DS OU
 RCT 1 month
 What do we expect?

46. Case #2
 F/U visit one month after Rx wear
 Aided VA’s at distance
OD 20/20
OS 20/20
At near 20/20 OD,OS,OU
 Cover Test: 2XP/8XP’
 BO: 10/12/5
 Pt. still symptomatic c/o HA’s and asthenopia

47. Case #2
 Treatment
 Vision Therapy
 Treat the compensating vergence

48. Binocular Vision Dysfunction
 3XP---10XP’
 Low AC/A (Convergence Insufficiency) CI
 8EP---3EP’
 Low AC/A Divergence Insufficiency (DI)
 3EP---6EP’
 High AC/A Convergence Excess (CE)
 10XP---5XP’
 High AC/A Divergence Excess (DE)

49. Conclusion
 AC/A is a tool that is often used in clinical
decision making
 Questions?