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- 1. The AC/A RatioCatherine 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 RatioOcular 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 RatioOcular 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 20/20At distance and near Cover Test: Ortho/5EP’ IPD=53mm
- 33. Case #1 AC/A=? High or low?
- 34. Case #1AC/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 20/50At 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 20/20At 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?

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