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Acaratio

  1. 1. The AC/A Ratio Catherine L. Heyman,O.D Assistant Professor
  2. 2. AC/A Ratio  Definition  Determination  Clinical significance
  3. 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. 4. AC/A Ratio  Why?
  5. 5. AC/A Ratio  Importance  Significant in  Diagnosis  Treatment
  6. 6. AC/A Ratio  Distance phoria dependant on  Tonic vergence  Near phoria dependant on  AC/A ratio
  7. 7. AC/A Ratio  Two ways to determine AC/A  Calculation  Gradient
  8. 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. 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. 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. 11. AC/A Ratio  Note  Be sure to use the correct signs for Eso and Exo  Be sure to use correct units
  12. 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. 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. 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. 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. 16. AC/A Ratio  Differences between the two methods  Proximal vergence  Lag of accommodation
  17. 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. 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. 19. AC/A Ratio  Lag of accommodation  Generally +0.25 to +0.50 D  This under accommodation can significantly effect your findings
  20. 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. 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. 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. 23. AC/A Ratio  Plug and chug
  24. 24. AC/A Ratio  Review  Clinical Examples
  25. 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. 26. AC/A Ratio  Two ways to measure the AC/A Ratio  Calculation  Gradient
  27. 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. 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. 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. 30. AC/A Ratio  Effect of uncorrected refractive errors on phorias  Myopia  Hyperopia  Astigmatism  Anisometropia
  31. 31. AC/A Ratio  Clinical Examples
  32. 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. 33. Case #1  AC/A=?  High or low?
  34. 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. 35. Case #1  High AC/A  More bang for your buck
  36. 36. Case #1  Gradient  Trial frame + 1.00 DS  Re measure CT at near find?
  37. 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. 38. Case #1  F/U one month after SRx  AC asymptomatic and loves her new glasses  The OD (YOU) are the hero!!
  39. 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. 40. Case #2  AC/A=?  High or low?
  41. 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. 42. Case #2  To accurately determine the AC/A we need to have the best refraction in place.  Do we have that?
  43. 43. Case #2  Unaided VA’s at distance OD 20/50 OS 20/50
  44. 44. Case #2  Examination findings:  Refraction  OD: -1.50 DS 20/20  OS: -1.50 DS 20/20
  45. 45. Case #2  Management  Rx -1.50 DS OU  RCT 1 month  What do we expect?
  46. 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. 47. Case #2  Treatment  Vision Therapy  Treat the compensating vergence
  48. 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. 49. Conclusion  AC/A is a tool that is often used in clinical decision making  Questions?

Editor's Notes

  • More purely AC/C
  • More purely AC/C
  • In this case more a combo of AC/C and tonic
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