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The AC/A RatioCatherine L. Heyman,O.D Assistant Professor

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AC/A Ratio Definition Determination Clinical significance

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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

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AC/A Ratio Why?

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

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

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

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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)

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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

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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

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

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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

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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

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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

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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

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

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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

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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

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

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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

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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

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AC/A RatioOcular Deviation AC/A Dist = Near  IPD (cm) More eso at near  >IPD (cm) More exo at near  <IPD (cm)

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AC/A Ratio Plug and chug

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AC/A Ratio Review Clinical Examples

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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

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

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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

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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

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AC/A RatioOcular Deviation AC/A Dist = Near  IPD (cm) More eso at near  >IPD (cm) More exo at near  <IPD (cm)

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

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AC/A Ratio Clinical Examples

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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

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Case #1 AC/A=? High or low?

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

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

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

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

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

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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

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Case #2 AC/A=? High or low?

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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

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

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

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

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

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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

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

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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)

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