Optometry Lecture: Accommodative Convergence to Accommodation Ratio (AC/A Ratio)
Introduction
- AC/A Ratio: Indicates the relationship between accommodative convergence and accommodation.
- Importance: Useful in diagnosing and managing binocular vision anomalies.
Key Concepts
- Accommodation and Convergence: Interrelated processes that develop together.
- Accommodative Convergence: Convergence occurring when the eyes accommodate.
Methods to Measure AC/A Ratio
- Gradient Method
- Calculated Method
Gradient Method
- Required Tools:
- Paint watch
- Maddox rod
- Trial frame
- Modified Thorington chart
- Procedure:
- Use a Maddox rod on a trial frame.
- Patient looks at the center of the chart at 40 cm.
- Stimulate accommodation with negative lenses (-1 or -2 diopters).
- Measure the change in phoria with the lenses.
- Example:
- Without lenses: 6 exophoria
- With -1 diopter: 2 exophoria
- Change: 4
- AC/A ratio: 4:1
- Alternative Example:
- Change from 6 exo to 4 eso with -1 diopter = AC/A ratio of 10:1
- If using -2 diopters: 10:2 or 5:1
Calculated Method
- Required Values:
- Interpupillary distance (IPD) in cm
- Near and distance phoria
- Formulas:
- ( AC/A = \frac{IPD + (Near Phoria - Distance Phoria)}{2.5} )
- ( AC/A = \frac{IPD + (Near Phoria - Distance Phoria) \times Near Functional Distance}{1} )
- Near Functional Distance: 0.4 meters for 40 cm
- Procedure:
- Measure IPD in cm.
- Perform cover-uncover test to find phoria type.
- Use prisms to neutralize phoria (base out for esophoria, base in for exophoria).
- Repeat for distance phoria.
- Example:
- IPD: 6 cm
- Near phoria: 5pd eso
- Distance phoria: 10 exophoria
- Both formulas yield AC/A ratio of 12:1
Conclusion
- Recap: Different methods to calculate the AC/A ratio.
- Application: Diagnostic tool for binocular vision issues.
Note: For hands-on demonstration, please refer to the video linked above.
Additional Resources
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