📏

Measuring ACA and PCA Ratios Guidelines

Aug 19, 2024

Clinically Measuring ACA and PCA Ratios

Types of Convergences

  • Accommodative Convergence
  • Proximal Convergence
  • Tonic Convergence
  • Fusional Convergence

Focus of the Video

  • Mainly affected in concomitant strabismus: Accommodative convergence and proximal convergence
  • Subtle and incomparable: Fusional and tonic convergences

Accommodative Convergence (AC/A Ratio)

  • Definition: Degree of convergence induced by accommodation
  • AC/A Ratio: Total accommodative convergence induced per diopter of accommodation
  • Normal Range: 3 to 5 prism diopters for every diopter of accommodation

Proximal Convergence (PCA Ratio)

  • Definition: Convergence stimulated by a sense of nearness
  • Normal Range: 1 to 3 prism diopters for every diopter of accommodation

Measuring ACA and PCA Ratios

Required Measurements

  • Three different PCT results:
    • Distance
    • Near
    • Either near or distance using plus or minus lenses

Distance Measurement Example

  • PCT Result: EXO phoria of 8 prism diopters at distance
  • Importance: Patient must be encouraged to accommodate in the distance

Near Measurement Example

  • PCT Result: EXO phoria of 2 prism diopters at near

Inducing Accommodation with Lenses

  • Use minus lenses at distance to induce accommodation
  • Example: Using -3 lenses, result is EXO phoria of 3 prism diopters

Calculating the Ratios

Methods

  • Gradient Method
  • Hetero Phoria Method

Gradient Method

  • Deviation:
    • Accommodating most: -3
    • Accommodating least: -8
  • Lens Accommodation: -3
  • Result: AC/A ratio = 1.7, outside normal range (3-5)

Hetero Phoria Method

  • Deviation at Near: -2
  • Deviation at Far: -8
  • Intercalary Distance: 6 cm
  • Result: Final value = 8

PCA Ratio Calculation

  • PCA Ratio Formula: Subtract gradient method result from hetero phoria method result
  • Result: PCA ratio of 6.3, higher than normal range

Implications for Diagnosis

  • Indications for Measuring Ratios:
    • When strabismus or eye turn is greater at a certain distance
    • Determine if variability in ACA/PCA ratio contributes
  • Diagnosis: ACA and PCA ratios alone do not provide a complete diagnosis

Conclusion

  • Holistic Approach Needed: Consider entire patient profile for diagnosis and treatment
  • Measure Ratios: When strabismus is noticeably different at various distances

Note: Observations made in the video discuss the process and importance of measuring ACA and PCA ratios to understand and diagnose strabismus better.

Full transcript