Understanding Positive and Negative Accommodation

Aug 19, 2024

Lecture Notes on Positive and Negative Relative Accommodation

Introduction

  • Positive Relative Accommodation (PRA) and Negative Relative Accommodation (NRA) measure the maximum ability to stimulate accommodation while maintaining binocular single vision.
  • They help determine:
    • Normal accommodative functions
    • Accommodation excess
    • Accommodation insufficiency
    • Status of fusional convergence and vergence
    • Refractive corrections

Terminology

  • Positive Relative Accommodation (PRA):

    • Accommodation is stimulated using minus lenses.
    • Called "positive" because minus lenses stimulate accommodation.

  • Negative Relative Accommodation (NRA):

    • Accommodation is relaxed using plus lenses.
    • Called "negative" because plus lenses relax accommodation.

Prerequisites for Testing

  • Refractive Status: Distance correction should be in the trial frame before starting.
  • Room Condition: Daylight conditions.
  • Target: Near vision chart or card with fixation size N6 or last line readable by the patient.
  • Test Distance: 40 cm in front of the patient.
  • Equipment: Trial box with positive and negative lenses, trial frame.

Measurement of Positive Relative Accommodation

  1. Patient sits comfortably, fixes on near vision target at 40 cm.
  2. Minus lenses added binocularly in 0.25 diopters steps until first persistent blur.
  3. Example:
    • Start with -0.25, increase stepwise till persistent blur.
    • Value causing first persistent blur = PRA.
  4. Expected PRA value between -1.5 to -2.5 diopters.
    • Less than -1.5: Accommodation insufficiency.
    • More than -2.5: Accommodation excess or inadequate fusional divergence.

Interpretation of PRA Test

  • High PRA values may indicate:
    • Overcorrection in hypermetropia.
    • Undercorrection in myopia.
  • Explanation: Uncorrected hyperopia and undercorrected myopia cause focus in front of the retina; extra minus power needed increases PRA.

Measurement of Negative Relative Accommodation

  1. Patient sits comfortably with near vision target at 40 cm.
  2. Plus lenses added binocularly in 0.25 diopters steps until first persistent blur.
  3. Value causing first persistent blur = NRA.
  4. Expected NRA value between +1.75 to +2.5 diopters.
    • More than +2.5: Accommodation insufficiency.
    • Less than +1.75: Inadequate fusional convergence.

Interpretation of NRA Test

  • High NRA values may indicate:
    • Overcorrection in myopia.
    • Undercorrection in hypermetropia.
  • Explanation: Uncorrected myopia and undercorrected hyperopia cause focus behind the retina; extra plus power needed increases NRA.

Conclusion

  • Both NRA and PRA are essential for assessing accommodative functions and determining refractive corrections.
  • Important in identifying over and under corrections in various refractive errors.

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