|
Negative Relative Accommodation/Positive Relative Accommodation (NRA/PRA)
PURPOSE To test the patient’s ability to increase and decrease accommodation under binocular conditions when the total convergence demand is constant. Under these conditions, changes in accommodative convergence are compensated for by changes in fusional vergence. The results of the NRA/PRA contribute to the functional analysis. They are part of the near refraction to determine an add for a presbyope.
EQUIPMENT
·Phoropter.
·Near point card.
·Near point rod.
·Illumination source.
SET-UP
·The phoropter contains the patient’s habitual correction or the best distance prescription from his most recent refraction if the patient is a nonpresbyope. If the patient is presbyopic, the patient’s tentative near prescription is placed in the phoropter.
·Put the near point card on the near point rod at 40 cm under bright illumination.
·Set the phoropter to the patient’s near PD. Make sure that both of the patient’s eyes are unoccluded.
STEP-BY-STEP PROCEDURE
1. Direct the patient’s attention to letters one or two lines larger than his best near VA on the near point card. Since the endpoint of the test occurs when the letters become blurry, ascertain that they are clear at the beginning. If they are not clear, add plus sphere power, +0.25 D at a time, until the patient reports that the letters are clear. This becomes the “tentative near prescription.” If the letters remain blurry despite the addition of plus, the NRA/PRA cannot be performed.
2. Do the NRA. Add plus lenses binocularly, +0.25 D at a time, until the patient reports the first sustained blur. “First sustained blur” means that the patient notices that the letters are not as sharp and clear as they were initially, even if the patient can still read them.
3. Note the total amount of plus added.
4. Return the lenses in the phoropter to the value at which you began, either the patient’s distance prescription or the tentative near prescription.
5. Again make certain that the letters are clear to the patient.
6. Do the PRA. Add minus lenses binocularly, -0.25 D at a time, until the patient reports the first sustained blur. Note the total amount of minus added.
RECORDING
·Record the amount of plus added for the NRA and the amount of minus added for the PRA, relative to the starting point of either the distance refraction or the tentative near prescription. Record the tentative add (the amount of plus added to the distance refraction to arrive at the tentative near prescription), when using the NRA/PRA as part of the near refraction of the presbyope.
EXAMPLES
·NRA/PRA: +2.25/-2.50
·NRA/PRA: +1.00/-1.00 over tent. add +1.25 |
