NRA/ PRA Flashcards
What is the significance of NRA and PRA
positive and negative relative accommodation represents the changes in accomm. which are elicited while the stimulus for vergence is held constant
for both NRA and PRA where is the endpoint taken
when pt resports FSSB
what the the magnitude of relative accommodation
the amount of additional sph power added to the distance correction
pt dist rx = -3.00 SPH
mesSure NRA: pt reports FSSB thru -1.00
What is their NRA
+2.00
think of it as a number line, going more +
pt dist rx = -3.00 sph
mesSure PRA: pt reports FSSB thru -5.25
What is their PRA
- 2.25
think of a number line, going more -
when viewing through a lens and the image and object are on the same visual axis , then what happens to vergence ?
vergence doesnt change
what are the expected values for NRA and PRA for a young pt ( < 30yr of age)
NRA +2.00 to +2.25
PRA -2.50 TO -2.75
the increase in blur driven accomm will stimulate
increase in accomm convergence
in PRA when an image doesn’t fall on the fovea, what do we expect
image appears double
what is retinal disparity
when the images appear double bc it doesnt fall directly on fovea
In PRA if the eye converged in more than needed, this can be fixed by
reducing disparity vergence or divergence, diverging the eyes back towards primary gaze
In PRA the divergence places the image on fovea but will
reduct the convergent accomm (reduces power of the eye) .. unfavorable in PRA bc the goal is to increase F eye
in PRA to compensate for reduced convergent accom we need to
increase blur driven accomm
what is the PRA mecahnism
minus lenses under power > increases blur driven accommodation > increases accomm convergen > decreases disparity divergence (eliminating double) > decrease convergence accom > increase blur driven .. recycle
What is the NRA mechanism
plus lenses under power > decreases blur driven accommodation > decreases accomm convergence > increases disparity convergence > increases convergence accom > decrease blur driven .. recycle
what are the factors to low PRA
Inability to either
1. increases blur driven accomm (common for presbyopes) or
3. no ability to decreased disparity diveregence (continues to see double)
for low PRA what are the common factors that lead to difficulty increasing blur accommodation
- presbyopia
- accommodative insufficiency
for low PRA what are the common factors with disparity divergence
- esotropia
- convergence excess
a low NRA finding is typically caused by
difficulty decreasing blur driven accomm. or difficulty with disparity convergence
for low NRA what are the common factors that lead to difficulty decreasing blur accommodation
- accommodative excess
- pesudomyopia
- latent hyperopia (pt always accommodating, hard to remove)
for low NRA what are the common factors with disparity convergence
- convergence insufficiency
- exotropia
which two conditions are associated with low and high NRA
psuedomyopia and latent hyperopia
if the patient is able to relax that AR then,
high NRA
if the patient was not able to relax the AR then
low NRA
