Add determination Flashcards
FCC/NRA &PRA/tentative add
lead
over-accommodating (more than the stimulus) - focusing in front of the target
lag
under-accommodating (less than the stimulus demands) - focusing behind the target
normal accommodative posture
+0.50 lag
FCC
fused crossed cylinder test - test performed to measure accommodative posture AND determine tentative add
FCC procedure
target at 40 cm, +/- 0.50 D auxillary lenses, vertical lines or horizontal lines darker
FCC horizontal lines darker
Add +0.25 until reversal (binocular)
FCC vertical lines darker
Add -0.25 until reversal (binocular) - stop after -1.00
positive FCC value
LAG
negative FCC value
LEAD
expected non-presbyope FCC
+0.50
expected presbyope FCC
approximately the patient ADD POWER
Relative Accommodation
evaluation of ability to maintain a clear and single image when changing accommodative demand
NRA
negative relative accommodation - adding plus - relaxing accommodation
PRA
positive relative accommodation - adding minus - stimulating accommodation
expected NRA values for non-presbyopes
+2.00 +/- 0.50 D
expected PRA values for non-presbyopes
-2.37 +/- 1.00 D
expected NRA/PRA values for presbyopes
balanced around ADD power (ex: NRA +2.00/PRA -2.00)
Plus Build Up Modification
adding plus to the patients distance prescription to get to the point where the patient is seeing clear for their near demands
Age Estimated Add
(Patient’s age - 35) x 0.1
what NRA/PRA values wouldn’t make sense?
PRA greater than NRA (FCC is likely incorrect)
During NRA, eyes are forced to
converge
NRA indirectly measures
positive fusional vergence
During PRA, eyes are forced to
diverge
PRA indirectly measures
negative fusional vergence
