- Positive relative accommodation
Positive relative accommodation (PRA) is a measure of the maximum ability to stimulate accommodation while maintaining clear, single
binocular vision[http://www.vision-therapy.com/Glossary_of_Terms.htm] . This measurement is typically obtained by an ophthalmologistor optometristduring an eye examinationusing a phoropter. After the patient's distance correction is established, he or she is instructed to view small letters on a card 40 cm from the eyes. The examiner adds lenses in -0.25 increments until the patient first reports that they become blurry. The total value of the lenses added to reach this point is the PRA value.
High PRA values (> or = 3.50
diopters) are considered to be diagnostic of disorders involving accommodative excessGarcia A, Cacho P, Lara F. "Evaluating relative accommodations in general binocular dysfunctions." "Optom Vis Sci." 2002 Dec;79(12):779-87. PMID 12512686.] . Those with accommodative insufficiencytypically have PRA values below -1.50 diopters [http://www.aoa.org/documents/CPG-18.pdf] .
Amplitude of accommodation
Negative relative accommodation
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