Does PRA indirectly test PFV or NFV?

Does PRA indirectly test PFV or NFV?

• A. PFV
• B. NFV
• C. Both
• D. Neither

Answer: (B)

PRA measures how far you can push accommodation toward near while keeping the vision clear and single. By adding minus lenses, you force the eyes to accommodate more, and because accommodation and vergence are linked, this also increases convergence demand. To keep the image single under this increased convergence, the fusional vergence system must provide counteracting divergence. The amount of divergence (negative fusional vergence) available limits how much minus power you can tolerate before breaking fusion, so PRA is essentially an indirect probe of NFV. If NFV reserves are poor, the patient will reach the limit sooner, yielding a smaller PRA value. This is why PRA is interpreted as reflecting NFV rather than PFV.

PRA measures how far you can push accommodation toward near while keeping the vision clear and single. By adding minus lenses, you force the eyes to accommodate more, and because accommodation and vergence are linked, this also increases convergence demand. To keep the image single under this increased convergence, the fusional vergence system must provide counteracting divergence. The amount of divergence (negative fusional vergence) available limits how much minus power you can tolerate before breaking fusion, so PRA is essentially an indirect probe of NFV. If NFV reserves are poor, the patient will reach the limit sooner, yielding a smaller PRA value. This is why PRA is interpreted as reflecting NFV rather than PFV.