CLINICAL OPTOMETRY III PROBLEM SET
Assume a 40 cm working distance.
1. A patient with a 60 mm PD has a distance phoria of 2 exo and a near phoria of 3 eso. Assuming no proximal convergence, calculate the AC/A ratio.
A patient with a 60 mm pd has a 15pd near point convergence demand. From distance fixation to near fixation, the amount of convergence changed from 2 exo at distance to 3 eso at near. The vergence angle changed 20pd (2pd to get to parallel, and then 3pd of overconvergence at near [15+3]).
Then for a 2.5 D change in accommodation, convergence changed 20pd. AC/A ratio = 20/2.5 = 8/1
2. The above patient has a - 1.00 gradient near point phoria of 5 eso. Calculate the PC/A ratio.
Near-far AC/A = 8/1 (we just calculated that). The near point phoria changed from 3 eso to 5 eso (a 2pd change in vergence) when retested through - 1.00.
Therefore gradient AC/A ratio = 2/ 1.
PC/A ratio = calculated AC/A - gradient AC/A= 8/1 - 2/1 = 6/1 (this is very large). Therefore the patient described in #1, has significant near point esophoria not because of a high AC/A ratio, but because of a dramatic convergence response to the awareness of nearness.
3. The above patient has a PRC of x/ 12/3 and an NRC of 6/10/-3. Calculate the amount of prism correction necessary to meet Sheard's criterion for visual comfort.
PRC= 12; NRC=6. The near point phoria = 3 eso. For esophoria, the BI vergence range (NRC) is the compensating vergence range. Sheard's criterion says that the vergence reserve range should be twice the compensating heterophoria. Therefore this patient fulfills Sheard's criterion, but minimally. This could be trouble when the patient is fatigued, stressed, etc.
4. Calculate the magnitude of the above patient's PFC and NFC ranges.
The fusional convergence ranges are determined from the heterophoria to the relative convergence point (relative convergence).
PRC=12; NRC=6; near point heterophoria = 3 eso.
Therefore, negative fusional convergence (NFC) = 6 + 3 = 9pd (i.e. 3pd to compensate for the eso with 6 additional prism diopters available.
positive fusional convergence (PFC) = 12 - 3 = 9pd
5. The PRC is re-tested while the above patient is looking through a total of 5pd BO prism. Calculate the repeated PRC value.
PRC = 12pd. When 5pd BO is held before the patient, 5pd of positive fusional convergence is required just to fuse the prism. If the PRC is repeated through this prism, the amount of available positive fusional convergence reserve is depleted by 5pd.
Therefore PRC= 7pd.
6. Another patient with a 60 mm PD has a distance phoria of 2 exo. If the AC/A ratio is 3/1 and the PC/A ratio is 2/1, calculate the near point phoria.
A patient with a 60 mm pd has a 15pd near point convergence demand. Distance phoria = 2 exo.
AC/A ratio = 3/1; PC/A ratio = 2/1. These two ratios have additive properties. For every 1 D change in accommodative demand, 5pd of convergence are generated (3 accommodatively and 2 proximally).