Comparison of Three Methods of Measuring CA/C Ratios

Background: Clinical determination of convergence accommodation to convergence (CA/C) ratios may be useful for analysis of accommodation and vergence disorders, but there are no standard clinical methods of measurement of CA/C ratios. This study compared two potential clinical CA/C measurement methods to a laboratory procedure. Methods: CA/C ratios were measured for 19 young adult subjects using procedures in which accommodation was measured with the binocular cross cylinder (BCC) test, Nott dynamic retinoscopy, and an eccentric photorefractor. Vergence was stimulated with prism, and the resultant change in accommodation was used to calculate CA/C ratios. Results: The mean CA/C ratios were 0.036 D/Δ with BCC and BI prism, -0.004 D/Δ with BCC and BO prism, 0.023 D/Δ with Nott retinoscopy and BI prism, 0.036 D/Δ with Nott retinoscopy and BO prism, and 0.115 D/Δ with eccentric photorefraction and BO prism. The differences between each pair of tests were statistically significant by paired t-test except for the comparison of the BCC BI prism CA/C ratio with the Nott BI prism CA/C ratio. The means and standard deviations of the paired differences were all high relative to the measurements themselves. Conclusion: The CA/C ratios obtained in the present study did not show good agreement with each other. The use of CA/C ratios in clinical analysis would therefore require further evaluation and standardization of measurement methods, establishment of norms, and development of analysis procedures in concert with AC/A ratios and other clinical findings.