Reliability of the response AC/A ratio determined using nearpoint autorefraction and simultaneous heterophoria measurement.

BACKGROUND: Clinicians frequently assess the accommodative convergence to accommodation (AC/A) ratio using near phoria measurements and accommodative stimuli. The purpose of this study was to evaluate the repeatability of AC/A ratio measurements and to compare the response AC/A ratio to stimulus AC/A ratios determined two different ways. METHODS: Heterophorias and accommodative responses to different stimuli were measured simultaneously on eight subjects at two visits. A Canon Auto Ref R-1 autorefractor was used to measure accommodative response, while vergence posture was measured using the modified Thorington heterophoria technique. These results were used to calculate response-slope, stimulus-slope and stimulus-gradient AC/A ratios and the means of these ratios for each session were compared. Correlations between the response AC/A ratios and the stimulus AC/A ratios were calculated. RESULTS: The mean response AC/A ratios for the two sessions were 3.8 prism dioptres per dioptre for each session. The correlation between the slope-determined ratios was 0.56, and between the response-slope and the stimulus-gradient AC/A ratios the correlation was -0.03. The regression equation for the slope-determined AC/A ratios was calculated. CONCLUSIONS: Means for the two sessions were not significantly different for any of the AC/A ratios. The response-slope and stimulus-slope AC/A ratios had a moderately significant correlation and a predictive regression equation was determined. The stimulus-gradient AC/A ratios were not correlated with the response-slope AC/A ratios.