Abstract
Background: Guidelines for predicting the amplitude of accommodation (AA) sometimes are based on measurements for the subjective push-up-to-blur method. However, the push-up-to-blur method often overestimates the accommodative response. Another method that is commonly used to measure AA is the minus-lens-to-blur technique. This method is routinely conducted at a viewing distance of 40 cm, with the addition of 2.50 D to correspond with the initial accommodative stimulus. Aim: Although the AA measured using the minus-lens-to-blur method can be performed with the target at 40 cm, this study investigated changing the viewing distance and then comparing the means for participants at 600 cm (6 m), 40 cm (0.4 m) and 33 cm (0.33 m). In this manner, the possibility of using the AA measured at 6 m to estimate AA at 40 cm or 33 cm could be explored. Setting: The study was conducted in the Optometry Department, University of Limpopo. Methods: The AA was measured using the minus-lens-to-blur method over the distance correction when the target was held at viewing distances of 6 m, 40 cm and 33 cm. The minus lens power used to stimulate accommodation was gradually increased in 0.25 D steps. All measurements were performed using a phoropter. Results: The mean AA was 8.46 ± 2.07 D for 6 m, 10.62 ± 2.05 D for 40 cm and 11.31 ± 2.07 D for 33 cm. The measured AA at 6 m was significantly lower than that with the target at 40 cm and 33 cm measurement, p < 0.01. The Bland–Altman plots showed that there was a weaker agreement between the measured AA with the target at 6 m and near; however, there was a better agreement when the target was at 40 cm or 33 cm. Conclusion: The findings of this study suggest that the results for AA measured using the minus-lens-to-blur method at 6 m are not comparable to the results with the target at 40 cm or 33 cm. Even though there was a positive correlation between 6 m and the near distances, the results were not interchangeable or useful to calculate AA at 40 cm and 33 cm. The difference between the AA measured with the target at 40 cm and 33 cm can vary on average by about 0.70 D.
Highlights
Accommodation can be defined as an increase in dioptric power of the eye for clearly focusing near objects of regard on the retina
The Bland–Altman plots showed that there was a weaker agreement between the measured AA with the target at 6 m and near; there was a better agreement when the target was at 40 cm or 33 cm
The findings of this study suggest that the results for AA measured using the minus-lens-to-blur method at 6 m are not comparable to the results with the target at 40 cm or 33 cm
Summary
Accommodation can be defined as an increase in dioptric power of the eye for clearly focusing near objects of regard on the retina. Accommodation results from a change in the shape of the crystalline lens.[1,2,3,4,5,6] The greatest increase in dioptric or refractive change an eye can undergo is called the amplitude of accommodation (AA).[1,2,3,4,5,6] The AA decreases with age; young individuals have the greatest amplitude that gradually decreases with age leading to presbyopia (which starts to affect performance with near-visual tasks around 40 years of age).[4,5] There is little or no accommodative ability left around the age of 55–60 years.[5]. The pushup-to-blur method often overestimates the accommodative response. Another method that is commonly used to measure AA is the minus-lens-to-blur technique. This method is routinely conducted at a viewing distance of 40 cm, with the addition of 2.50 D to correspond with the initial accommodative stimulus
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