Original Research

Agreement between distance and near minus-lens-to-blur amplitude of accommodation in pre-presbyopic subjects

Mbali Z.G. Ndlovu, Kgaogelo T. Mothapo, Makgatha H. Kgatla, Muano S. Tshihomu, Solani D. Mathebula
African Vision and Eye Health | Vol 78, No 1 | a491 | DOI: https://doi.org/10.4102/aveh.v78i1.491 | © 2019 Mbali Z.G. Ndlovu, Kgaogelo T. Mothapo, Makgatha H. Kgatla, Muano S. Tshihomu, Solani D. Mathebu | This work is licensed under CC Attribution 4.0
Submitted: 27 November 2018 | Published: 24 June 2019

About the author(s)

Mbali Z.G. Ndlovu, Department of Optometry, University of Limpopo, Sovenga, South Africa
Kgaogelo T. Mothapo, Department of Optometry, University of Limpopo, Sovenga, South Africa
Makgatha H. Kgatla, Department of Optometry, University of Limpopo, Sovenga, South Africa
Muano S. Tshihomu, Department of Optometry, University of Limpopo, Sovenga, South Africa
Solani D. Mathebula, Department of Optometry, University of Limpopo, Sovenga, South Africa


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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.


Keywords

amplitude of accommodation; blur; dioptre; minus-lens-to-blur method; push-up-to-blur method; subjective method

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