Investigation of a new clinical method of measuring amplitude of accommodation

Amplitude of Accommodation (AoA) is the extent of the eye’s ability to accommodate, or
focus over a range of distances. This thesis describes the mechanism of accommodation
and reviews the literature concerning the measurement of accommodation’s amplitude,
its maximum range. The reasons for measuring the amplitude of accommodation, in
routine clinical practice, are outlined. The aim of this study is to investigate the accuracy
of a new clinical method of measuring amplitude of accommodation and to compare it to
the prevalent method.
Methods by which the amplitude of accommodation has been measured clinically are
described and compared. Each method has inherent sources of error and these are
examined individually to show how they affect the results of measurement. The new
method of measurement, developed by the author, is introduced. Its basis, which may
lead to a redefinition of amplitude of accommodation, is explained and contrasted with
the rationale of existing methods. Experimental work is reported comparing accuracy of
measurement using the new method (the TRU) with that of the prevalent method (Push-
Up with the RAF Rule). Two techniques for using the TRU, distance-measurement and
acuity-measurement, were examined and distance-measurement was shown to be more
precise.
The method-comparison was by repeated measures of results with both methods and
with those of an objective reference method, the WAM-5500 autorefractor. The estimated
95% limits of agreement between the two test methods spanned 6.36 D (dioptres). The
disparity of results appeared due more to differences between the test methods’ trueness
than their precision. The RAF Rule gave results that averaged 2.10 D higher than results
with the TRU and 2.19 D higher than results with the WAM-5500 autorefractor.
Measurements of AoA with the autorefractor were 67% more repeatable than
measurements with the TRU and 114% more repeatable than measurements with the
RAF Rule, although of questionable trueness.
The estimated 95% limits of agreement of reproducibility between sessions spanned
6.57D for the established method but 2.89 D for the new method, and reproducibility
between investigators similarly spanned 5.10 D for the established method and 2.56 D
for the new method.
The significance of these findings for clinical vision science is discussed and examined
in the light of theoretical considerations of each method’s validity. Suggestions are made
for improving the accuracy of measurement of the amplitude of accommodation, which
should improve the reliability of normative values in current clinical use.