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Background: Deficient amplitude of accommodation is the most frequently used criteria in an optometric practice in diagnosing whether a patient has accommodative insufficiency. This deficiency is determined based on an age-related expected finding calculated using Hofstetter's equation derived from Donder's and Duane's data. The aim of the present study was to investigate the amplitude of accommodation among Ghanaian school children and to compare the findings with age-expected norms predicted by Hofstetter's equation.
Methods: The amplitude of accommodation was measured using the push-up method in a random sample of 435 school children from the Cape Coast Municipality. The mean amplitude of accommodation was compared with the age-expected amplitude of accommodation as predicted by Hofstetter's equation for average amplitude of accommodation.
Results: The mean amplitude of accommodation was 16.86 ± 3.07 D (95% CI = 16.57, 17.15). This is significantly higher than age-expected norms calculated using Hofstetter's equation. The amplitude of accommodation showed the characteristic decline with age.
Conclusion: From the results, we conclude that the age-expected norms for amplitude of accommodation using Hofstetter's equation might not be accurate for Ghanaian children.
The amplitude of accommodation defines the maximum amount of accommodation the visual system can elicit and is one of the commonly assessed visual functions during an eye examination. It is valuable when investigating the accommodative status of a patient. Clinically, it is used to diagnose accommodative anomalies, as well as estimating the additional power required to correct presbyopia. In these instances, the predicted average amplitude of accommodation (AOA) is calculated for a patient by using Hofstetter's equation1 (AOA = 18.5 - 0.3 × age in years). This equation was derived from the original work of Duane and is routinely used in determining whether a patient has sufficient AOA, and this is often combined with other accommodative findings to diagnose accommodative anomalies.2–6 The reliability of the norms for children aged eight to 12 years has been discussed.7,8 Both Wold7 and Turner8 queried the reliability of Duane's norms because only 35 of the 1,000 subjects in Duane's study were aged eight to 12 years. The indiscriminate application of Duane's norms derived from a Caucasian population has also been queried on the basis of race,9 as race might influence AOA.10,11 Sterner, Gellerstedt and Sjöström12 have also remarked that the AOA using Duane's norms is flawed in predicting the accommodative amplitude for children of a younger age.
We investigated AOA of junior secondary school children in Ghana. The aim of the study was to determine if there is a difference between the AOA predicted by Hofstetter's equation and the actual AOA of Ghanaian children. The data from this study would lead to better assessment of the accommodative function of school children in Ghana.
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This is the first investigation of the AOA in Ghanaian school children, which is important in diagnosing accommodative insufficiency. As expected, amplitudes for the right and left eyes were similar13 and decreased significantly with age. The mean AOA for the subjects in the present study was 16.86 D. This finding was higher than the 12.40 ± 3.7 D obtained for Swedish children aged six to 10 years12 and the 13.29 ± 2.05 D for Austrian children aged six to 14 years14 despite the younger age ranges of those cohorts. These studies used the push-up method for determining the AOA. It was higher than the mean AOA for Korean children,15 among whom the mean AOA was 14.55 D in children aged six to 10 years decreasing to 12.36 D in those aged 11 to 15 years,15 using the minus lens to blur technique. From the foregoing, it is clear that irrespective of the technique used in measuring AOA, there appears to be some racial variation in mean AOA.
From the present study, it appears that the AOA of Ghanaian children might be higher than those of European and Korean children. There could be several reasons for this observation. We could not find supporting evidence that genetic factors might influence AOA. We could infer that the environment might be a factor in this racial variation in AOA. Miranda16 indicated that populations living in warmer regions have an earlier onset of presbyopia compared with those living in colder regions. In contrast, Edwards and colleagues17 have also argued that the lower AOA among the Chinese population might be due to factors other than long-term environmental effects.
This higher accommodation observed for Ghanaian children cannot be conclusive until various techniques for determining the AOA including objective methods have consistently shown higher amplitudes of accommodation for these subjects.
As shown in Table 4, there was a significant difference between the expected AOA predicted by Hofstetter's equations and the measured AOA in the present study. Our values were higher than the predicted average (expected) and minimum amplitudes of accommodation but lower than the maximum amplitude. Sterner, Gellerstedt and Sjöström12 have also demonstrated a discrepancy between the predicted and actual measured amplitudes of accommodation for Swedish children aged six to 10 years; however, their values were lower than the predicted AOA.
An interesting finding in the present study was the significant difference between the mean AOA between male and female participants, although there was no difference between their ages. We found no literature demonstrating gender difference with AOA. As the procedures and the test conditions were similar for both male and female subjects and a physiological difference in ocular mechanisms is unlikely, we can only suggest that the difference might be due to a higher proportion of males in the younger age group (eight- and nine-year groups) compared with the higher proportion of females in the older age group (10-, 11-, 12- and 14-year groups). The present study also demonstrated the decline in AOA with age. In the present study, the mean AOA declined from 19.00 D at age eight years to 15.47 D at age 14 years. This represented an 18 per cent decline. Using Hofstetter's equation for expected average AOA, there would be an 11 per cent decline in AOA from age eight years to 14 years.
In the present study, refraction was not performed. This might have led to a misclassification of low-grade ametropia as emmetropia. It is possible that this misclassification could affect our conclusion. Notwithstanding, we do not expect such bias to be significant or to negate the observed findings. This is because a visual acuity cut-off point of 6/5 was used in selecting subjects. It is not likely that the low-grade refractive error of the order of say ±0.25 D would significantly negate the observed findings. The subjective method of determining the AOA (push-up technique) used in this study also has some limitations. It is thought to overestimate AOA.18