Axial length of cataract eyes: a comparison of two cohorts over a span of 10 years apart

Various studies have shown an unprecedented rise in myopia prevalence for over half a century, especially in East Asian countries [7]. As the underlying defect of myopia is the elongated eyeball, which is also the cause of increased risk of pathological ocular complications in high myopias [27], we documented axial lengths of two cohorts over a ten-year period in an East Asian adult population. We observed an overall age-related elongation over the ten-year period, with doubling of percentage of high myopic eyes.

There has been considerable speculation about time trends in the prevalence of myopia. Although no strong evidence of time trends in myopia were found among the white population, it increased by 23 % in East Asians over the last decade [28]. In Taiwan, nationwide surveys of myopia and axial lengths in schoolchildren have been performed every 5 years since 1983, and the prevalence of myopia and high myopia has continued to increase [5]. However, there is limited data in an older, adult population. In our study, the mean age of both cohorts were in the mid-sixties, with a shift of 0.58 mm on axial length over the ten-year period. In particular, the percentage of high myopias doubled in the span of ten years from 8.1 to 16 %. Rapid economic development and stringent academic programs may have impacted the steep rise in myopia prevalence in Taiwan [5].

Population-based studies on mean ocular axial length have been performed in various regions. In a Chinese population in Singapore, Wong et al. examined 1004 phakic subjects aged 40 to 81 years of age, and a mean axial length of 23.23 ± 1.17 mm was found [14]. This was supported by the results of the Beijing Eye Study, which investigated the axial length of 3159 older individuals aged 50- to 93-year-old, and reported an average axial length of 23.25 ± 1.14 mm [13]. In 2010, Nangia et al. analyzed an adult population in rural central India, with the mean axial length at 22.6 ± 0.91 mm. The rural character of the study may be an explanation for the shorter axial length when comparing between the Indian study with previous studies [13]. A survey conducted in a general Japanese population age 40 years and older reported the trend of average axial length to be 23.4 mm in 2005 and 23.8 mm in 2017 [29]. In our study, the axial length was found to be longer than other studies at 23.65 ± 1.80 mm in the 2008 cohort and 24.23 ± 1.88 mm in 2018.

Some studies have reported age-related reductions in axial length which serves as an emmetropizing mechanism in view of the fact that the refracting power increases in the adult eye [30]. According to Gudmundsdottir et al., they observed a decrease in axial length when comparing those of 50 to 59 years of age at 23.6 ± 1.1 mm to those aged 70 years or older at 23.2 ± 1.4 mm [31]. Further verified by a study examining the 10-year change of axial length in older individuals in the Blue Mountains Eye Study, it was found that the axial length showed a decrease from 23.6 mm in 59 to 64 year olds to 23.2 mm in those aged 85 years or older [32]. In our study, we observed a significant age-related decrease in axial length across all age groups, with the difference between the youngest and oldest age group greater in the 2018 cohort than in the 2008 cohort. Furthermore, a birth cohort effect was noted when we compared the mean axial lengths of people of the same operation age group born in different periods. In the Beaver Dam Eye study, those born in more recent years were more myopic than those born in earlier years [33]. Another longitudinal study from an older population demonstrated similar birth cohort influences on spherical equivalent refraction [32].

On account of the results of this study which were obtained over a long period, only the applanation A-scan ultrasound was available in the year 2008, while the noncontact IOLMaster was more routinely used on the 2018 cohort. Some studies concluded that both contact ultrasound biometry and the IOLMaster were similar in their predictive capabilities, and while IOLMaster was an easier and faster tool to use, in eyes with significant posterior subcapsular cataracts, ultrasound biometry was still needed for accurate axial length measurement [23, 25, 34]. We analyzed axial lengths that were measured with both techniques in our data, and the mean difference in measurement results (0.08 mm) were similar to previous studies [21‐26]. After axial length measurements assessed with the IOLMaster were corrected, our data in all categories still remained significant.

Strengths of our study included the standardization of the methods and the same physicians enrolled during the 10-year study period. One important limitation was the consistency in observing axial lengths across different study populations. Given the large number of participants, inclusion of all individuals who underwent cataract surgery during the study period at our institution, and our focus on a long study period of ten years, we believe our data will be generalizable to all adults seeking cataract in this population. Another was the small number of patients who had axial lengths measured with both techniques. However, as presented in Table 3, the majority of mean difference in axial length measurements with both techniques was 0.2 mm, and our difference was smaller than the majority, therefore we used the results from our data to make the axial length corrections. Third, as technological improvements and surgeon experiences may improve over time, there may be differences in the complexity of cataract surgeries between the two populations. As Kaohsiung Chang Gung Memorial Hospital is a large tertiary referral center in Southern Taiwan, patients that undergo cataract surgery at our hospital are more complex cases, both in year 2008 and 2018. We chose the same experienced ophthalmologists that performed cataract surgeries in both years to reduce the differences in surgery techniques. Lastly, further studies on axial lengths may need to be performed to correlate with the older yet general population in Taiwan.