Study population
A total of 3438 eyes were analysed. The mean age of the subjects was 62.2 ± 10.6 years (range, 18–94 years; median, 63.0 years), and 56.4% were female. There were 2665 patients (77.5%) with high axial myopia (AL, ≥26.5 mm) and 773 patients without (AL, < 26.5 mm). The mean AL of the study population was 28.54 ± 2.74 mm (range, 21.58–37.11 mm; median, 28.25 mm). Table 1 presents the Q value, TCRP, TCA, WTW, central CT (CCT), ACD, IOP, and AL data for the subjects (mean ± SD) according to age and sex.
Correlation of ocular biometry
A Pearson’s correlation analysis was made to investigate the basic associations between the variables and to give hints for further multivariate-adjusted analysis (Table 1). Younger age was correlated with a more negative Q value, greater TCRP, greater WTW, deeper ACD, higher IOP, and greater AL. Women showed lower WTW, CCT, ACD, and IOP than men, whereas TCRP and TCA values were lower in men than in women.
Pearson’s correlation analysis also revealed the relationships between the anterior segment biometric parameters. A very low correlation was observed between a more negative Q value and a lesser WTW (r, 0.14; P < 0.001). Low correlations were detected between the CCT and IOP (r, 0.26; P < 0.001) and between the ACD and WTW (r, 0.25; P < 0.001). A moderate inverse correlation was detected between TCRP and WTW (r, − 0.41; P < 0.001).
Furthermore, the analysis for the correlations between the AL and anterior segment biometric parameters was made within subgroups categorized according to AL. In the high axial myopia subgroup (AL, ≥ 26.5 mm), the AL showed a statistically significant inverse relationship with Q value (r, − 0.11; P < 0.001) and NCT (r, − 0.08; P < 0.001), and a significant direct association with TCT (r, 0.12; P < 0.001) and IOP (r, 0.07; P < 0.001). In the non-high axial myopia subgroup (AL, < 26.5 mm), the effect of ocular elongation on Q value (r, − 0.08; P < 0.05) and TCT (r, 0.21; P < 0.001) remained significant. Besides, greater AL was correlated with lower TCRP (r, − 0.18; P < 0.001), thicker CCT (r, 0.15; P < 0.001), wider WTW (r, 0.21; P < 0.001), and deeper ACD (r, 0.32; P < 0.001).
Comparison of anterior segment biometry between axial length subgroups
Table 2 shows the results of one-way analysis of variance and Pearson’s χ2 test for the comparison of anterior segment biometry between three subgroups separated according to AL. The mean age of the subgroup with normal axial dimensions was significantly greater than that of the subgroups with medium-long and long axial dimensions. The subgroup of eyes with a long AL had more negative Q values, lower TCRP, greater TCA, thicker CCT and TCT, thinner NCT, wider WTW, deeper ACD, and higher IOP than the subgroup with a normal axial dimension (Fig. 1).
Table 2 also shows the Pearson’s χ2 test results for the qualitative parameters. The eyes for which toric intraocular lenses were indicated (TCA, ≥1.5 D) included a higher proportion of eyes with long ALs than of eyes with medium-long ALs. However, the proportions of eyes with with-the-rule astigmatism, against-the-rule astigmatism, and oblique astigmatism did not differ significantly between the three AL subgroups. The long axial dimension subgroup had a higher prevalence of ocular hypertension (IOP, ≥21 mmHg) than the normal axial dimension subgroup.
Table 3 shows the results of Student’s t test and Pearson’s χ2 test for the comparisons between non-high axial myopia subgroup (AL, < 26.5 mm) and high axial myopia subgroup (AL, ≥ 26.5 mm). According to the Student’s t test, the subgroup with high axial myopia had more negative Q values, lower TCRP, greater TCA, thicker CCT and TCT, thinner NCT, wider WTW, deeper ACD, and higher IOP than the subgroup of non-high axial myopia. According to the Pearson’s χ2 test, the eyes for which toric intraocular lenses were indicated (TCA, ≥1.5 D) included a higher proportion of eyes with high axial myopia than of eyes without. However, the proportions of eyes with with-the-rule astigmatism, against-the-rule astigmatism, and oblique astigmatism did not differ significantly between the two subgroups. The highly axial myopia subgroup had a higher prevalence of ocular hypertension (IOP, ≥21 mmHg) than the non-high axial myopia subgroup.
Association between high axial myopia and anterior segment biometry
A multivariate-adjusted logistic regression model was used to investigate the associations between the presence of high axial myopia and anterior segment biometric parameters. Table 4 summarizes the relationship results of anterior segment biometric parameters with high axial myopia in a multivariate-adjusted logistic model. After adjusting for age, sex, diabetes mellitus, and arterial hypertension, the characteristics independently associated with high axial myopia included a more negative Q value (OR, 0.42; 95% CI, 0.23–0.79; P < 0.01 per unit increase), lower TCRP (OR, 0.86; 95% CI, 0.79–0.94; P < 0.01 per diopter increase), greater TCA (OR, 1.20; 95% CI, 1.03–1.41; P < 0.05 per diopter increase), wider WTW (OR, 1.38; 95% CI, 1.03–1.85; P < 0.05 per millimeter increase), deeper ACD (OR, 3.25; 95% CI, 2.51–4.22; P < 0.001 per millimeter increase), and higher IOP (OR, 1.06; 95% CI, 1.02–1.09; P < 0.01 per mmHg increase). After the values were categorized into quartiles, the associations remained significant in a trend analysis. No association was found between CCT and high axial myopia in the multivariate-adjusted logistic model (OR, 1.00; 95% CI, 0.99–1.01; P = 0.07 per micrometer increase).
Association between axial length and anterior segment biometry
A multivariate-adjusted linear regression model was used to investigate the associations between AL and anterior segment biometric parameters. After adjustment for age, sex, arterial hypertension and diabetes mellitus, AL increased with significantly thicker TCT, thinner NCT, more negative Q value, lower TCRP, greater TCA, deeper ACD, and higher IOP.
For each millimeter increase in AL, the mean variation in TCT was + 2.3 μm (95% CI, 1.5–3.0 μm; β, 0.14; P < 0.001), in NCT was − 2.1 μm (95% CI, − 2.9 to − 1.4; β, − 0.13; P < 0.001), in the Q value was − 0.007 (95% CI, − 0.010 to − 0.004; β, − 0.11; P < 0.001), in flat TCRP was − 0.03 D (95% CI, − 0.05 to − 0.02; β, − 0.08; P < 0.01), and the mean TCRP was − 0.03 D (95% CI, − 0.04 to − 0.01; β, − 0.06; P < 0.05), in TCA was + 0.02 D (95% CI, 0.01–0.03; β, 0.08; P < 0.001), ACD was + 0.03 mm (95% CI, 0.02–0.04; β, 0.17; P < 0.001), and IOP was + 0.08 mmHg (95% CI, 0.03–0.13; β, 0.07; P < 0.01). No correlation was found between AL and steep TCRP in the multivariate-adjusted linear regression model (B, − 0.02; 95% CI, − 0.04 to + 0.01; β, − 0.03; P > 0.05).