Relationship Between Axial Length and Retinal Oxygen Dynamics in Adults With Myopia

Abstract

Purpose: The purpose of this study was to evaluate the correlation between axial length (AL) and retinal oxygen dynamic parameters in adult patients.

Methods: This was an observational cross-sectional study with 79 Chinese adults with myopia aged 18 to 37 years. All participants underwent AL measurements, cycloplegic refraction, and other ophthalmic examinations. Additionally, the retinal oxygen kinetics imaging and analysis (ROKIA) system was utilized to obtain the retinal oxygen dynamic parameters of all patients. Simple and multiple linear regression tests were used to assess the correlation between various oxygen dynamic parameters and AL.

Results: The mean age, AL, and spherical equivalent (SE) of subjects were 26.32 ± 5.4 years, 25.78 ± 1.06 mm, and −5.13 ± 2.1 diopters (D), respectively. The Pearson correlation coefficients among AL and retinal oxygen delivery (DO₂) and retinal oxygen metabolism (MO₂) were −0.44 (95% confidence interval = −0.24 to −0.60, P < 0.001), −0.26 (95% confidence interval = −0.04 to −0.46, P = 0.02), respectively. The group with high myopia exhibited lower DO₂ and higher oxygen extraction fraction (OEF) compared with the group with moderate myopia, and no significant difference was observed in MO₂ between the two groups. In multivariate analyses adjusting for age, sex, intraocular pressure (IOP), and anterior chamber depth (ACD), a longer AL was significantly associated with decreased DO₂ (standardized regression coefficient B = −0.47, P < 0.001).

Conclusions: Retinal oxygen dynamic parameters, including DO₂ and MO₂, were decreased with longer AL in myopic eyes. Patients with high myopia demonstrated an elevated OEF than those with moderate myopia.

Translational Relevance: This study demonstrated that the retinal oxygen metabolism changes in myopia, as confirmed using a novel device that quantifies retinal oxygen dynamic parameters and provides a new monitoring approach for other hypoxic retinal diseases.

Introduction

Myopia has remained one of the most concerning ophthalmologic issues. In some regions of East and Southeast Asia, the prevalence of myopia has even reached 80% to 90%.^1,2 During the development of axial myopia, the retina and choroid are stretched, resulting in extensive changes to the retina and its vascular system, which further affect the oxygen demand of and supply to retinal tissue, respectively.^3–5 

Several studies have reported changes in retinal oxygen saturation (SO₂) among myopic individuals, but these studies have yielded inconsistent conclusions.^6–11 The lack of individual data on retinal blood flow (RBF) has hindered the ability to calculate oxygen dynamic parameters, including retinal oxygen delivery (DO₂) and retinal oxygen metabolism (MO₂), which are used to describe the retinal tissue's oxygen supply and oxygen consumption per unit of time. This limitation restricted further interpretation of their results. 

The combination of dual-wavelength imaging and Doppler optical coherence tomography (OCT) technologies is recognized as a standard method for measuring hemodynamic parameters and has been applied to assess DO₂ and MO₂ in patients with diabetes, glaucomatous retinopathy, and sickle cell retinopathy.^12–14 However, the need to switch between the two instruments can cause patient discomfort and increase image processing complexity and time, thereby limiting the clinical application of this technology in larger patient samples. To this end, the present study has developed the retinal oxygen kinetics imaging and analysis (ROKIA) system, which assesses retinal hemodynamics through laser speckle contrast imaging (LSCI) and further calculates retinal oxygen dynamics parameters. The ROKIA system acquires images on a single machine via a custom optical path.¹⁵ This technology has been applied to both healthy individuals and patients with non-proliferative diabetic retinopathy, demonstrating great stability and reproducibility.^15,16 

The present study aimed to measure DO₂ and MO₂ in individuals with myopia and examine their relationship with axial length (AL). 

Materials and Methods

Study Population

This cross-sectional study recruited all patients from the Ophthalmology Department at the First Medical Center of the General Hospital of the People's Liberation Army, China. The study consecutively enrolled 79 patients with varying degrees of simple myopia, all with a best-corrected visual acuity equal to or better than 20/20 and cylinder refraction less than 2 diopters (D). Patients completed a questionnaire on their overall health status. Those who exhibited any of the following comorbidities and risk factors were excluded from the study: significant ocular symptoms; signs of non-myopia-related ocular diseases; systemic diseases, such as hypertension or diabetes; a history of sleep apnea; and smokers or patients on long-term medication affecting blood oxygenation or hemodynamics. All research procedures followed the principles of the 1989 revised Declaration of Helsinki after obtaining written informed consent from the participants. This study was also approved by the Ethics Review Committee of the General Hospital of the People's Liberation Army (approval number: S2024-025-01). 

Ophthalmic Examination

A comprehensive ophthalmic examination was performed on all subjects, including slit lamp examination, visual acuity testing, intraocular pressure (IOP) measurement (Canon Full Auto Tonometer TX-F; Canon, Tokyo, Japan), and anterior chamber depth (ACD) and white-to-white (WTW) measurements using a Pentacam comprehensive anterior segment analyzer (OCULUS, Wetzlar, Germany). The spherical equivalent (SE) was computed by the same optometrist through computerized optometry (AUTO REFRACTOMETER RM-8800; TOPCON, Tokyo, Japan) after a 10-minute interval following the administration of the compound tropicamide twice (Mydrin-P; Santen, Osaka, Japan). SE was defined as the sum of the spherical power and half of the cylindrical power. The AL was measured using an optical biometer (IOLMaster version 5.4, Carl Zeiss; Meditec AG, Jena, Germany), and individuals with myopic AL < 26.5 mm were classified into the group with moderate myopia, whereas those with AL ≥ 26.5 mm were classified into the group with high myopia for subsequent subgroup analysis. 

Retinal Oxygen Dynamic Parameters Measurement

The SO₂ measurements in the ROKIA system are achieved using dual-wavelength (548 nm and 605 nm) retinal oximetry, as detailed in a previously published study.³ Each subject is photographed to obtain fundus images centered on the fixation point with a 45 degree field of view at the 2 wavelengths, as shown in Figures 1a and 1b. A custom-made software encoded in the matrix laboratory (MATLAB; MathWorks, Natick, MA, USA) is used to first generate a pseudocolor distribution map of SO₂ (Fig. 1c). Subsequently, it calculates the SO₂ and vessel diameters of the 8 arteriovenous vessel segments in 4 quadrants (superior temporal, inferior temporal, superior nasal, and inferior nasal) selected sequentially by the analyst within a range of 1.5 to 3 times the radius of the optic disc and averages these values (Fig. 1d). The vessel selection principle is as follows: prioritize selecting principal branch vessels within the region of interest (ROI) with a diameter greater than 70 µm; aim to include the entire segment of retinal vessels within the ROI to minimize selection error; exclude areas where other vessels intersect, as this can disturb the SO₂ measurement.¹⁷ The oxygen concentration (CO₂) is then calculated based on SO₂ using the following formula: C_O2 = c_HB × γ × SO₂, where C_HB represents the concentration of hemoglobin in human blood (set at 14.5 g/dL for male patients and 13 g/dL for female patients), and γ denotes the oxygen-binding capacity of hemoglobin, set at 1.368 mL/g.¹⁸ 

Figure 1.

 

Images and analytical process of the ROKIA system. (a) The fundus image at 548 nm wavelength. (b) The fundus image at 605 nm wavelength. (c) Retinal vessel oxygen saturation distribution map. (d) Eight vessel segments in four quadrants selected around the optic disc for oxygen dynamics analysis (4 arteries and 4 veins in each quadrant). (e) Fundus laser speckle pattern images. (f) Retinal blood flow velocity (BFV) distribution map. (g, h) Alignment of the laser speckle pattern images with the fundus image at 548 nm wavelength, with 4 non-collinear alignment points shown in the images, respectively.

The vessel diameter was determined by fitting the cross-section curve of the vessel with the Gaussian function, using the full width at half maximum as the measurement metric.¹⁹ Before incorporating the vessel diameter into further calculations, magnification errors stemming from refractive factors were adjusted using the Bengtsson formula, yielding the corrected vessel diameter (CVD).²⁰ 

For measuring the retinal hemodynamic parameters, each subject was required to fixate on a visual target for 5 seconds to record 400 retinal speckle pattern images (SPIs) for subsequent analysis. Based on the laser speckle phenomenon, the velocity of the scatterers is inversely proportional to the laser speckle contrast values within a defined spatiotemporal window, and the actual blood flow velocity (BFV) can be further estimated by the ratio of the mean to the standard deviation of laser speckle contrast values. In the ROKIA system, speckle images are first analyzed by customized software in MATLAB (MathWorks, Natick, MA, USA) to obtain the velocity distribution map of the retinal vessels through speckle patterns (Figs. 1e, 1f). Subsequently, researchers use at least three non-collinear alignment points to align the SPIs with the fundus photographs captured in SO₂ imaging (Figs. 1g, 1h) to identify the vascular segments measured for SO₂ on the SPIs, following which the BFV within this region are calculated and recorded. 

The RBF was calculated and averaged for each vessel segment using the following equation: \({\rm{RBF}} = {\rm{BFV\ }} \times {\rm{\ \pi \ }} \times {{{\rm{r}}}^2}( {{\rm{r}} = \frac{{{\rm{CVD}}}}{2}} )\), where r represents half of the average CVD for four segments of arterioles or venules. DO₂ was calculated as the product of average arterial oxygen concentration (\({\rm{C}}_{{\rm{O}}2}^{\rm{a}}\)) and average venular RBF (RBF^v) using the equation \({\rm{D}}{{{\rm{O}}}_2} = {\rm{C}}_{{\rm{O}}2}^{\rm{a}} \times {\rm{RB}}{{{\rm{F}}}^{\rm{v}}}\). Retinal MO₂ was computed as the product of the average difference in arteriovenous concentration and RBF^v using the following equation: \({\rm{M}}{{{\rm{O}}}_2} = ({\rm{C}}_{{\rm{O}}2}^{\rm{a}} - {\rm{C}}_{{\rm{O}}2}^{\rm{v}}){\rm{\ }} \times {\rm{RB}}{{{\rm{F}}}^{\rm{v}}}\). The oxygen extraction fraction (OEF) was defined as the ratio of DO₂ to MO₂. 

Retinal Oxygen Dynamics Imaging and Analysis Consistency

The preliminary experiments consecutively imaged 20 eyes with an average SE of –6.16 ± 3.5 D twice with a 30-minute rest interval in between, and the images were annotated by the same analyst to assess the repeatability of the imaging system. The intraclass correlation coefficients (ICCs) for DO₂, MO₂, and OEF were 0.948, 0.967, and 0.980, respectively. Additionally, the study performed back-to-back vessel annotation and analysis of the same set of images by two analysts for another 20 eyes with an average SE of –4.89 ± 1.6 D to assess inter-analyst consistency. The ICCs for DO₂, MO₂, and OEF were 0.962, 0.944, and 0.917, respectively. 

Statistical Analysis

All statistical analyses were performed using the Statistical Package for the Social Sciences (SPSS) software 26.0 (SPSS, Inc., Chicago, IL, USA). Data from the right eyes of all participants were included in the final analysis. Pearson correlation coefficients were calculated to assess the correlation between AL and oxygen parameters, including SO₂, BFV, CVD, RBF, DO₂, MO₂, and OEF. Multivariate linear regression analysis was conducted to investigate independent factors associated with AL and SE, adjusting for age, gender, IOP, and ACD. Correlation analysis was used for collinearity diagnostics before applying the multivariate regression models. An independent sample t-test was performed to compare the differences between groups with moderate and high myopia. The ICCs were used to evaluate the repeatability of imaging and analysis. The P values < 0.05 (2-tailed) were considered statistically significant. 

Results

The study included a total of 79 Han Chinese patients with myopia. The average age of the subjects was 26.32 ± 5.4 years. The average AL and SE were 25.78 ± 1.06 mm and –5.13 ± 2.1 D, respectively. Table 1 presents the basic demographic and ocular parameters of all subjects. 

Table 1.

 

Demographic Characteristics and Basic Ocular Parameters of Subjects Recruited

 

Univariate linear regression demonstrated that both arteriolar SO₂ (SaO₂; P < 0.05, R² = 0.07) and venular SO₂ (SvO₂) significantly decreased with increasing AL (P < 0.05, R² = 0.08), whereas arteriolar-venular SO₂ difference (SavO₂) was not significantly correlated with AL (P = 0.37; Fig. 2a). Both arteriolar BFV (BFV^a; P < 0.05, R² = 0.07) and venular BFV (BFV^v; P < 0.05, R² = 0.06) significantly decreased with an increase in AL (Fig. 2b). Before the correction, the diameters of the retinal arterioles (P < 0.001, R² = 0.17) and venules (P < 0.001, R² = 0.19) significantly decreased with increasing AL. After correction using the Bengtsson formula, the corrected retinal arteriolar diameter (CVD^a) remained significantly correlated with AL (P < 0.05, R² = 0.07), whereas the corrected retinal venular diameter (CVD^v) showed a marginally significant correlation (P = 0.051). 

Figure 2.

 

Scatterplots illustrating the relationship among (a) AL and SO₂, (b) BFV, (c) RBF, and (d) oxygen dynamic parameters.

The Bland–Altman plot demonstrated satisfactory consistency between arteriolar RBF (RBF^a) and RBF^v (Fig. 3). The 95% limits of agreement ranged from –16.15 to 20.37 µL O2/min. Both RBF^a and RBF^v significantly decreased with increasing AL (arteriole = P < 0.01, R² = 0.11 and venule = P < 0.001, R² = 0.16; Fig. 2c). Both DO₂ and MO₂ significantly decreased with increasing AL (DO₂ = P < 0.001, R² = 0.19 and MO₂ = P < 0.05, R² = 0.07; Fig. 2d). The OEF was not significantly correlated with AL (P = 0.12). 

Figure 3.

 

The Bland–Altman plot illustrating the consistency between arteriolar RBF and venular RBF.

As shown in Table 2, after adjusting for age, IOP, and ACD, the multiple linear regression analysis with AL as the dependent variable demonstrated that a longer AL was significantly related to the decrease of DO₂ (regression coefficient B = –0.387, P < 0.001). DO₂ was also an independent factor associated with SE after adjusting the above factors (B = 0.663, P < 0.001; Table 3). 

Table 2.

 

Multivariate Regression Analysis of Associations Between AL and DO₂

 

Table 3.

 

Multivariate Regression Analysis of Associations Between SE and DO₂

 

The group with high myopia exhibited lower DO₂ and higher OEF compared to the group with moderate myopia (DO₂ = mean difference = 0.92, 95% confidence interval [CI], 0.32 to 1.51, P < 0.01 and OEF = mean difference = –0.04, 95% CI = –0.07 to –0.02, P < 0.01), and there was no significant difference in MO₂ between the two groups (P = 0.49; Fig. 4). Regarding hemodynamic parameters, no significant difference was observed in BFV between the two groups (BFV^a: P = 0.17 and BFV^v: P = 0.44), whereas CVD was significantly narrower in the group with a longer AL (CVD^a and CVD^v: P < 0.05). 

Figure 4.

 

Comparison of DO₂ and MO₂ between the groups with moderate myopia (MM) and high myopia (HM).

Discussion

The present study used the ROKIA system to measure and compare the hemodynamic parameters in young and middle-aged patients with varying degrees of myopia. Our findings indicate that with the increase in AL and the SE becoming more myopic, the DO₂ significantly decreased. Furthermore, eyes with longer AL exhibited significantly lower DO₂ and higher OEF compared to the control group, whereas no significant difference in MO₂ was observed between the two groups. 

The trend of DO₂ decreasing with increasing AL is predictable as numerous previous studies have indicated that both SaO₂ and RBF significantly decrease with increasing myopia in adults with myopia.^4,6,21,22 This study hypothesized that the reduction in DO₂ reflects the disruption of the retinal vascular system during the process of AL increase, which subsequently affects the oxygen supply to the retina. Shimada et al.²³ utilized laser Doppler ultrasound technology to measure the BFV and retinal vessel diameter in individuals with different degrees of myopia and found that the vessel diameter was narrower in subjects with severe myopia, whereas there was no significant change in BFV. The present study extended the measurements to include nasal vessels and reached a consistent conclusion: the reduction of RBF in myopia is primarily attributed to narrower vessel diameter rather than decreased BFV. 

Several studies suggest degenerative changes in the retina of myopic eyes, including structural thinning of the average retinal nerve fiber layer and ganglion cell complex^24,25 and functional decreases in multifocal electroretinogram amplitude.^11,26 This strongly suggests a reduction in retinal oxygen demand. As direct measurement of retinal oxygen demand is not feasible, previous studies have explored retinal oxygen consumption using SO₂, but these have yielded conflicting conclusions. Man et al.¹¹ found that a longer AL was associated with SavO₂ and reduced multifocal electroretinogram amplitude. However, a recent study by Ge et al.⁶ involving 1373 individuals with myopia with an average age of 26 years reported that SavO₂ was negatively correlated with SE and was not significantly correlated with AL. Furthermore, in another study involving 124 adults with anisometropia, Ge et al.⁷ found that for individuals with myopia who exhibited interocular differences in SE greater than 3 D, there was no significant difference in SavO₂ between the 2 eyes. These studies suggest SavO₂ may not sensitively reflect decreased retinal oxygen consumption associated with AL growth. On the other hand, MO₂ incorporates hemodynamic parameters based on SavO₂ and reflects the amount of oxygen extracted by retinal tissue from the retinal vascular system per unit of time, thus providing a more comprehensive assessment of the overall decreased retinal oxygen metabolic state in myopia. 

Interestingly, this study found that despite an overall trend of decreasing MO₂ with an increase in AL, there was no significant difference in the average MO₂ between the groups with high and moderate myopia classified by a 26.5 mm AL cutoff, and the group with high myopia exhibited a higher OEF. The OEF defined in this study is independent of RBF and can be calculated as SavO2/SaO2, making this study's OEF values comparable to those from studies that did not measure hemodynamic parameters but only measured SO₂. The research by Ge et al.⁶ reported that SavO₂/SaO₂ also showed an increase in eyes with longer AL, further validating our findings. Among the two parameters for calculating MO₂, RBF strictly decreases with AL growth. Therefore, the relatively increased MO₂ in the group with high myopia can only be attributed to increased oxygen consumption. Consequently, we can reasonably propose the following hypothesis: during the increase of AL in myopic eyes, retinal tissue needs to extract more oxygen per unit of blood flow from retinal vessels to compensate for the reduced hemodynamics. 

Previous studies have demonstrated that choroidal thickness²⁷ and blood flow^22,28 decrease with the elongation of the AL in myopia. However, Kristjansdottir et al.²⁹ used oximetry to reveal that both the choroidal vasculature and vortex veins exhibit comparably high oxygen saturation levels, indicating an adequate reserve of oxygen supply in the choroid of normal individuals. This suggests that despite a certain degree of AL elongation affecting the choroidal structure, the choroid is still able to provide sufficient oxygen to the outer retina. Given that the retina is supplied by both the choroid and the retinal vascular system, the stable oxygen consumption this study observed in the inner retina implies that the oxygen supplied to the outer retina from the choroid also remains relatively constant, thus supporting the above hypothesis. Nevertheless, as this study did not involve any direct measurements of the choroidal vasculature, the validation of this hypothesis lies beyond the scope of the current investigation. 

All subjects included in this study had good corrected visual acuity and no significant choroidal atrophy. The stability observed in MO₂ can be partially attributed to these factors. For individuals with extremely high myopia who were not included in this study, the further atrophy of retinal tissue and vascular systems could disrupt the balance between oxygen consumption and supply within the inner and outer layers of the retina or across the entire retina. This disruption may serve as a potential underlying cause for the progression of pathological myopia. Therefore, future studies can include this population to further elucidate the changes in retinal oxygen metabolism during the progression of myopia. 

The advantage of this study lies in the inclusion of healthy young adult subjects, which not only provides a clear refractive medium that minimizes interference with image quality, but also decreases the probability of age-related systemic factors affecting hemodynamic parameters. This study also tried to control factors that may affect imaging, including pupil size and gaze direction. Additionally, all image analyses were performed without the analysts being aware of the AL or refractive status of the imaged eyes. 

The present study also poses several limitations. First, due to the currently established equipment and analysis system, the study did not measure the total retinal blood flow (TRBF) from all vessels emanating from the optic disc but instead used the average RBF as a surrogate. Although this approach could enhance the stability of hemodynamic data measurement, it also resulted in the loss of some descriptions of retinal vascular changes. Nonetheless, as the retina is a typical end-organ for blood perfusion, the consistency between average RBF^a and RBF^v demonstrates the stability of the measurement system and vessel selection protocol, lending credibility to the research conclusions. Second, the study did not obtain the average arterial blood pressure levels of the patients, thus hindering the assessment of the impact of ocular perfusion pressure on oxygen dynamic parameters and its involvement in the pathophysiological process of AL growth, which can be further explored in future studies. Finally, this was a relatively small sample cross-sectional study. Thus, further investigations into the correlation between AL and retinal oxygen dynamics with a larger sample size and additional research into their causal relationship are warranted. 

In conclusion, the present study has demonstrated that within a certain range of AL, an elevation of the oxygen extraction fraction OEF can adequately compensate for reduced oxygen delivery, thereby maintaining a relatively stable retinal oxygen consumption. Although further validation is necessary to generalize this conclusion to individuals with severe pathological myopia, this study presents a convenient method for measuring retinal oxygen dynamics indicators, which holds potential for future applications in the prevention and treatment of myopia and even the assessment of other hypoxic retinal diseases. 

Acknowledgments

Supported by Equipment Comprehensive Scientific Research Project of China (Grant No. LB20201A010024). 

Disclosure: G. Xiao, None; M. Ge, None; G. Qiao, None; S. Liu, None; N. Li, None; F. Liu, None; Y. Lu, None; Q. Ren, None; L. Wang, None