|Year : 2022 | Volume
| Issue : 3 | Page : 235-239
An observational study of the correlation between axial length and retinal nerve fibre layer thickness in myopic eyes in a tertiary care centre in south india
A Rajeshwari, VK Malathi
Dr. MGR Medical University, ESIC Medical College and Postgraduate Institute of Medical Science and Research, K.K.Nagar, Chennai, Tamil Nadu, India
|Date of Submission||18-Nov-2021|
|Date of Decision||17-May-2022|
|Date of Acceptance||20-May-2022|
|Date of Web Publication||26-Sep-2022|
V K Malathi
Plot No. 76, Door No. 14, Balakrishnapuram 4th Street, Adambakkam, Chennai, Tamil Nadu - 600 088
Source of Support: None, Conflict of Interest: None
Objective: To assess the correlation between axial length and retinal nerve fibre layer (RNFL) thickness in myopic eyes in a tertiary care hospital in a two-year period (August 2017 to June 2019). Methods: Institutional ethics committee approval was obtained. Written and informed consent was obtained from all the participants. One hundred eyes of 58 patients were screened. Myopic patients with > -0.50 D myopia in the age group of 18 to 40 years with intraocular pressure (IOP) <20 mm Hg were included in the study. Patients with a history of ocular trauma, glaucoma, ocular surgeries, optic nerve or macular diseases, pathological myopia, and media opacities were excluded from the study. An ophthalmic evaluation was done by assessing the visual acuity, IOP, slit-lamp examination, and perimetry using Humphrey field analyser II (Swedish interactive threshold algorithm SITATM testing strategy). After dilating the pupils with Tropicamide plus, posterior segment evaluation was done using a 90 D lens with slit lamp bio-microscopy as well as indirect ophthalmoscopy and 20 D lens. The axial length was measured using Lenstar LS 900. Optical Coherence Tomography (OCT) images were obtained using a 3 D OCT 1 Maestro machine by a single observer. Results: There was a statistically significant association between axial length and RNFL thickness in myopic eyes (P = 0.0003). As the axial length of the eyeball increases, the RNFL thickness decreases in all four quadrants except the temporal quadrant, producing a statistically significant negative correlation (r = -0.3551). Conclusion: There is a statistically significant association between axial length and RNFL thicknesses in all quadrants except the temporal quadrant. The negative correlation between axial length and RNFL thickness would be useful in avoiding misdiagnosis of glaucoma since RNFL thickness is low in both glaucoma and myopia.
Keywords: Axial length, optical coherence tomography (OCT), Retinal nerve fibre layer (RNFL) thickness
|How to cite this article:|
Rajeshwari A, Malathi V K. An observational study of the correlation between axial length and retinal nerve fibre layer thickness in myopic eyes in a tertiary care centre in south india. TNOA J Ophthalmic Sci Res 2022;60:235-9
|How to cite this URL:|
Rajeshwari A, Malathi V K. An observational study of the correlation between axial length and retinal nerve fibre layer thickness in myopic eyes in a tertiary care centre in south india. TNOA J Ophthalmic Sci Res [serial online] 2022 [cited 2022 Nov 29];60:235-9. Available from: https://www.tnoajosr.com/text.asp?2022/60/3/235/357110
| Introduction|| |
Myopia is one of the most common correctable ocular refractive errors in the world. About 30% of the world population will be affected by the year 2020 and 50% by 2050. Axial length (anterior surface of the cornea to retinal pigment epithelium/Bruch's membrane) is one of the commonest determinants of myopia., Normal axial length in an adult ranges from 22–25 mm. Elongation of the eyeball starts from the age of 6 months. Emmetropic axial length is reached by 13 years of age. The retinal nerve fibre layer (RNFL) is formed by the expansion of the fibres of the optic nerve. RNFL thickness in the various quadrants of optic disc increases as follows: Temporal < Supero-temporal and inferotemporal < nasal < Supero-nasal and inferonasal. In high myopia, there is thinning of the sclera and RNFL. Myopes of >4 D have a 2 to 3-fold higher risk of glaucoma.,, Thinning of RNFL can be misdiagnosed as glaucoma in the preclinical stage as RNFL thinning occurs in both glaucoma and myopia. Hence, this study is important to correlate with the refractive status of the eye before diagnosing glaucoma in everyday practice.
| Subjects and Methods|| |
Study type: Prospective observational study
Sample size calculation
The reference article by Akram et al. showed the significant correlation between axial length and RNFL thickness in myopic eyes with Pearson coefficient of correlation (r) equal to -0.328. With an assumption of 80% power, 95% of confidence interval [Table 1], [Table 2], [Table 3], [Table 4], and 5% level of significance, a minimum sample of 65 is needed for the study. The final sample size of this study is 100. The research was started after obtaining the approval from the ethics committee. date of approval: 24 August 2017.
|Table 3: Mean superior RNFL thickness according to axial length in the study population|
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The study was conducted after getting clearance from the institutional ethics committee. The purpose of the study was explained to the study participants and written informed consent was obtained from all the participants. One hundred eyes of 58 patients were studied. Patients with the myopia of > -0.50 D and < -6 D, in the age group of 18 to 40 years, with intraocular pressure less than 20 mm Hg, and a normal visual field, were included in the study.
Patients with a history of glaucoma, ocular trauma, ocular surgeries, optic nerve disease, macular disease, pathological myopia, media opacities, diabetes mellitus, and hypertension, were excluded from the study.
Ophthalmic examination included distant vision using Snellen's chart, near vision using Jaeger's chart, and intraocular pressure measurement using a non-contact tonometer. The anterior segment was examined using a slit lamp biomicroscope, the visual field was analysed using the Humphrey field analyser II, following which pupils were dilated with 0.8% Tropicamide and 5% Phenylephrine eye drops. Fundus examination was done with an indirect ophthalmoscope and 20 D lens as well as 90 D with slit lamp biomicroscopy.
Axial length was measured using Lenstar LS 900, a non-contact optical biometry device. A minimum of three measurements were taken to be more accurate.
Based on the axial length, the study population was grouped into group A: 23–23.99 mm axial length, Group B: 24–24.99 mm axial length, Group C: 25–25.99 mm axial length, Group D: 26–26.99 mm axial length [Table 5], and Group E: ≥ 27 mm axial length.
Based on the degree of myopia, the patients were sub-grouped as low myopia (spherical equivalent of -0.5 to -3 D), moderate myopia (-3 to -6 D), and high myopia (> -6 D). Astigmatism of up to -1 D was included in the study and transposed to spherical equivalent.
The visual field screening [Table 6], [Table7], [Table 8], [Table 9], [Table 10] test was done for all participants using the Humphrey field analyser II. With the statistical software STATPACTM immediate system analysis of visual field test results was done. The SITATM testing strategy was used, which allows precise visual field measurement with increasing speed and accuracy.
Optical coherence tomography (OCT) images were obtained using a 3 D OCT 1 Maestro machine in dilated pupils. A single person has done the investigation.
Statistical analysis was done by the statistical software STATA 11.0. Continuous variables were represented as mean and categorical variables were represented as frequency (%). The Chi-square test was used to assess the differences in categorical data. Pearson's correlation coefficient was used to assess the relationships between two continuous variables. The P value of < 0.05 was considered significant.
| Results|| |
The average RNFL thickness of the study population was 101.4 ± 0.11 μm. The average RNFL thickness of the superior quadrant was 128.68 ± 18.01 μm, the inferior quadrant was 130.23 ± 18.19 μm, the nasal quadrant was 74.43 ± 12.23 μm, and the temporal quadrant was 72.13 ± 12.05 μm. The RNFL thickness in the four quadrants follows the Inferior Superior Nasal Temporal (ISNT) rule for RNFL thickness, with the inferior quadrant showing more thickness followed by the superior, the nasal, and the temporal quadrants [Table 7], [Table 8], [Table 9].
| Discussion|| |
Ocular axial length is one of the important indicators of the refractive status of the eye. The axial length is influenced by various factors such as age, gender, race, ethnicity, the refractive status of the eye, and genetics. The axial length of the eye increases in myopia. The genetic expression of the Axial Length (AL) gene activated by environmental factors leads to an increase in axial length, which causes stretching and thinning of the retina leading to a decrease in RNFL thickness.
The average RNFL thickness in this study is 101 ± 10.93 μm, which is close to the value of 102 μm, obtained by Zhao et al. and Alasil et al. The population in these studies comprised of Asian ethnicity.
In our study, the average RNFL thickness of group A was 101.64 ± 11.09 μm, group B was 105.86 ± 12.09 μm, group C was 98.40 ± 7.01 μm, group D was 93 ± 5.36 μm, and group E was 90 ± 5.65 μm. These changes in average RNFL thickness, when compared to the axial length, were found to be statistically significant, with a negative correlation of r = -0.3551.
This study shows that the total and quadrant RNFL thickness decreases as the axial length of the eyeball increases, except the temporal quadrant (total r = 0.3551, P = 0.0003, superior r = -0.2881, P = 0.0037, inferior r = 0.3751, P = 0.0001, nasal r = 0.2203, P = 0.0227). A significant negative correlation (r = -0.3551) was found between axial length and RNFL thickness in myopic eyes in all quadrants, except in the temporal quadrant. There was a significant association between axial length and RNFL thickness in the superior (P = 0.0037) and inferior quadrants (P = 0.0001), which was similar to the results of a study done in South India in 2015.
Of the 100 eyes evaluated in the study, 11 eyes showed pronounced thinning of RNFL thickness in the inferior quadrant with an increase in axial length of the eye. All the 11 eyes showed RNFL thickness outside the normal limit, with seven eyes showing severe RNFL thinning and four eyes showing moderate RNFL thinning. Of those 11 eyes showing RNFL thickness outside the normal limit, eight (72.72%) eyes had an axial length >25 mm, and three (27.27%) eyes had an axial length >24.50 mm, indicating a more pronounced thinning of RNFL thickness in inferior quadrant when the axial length of the eye was > 24.50 mm.
In this study, the average RNFL thickness decreased in high myopes (R = 94 ± 7.39 μm) than those in moderate myopes (R = 105.34 ± 7.32 μm), which indicates that there is a significant negative correlation of RNFL thickness (P = 0.0001, r = -0.4090) with the myopic refractive error.
The present study also shows that the average RNFL thickness decreases by 4.15 μm for every one dioptre increase in spherical equivalent. The decrease in RNFL thickness with axial length (4.15 μm/mm) was lower in this study population when compared to Kumar et al. (6 μm/mm) but higher than Budenz et al. (2 μm/mm), Leung et al. (2.75 μm/mm), and Salchow et al. (1.7 μm/mm) study population.
The discrepancies in the values of various ocular parameters in the study with other studies is due to the difference in age, ethnicity, newer generation OCT, and software used in this study when compared to other studies.
In this study, although there was a sectorial variation in the RNFL thickness measured by OCT-1 maestro in low and moderate myopic eyes, the average RNFL thickness decreased with increasing degree of myopia which was similar to the Wang et al. study.
In myopic eyes, the elongation of the globe leads to mechanical stretching and thinning of the retina. Therefore, the amount of RNFL thinning is related to the extent of elongation of the globe, though it is yet to be ascertained whether RNFL thickness decreases at the histological level.
Rauscher et al. found a significantly strong association between axial length and RNFL thickness in their study, and they reported a less strong association between axial length and RNFL thickness. They also reported a less strong relationship between spherical equivalent and RNFL thickness. They found thin RNFL in myopic subjects preferentially at the superior (r = 0.60, P = 0.001) and inferior poles (r = 0.60, P = 0.001), with an overall decrease in RNFL thickness of 7 μm for every 1 mm increase in axial length and 3 μm for every 1 dioptre of the sphere, which is similar to the present study.
| Conclusion|| |
This study found a statistically significant association between axial length and RNFL thickness in all except the temporal quadrant with a statistically significant negative correlation.
Although both high and moderate myopic eyes have similar double hump RNFL profiles in this study, high myopic eyes had significantly lower RNFL thickness than moderate myopic eyes.
In this study, myopic refractive error and RNFL thickness found a statistically significant association with a significant negative correlation. This correlation of axial length and degree of myopia with RNFL thickness would be useful before diagnosing glaucoma since RNFL thickness decreases in both glaucoma and myopia.
Limitations of the study
The study has a small sample size. Less number of high myopes have been included in the study. Magnification factor was not used during the measuring of RNFL thickness. A narrow range of age was taken in this study (18 to 40 years). Hence, the results of this study cannot be used as a reference for older myopic individuals.
Dr. S. Ramakrishnan, Senior Consultant, Vijaya Eye Foundation, Chennai; Dr. P. Janakiraman, Director, Vijaya Eye Foundation, Chennai; Dr. Babu Rajendran; Dr. Archana; Dr. Suganya; Mrs. Gothai Natchiyar; Dr. M. Atchaya Arun Kumar.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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[Table 1], [Table 2], [Table 3], [Table 4], [Table 5], [Table 6], [Table 7], [Table 8], [Table 9], [Table 10]