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ORIGINAL ARTICLE
Year : 2020  |  Volume : 33  |  Issue : 2  |  Page : 534-539

Determining the correlation between axial length/spherical equivalent and macular thickness in myopia


Department of Ophthalmology, Faculty of Medicine, Menoufia University, Menoufia, Egypt

Date of Submission28-Mar-2016
Date of Decision21-Jun-2016
Date of Acceptance26-Jun-2016
Date of Web Publication27-Jun-2020

Correspondence Address:
Basma A. E. Hassan
Benha, Kalubia
Egypt
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/mmj.mmj_172_16

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  Abstract 


Objective
The aim was to assess the relationship between macular thickness in each of the nine regions and the axial length (AL) and spherical equivalent (SE) in myopes.
Background
Myopia is the most common error of refraction and, in many countries; complications related to high myopia are a major cause of blindness. The relation between macular thickness and AL in myopia has been one of much interest over the past few years. Many researchers believe that since in axial myopia, the globe elongates and the sclera stretches, so the macula must thin.
Patients and methods
One hundred eyes of 50 myopes, free of any other ocular abnormalities were assessed. All were above 18 years of age. Each patient was subjected to a full clinical examination, refraction, AL assessment by A scan ultrasonography and macular thickness was determined by spectral domain optical coherence tomography. The patients were subdivided into three diagnostic groups into high myopia (>–6.00 D), moderate myopia between (–3.00 and –6.00 D), and low myopia (< –3.00 D).
Results
The study revealed a highly significant negative correlation between increasing AL and macular thickness in all quadrants except in the fovea (positive correlation found) and outer temporal quadrant, and a highly significant positive correlation between SE and macular thickness in all quadrants except in the fovea (negative correlation found) and outer temporal quadrant. Macular thickness was significantly less in high myopes than low and moderate myopes except in the fovea (more thickness in high myopes). The SE was significantly correlated with increasing AL.
Conclusion
In myopes, increasing AL and degree of myopia can be correlated with a decrease in macular thickness except in the foveal region.

Keywords: axial length, macular thickness, myopia, optical coherence tomography


How to cite this article:
Sarhan AR, Zaky MA, Hassan BA. Determining the correlation between axial length/spherical equivalent and macular thickness in myopia. Menoufia Med J 2020;33:534-9

How to cite this URL:
Sarhan AR, Zaky MA, Hassan BA. Determining the correlation between axial length/spherical equivalent and macular thickness in myopia. Menoufia Med J [serial online] 2020 [cited 2020 Oct 28];33:534-9. Available from: http://www.mmj.eg.net/text.asp?2020/33/2/534/287753




  Introduction Top


Myopia is the most common error of refraction and, in many countries; complications related to high myopia are a major cause of blindness. It is a refractive defect of the eye in which collimated light produces an image which comes into focus in front of the retina when accommodation is relaxed. Thus the image one sees comes to focus only on approximation of the object[1]. Myopia, which is measured in diopters, has been classified by degree. Low myopia usually describes myopia of −3.00 D or less. Medium myopia usually describes myopia of between −3.00 and 6.00 D. High myopia usually describes myopia of −6.00 or more[2]. Similarly, myopia has also been classified according to the axial length (AL) into low to moderate myopia (AL 25.1 ± 1.1 mm) and high myopia (AL 27.1 ± 1.1 mm),[3].

It is well known that individuals with high myopia have an increased risk of retinal complications such as peripheral retinal degeneration, retinal detachment, posterior staphyloma, chorioretinal atrophy, retinal pigment epithelial atrophy, and macular hemorrhage[4].

The macula is an oval shaped highly pigmented yellow region near the center of the retina of the human eye. It has a diameter of around 5 mm and is often histologically defined as having two or more layers of ganglion cells. Near its center is the fovea, a small pit that contains the largest concentration of cone cells in the eye and is responsible for central, high-resolution vision. With the availability of modern imaging technologies, such as the optical coherence tomography (OCT), in-vivo measurement of retinal thickness has been made possible[5]. In myopic eyes, the elongation of the globe leads to mechanical stretching and thinning of the retina[3]. It is conceivable that the extent of the elongation would be related to the degree of retinal thinning. Thus, macular thickness both in the region of the fovea and parafovea may be correlated with the AL and the degree of refractive error[6].

With OCT, the exact localization of pathologic features can be visualized in segmentation maps of the retina, and this has allowed OCT to be used to evaluate specific features that may serve as predictive factors in the prognosis and follow-up of these pathologies[7].


  Patients and Methods Top


The current study was a cross sectional comparative study which was conducted after approval of the ethics committee of the faculty and a written consent was taken from every patient. The study included 100 myopic eyes of 50 participants for correlation between the AL and macular thickness. They were subdivided into three diagnostic groups according to refractive status into: high myopia [spherical equivalent (SE) >−6.00 D], moderate myopia (SE between −3.00 and −6.00 D), and low myopia (SE < −3.00 D). The cases were selected from the outpatient clinic of Menoufia University Hospital. All were above 18 years of age. Each case was subjected to detailed ocular examination. This assessment included uncorrected visual acuity, refraction (automated), and then BCVA. The intraocular pressure was determined by Goldmann Applanation Tonometry. Anterior segment was examined by the slit-lamp and posterior segment examination by both indirect ophthalmoscopy and the 90 D auxiliary lens. The AL s were determined by A-scan ultrasound. The device used was the Sonomed PAcScan 300AP (Sonomed Inc., New York, New York, USA). The program was set to 'normal cataract.' Macular mapping was obtained by spectral domain OCT (Heidelberg Engineering GmbH., Max-Jarecki-Strasse, Heidelberg, Germany). The macular map is 1, 3, 6 mm in diameter. The central 1 mm was considered as the fovea. Extending from the central 1–31 mm is the parafoveal region and extending from 3 to 61 mm is the perifoveal region. Both the para and perifoveal regions were subdivided into four quadrants: superior, inferior, temporal, and nasal. The exclusion criteria were: any participant younger than 18 years, history of prior ocular surgery, those whose vision did not correct to at least 6/36 by the Broken C charts, and any ocular pathology such as glaucoma, cataract, or diabetic retinopathy.

Statistical analysis

Data were collected, tabulated, and statistically analyzed using an IBM personal computer with the Statistical Package for the Social Sciences (SPSS) Version 20 (IBM Corp, Armonk, NY, USA) where the following statistics were applied. Descriptive statistics in which quantitative data were presented in the form of mean, SD, range, and qualitative data were presented in the form of numbers and percentages. Analytical statistics was used to find out the possible association between the studied factors and the targeted disease. The used tests of significance included: χ2 test, analysis of variance (F) test, Pearson's correlation (r). P value of greater than 0.05 was considered statistically nonsignificant. A P value of less than 0.05 was considered statistically significant. A P value of less than 0.001 was considered statistically highly significant.


  Results Top


One hundred eyes of 50 cases enrolled in the study were available for analysis. They were divided according to SE into low myopes (<−3.00 D), moderate myopes (between −3.00 and −6.00 D), and high myopes (>−6.00 D). There were 44 eyes belonging to the low myopic group, 29 eyes to the moderate myopic group, and 27 eyes to the high myopic group. The refraction ranged from −0.25 to −21.5. The AL ranged from 22.1 to 30.0 mm. Examination of the fundus of these cases revealed that 18 had a tigroid pattern (moderate and high myopes) and three had a temporal crescent (high myopes).

Our study shows the mean and SD of all variables studied (AL, SE, and macular thickness) in all studied groups. Mean AL is 24.6 ± 1.83., while the mean refractive error of all participants is −4.48 ± 5.05 and the mean foveal thickness of all studied participants is 271.7 ± 29.2 [Table 1].
Table 1: Mean and SD of axial length, refractive error, and optical coherence tomography measurements in the studied group

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Also our study shows that thicknesses in all quadrants of the macula, except for the fovea, were less for high myopes than for low and moderate myopes. This finding was highly significant in the foveal, outer nasal, inner superior, and inner inferior quadrants while was significant in the inner temporal, inner nasal, outer superior, and outer inferior quadrants [Table 2].
Table 2: Optical coherence tomography measurement of the studied group

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Correlations between the AL and the thickness of the macula in each of the nine quadrants are shown in [Table 3]. It revealed that the fovea was the only quadrant to show positive correlation (an increase in thickness with increasing AL) [Figure 1]. This finding was highly statistically significant. In the remaining quadrants there was a negative correlation (decrease in thickness as the AL increased). This correlation was highly significant except in the outer temporal quadrant (P < 0.001).
Table 3: Correlation between macular thickness, axial length, and spherical equivalent and optical coherence tomography measurements

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Figure 1: Correlation between axial length and central macular thickness.

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The correlation between the degree of myopia and the macular thickness was assisted. The SE used for subjective refraction showed negative highly significant correlation with the thickness of the fovea [Table 3]. On the other hand, there was a significant positive correlation in all the remaining quadrants, which was highly significant only in the inner temporal, outer nasal, inner superior, and inner inferior quadrants and nonsignificant in the outer temporal quadrant [Figure 2].
Figure 2: Correlation between spherical equivalent and central macular thickness.

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Macular mappings are shown in [Figure 3],[Figure 4],[Figure 5]. Macular mapping of low myopic patients with SE is −2.00 D and AL is 22.64 mm [Figure 3]. mapping of moderate myopia with SE is −4.00 D and AL is 23.87 mm [Figure 4], and mapping of high myopia with SE is −7.25 D and AL is 25.20 mm [Figure 5].
Figure 3: Case no 1: low myopic patient.

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Figure 4: Case no 2: moderate myopic patient.

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Figure 5: Case no 3: high myopic patient.

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  Discussion Top


Data obtained from the macular maps were correlated in the study to the AL and refractive error. In the fovea it was found that there was high statistically significant correlation between its increasing thickness and the increasing ALs and refractive error. Also, when comparing the thickness of the fovea in low, moderate, and high myopes, the high myopes had a greater thickness. In the following studies: of Lam et al.[3], and Song et al.[8], there was also an increase of foveal thickness with increase in AL and SE but this finding was found to be statistically nonsignificant. In other studies, there was also an increase of foveal thickness with increase in AL and refractive error but this finding was found to be statistically significant. These studies include those of Luo et al.[9], Othman et al.[10], Chen et al.[5], Chew et al.[11], Aung et al.[12], and Chan et al.[13]. There are several proposed theories for this finding: increasing foveal thickness with increasing AL and refractive error. In Chan et al.[13], the authors hypothesized that this finding may be due to subclinical vitreoretinal traction not yet detected by OCT. In Aung et al.[12], the authors proposed that this increase in thickness may be due to pathologic subfoveal chorioretinal changes. In other studies, the fovea was not correlated with increasing AL. These studies include Ido et al.[7] and Kim et al.[14], in which there was no statistically significant change in retinal thickness with increasing AL in any macular quadrants. In this study, contrary to the other ones, the OCT used was the Humphrey (2000), which is one of the first-generation time-domain OCTs. This may attribute to its different findings. In those studies the fovea was considered as the point of minimum thickness. In our study it was considered as a region 1 mm in diameter, which would extend into what was presumed to be the parafoveal region in Chew et al.[11] and Aung et al.[12]. Therefore, our results cannot be compared with theirs. In Lam et al.[3], Othman et al.[10], Chen et al.[5], and Chan et al.[13], the area considered to be the fovea was, as in our study, the central 1 mm. In the remaining eight quadrants of the macula, our study found a significant negative correlation between AL and the thickness when all study cases were analyzed as one group [Table 3]. This correlation was highly significant in all the quadrants except in the outer temporal quadrant. This was similar to the results in the study by Luo et al.[9] and Lam et al.[3], there was a negative correlation between total macular thickness and AL (positive correlation with SE) and a positive correlation between foveal thickness and AL (negative correlation with SE). In Luo et al.[9], the minimum macular thickness at the foveal region increased, whereas overall macular thickness decreased with increasing myopic refraction and AL. It is possible that the increase in foveal thickness, which was associated with increasing myopic SE and AL in this study, may be due to pathologic subfoveal chorioretinal changes. The absence of vasculature in the foveal area may lead to foveal pits that are very deformable in response to intraocular pressure, and ocular growth-induced retinal stretch may contribute to the formation of foveal pits in primates. The significance of these OCT findings may require further research.

In this study, we found that the outer ring (3–6 mm) and the inner ring (1–3 mm), macular thicknesses decreased in eyes with a greater degree of myopia. Similarly, negative correlations were found between the inner and the outer ring but not the central macular thicknesses and the AL agreed with the hypothesis that macular thickness in the most central area is preserved in high myopia. In Chew et al.[11], there was statistically significant negative correlation in the parafoveal region, while in Kim et al.[14], the significance was in the perifoveal region only. In Chan et al.[13], the significant negative correlation was found only in the perifoveal region and not in the parafoveal region. In Song et al.[8], the significant negative correlation was found in the outer macular quadrants. Also, this finding was in Othman et al.[14] in the outer macular quadrants. In Song et al.[8], as AL increased, average outer macular thickness, overall average macular thickness, and overall macular volume decreased. This finding affirms histopathologic studies that have demonstrated increasing retinal thinning with myopia and a clinical study that demonstrated more frequent chorioretinal atrophy in the posterior pole in eyes with longer AL. The Othman et al.[10] study showed that thinning of the outer macula in most of the quadrants is correlated with increasing myopia and elongation of the globe. This study also showed that there is a trend toward thickening of the foveal area with increasing degrees of myopia. In Chen et al.[5], there was significant negative correlation of average macular thickness with AL and positive correlation with SE. This study showed that, as a whole, the fovea was the thinnest region, followed by the outer ring, while the inner ring was the thickest.

Regardless of age, sex, SE, and AL, there was no wide variation in macular thickness among the inner four quadrants, whereas there was a large diversity among the outer four quadrants, with the macular thickness in the outer nasal quadrant being much thicker than the other three quadrants. It has been hypothesized that the thinning in the inner and outer rings is due to the mechanical stretching of a similar volume of retina over a larger area, a decrease in the number of photoreceptors, and/or even the early occurrence of chorioretinal atrophy in highly myopic eyes. In contrast, the thickening in the fovea is likely due to myopia-induced pathologic subfoveal chorioretinal changes. Another possibility supported by myopia modeling in young primates is the alteration of anatomic characteristics of the macula, such as the absence of vasculature leaving the foveal pits deformable in response to retinal stretching induced by ocular growth[5].

In our study of the patients when comparing the three groups (low myopes, moderate myopes, and high myopes), we found that thicknesses in all quadrants of the macula, except for the fovea, were less for high myopes than for low and moderate myopes. This finding was highly significant in the outer nasal, inner superior, and inner inferior while in the inner temporal, inner nasal, outer superior, and outer inferior quadrants the differences between the three groups were significant. In Ido et al.[7], no significant difference was found among their three groups (emmetropes and low myopes, medium and high myopes). In Chan et al.[13], there was significant difference between low and high myopes in the outer macular area only (thinner in high myopes). In Chen et al.[5], there was significant difference between emmetropes, low myopes, and moderate to high myopes groups in outer macular quadrants only (thinner in high myopes).

When correlating age with macular thickness, SE and AL, there was no significant correlation found except for outer nasal and inner inferior quadrants and SE with negative correlation. In Song et al.[8] study, the average inner, outer, overall macular thickness, and overall macular volume were shown to decrease with the participants' mean age in this study. These results correspond to histologic human retina studies that have demonstrated a decrease in the density of photoreceptors, ganglion cells, and retinal pigment epithelial cells with age.


  Conclusion Top


A significant negative correlation was found between increasing AL and macular thickness in all quadrants of the macula except for the fovea and the outer temporal quadrant. The SE correlated significantly with thickness in the overall macular area and its nine quadrants except for the outer temporal quadrant. Positive correlation was found between foveal thickness and AL and negative correlation with SE.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
  References Top

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    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]
 
 
    Tables

  [Table 1], [Table 2], [Table 3]



 

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