|Ahead of print publication
A study of retinal nerve fiber layer thickness and other optic nerve head parameters in cases of amblyopia
Sanjay Kumar Dhar, Kurumkatil Raji, Vijay Kumar Sharma, Prakhar Kumar Singh
Department of Ophthalmology, Army Hospital (R and R), New Delhi, India
|Date of Submission||23-Jul-2020|
|Date of Decision||07-Oct-2020|
|Date of Acceptance||27-Dec-2020|
|Date of Web Publication||01-Apr-2021|
Sanjay Kumar Dhar,
Department of Ophthalmology, Army Hospital (R and R), Delhi Cantt, New Delhi - 110 010
Source of Support: None, Conflict of Interest: None
Introduction: Amblyopia is a disorder of decreased visual acuity and contrast sensitivity, studies in past have shown retinal changes in amblyopic eyes, which was refuted by other studies. Studies in the recent past have reported variable results on retinal nerve fiber layer (RNFL) thickness in amblyopic eyes. Therefore, there is still a gap in present knowledge. This study was conducted to study RNFL thickness and other optic nerve head (ONH) parameters in amblyopic and better fellow eyes. Materials and Methods: A total of 30 diagnosed cases of amblyopia (four cases of meridional, 11 cases of anisometropic, three cases of strabismic, eight cases of ammetropic, and four cases of form deprivation amblyopia) were divided into two groups: Group A (amblyopic eyes – 30 eyes) and Group B (better fellow eyes – 30 eyes). All the eyes underwent visual acuity assessment, refraction (under cycloplegia), anterior segment evaluation, and fundus evaluation. All the patients underwent ONH evaluation by Cirrus 500 spectral-domain high-definition optical coherence tomography, before and after occlusion therapy. The ONH parameters studied were average RNFL thickness, rim area, disc area, and cup volume. The data were analyzed using SPSS version 25:00. Results: The RNFL thickness was less in amblyopic eyes as compared to normal fellow eyes, (P = 0.002). There was a positive correlation between RNFL thickness and rim area in both the groups, but it was significant (P = 0.02) in Group A only. No statistically significant difference was found between the rim area, disc area, and cup volume between the two groups. Conclusion: The RNFL thickness was found to be significantly less in amblyopic eyes, also there was a significant positive correlation between RNFL thickness and rim area in Group A (P = 0.02).
Keywords: Amblyopia, optic nerve head, retinal nerve fiber layer thickness
| Introduction|| |
Amblyopia is a disorder of decreased visual acuity and contrast sensitivity, either unilateral or bilateral, due to visual deprivation or defective binocular interactions. Previous studies have shown retinal changes in amblyopic eyes;, however, this was refuted later by other studies., Repeated investigations and research in the recent past have pointed toward the presence of organic changes in amblyopic eyes; however, further investigations are required to establish whether there are any associated retinal changes in amblyopia. Studies in the recent past with the help of spectral domain (SD)-optical coherence tomography (OCT), which has better speed and resolution, have reported variable results on RNFL thickness in amblyopic eyes., The reported result in these studies ranged from no difference to thicker or thinner retinal thickness between amblyopic and fellow eye. Therefore, this is still an area of exploration which requires to be studied and investigated further. With this background, we carried out this study to compare the retinal nerve fiber layer (RNFL) thickness and other optic nerve head (ONH) parameters (disc area, rim area, and cup volume) in amblyopic and better fellow eyes.
| Materials and Methods|| |
This prospective observational study was carried out at a tertiary care center, over a period of 6 months. All diagnosed patients of unilateral amblyopia were included in the study, after approval from the Institutional Ethical Committee and consent from the patients. A total of 30 patients (four patients with meridional amblyopia, 11 patients with anisometropic amblyopia, three patients with strabismic amblyopia, eight patients with ammetropic amblyopia, and four patients with form-deprivation amblyopia) diagnosed as amblyopic were divided into two groups: Group A (amblyopic eyes –30) and Group B (fellow eyes as controls –30). Patients with established amblyopia with or without strabismus were included in the study. Unilateral amblyopia was defined as a best-corrected visual acuity (BCVA) difference of at least two lines between the amblyopic and fellow eye. Inclusion criteria were (1) children with unilateral amblyopia (difference in BCVA of ≥0.2 LogMAR between two eyes), (2) children with normal neurological, ocular, and systemic examination, and (3) parents or patients willing for follow-up. Patients having any neurological disease (cerebral palsy, cortical visual impairment, and optic neuritis) were excluded from the study. All the eyes underwent visual acuity assessment (uncorrected visual acuity and best corrected visual acuity), refraction (under cycloplegia), anterior segment evaluation, and fundus evaluation. All the patients underwent evaluation of ONH parameters, namely RNFL thickness (peripapillary), disc area, rim area, and cup volume by Cirrus 500 SD high definition OCT, before and after occlusion therapy. The duration of occlusion was decided as per the severity of amblyopia and all patients were advised daily part-time occlusion only, patients with severe amblyopia (20/100–20/400) were advised 6 h of patching, with moderate amblyopia (20/40–20/80) were advised 4 h, and patients with mild amblyopia (<20/40) were advised 2 h of daily patching. Once an increase in visual acuity was observed, patching was gradually tapered over a period of 2 months and all patients were observed for development of any occlusion amblyopia in normal fellow eye during follow-up visits. The mean BCVA in amblyopic eyes was +0.67 LogMAR and +0.09 LogMAR in fellow eyes before amblyopia treatment and +0.47 LogMAR in amblyopic eyes and +0.07 LogMAR in better fellow eyes after amblyopia treatment. Statistical analysis was done using SPSS Inc,Version 25.
| Results|| |
The study included 20 males and 10 females. The youngest patient was 5 years' old and the eldest was 21 years old [Graph 1]. There were four cases (13.5%) of meridional amblyopia, 11 (36.5%) cases of anisometropic amblyopia, 3 (10%) cases of strabismic amblyopia, 8 (26.5%) cases of ammetropic amblyopia, and 4 (13.5%) cases of form-deprivation amblyopia. The mean and standard deviation for RNFL thickness was 83.37 μ ± 20.81 μ in Group A and 89.67 μ ± 16.74 μ in Group B, respectively. There was a trend of decreased RNFL thickness in amblyopic eyes, which was statistically significant (P = 0.002) [Graph 2]. The mean rim area in Group A was 1.58 mm2 and in Group B was 1.58 mm2 (P = 0.87) [Graph 3]. The mean disc area in Group A was 2.11 mm2 and in Group B was 2.23 mm2 (P = 0.54) [Graph 4]. The mean cup volume in Group A was 0.13 mm3 and in Group B was 0.14 mm3 (P = 0.97) [Graph 5]. Correlation between various ONH parameters was also calculated [Table 1].
Correlation between RNFL thickness and rim area in Group A was positive (0.407) as well as statistically significant (P = 0.02). Other ONH parameters which were evaluated and found to have a positive correlation were RNFL thickness and disc area in Group A and RNFL thickness and rim area, disc area, and cup volume in Group B, none of these correlations were statistically significant. ONH parameters which had a negative correlation were RNFL thickness and cup volume in Group A. RNFL thickness and postocclusion vision in Group A had a positive correlation.
RNFL values, disc area, and cup volume showed a positive correlation with the actual visual acuity measured both pre and post occlusion, but the same were not statistically significant. Rim area had a negative correlation with the pre- and post-occlusion actual visual acuity but was not statistically significant [Table 2].
|Table 2: Correlation between optic nerve head parameters in Group A with pre- and postocclusion vision|
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Parameters compared in the two groups were,RNFL thickness,rim area,disc area and cup volume, only the RNFL thickness in the two groups was found to have a statistically significant difference (P = 0.002, Wilcoxon signed-rank test), and the rest of all the other ONH parameters showed no statistically significant difference [Table 3].
| Discussion|| |
This study was done to find the difference between RNFL thickness and other ONH parameters (disc area, rim area, and cup volume) between Group A (amblyopic eyes) and Group B (normal fellow eyes).
RNFL thickness in Group A was found to be less as compared to Group B, and the difference was statistically significant (P = 0.002). This was different from the study by Anaki et al. and Wu et al., who observed thicker RNFL in cases of amblyopia. Our study results are also different from Al-Haddad et al. who observed no difference in the RNFL thickness in two groups. Our results are also different from that of Alotaibi and Al Enazi who observed thicker peripapillary RNFL in anisometropic amblyopia eyes. In addition, RNFL thickness had a positive correlation (Pearson's correlation coefficient 0.204) with postocclusion visual acuity in the amblyopia group.
There was a positive correlation between RNFL thickness and rim area in both the groups, but it was significant (P = 0.02) in Group A only, which was similar to the study done by Araki et al. who have also found a positive correlation between RNFL thickness and rim area in cases of amblyopia.
RNFL thickness and disc area had a positive correlation in both the groups, but the correlation was stronger in Group A (0.319 in Group A and 0.092 in Group B), whereas RNFL thickness and cup volume had a negative correlation (−0.050) in Group A and positive (0.163) in Group B. No statistically significant difference was found between the rim area, disc area, and cup volume between the two groups. A study done by Kim et al. also observed no statistically significant difference between ONH parameters of amblyopic and normal fellow eyes, although the study group was that of deprivational amblyopia, indicating the need to study different types of amblyopia separately.
A study done by Araki et al. found rim area to be significantly larger in amblyopia group; however, the study had some limitations, first, the study population was not segregated as per the type of amblyopia and neither they divided the patients into those who responded to amblyopia therapy and those who did not. Although we segregated the various types of amblyopia, the groups were too small in number to study them separately.
| Conclusion|| |
RNFL thickness in amblyopic eyes was less as compared to normal fellow eyes, which was statistically significant (P = 0.002). RNFL thickness showed a positive correlation with postocclusion vision, meaning the thicker the RNFL, the better will be the vision, post occlusion therapy, suggesting a better prognosis for visual gain from occlusion therapy for eyes with thicker RNFL, and a slower and suboptimal visual recovery in patients with relatively thinner RNFL. None of the other ONH parameters showed any statistically significant difference in the two groups. No consensus has reached so far in various studies regarding changes in the various ONH parameters in amblyopic eyes; hence, there is a further need of a larger study, with subgroups of different types of amblyopia, to explore the ONH parameters in cases of different types of amblyopia which may hold a key to understanding the prognosis and treatment in these cases.
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Conflicts of interest
There are no conflicts of interest.
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[Table 1], [Table 2], [Table 3]