|Year : 2014 | Volume
| Issue : 1 | Page : 15-17
Cross-sectional study of macular thickness variations in unilateral amblyopia
Siddharth Agrawal, Vinita Singh, Vivek Singhal
Department of Ophthalmology, King Georges' Medical University, Lucknow, Uttar Pradesh, India
|Date of Submission||15-Mar-2013|
|Date of Acceptance||07-Jun-2013|
|Date of Web Publication||3-Dec-2013|
Department of Ophthalmology, King Georges' Medical University, Lucknow - 226 003, Uttar Pradesh
Source of Support: None, Conflict of Interest: None
Aim: The aim of this study is to compare the mean macular thickness between normal and amblyopic eyes in unilateral strabismic and anisometropic amblyopia. Materials and Methods: A spectral domain-optical coherence tomography (Carl Zeiss Meditech, Inc.) was used to evaluate 51 patients (mean age 11.63 ± 2.84 years) with unilateral strabismic (n = 29) and anisometropic (n = 22) amblyopia. Data between the two eyes were compared using a paired t-test and a P value <0.05 was considered as significant. Results: Mean best-corrected visual acuity was +0.53 LogMAR (range 0.2-1.0) in the amblyopic eye. Mean macular thickness in the amblyopic eyes was 277.5 μ ± 15.3 and in the fellow normal eyes was 272.4 μ ± 13.1 (P < 0.05). On subgroup analysis, the difference was statistically significant in strabismic (P = 0.01) and not significant in anisometropic amblyopia (P = 0.08). Conclusion: The mean macular thickness was greater in amblyopic eyes as compared to the normal fellow eyes, and this difference was significant in strabismic amblyopia, but not significant in anisometric amblyopia.
Keywords: Amblyopia, anisometropia, macular thickness, optic coherence tomography, strabismus
|How to cite this article:|
Agrawal S, Singh V, Singhal V. Cross-sectional study of macular thickness variations in unilateral amblyopia. J Clin Ophthalmol Res 2014;2:15-7
|How to cite this URL:|
Agrawal S, Singh V, Singhal V. Cross-sectional study of macular thickness variations in unilateral amblyopia. J Clin Ophthalmol Res [serial online] 2014 [cited 2022 Aug 17];2:15-7. Available from: https://www.jcor.in/text.asp?2014/2/1/15/122630
The prevalence of amblyopia among children considered to be 1.6-3.5% varies depending upon selection criteria and population. , Anisometropic and/or strabismic amblyopia account for the majority, nearly over 90% of all amblyopia. The deleterious effect of amblyopia on the cell growth of the lateral geniculate body has been well-established by quantitative histologic studies in several animal species and in humans. ,, Retinal involvement accompanying amblyopia is poorly understood. It was hypothesized that amblyopia may affect the postnatal maturation of the retina including the postnatal reduction of retinal ganglion cells.  The process of postnatal reduction of ganglion cells requires sharply focused objects as appropriate stimuli.  However, the initial neural site of the visual deficit in this condition is still under investigation and studies that have observed the presence of retinal modifications in amblyopic eyes remain inconclusive and controversial.
Optical coherence tomography (OCT) is a technology with good reproducibility in measuring macular thickness. , Its application in young children has been demonstrated. ,, Some studies have found increased mean macular thickness in amblyopic eyes while others have found no significant differences. ,,,, Few studies even observed macular thinning in amblyopic eyes.  This study was aimed to assess the effect of amblyopia on the mean macular thickness. This information may be useful in understanding the anatomical changes in amblyopia and in predicting treatment outcomes.
| Materials and Methods|| |
This was a prospective cross-sectional study, conducted after clearance from the local ethics committee. Patients in the age group of 5-15 years attending the pediatric vision clinic of our Institute and having unilateral amblyopia were included over a 1 year period (July 2011 to June 2012). Amblyopia was defined as the difference of ≥0.2 between the two eyes on LogMAR scale. Anisometropic amblyopia was defined as ≥1.00 D interocular difference in refractive error with no manifest squint while strabismic amblyopia was defined as manifest squint, near or distance and/or eccentric fixation and ≤0.75 D interocular difference. 
All patients underwent a detailed history and ocular examination, including orthoptic evaluation and dilated fundus examination. Subjects with any other ocular disease or having a history of any surgical intervention were excluded. Patients with a refractive error of greater than 5.0 D, uncooperative for OCT or having signal strength less than 5 (on a 10-point scale) in either eye were also excluded from the study.
OCT scanning was performed using OCT Cirrus high definition OCT (Carl Zeiss, Meditech Inc., CA, USA). In every case right eye was tested first, followed by the left eye.
The macular thickness map scan protocol was used for macular thickness measurements [Figure 1]. The mean of these values was used to calculate the average macular thickness. Measurement of retinal thickness at selected points on the tomographs was obtained automatically by means of a computer algorithm, which assumes that the first highly reflective band corresponds to the vitreo-retinal interface and the second corresponds to the retinal pigment epithelium.
|Figure 1: Map diameters centered on fovea (left) and nine standard early treatment diabetic retinopathy study regions (right). The macula is divided into nine regions with three concentric rings measuring 1 mm (innermost ring), 3 mm (inner ring) and 6 mm in diameter (outer ring) centered on the fovea with inner and outer ring being divided into four quadrants each|
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All measurements were performed by a single person trained in performing OCT who was blind to the diagnosis.
Data was compiled at the end of 12 months and subjected to statistical analysis. Data was presented as mean ± standard deviation. A 95% of the confidence interval and 5% level of significance were adopted; therefore, results with a P value ≤ 0.05 were considered as significant. All descriptive statistics were performed using the Statistical Package for Social Sciences (SPSS) software for Windows (version 17.0, SPSS, Chicago, IL).
| Results|| |
During the study period, 102 eyes of 51 patients were taken up for this cross-sectional analysis. A total of 29 patients were having strabismic and 22 were having anisometropic amblyopia. Out of 51 patients, 26 were males and 25 were females. The mean age of the patients was 11.63 ± 2.84 years. The age distribution of the patients in the two categories is described in [Table 1]. The mean best corrected visual acuity in amblyopic eyes was 0.53 ± 0.35 and in fellow normal eyes was −0.02 ± 0.05. The distribution of the same in strabismic and anisometropic amblyopic patients is described in [Table 1].
|Table 1: Demographic and visual acuity data the mean age and visual acuity of amblyopic eyes in both groups are statistically similar|
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The overall mean macular thickness in amblyopic and fellow normal eyes and distribution in strabismic and anisometropic patients has been depicted in [Table 2].
|Table 2: Macular thickness measurements comparing the amblyopic with the normal eye|
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| Discussion|| |
The amblyopic process may have an effect on various levels of the visual pathway. Atrophy involving the cells in the lateral geniculate nucleus that receive input from the amblyopic eye has been reported.  However, retinal involvement in the amblyopic eye is controversial. 
Several experiments have demonstrated that retinal ganglion cells can suffer modifications with visual stimulus deprivation from birth; including cell loss, mean nucleolar volume diminution in ganglion cell cytoplasm and internal plexiform layer thinning in rats and cats with reduction in the optic nerve size area in mice.  Arden and Wooding reported that electroretinograms elicited by patterned stimuli in humans with various types of amblyopia were significantly reduced.  These results suggest that in humans, amblyopia may be associated with changes in retinal function at the level of production of the pattern electroretinograms, which is presumed to be pre-ganglionic.
Differences in magnification between the eyes have been a concern with optic nerve imaging including OCT.  When the amblyopic eye is more hypermetropic, magnification would increase the retinal nerve fiber layer (RNFL) thickness measurement. A recent cross-sectional study in children found that the RNFL was 1.67 microns thicker per D of hypermetropia. 
To reduce this potential concern, we performed intra-patient comparisons and did not enroll patients with myopia or hypermetropia >5.00 D.
Mean macular thickness of amblyopic eyes was found to be higher as compared with fellow normal eyes, the difference was significant statistically (P < 0.05), similar to other studies. ,,
The difference in mean macular thickness between amblyopic and fellow normal eye in our study was 4.4 μm. This difference is statistically significant (P < 0.05), but lower in comparison with few other reports. ,
The difference in mean macular thickness between normal and amblyopic eye was found to be significant statistically in strabismic (P = 0.01) but, not in anisometropic (P = 0.08) amblyopia. Other reports provide varied results. ,,
The variations in results of these reports are probably due to different study populations, defining criteria and type of amblyopia, different instruments used to measure the macular thickness and co-existent factors like refractive errors.
It is unclear why there is a difference in macular thickness between amblyopia associated with strabismus versus refractive error when the visual acuity in both groups are comparable (P = 0.69). Although strabismic and uncorrected refractive amblyopias are both characterized by decreased visual acuity, psycho-physical investigations have revealed substantial differences in the visual characteristics of humans with different types of amblyopia. ,, The findings suggest that different neural losses are associated with amblyopias of different etiologies. In this study, the mean macular thickness was thicker in eyes with strabismic amblyopia, suggesting that patients with refractive amblyopia might have enjoyed better binocular vision during their life and thus might have a better prognosis for recovery.
| Conclusion|| |
The mean macular thickness is greater in amblyopic eyes as compared to the normal fellow eyes. On subgroup analysis, the difference is significant in strabismic amblyopia, and not significant in anisometropic amblyopia.
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[Table 1], [Table 2]