The reason of amblyopia is still certainly unknown. There are many possible causes that might promote for amblyopia development. It was once considered as a situation associated with an abnormality of the retina [13
]. However, amblyopia-induced cerebral changes were later shown in the visual cortex and the lateral geniculate body.
In 1977, von Noorden et al. reported that, several changes occured after amblyopia development with suturing one lid, such as; an arrest in the lateral geniculate body cell growth, an abnormal distribution of the cerebral cortex, and a decrease in the size and density of parafoveal ganglion cells [14
]. Wiesel and Huble have reported that atrophy of the neurons in the cerebral cortex and lateral geniculate body was detected; nevertheless, it had no influence on the retina [3
One of the major consequences of amblyopia is reduction in the ganglion cells and optic disc. Lempert et al. showed that in a presumed amblyopia, visual impairment may cause optic disc hypoplasia [4
]. Such changes may affect retinal nevre fiber layer and foveal thickness after all.
In several studies, retinal changes were investigated in amblyopic patients. Both RNFL and macular thickness were evaluated and different results were obtained from those studies. RNFL thickness in amblyopic patients were found either thicker, thinner or unchanged comparing to control groups.
Some studies showed that RNFL was found thicker in amblyopic patients. Such as, Wu et al. reported that anisometropic amblyopes had thicker RNFL values in the amblyopic eye than in the fellow eye based on OCT [15
]. Also Yen et al. hypothesized that amblyopia affects the postnatal maturation of the retina, including the postnatal reduction of retinal ganglion cells, which would cause an increase in the RNFL thickness in amblyopic eyes [16
]. In their study, they measured RNFL thickness in patients with unilateral amblyopia (strabismic and refractive amblyopia) and found no significant difference between strabismic amblyopic and normal eyes, although the RNFL was significantly thicker in eyes with refractive amblyopia.
Similarly Chen et al. found that, in amblyopic group, RNFL were significantly thicker than the emmetropia group. However, they explained that the significance of this difference disappeared after adjustment for axial length and refractive error [17
]. Yoon et al. measured the peripapillary RNFL in patients with anisometropic amblyopia. They reported that the RNFL in patients with amblyopia was significantly thicker [6
Unlike these studies, some others showed no significant difference in RNFL thickness between amblyopic groups and controls. Fırat et al. found no statistically significant difference for RNFL thickness among the amblyopic and control eyes although it was slightly thicker in amblyopic patients. They suggested that amblyopia does not affect on RNFL [10
]. Yalcin et al. reported that the difference in RNFL thickness between amblyopic eyes, fellow eyes of the amblyopic patients, and normal eyes of the emmetropic subjects was not clinically significant . On the other hand, Ersan et al found that, RNFL thickness did not differ between strabismic amblyopic and anisometropic amblyopic patients and fellow eyes but they found temporal RNFL quadrant in the hypermetropic anisometropic group, and superior RNFL quadrant in the myopic anisometropic group were significantly thinner in amblyopic eyes compared to their fellow eyes [9
In our study, RNFL was found thicker in strabismic group comparing both anisometropic group and fellow eyes. Also, all amblyopic patients had slighlty thicker RNFL comparing all controls. But none of the differences were not statistically significant. Similar to Yalcin et al., Altintas et al. and Fırat et al., our study revealed no significant difference in RNFL between amblyopic and fellow eyes [8
There is another region in retina that has been investigated with imaging devices in amblyopia. Macular thickness was also compared in amblyopic eyes and control groups. There are various results obtained from different studies in literature.
Another study performed by Yoon et al. used OCT to measure the peripapillary RNFL and foveal thickness in patients with anisometropic amblyopia. Even though RNFL in patients with amblyopia was found significantly thicker, there was no significant difference in macular thickness [6
In another study, Xu et al. reported the fovea and the central sector of the retina in amblyopic eyes were slightly but not significantly thicker than those in the normal fellow eyes in children aged seven to 14 years [19
]. Huynh et al. found increase in macular region among amblyopic patients but it was not statistically significant [20
]. In another study Yalcin et al. reported that the mean foveal thickness for amblyopic patients was 220 ± 38.25 microns; for fellow eyes, it was 202.87 ± 31.01 microns, and for healthy subjects, 198.91 ± 22.50 microns. They found a statistical difference between groups (P=0.025). The difference between amblyopics and fellow eyes was statistically significant (P=0.038). There was also a significant statistical difference in macular thickness between amblyopics and healthy subjects (P=0.028) [8
Tugcu et al. conducted similar study about amblyopic and control patients but they both evaluated persistant and resolved amblyopia. They found that foveolar thickness was significantly increased in both resolved and persistent amblyopia groups compared with the control group (p=0.031). However, there was no difference between amblyopic groups [21
]. Al-Haddad et al. demonstrated that the mean macular thickness was significantly increased in amblyopic eyes versus the fellow eye while the mean the RNFL thickness was similar. The mean macular thickness was significantly increased in the amblyopic (273.8 μm) vs fellow eyes (257.9 μm) in their study (p=0.001). This difference remained significant in the anisometropic group (p=0.002) but not the strabismic group [7
In our study, we compared macular thickness of strabismic amblyopes, anisometropic amblyopes and fellow eyes. The anisometropic subgroup had slightly thicker macula, while the strabismic subgroup was thinner comparing their fellow eyes. When comparing all amblyopic patients to their fellow eyes, the mean macular thickness is 265 µm in amblyopic group and 266 µm in fellow eyes, respectively. But the difference was not significant (p: 0.932).
Our study aimed to investigate to report the difference in RNFL and macular thickness between ambliyopic and fellow eyes. OCT was used to obtain the data like other studies but the differences between devices and the examiners, the patients ages and their refractive situations may explain the differences between studies. The only statistically significant result came out of our study is the difference in macular thickness between strabismic and anisometropic amblyopes, which the anisometropic group was found significantly thicker.
One of the limitations of our study is not evaluating the relationships between RNFL, macular thickness, and age/refractive errors. We did not obtain data from patients after they overcome amblyopia with treatment. The mechanism of amblyopia and the differences between normal eyes is not totally understood yet. Further studies, including histopathological and instrumental studies with a greater number of patients, are required to investigate the differences between amblyopic and normal eyes.