152
D. Cabrera DeBuc et al.
macular locations and at the outer superior and inferior locations. The ONL was
thinner at the inner and outer temporal locations and thicker at the inner and outer
superior and inner nasal locations. The NFL and OPL were thicker in the amblyopic
eyes than in the fellow eyes in some areas and thinner in other areas [182]. It should
be noted that none of these studies corrected their results for either age or AL.
Szigeti et al. [192] used OCT image segmentation methodology involving the
entire macular area, extracting seven retinal layers. They found that subtle changes
may be present in the retina in unilateral amblyopia. As there was evidence showing
the confounding effect of AL and age they used state-of-the-art statistical methodology to keep these variables under control. As most of the similar previous studies
in the field were using basic comparisons, they also performed such analyses. Interestingly, the basic pairwise comparisons indicated significant changes in the GCC
r in the pericentral region and in the OPL layer calculated for the total macula
and measured in the peripheral region. However, after applying rigorous statistical
methodology to account for the effects of AL and age, these differences disappeared,
while a significant difference was revealed for the central (foveal) ONL. This implies
that the photoreceptors and not the ganglion cells could be affected by amblyopia,
this being the opposite of what was earlier speculated.
Previous studies reported that there was a correlation between retinal thickness
and AL, age, or even race in healthy eyes [51, 59, 60, 193, 194] and that AL may
influence the thickness of the intraretinal layers in the macula [13]. Song et al. found
that AL correlated negatively with average outer macular thickness, overall average
macular thickness and macular volume [59].
In unilateral amblyopia, Szigeti et al. found significant difference in AL between
amblyopic and fellow eyes (not only in the anisometropic subgroup), and multiple
regression showed a statistically significant correlation between IOD in the thickness
of most retinal layers and IOD in AL [192]. For this reason, the effect of AL must
be taken into consideration in statistical analyses to obtain reliable results, just as we
did in our study. In contrast, most of the previous studies about amblyopic retinal
OCT measurements were not considering this potential effect of AL, which could
influence their results. A summary of these studies, also mentioned above, can be
found in Table 6.2.
In support of Szigeti et al. results, there is early evidence showing that photoreceptors may be affected in amblyopia. First, Enoch suggested that photoreceptor
orientation is abnormal in amblyopic eyes using the Stiles-Crawford function [189],
while others did not found indication of retinal dysfunction at the level of the cone
photoreceptors in amblyopic eyes. Later, three groups described electro-oculographic
abnormalities in amblyopic patients, with their results providing evidence for a retinal abnormality in amblyopia [195–197]. These results were also suggestive of the
RPE being involved in the process. Indeed, the RPE plays an important role in maintaining visual pigment density and perhaps also in the maintenance of photoreceptor
orientation.
Leone et al. reviewed the literature on measuring macular thickness in amblyopes
and proposed that increased macular thickness found by several studies may be due
to inadvertent measurement of a parafoveal eccentric point in amblyopia [198]. To
D. Cabrera DeBuc et al.
macular locations and at the outer superior and inferior locations. The ONL was
thinner at the inner and outer temporal locations and thicker at the inner and outer
superior and inner nasal locations. The NFL and OPL were thicker in the amblyopic
eyes than in the fellow eyes in some areas and thinner in other areas [182]. It should
be noted that none of these studies corrected their results for either age or AL.
Szigeti et al. [192] used OCT image segmentation methodology involving the
entire macular area, extracting seven retinal layers. They found that subtle changes
may be present in the retina in unilateral amblyopia. As there was evidence showing
the confounding effect of AL and age they used state-of-the-art statistical methodology to keep these variables under control. As most of the similar previous studies
in the field were using basic comparisons, they also performed such analyses. Interestingly, the basic pairwise comparisons indicated significant changes in the GCC
r in the pericentral region and in the OPL layer calculated for the total macula
and measured in the peripheral region. However, after applying rigorous statistical
methodology to account for the effects of AL and age, these differences disappeared,
while a significant difference was revealed for the central (foveal) ONL. This implies
that the photoreceptors and not the ganglion cells could be affected by amblyopia,
this being the opposite of what was earlier speculated.
Previous studies reported that there was a correlation between retinal thickness
and AL, age, or even race in healthy eyes [51, 59, 60, 193, 194] and that AL may
influence the thickness of the intraretinal layers in the macula [13]. Song et al. found
that AL correlated negatively with average outer macular thickness, overall average
macular thickness and macular volume [59].
In unilateral amblyopia, Szigeti et al. found significant difference in AL between
amblyopic and fellow eyes (not only in the anisometropic subgroup), and multiple
regression showed a statistically significant correlation between IOD in the thickness
of most retinal layers and IOD in AL [192]. For this reason, the effect of AL must
be taken into consideration in statistical analyses to obtain reliable results, just as we
did in our study. In contrast, most of the previous studies about amblyopic retinal
OCT measurements were not considering this potential effect of AL, which could
influence their results. A summary of these studies, also mentioned above, can be
found in Table 6.2.
In support of Szigeti et al. results, there is early evidence showing that photoreceptors may be affected in amblyopia. First, Enoch suggested that photoreceptor
orientation is abnormal in amblyopic eyes using the Stiles-Crawford function [189],
while others did not found indication of retinal dysfunction at the level of the cone
photoreceptors in amblyopic eyes. Later, three groups described electro-oculographic
abnormalities in amblyopic patients, with their results providing evidence for a retinal abnormality in amblyopia [195–197]. These results were also suggestive of the
RPE being involved in the process. Indeed, the RPE plays an important role in maintaining visual pigment density and perhaps also in the maintenance of photoreceptor
orientation.
Leone et al. reviewed the literature on measuring macular thickness in amblyopes
and proposed that increased macular thickness found by several studies may be due
to inadvertent measurement of a parafoveal eccentric point in amblyopia [198]. To