Abstract
Purpose: :
The presence of a foveal pit is associated with high visual acuity but subjects with a flat fovea can have normal acuity (Marmor et al., Arch. Ophthalmol., 2008). Several optical coherence tomography (OCT) studies have shown that myopic eyes exhibit increased foveal thickness and decreased perifoveal thickness (Lim et al., IOVS, 2005; Luo et al., IOVS, 2006; Lam et al., IOVS, 2007; Wu et al., 2008; Coletta et al., OSA Fall Vision Meeting, 2008). These results indicate that the foveal pit should become shallower with increasing myopia. The previous OCT studies have not reported the relationship of foveal dimensions to foveal acuity in myopia. We examined acuity and foveal retinal thickness in a group of 18 subjects whose spherical equivalent refractions ranged from plano to -10.60 D.
Methods: :
The subjects’ best spectacle-corrected acuity was measured using a Bailey-Lovie LogMAR chart under photopic conditions. The retinal magnification factor (RMF) was calculated for each eye from corneal topography and axial eye dimensions. The RMF was used to convert acuity to retinal units of cycles/mm to compensate for differences in retinal image size across refractions. Retinal thickness was measured with an Optovue RTVue FD-OCT using the MM6 scan pattern and is reported as the full thickness from the inner limiting membrane to the retinal pigment epithelial layer. For each of the twelve 6-mm length scans in the MM6 pattern, the foveal pit depth was estimated as the difference between the minimum foveal thickness and the average maximum thickness on either side of the foveal pit.
Results: :
The average maximum retinal thickness was 345.85µm ±11.78 s.d. and the average foveal pit depth was 130.66µm ±20.04 s.d. The pit depth ranged from 86.50 to 161.79µm and became shallower with increasing myopia (r=0.48; p=0.03). There was a trend of decreasing retinal acuity with decreasing pit depth; the strongest correlations between acuity and pit depth were for scans taken near the vertical meridian but these did not reach significance. Subjects with a foveal pit depth in the vertical dimension that was less than 130µm had a mean retinal acuity of 107.14 cyc/mm±19.53 s.d., while subjects with a vertical pit depth greater than 130µm had a mean acuity of 123.31 cyc/mm ±12.82 s.d. (p=0.025 in one-tailed t-test). These acuities in cyc/deg were 31.57±6.01 s.d. and 36.11±4.02 s.d. in the two groups, respectively.
Conclusions: :
Myopes showed a decrease in foveal pit depth that was associated with decreased photopic retinal acuity. The decrease in pit depth and lowered acuity may have a common underlying cause, perhaps related to forces associated with retinal stretching (Kitaguchi et al., Jpn J Ophthalmol., 2007; Chui et al., IOVS, 2008; Wu et al., Eye, 2008).
Keywords: myopia • imaging methods (CT, FA, ICG, MRI, OCT, RTA, SLO, ultrasound) • visual acuity