Evaluating outer segment length as a surrogate measure of peak foveal cone density

Abstract

Adaptive optics (AO) imaging tools enable direct visualization of the cone photoreceptor mosaic, which facilitates quantitative measurements such as cone density. However, in many individuals, low image quality or excessive eye movements precludes making such measures. As foveal cone specialization is associated with both increased density and outer segment (OS) elongation, we sought to examine whether OS length could be used as a surrogate measure of foveal cone density. The retinas of 43 subjects (23 normal and 20 albinism; aged 6-67years) were examined. Peak foveal cone density was measured using confocal adaptive optics scanning light ophthalmoscopy (AOSLO), and OS length was measured using optical coherence tomography (OCT) and longitudinal reflectivity profile-based approach. Peak cone density ranged from 29,200 to 214,000cones/mm² (111,700±46,300cones/mm²); OS length ranged from 26.3 to 54.5μm (40.5±7.7μm). Density was significantly correlated with OS length in albinism (p<0.0001), but not normals (p=0.99). A cubic model of density as a function of OS length was created based on histology and optimized to fit the albinism data. The model includes triangular cone packing, a cylindrical OS with a fixed volume of 136.6μm³, and a ratio of OS to inner segment width that increased linearly with increasing OS length (R²=0.72). Normal subjects showed no apparent relationship between cone density and OS length. In the absence of adequate AOSLO imagery, OS length may be used to estimate cone density in patients with albinism. Whether this relationship exists in other patient populations with foveal hypoplasia (e.g., premature birth, aniridia, isolated foveal hypoplasia) remains to be seen.

Keywords: Adaptive optics; Cone density; Cone photoreceptor; Fovea; Outer segment.

Fig. A.1

OCT Reflectivity Analytics (ORA) user interface. The main user interface is shown on the left with the corresponding, free-floating LRP graph on the right. The green box on the OCT image marks the region used for the LRP on the right. The second derivative peak detection algorithm detects local maxima as well as less apparent inflection points in the LRP that could be considered a peak (LRP - pink squares). The O(n) one dimensional peak detection algorithm detects elements where the neighboring elements are less than the given element (LRP - blue dots). Black lines denote the full width at half maximum for the peaks marked with blue dots.

Fig. 1

OS length measured using LRP. (A) OCT in the linear display. Consecutive LRPs were created every 25 μm over a 500 μm region. By selecting the peaks for the EZ (blue) and IZ (orange), ORA creates the corresponding segmentation lines. The green box denotes the location of maximum OS length as determined from the Gaussian fit to the difference between the blue and orange segmentation points. (B) Foveal region outlined in green in (A) showing the location of peak OS length. An LRP (right) was generated over the width of the selection, allowing identification of the EZ (blue arrow) and IZ (orange arrow), with the OS length (h_OS) being the distance between them (dashed line). Scale bars = 100 μm.

Fig. 2

Range in normal foveal cone specialization. Left panel shows the foveal cone mosaic and OCT from a normal subject with low peak cone density (JC_10119, peak density = 108,100 cones/mm²). The images on the right are from the normal subject with highest peak cone density (JC_0654, peak density = 214,000 cones/mm²). Despite differences in peak cone density, the OS length for both of these subjects is 47.6 μm. The asterisk (*) in the top panel marks the estimated location of peak cone density for each subject. AO image scale bars = 50 μm; OCT scale bars = 200 μm.

Fig. 3

Variability in foveal cone specialization in patients with albinism. Left panel shows the foveal cone mosaic and OCT from a patient with albinism who had the lowest peak cone density (JC_0103, peak density = 29,200 cones/mm²). The images on the right are from the subject with albinism who had the highest peak cone density (KS_0935) of 126,400 cones/mm². OS lengths for these subjects are 26.3 and 39.4 μm, respectively. The asterisk (*) in the top panel marks the location of peak cone density for each subject. AO image scale bars = 50 μm; OCT scale bars = 200 μm.

Fig. 4

Peak cone density and foveal OS length are correlated in albinism but not in normal retinas. Peak cone density is plotted against OS length. Orange squares represent subjects with albinism and open black squares are normal subjects. Peak cone density is significantly correlated with OS length in patients with albinism (Spearman r = 0.83; p < 0.0001) but not normal subjects (r = 0.006; p = 0.98). The model of the relationship between OS length and cone density in albinism is shown as the dashed black line, which fits the albinism data with an R² of 0.72. The OS lengths of normal subjects are generally greater than would be expected based on this model.