title: 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.
authors.affiliation: Department of Cell Biology, Neurobiology & Anatomy, Medical College of Wisconsin, 8701 Watertown Plank Road, Milwaukee, WI 53226, USA. Electronic address: mwilk@hudsonalpha.org.
authors.forename: Melissa A
authors.initials: MA
authors.lastname: Wilk
authors.affiliation: Department of Ophthalmology, Medical College of Wisconsin, 925 N. 87th Street, Milwaukee, WI 53226, USA. Electronic address: wilkb777@gmail.com.
authors.forename: Brandon M
authors.initials: BM
authors.lastname: Wilk
authors.affiliation: Department of Cell Biology, Neurobiology & Anatomy, Medical College of Wisconsin, 8701 Watertown Plank Road, Milwaukee, WI 53226, USA. Electronic address: clanglo@mcw.edu.
authors.forename: Christopher S
authors.initials: CS
authors.lastname: Langlo
authors.affiliation: Department of Biomedical Engineering, Marquette University, 1250 W Wisconsin Avenue, Milwaukee, WI 53233, USA. Electronic address: rfcooper@sas.upenn.edu.
authors.forename: Robert F
authors.initials: RF
authors.lastname: Cooper
authors.affiliation: Department of Cell Biology, Neurobiology & Anatomy, Medical College of Wisconsin, 8701 Watertown Plank Road, Milwaukee, WI 53226, USA; Department of Ophthalmology, Medical College of Wisconsin, 925 N. 87th Street, Milwaukee, WI 53226, USA; Department of Biomedical Engineering, Marquette University, 1250 W Wisconsin Avenue, Milwaukee, WI 53233, USA; Department of Biophysics, Medical College of Wisconsin, 8701 Watertown Plank Road, Milwaukee, WI 53226, USA. Electronic address: jcarroll@mcw.edu.
authors.forename: Joseph
authors.initials: J
authors.lastname: Carroll
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D016117:Albinism, Ocular / Q000000981:diagnostic imaging / Q000473:pathology*; D016022:Case-Control Studies; D002648:Child; D005260:Female; D005584:Fovea Centralis / Q000000981:diagnostic imaging*; D006801:Humans; D008297:Male; D008875:Middle Aged; D009887:Ophthalmoscopy / Q000379:methods*; D055095:Optics and Photonics / Q000379:methods*; D017949:Retinal Cone Photoreceptor Cells / Q000473:pathology*; D055214:Retinal Photoreceptor Cell Outer Segment / Q000473:pathology*; D041623:Tomography, Optical Coherence / Q000379:methods; D055815:Young Adult
keywords: Adaptive optics; Cone density; Cone photoreceptor; Fovea; Outer segment