Longitudinal imaging of microvascular remodelling in proliferative diabetic retinopathy using adaptive optics scanning light ophthalmoscopy.

PURPOSE: To characterise longitudinal changes in the retinal microvasculature of type 2 diabetes mellitus (T2DM) as exemplified in a patient with proliferative diabetic retinopathy (PDR) using an adaptive optics scanning light ophthalmoscope (AOSLO). METHODS: A 35-year-old T2DM patient with PDR treated with scatter pan-retinal photocoagulation at the inferior retina 1 day prior to initial AOSLO imaging along with a 24-year-old healthy control were imaged in this study. AOSLO vascular structural and perfusion maps were acquired at four visits over a 20-week period. Capillary diameter and microaneurysm area changes were measured on the AOSLO structural maps. Imaging repeatability was established using longitudinal imaging of microvasculature in the healthy control. RESULTS: Capillary occlusion and recanalisation, capillary dilatation, resolution of local retinal haemorrhage, capillary hairpin formation, capillary bend formation, microaneurysm formation, progression and regression were documented over time in a region 2° superior to the fovea in the PDR patient. An identical microvascular network with same capillary diameter was observed in the control subject over time. CONCLUSIONS: High-resolution serial AOSLO imaging enables in vivo observation of vasculopathic changes seen in diabetes mellitus. The implications of this methodology are significant, providing the opportunity for studying the dynamics of the pathological process, as well as the possibility of identifying highly sensitive and non-invasive biomarkers of end organ damage and response to treatment.

Local flicker stimulation evokes local retinal blood velocity changes.

We investigated the effect of localized visual stimulation on human retinal blood velocity using an adaptive optics scanning laser ophthalmoscope (AOSLO). To measure the blood velocity response, the AOSLO scanning raster was moved over the target arteries and red blood cell velocity was measured. Localized visual stimuli were delivered by projecting flicker patterns inside or outside the target artery's downstream region. The blood velocity increased in the presence of a flicker stimulus in the downstream region but not when outside the downstream region. The blood velocity increased more with larger area of stimulation. This increase was significant even when the stimulus was smaller than 600 µm × 600 µm. These findings suggest that when the retina regulates its blood flow to metabolic demands, it regulates blood velocity in the vascular system selectively, according to activity of neurons within its field of influence.

Variation of cone photoreceptor packing density with retinal eccentricity and age.

PURPOSE: To study the variation of cone photoreceptor packing density across the retina in healthy subjects of different ages. METHODS: High-resolution adaptive optics scanning laser ophthalmoscope (AOSLO) systems were used to systematically image the retinas of two groups of subjects of different ages. Ten younger subjects (age range, 22-35 years) and 10 older subjects (age range, 50-65 years) were tested. Strips of cone photoreceptors, approximately 12° × 1.8° long were imaged for each of the four primary retinal meridians: superior, inferior, nasal, and temporal. Cone photoreceptors within the strips were counted, and cone photoreceptor packing density was calculated. Statistical analysis (three-way ANOVA) was used to calculate the interaction for cone photoreceptor packing density between age, meridian, and eccentricity. RESULTS: As expected, cone photoreceptor packing density was higher close to the fovea and decreased with increasing retinal eccentricity from 0.18 to 3.5 mm (∼0.6-12°). Older subjects had approximately 75% of the cone density at 0.18 mm (∼0.6°), and this difference decreased rapidly with eccentricity, with the two groups having similar cone photoreceptor packing densities beyond 0.5 mm retinal eccentricity on average. CONCLUSIONS: Cone packing density in the living human retina decreases as a function of age within the foveal center with the largest difference being found at our most central measurement site. At all ages, the retina showed meridional difference in cone densities, with cone photoreceptor packing density decreasing faster with increasing eccentricity in the vertical dimensions than in the horizontal dimensions.

Noninvasive measurements and analysis of blood velocity profiles in human retinal vessels.

PURPOSE: To quantitatively model the changes in blood velocity profiles for different cardiac phases in human retinal vessels. METHODS: An adaptive optics scanning laser ophthalmoscope (AOSLO) was used to measure blood velocity profiles in three healthy subjects. Blood velocity was measured by tracking erythrocytes moving across a scanning line. From the radial position of the cells within the lumen, the blood velocity profile was computed. The cardiac pulsatility was recorded with a cardiac signal monitor. RESULTS: The shape of the blood velocity profile in retinal arteries changed systematically during the cardiac cycle, with the flattest profile occurring during the diastolic phase. The measured blood velocity profiles were typically flatter than the commonly assumed parabolic shape. The flatness increased with decreasing vessel size. For the large veins (>80 µm), the ratio of the centerline velocity to the cross-sectional average velocity was between 1.50 and 1.65. This ratio decreased to 1.36 in the smallest vein studied (32 µm). Velocity profiles downstream from a venous confluence showed two peaks at 120 µm from the confluence, but a single velocity peak 500 µm downstream from the confluence. CONCLUSIONS: The cardiac cycle influences the blood flow velocity profiles systematically in retinal arteries but not in veins. Parabolic flow was not found in even the largest vessels studied, and deviations from parabolic flow increased in smaller vessels. The measurements are sensitive enough to measure the dual-humped blood velocity profile at a vein confluence.

Individual variations in human cone photoreceptor packing density: variations with refractive error.

PURPOSE: To measure the variation in human cone photoreceptor packing density across the retina, both within an individual and between individuals with different refractive errors. METHODS: A high-resolution adaptive optics scanning laser ophthalmoscope was used to image the cones of 11 human eyes. Five subjects with emmetropia and six subjects with myopia were tested (+0.50 to -7.50 D). For each subject, four approximately 10 degrees x 1.5 degrees strips of cone images were obtained. Each strip started at the fovea and proceeded toward the periphery along the four primary meridians. The position of each cone within the sampling windows was digitized manually by the investigator. From these cone counts, the density of the cones was calculated for a set of fixed distances from the fovea at locations throughout the image. RESULTS: Cone photoreceptor packing density decreased from 27,712 cells/mm(2) to 7,070 cells/mm(2) from a retinal eccentricity of 0.30 to 3.40 mm along the superior meridian in five emmetropic eyes. Cone photoreceptor packing density (cells per square millimeter) was significantly lower in myopic eyes than in emmetropic eyes. At a given location, there was considerable individual variation in cone photoreceptor packing density, although more than 20% of the variance could be accounted for by differences in axial length. CONCLUSIONS: The results provide a baseline analysis of individual difference in cone photoreceptor packing density in healthy human eyes. As predicted by retinal stretching models, cone photoreceptor packing density is lower in highly myopic eyes than in emmetropic eyes.