Non–Invasive Assessment of Parafoveal Capillary Leukocyte Pulsatility

Abstract

Abstract: : Purpose: The addition of adaptive optics to a confocal scanning laser ophthalmoscope (AOSLO) has allowed for long–term imaging of parafoveal capillary leukocyte movement and measurement of leukocyte velocity without contrast dyes. We used the AOSLO to investigate the possible role of the cardiac cycle, systole and diastole, on capillary leukocyte velocity and directly measured capillary leukocyte pulsatility. Methods: Three normal healthy subjects with clear ocular media were imaged. All subjects were dilated. An imaging wavelength of 532 nm and a frame rate of 30 Hz were used. The AOSLO field of view was 1.4 x 1.5 degrees. A photoplethysmograph was used to record a subject’s pulse synchronously with the AOSLO imaging session. The subject’s pulse was encoded onto the corresponding AOSLO video frame as a white bar at the bottom of the frame. Leukocyte velocities were measured from the encoded videos and the measurements were divided into five equal bins, each corresponding to the time in which they were observed relative to the subject’s pulse. The average leukocyte velocity and SEM for each of these five bins of the relative pulse cycle were determined. Pulsatility for each capillary was calculated as P = (V_max –V_min)/V_mean where V_max and V_min are the maximum and minimum mean leukocyte velocities calculated from the five equal segments of the pulse cycle and V_mean is the mean of all leukocyte velocities. Results: Leukocyte velocity and pulsatility were determined for all three subjects. There was a statistically significant difference between the V_max and V_min for each subject, p<0.05. The mean parafoveal capillary leukocyte velocity for subjects 1,2, and 3 are respectively: V_mean=1.19, 1.16, and 1.99 mm/sec and Pulsatility = 0.30, 0.29, and 0.52. Conclusions: Parafoveal capillary leukocyte pulsatility can be directly and non–invasively measured without the use of contrast dyes using an AOSLO. Variations in leukocyte velocity are due in part to the cardiac cycle. By controlling for variations in leukocyte velocity caused by the cardiac cycle, we can better detect other changes in leukocyte velocity induced by disease or pharmaceutical agents near the fovea.

Keywords: blood supply • imaging methods (CT, FA, ICG, MRI, OCT, RTA, SLO, ultrasound) • retina