We evaluated in ocular hypertension (OHT) and early glaucoma (EOAG) patients the optic nerve head (ONH) blood flow response (RF onh ) to chromatic equiluminant flicker. This stimulus generates neural activity dominated by the parvo-cellular system. Eleven EOAG, 20 OHT patients, and 8 age-matched control subjects were examined. The blood flow (F onh ) at the neuroretinal rim was continuously monitored by laser Doppler flowmetry before, during, and after a 60-s exposure to a 4 Hz, red-green equiluminant flicker stimulus (30 deg field). RF onh was expressed as percentage F onh -change during the last 20 s of flicker relative to baseline F onh . Responses were collected at a number of temporal sites. The highest RF onh value was used for subsequent analysis. As compared to controls, both OHT and EOAG patients showed a decrease (p<0.01 ) in mean RF onh . We conclude that RF onh elicited by chromatic equiluminant flicker is abnormally reduced in OHT and EOAG patients indicating an impairment of the parvo-cellular-mediated vasoactivity. This decrease of vascular response may occur independently of neural activity loss early in the disease process.
The measurement of blood flow in the ocular fundus is of scientific and clinical interest. Investigating ocular blood flow
in the choroid may be important to understand the pathogenesis of numerous ocular diseases, such as glaucoma or agerelated
macular degeneration (AMD). Laser Doppler flowmetry (LDF) was applied to measure relative velocity, volume
and flux of red blood cells in the tissues of human eye. Its main application lies in the possibility of assessing alterations
in blood flow early in the course of diseases. The purpose of the present study was to investigate the effect of pupil
dilatation with one drop of 1% tropicamide on blood flow in the foveal region of the choroid of the human fundus. The
blood flow parameters were measured in 24 eyes during 30 minutes (one measurement in every 3 minutes) after the
application of the drop. Since the Doppler parameters depend on the scattering geometry, which may change as the pupil
dilates; an artificial pupil of 4mm in diameter was placed directly in front the eye. Following the administration of
tropicamide the mean pupil diameter was increased from 3.29 mm to 8.25 mm (P<0.0001, Paired student t-test). In
comparison to the baseline values, the data shows no significant increases were observed in velocity, volume, and flow
with 4 mm artificial pupil (0.2%, 1.3%, 0.8% respectively) and a statistically significant increases were observed
without artificial pupil (10.7%, 13.9%, 12.8% respectively) following the application of tropicamide.
Laser Doppler flowmetry (LDF) is a technique used to measure relative average velocity, number and flux (number
times velocity) of red blood cells in vessels or capillaries. In this study, the effect of topical timolol on the choroidal
circulation was investigated in 12 healthy subjects. Maximum velocity of red blood cells and volumetric blood flow rate
in sub-foveal choroids are determined in each eye just before instillation of drops and then every 30 min upto 2 hours.
Average intraocular pressure (IOP) decreased significantly in the timolol-treated eyes compared to that of placebo-treated
eyes. Nevertheless no significant differences in choroidal blood hemodynamic between timolol and placebo-treated
eyes were observed.
Ocular fundus reflectometry is a technique aimed at the in-vivo measurement of the reflectance of the tissues of the
ocular fundus. Studies have demonstrated a correlation between optical and physiological properties of such tissues in
humans and the existence of a control mechanism, called neuro-vascular coupling (NC), which adjusts local blood
perfusion to support vision-induced neural activity. We developed an instrument for functional imaging of the neural
tissues of the ocular fundus based on reflectance measurements to study the NC. The images acquired with the
instrument needed processing to work out reflectance time-courses. The algorithm exploited previously requires long
computational time, provides poor discrimination of objects and need manual intervention. We have developed a fully
automatic algorithm based on differential multiscale framework for the processing of the images of the ocular fundus
with reduced computational time. This algorithm is reasonably efficient to determine relative translational displacement
(translation and rotation) between the images and also to remove the geometric distortion. Simulation results performed
on the fundus images show that differential multiscale framework based image registration reduces computational times
up-to one fourth of the time required by the general purpose algorithm, and provides better alignment precision.
Neurovascular coupling in the optic nerve is the physiological mechanism that adjusts the blood perfusion in the
microcirculation of the optic nerve to support the neural activity induced by vision. The resulting variations in blood
flow and thus in neural tissue oxygenation induce changes in the optical properties of the tissues. These variations can
be detected optically as reflectivity changes in the neural tissues, i.e. the retina and optic nerve disk. To achieve a better
understanding of the mechanisms underlying the neurovascular coupling, our study is aimed at the evaluation of
reflectance changes of the optic nerve induced by visual stimulation. For this purpose, an ocular fundus reflectometer
has been developed.
The technique of phosphorescence quenching by O2 (PQ) allows the non-invasive measurement of the partial pressure of oxygen in blood (pO2blood). This technique and its application to the investigation of the pO2blood in the microvasculature of the retina and optic nerve head (ONH) of two animal species is described. Using the imaging mode of PQ, 2-dimensional pO2blood maps were obtained to investigate the response of the pO2blood to various physiological stimuli in miniature pigs and the effect of experimental glaucoma in monkeys. Applied in its focal mode, PQ allows measurements of the pO2blood with a time resolution of 1 second and is adequate to investigate the pO2blood time course during light stimulation.
A compact laser Doppler flowmeter (35x80x210mm3) for the measurement of subfoveal choroidal blood flow parameters (ChBF) was mounted on a helmet. This device allows the measurement of ChBF during dynamic exercises or in supine position, without the need for pupil dilatation. Its optical system is based on a Schlieren arrangement by which the surface of light collection and that of the illumination are spatially separated by an obscuration. The laser probing beam ((lambda) equals 790 nm, 100 (mu) W at the cornea) is focused at the fovea by having the tested subject look directly at the beam. Computer analysis of the photocurrent produced by the scattered light provides a relative measure of the mean blood velocity, number and flux of the red blood cells in the choriocapillaris. Measurements were performed to assess the reliability of the flow parameters measurements in normal volunteers: reproducibility and sensibility when subjects are sitting or standing; measurement of changes in ChBF in the case of isometric and dynamic exercises. Results demonstrate that this new helmet-mounted device provides data comparable to the conventional device. It allows for the first time, however, the continuous measurement of choroidal hemodynamics in humans during various types of exercises.
A method is described that provides measurements of blood flow (ChBF) in discrete regions of the choroidal vascular system. It is based on the technique of laser Doppler flowmetry (LDF). A diode laser beam (wavelength = 810 nm, 60 μW at the cornea) is delivered to the eye through a fundus camera. Light scattered by red blood cells in the tissue volume illuminated by the incident laser beam is detected at the fundus image plane of the camera by an optical fiber. Two analysis schemes of the Doppler signal are developed: one uses commercial skin blood flowmeters and the other a NeXTstation (Motorola 68040 based) computer system. Responses of ChBF to various stimuli are in good agreement with previously published findings in animals. LDF is a valid technique for obtaining noninvasive, continuous, and sensitive recordings of ChBF at discrete regions of the cat eye and in the foveal region of the human fundus, without the need to dilate the pupil.
Laser Doppler flowmetry (LDF) was applied to measure blood flow in discrete regions of the optic nerve head (ONHBF) and in the foveal region of the choroid (ChBF) in humans. LDF is based on the Doppler effect. For its ocular application, a diode laser beam (wavelength equals 810 nm, 60 mW at the cornea) was delivered to the eye through a fundus camera. For ONHBF the beam was directed at regions of the optic disk with no apparent individual vessels. For ChBF in the foveal region subjects were asked to look directly at the beam. Light scattered by red blood cells in the tissue volume illuminated by the incident laser beam was detected at the fundus image plane of the camera by an optical fiber. Two analysis schemes of the Doppler signal were developed: one uses commercial skin blood flowmeters, the other a NeXT station (Motorola 68040 based) computer system. Responses of ONHBF and ChBF to various physiological and pharmacological stimuli were obtained and shown to be in agreement with previously published findings. LDF is a valid technique for obtaining non- invasive, continuous and sensitive recordings of ONHBF and foveal ChBF, the latter without the need to dilate the pupil.