Fixation stability and scotoma mapping for patients with low vision.

Purpose: To develop a simplified device that performs fundus perimetry techniques such as fixation mapping and kinetic perimetry.

Methods: We added visual stimulation to a near-infrared retinal imager, the laser scanning digital camera (LSDC). This device uses slit scanning illumination combined with a two-dimensional CMOS (complementary metal oxide semiconductor) detector, with continuous viewing of the retina.

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.

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.

Delayed appearance of high altitude retinal hemorrhages.

Background: Retinal hemorrhages have been described as a component of high altitude retinopathy (HAR) in association with altitude illness. In this prospective high altitude study, we aimed to gain new insights into the pathophysiology of HAR and explored whether HAR could be a valid early indicator of altitude illness.

Methodology/principal Findings: 28 mountaineers were randomly assigned to two ascent profiles during a research expedition to Mt.

New insights into changes in corneal thickness in healthy mountaineers during a very-high-altitude climb to Mount Muztagh Ata.

Objective: To investigate the effect of very high altitude and different ascent profiles on central corneal thickness (CCT).

Methods: Twenty-eight healthy mountaineers were randomly assigned to 2 different ascent profiles during a medical research expedition to Mount Muztagh Ata (7546 m) in western China. Group 1 was allotted a shorter acclimatization time prior to ascent to 6265 m.

Ocular blood flow assessment using continuous laser Doppler flowmetry.

This article describes the technique of continuous laser Doppler flowmetry (LDF) as applied to the measurement of the flux of red blood cells in the optic nerve head, iris and subfoveal choroid. Starting with the exposition of the physical principles underlying LDF, we first describe the various devices developed to perform LDF in these vascular beds. We then discuss the clinical protocols, blood flow parameters, calibration procedures, reproducibility and limitations of the LDF technique.

New insights into ocular blood flow at very high altitudes.

Little is known about the ocular and cerebral blood flow during exposure to increasingly hypoxic conditions at high altitudes. There is evidence that an increase in cerebral blood flow resulting from altered autoregulation constitutes a risk factor for acute mountain sickness (AMS) and high-altitude cerebral edema (HACE) by leading to capillary overperfusion and vasogenic cerebral edema. The retina represents the only part of the central nervous system where capillary blood flow is visible and can be measured by noninvasive means.

In vivo measurement of erythrocyte velocity and retinal blood flow using adaptive optics scanning laser ophthalmoscopy.

In vivo measurement of retinal blood flow is obtained by measuring the blood velocity of erythrocytes and lumen diameters of the blood vessels using an adaptive optics scanning laser ophthalmoscope. Erythrocyte velocity is measured by tracking erythrocytes moving across a horizontal scanning line. This approach provides high temporal bandwidth measurements, allowing the fluctuation of blood flow during cardiac cycles to be measured.

Regulation of subfoveal choroidal blood flow in age-related macular degeneration.

Purpose: To assess the capability of the subfoveal choroidal circulation to regulate its blood flow in response to an acute increase in ocular perfusion pressure in the eyes of healthy elderly persons or of subjects with neovascular age-related macular degeneration (AMD).

Methods: Changes of subfoveal choroidal blood velocity (ChBVel), volume (ChBVol), and flow (ChBF) induced by isometric exercise were determined using laser Doppler flowmetry (LDF) in 19 young healthy volunteers (group 1), 24 elderly healthy volunteers with mild macular pigment distribution changes (group 2), and 23 subjects with subfoveal classic neovascularization caused by AMD (group 3).

Results: Isometric exercise induced significant increases in mean ocular perfusion pressure (PPm) of 19.

Ray tracing model for the estimation of power spectral properties in laser Doppler velocimetry of retinal vessels and its potential application to retinal vessel oximetry.

A new model based on ray tracing was developed to estimate power spectral properties in laser Doppler velocimetry of retinal vessels and to predict the effects of laser beam size and eccentricity as well as absorption of laser light by oxygenated and reduced hemoglobin. We describe the model and show that it correctly converges to the traditional rectangular shape of the Doppler shift power spectrum, given the same assumptions, and that reduced beam size and eccentric alignment cause marked alterations in this shape. The changes in the detected total power of the Doppler-shifted light due to light scattering and absorption by blood can also be quantified with this model and may be used to determine the oxygen saturation in retinal arteries and veins.

Choroidal blood flow during exercise-induced changes in the ocular perfusion pressure.

Purpose: The high metabolic rate of the human retina is supported by the choroidal vasculature. Knowledge of the normal choroidal blood flow (ChBF) responses to various physiological stimuli is therefore highly important if the pathophysiology of ocular diseases involving the choroid is to be understood better. In the present study, the hemodynamic responses of the subfoveal ChBF were examined during and after an exercise-induced increase in the ocular perfusion pressure (OPP).

Blood pO2 and blood flow at the optic disc.

A fundus camera-based phosphorometer to noninvasively and quasicontinuously measure the blood partial pressure of oxygen (pO(2,blood)) in the microvasculature of the pig optic nerve using the principle of the phosphorescence quenching by O(2) is described. A porphyrin dye is injected into the venous circulation and the decay of its phosphorescence emission is detected locally in the eye, after excitation with a flash of light. Combined with blood flow measurements by means of a laser Doppler flowmeter mounted on the phosphorometer, we demonstrate the capability of the instrument to determine the time course of optic nerve blood flow and pO(2,blood) in response to various physiological stimuli, such as hyperoxia and hypercapnia.

Feasibility of LDF measurements of optic nerve head blood flow in children with cerebral malaria.

The pathogenesis of cerebral malaria (CM), a significant cause of death in the tropics, is still not understood. Cerebral blood flow measurements would be important but are difficult under the conditions prevailing in CM clinics in the tropics. With the goal of using optic nerve head (ONH) blood flow (F(onh)) instead of cerebral blood flow to help outcome prediction, we have tested the feasibility of performing F(onh) measurements in comatose CM children, using a portable ocular laser Doppler flowmeter (LDF).

Effect of isovolumic hemodilution on oxygen delivery to the optic nerve head.

Background: To investigate the effect of isovolumic hemodilution on the tissue oxygenation of the optic nerve head (ONH).

Material And Methods: In 9 miniature pigs (6 - 12 kg), hemodilution was performed by replacing 100 - 140 ml of blood by an equivalent volume of 6 % hydroxyethyl starch in saline. The blood flow parameters in the ONH microcirculation, namely velocity (BVel), volume (BVol) and flow (BF), were measured by laser Doppler flowmetry.