Use of colour Doppler imaging in ocular blood flow research.

The main objective of this report is to encourage consistent quality of testing and reporting within and between centres that use colour Doppler imaging (CDI) for assessment of retrobulbar blood flow. The intention of this review is to standardize methods in CDI assessment that are used widely, but not to exclude other approaches or additional tests that individual laboratories may choose or continue to use.

Use of the retinal vessel analyzer in ocular blood flow research.

The present article describes a standard instrument for the continuous online determination of retinal vessel diameters, the commercially available retinal vessel analyzer. This report is intended to provide informed guidelines for measuring ocular blood flow with this system. The report describes the principles underlying the method and the instruments currently available, and discusses clinical protocol and the specific parameters measured by the system. Unresolved questions and the possible limitations of the technique are also discussed.

Seven-degree head-down tilt reduces choroidal pulsatile ocular blood flow.

INTRODUCTION: We investigated the effects of head-down tilt (HDT), which simulates microgravity during spaceflights, on the choroidal pulsatile ocular blood flow (POBF). This investigation is important because alterations in the choroidal blood flow can affect the function of retinal rods and cones that rely totally on the choroid for metabolites. METHODS: Nineteen healthy adults between 20 and 38 yr of age participated in this study. The POBF was compared for: 1) baseline, wherein subjects were declined 30 degrees from vertical; 2) microgravity simulation where subjects were in a 7 degrees HDT for 2 min; 3) 90 min of the 7 degrees HDT; and 4) recovery, i.e., back at 30 degrees for 2 min. RESULTS: The group averaged POBF (Mean +/- SEM values: 828.43 +/- 48.88 microL x min(-1)) decreased immediately during the 2-min microgravity simulation (582.18 +/- 43.62 microL x min(-1)), remained at that inferior level at the 90-min mark of HDT (542.26 +/- 45.35 microL x min(-1)), and came back toward baseline POBF during the recovery period (760.11 +/- 46.03 microL x min(-1)) (p = 0.0001). DISCUSSION: The results show that simulated-microgravity of relatively short duration induces retinal hypoperfusion throughout the microgravity interval through the reduction in the POBF. This finding may have important implications regarding visual performance in space crewmembers placed in prolonged microgravity environments.

Experimentally reduced perfusion of one eye impairs retinal function in both eyes.

PURPOSE: The oscillatory potential index of scotopic white flash electroretinograms is reversibly enhanced in the contralateral eye when the ocular perfusion pressure (OPP) to the test eye is transiently reduced. A transient increase in the intraocular pressure (IOP) and decrease in the OPP in the test eye induced quantifiable vascular changes in the optic nerve head of the contralateral eye. We explored this contralateral phenomenon looking at ganglion cell function in both eyes during elevated IOP and decreased OPP in the test eye only. Our specific objective was to characterize the effects that transient hypoperfusion had on the neural generators of the pattern-reversal electroretinograms (pERGs), the ganglion cells, and preganglion neurons. METHODS: A transient elevation in the IOP was sustained in 10 healthy subjects by scleral suction to reduce the baseline OPP by 15, 30, 45, and 60% for 2-min intervals. For each level of OPP, pERGs were evoked by a checkerboard with 75 minarc high-contrast black-white checks reversing at 5 Hz and recorded bilaterally using DTL fiber electrodes. The pERGs were also recorded immediately after removal of scleral suction and at 2-min intervals thereafter for an 8-min recovery interval. RESULTS: The unilateral decrease in OPP differentially reduced the pERG in the test and contralateral eyes. The pERG for the test eye returned to baseline amplitude within 2 min of removing the suction cup. In contrast, the pERG in the contralateral eye remained below baseline throughout the entire 8-min recovery interval. CONCLUSIONS: The observation of a bilateral decrease in the pERGs while the OPP was decreased in the test eye only suggested that these neuronal changes were modified at more central visual centers for retinal function to be compromised bilaterally. This latter effect may have been mediated by the transiently altered OPP or yet unknown neurohormonal mechanisms.

Effects of transient mild systemic hypoxia on the pulsatile choroidal blood flow in healthy young human adults.

PURPOSE: To investigate the effects of transient mild systemic hypoxia on the pulsatile ocular blood flow (POBF) in the healthy young adult. METHODS: Two measurements of the intraocular pressure (IOP) pulses, used to derive the POBF, were recorded from 19 subjects before, during, and after breathing 12% oxygen in nitrogen. Many physiological variables were also assessed throughout testing. RESULTS: Transient mild systemic hypoxia decreased the hemoglobin oxygen saturation and end-tidal carbon dioxide, increased the heart rate, but did not change the respiratory rate, systemic blood pressure, or ocular perfusion pressure. Mild systemic hypoxia decreased the intraocular pulse volume and the systolic and diastolic times but did not alter the pulse amplitude or the POBF. The IOP was not altered during mild systemic hypoxia. CONCLUSIONS: The absence of change in the POBF during transient mild systemic hypoxia indicated that the global pulsatile choroidal blood flow was not vulnerable to the effects of this transient mild systemic hypoxic stress in the healthy young adult.

Blue flicker modifies the subfoveal choroidal blood flow in the human eye.

The objective of the present study was to reveal an interaction between choroidal blood flow (ChBF) and light-induced photoreceptor activity, a physiological coupling that has been already demonstrated for retinal blood flow but rejected for ChBF. Ten healthy adults volunteered for this study. A real-time recording near-infrared laser-Doppler flowmeter was used to quantify the subfoveal ChBF while the luminance of blue flicker between 1 and 64 Hz was first increased and then decreased by 4.0 log units in 1.0-log unit steps between 0.0375 and 375 cd/m2. In separate testing, flash electroretinograms (ERGs) provided electrophysiological indexes of the relative response of short-wave cones (s-cones) and rods to blue light stimulation. Group-averaged, normalized ChBF measurements revealed that it was modulated by approximately 9% by flicker frequency. Increasing the blue flicker luminance from low to high attenuated the subfoveal ChBF, volume, and velocity by approximately 32%, approximately 30%, and approximately 5%, respectively. Decreasing the luminance from high to low over the same range had no effect on the subfoveal choroidal hemodynamics. The markedly different effects of reversed directions of change in blue flicker luminance on the subfoveal ChBF were linked to transitions between rod-dominated and s-cone-dominated retinal responses. Collectively, these findings indicate that the blue light-induced photoreceptor response is associated with a differential distribution of the ChBF across the ocular fundus according to the degree and type of retinal photoreceptor stimulated.

Systemic hyperoxia and retinal vasomotor responses.

PURPOSE: Several studies have investigated the changes in retinal vessel diameter during physiological stress or pathologic conditions. These studies were principally based on individual fundus photographs and as such did not allow the evaluation of vessel dynamics over time. The research objective was to detail the time course and amplitude changes in the diameter of arteries and veins across all retinal quadrants, during and after hyperoxic vascular stress. METHODS: The dynamics of changes in retinal vessel diameter were quantified with a retinal vessel analyzer, which digitizes fundus images in real time and simultaneously quantifies vessel diameter. The arterial and venous diameters within one disc diameter of the optic nerve head in each quadrant were studied. Twenty young adults participated in this study in which the vessel diameters were measured during successive phases of breathing either room air or pure oxygen. The oxygen saturation level (SaO(2)), end-tidal carbon dioxide (EtCO(2)), pulse rate (PR), respiratory rate (RR), and blood pressure (BP) were also monitored throughout testing. RESULTS: Breathing 100% O(2) caused an increase in SaO(2) and a decrease in the EtCO(2). All other systemic parameters measured (PR, RR, BP, and ocular perfusion pressure [OPP]) remained unchanged. However, the retinal veins and arteries constricted by approximately 14% and approximately 9% respectively, in all retinal quadrants. After experimental hyperoxia, inhalation of room air was associated with a progressive increase in the caliber of vessels toward their pretest size. The amplitude and overall profile of vessel reactivity to and recovery from hyperoxia was the same across retinal quadrants. CONCLUSIONS: These data indicate that, during systemic hyperoxic stress, the retinal vessels change in caliber uniformly across retinal quadrants in healthy young adults. This type of physiological vascular provocation could be used to investigate the quality of vascular regulation during aging and in vascular diseases of the eye.

Consequences of an increase in the ocular perfusion pressure on the pulsatile ocular blood flow.

PURPOSE: In previous studies we have demonstrated that physical exertion constricted the major retinal arteries, increased the redness of the optic nerve head, and attenuated scotopic white flash oscillatory potentials. These anatomic and functional changes in the human retina suggested that blood flow to the innermost retinal layers was modified in some way to cause these changes. The objective of this study was to determine whether physical exertion also affected blood flow in the choroid, the exclusive source of blood to the outermost layers of the retina. METHODS: Eighteen healthy adults volunteered for this study. An OBF Tonograph system (OBF Laboratories UK Ltd., Malmesbury, Wiltshire, U.K.) was used to derive the pulsatile ocular blood flow, which reflects the pulsatile component of blood flow in the choroid, at rest and 20 minutes after biking at a heart rate of 140 beats/min. RESULTS: At the systemic level, biking increased the blood pressure and the heart rate. At the ocular level, the duration of the systolic and diastolic phases of the intraocular pulse was shortened, and the pulse amplitude and volume were reduced. Despite the attenuation of the intraocular pulse parameters, the pulsatile ocular blood flow increased by some 18% after exercise largely because of the much larger increase in heart rate. The ocular perfusion pressure increased, whereas the intraocular pressure decreased. CONCLUSIONS: Physical exertion in the form of aerobic exercise increased the pulsatile component of blood flow in the choroid. Because the choroid is the sole blood supply to the outer retina, it was concluded that the degree of perfusion of the photoreceptors necessary for vision is increased by physical exertion. This increased choroidal blood flow is presumably to sustain vision as blood is directed to the large muscle groups involved in the physical activity.

Laser in situ keratomileusis for myopia and the contrast sensitivity function.

PURPOSE: To characterize the clinical effects of laser in situ keratomileusis (LASIK) on the cornea and visual performance by the contrast sensitivity function (CSF). SETTING: Clinique d'Ophtalmologie IRIS, Laval, Quebec, Canada. METHODS: Thirty-four patients aged 18 to 50 years volunteered for this prospective study. All patients had bilateral LASIK to correct myopia between -1.00 and -6.75 diopters (D) and refractive astigmatism less than 2.50 D. The mean spherical equivalent (SE) refractive error in the 68 eyes was -3.93 D +/- 1.67 (SD). A Hansatome microkeratome (Bausch & Lomb) and a Technolas 217C excimer laser (Bausch & Lomb) driven by the PlanoScan program were used. The monocular CSF for spatial frequencies of 3, 6, 12, and 18 cycles per degree (cpd) for both day and night vision simulations were made with a CSV-1000E system (Vector Vision) before and 1 month and up to 9 months after LASIK. All patients wore their best spectacle correction for the baseline CSF. RESULTS: The group (n = 68) averaged preoperative and postoperative CSFs did not differ at 1 month (analysis of variance [ANOVA], P>.05). In a subgroup of 11 eyes that had corneal microstriae, however, there was a significant reduction in the photopic and mesopic CSF at 6, 12, and 18 cpd (ANOVA, P<.05) despite normal Snellen visual acuities. The CSF normalized in 6 to 9 months as the microstriae became less visible. CONCLUSIONS: Subtle central corneal microstriae after LASIK can reduce the baseline CSF at medium to high spatial frequencies even with 20/20 visual acuity. The CSF normalizes as the microstriae fade over time.

Reproducibility of retinal and optic nerve head perfusion measurements using scanning laser Doppler flowmetry.

BACKGROUND AND OBJECTIVE: To evaluate the reproducibility of full-field perfusion analysis using scanning laser Doppler flowmetry (SLDF) for perfusion measurements of the neuroretinal rim of the optic nerve head and the peripapillary retina in patients with open-angle glaucoma or ocular hypertension, and in normal subjects. PATIENTS AND METHODS: SLDF perfusion measurements of the neuroretinal rim and the peripapillary retina were performed on 20 patients with open-angle glaucoma or ocular hypertension (group G) and 20 normal volunteers (group N). Each subject underwent two independent sessions, 30 minutes apart, each involving 5 high quality images. Intrasession and intersession reproducibility coefficients for flow, volume, and velocity were calculated for a single image and for means of 3 and 5 images using analysis of variance models. flow, the intrasession coefficient of reliability was 0.99 each for the rim, nasal retina, and temporal retina in group G and 0.93, 0.93, and 0.95, respectively, in group N. The intersession coefficient of reliability for flow was 0.99 for the rim, 0.95 for the nasal retina, and 0.87 for the temporal retina in group G and 0.87, 0.82, and 0.80, respectively, in group N. Compared with single image analysis, intrasession and intersession reproducibility were generally better when a mean of 3 images and substantially better when a mean of 5 images was used. CONCLUSION: SLDF full-field perfusion analysis is markedly more reproducible than the original software using 10 x 10 pixel windows. Obtaining mean values for at least 3 images improves the intrasession and intersession reproducibility of this technique.

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). METHODS: Twenty-six healthy volunteers, 19 to 55 years of age participated in this two-phase study. Each subject increased resting OPP through stationary biking at a heart rate (HR) of 140 beats per minute (bpm) over 20 minutes. The ChBF was measured by laser Doppler flowmetry (LDF), the systemic BP by electronic sphygmomanometry, and the resting intraocular pressure (IOP) by applanation tonometry. RESULTS: The OPP increased by approximately 43% at the onset of biking, and then decreased biphasically to approximately 12% above resting value by the end of biking. The ChBF remained within 10% of its basal value throughout biking. Immediately after biking, the OPP decreased twice as much as the ChBF in the same time frame. CONCLUSIONS: The dissociation between the OPP and the ChBF during biking and recovery suggests that some mechanism keeps the ChBF close to its basal value, an observation that indicates blood flow regulation.

Neuroretinal function during mild systemic hypoxia.

PURPOSE: Few studies have investigated the effect of systemic hypoxia on the vascular and neural function of the human retina. Such studies can help us understand physiological responses to environmental hypoxia, as well as the pathophysiological mechanisms underlying certain ocular diseases. The objective of this study was to investigate the impact of mild systemic hypoxia on neuroretinal function through photopic flash electroretinogram (fERG) and oscillatory potential (OP) recordings. METHODS: The photopic fERGs and OPs were recorded in 18 healthy adults under conditions of mild systemic hypoxia. The retinal responses were recorded before, during, and after a 5-min period of breathing 12% oxygen (O2) in 88% nitrogen. The heart rate (HR), respiratory rate (RR), end-tidal carbon dioxide pressure (Petco2), and O2 saturation (SaO2) were measured throughout testing. The systemic blood pressure (BP) and intraocular pressure (IOP) were measured to derive the ocular perfusion pressure (OPP). RESULTS: Systemic hypoxia reduced SaO2 and PetCO2, increased HR but did not alter the RR or OPP. The a-wave amplitude and latency were not altered throughout testing. The b-wave amplitude decreased with hypoxia, whereas its latency was not affected. The amplitude of OP1, OP2, and OP4 remained stable throughout testing, whereas the amplitude of OP3 tended to decrease with hypoxia and was increased at the end of testing. The latency of OP1, OP3, and OP4 did not vary. The latency of OP2 was reduced at the end of testing. CONCLUSIONS: Our results show that mild systemic hypoxia alters the fERG b-wave and OPs but not the a-wave. This suggests that the outer retina in humans is more resistant to a mild systemic hypoxic stress than the inner retinal layers.