[Clinic use of Laser Doppler Flowmetry (LDF) in the study of subfoveal choroidal circulation]. / Impiego clinico del Laser Doppler Flussometria (LDF) nello studio della circolazione coroideale sottofoveale.

Today our knowledge of the choroidal circulation is limited: we know its anatomy, but, on the other hand, its physiopathology remains to be fully. The choroid is involved in a number of important ocular diseases. The Laser Doppler Flowmetry (LDF) is a technique that allows non-invasive measurement of haemodynamic parameters of the subfoveal choroidal circulation. It is easy to use in daily clinic activity. The aim of this mini-review is to describe LDF studies of the choroidal circulation performed in healthy subjects under different environmental conditions, in subjects with ocular diseases, as well as studies of the effects of various drugs can induce on this circulation.

Changes in optic nerve head blood flow in children with cerebral malaria and acute papilloedema.

OBJECTIVE: To investigate capillary blood flow in the optic nerve head (ONH) of children with cerebral malaria. METHODS: Malawian children with cerebral malaria admitted to a paediatric research ward were examined by direct and indirect ophthalmoscopy. ONH blood flow was measured using laser Doppler flowmetry (LDF) in suitable patients. Mean blood volume and velocity were obtained from 30 to 60 s recordings from the temporal ONH and used to calculate blood flow. These were compared with admission variables, funduscopic findings and disease outcomes. RESULTS: 45 children with cerebral malaria had LDF recordings; 6 subsequently died and 5 survivors had neurological sequelae. 12 (27%) had papilloedema. The mean microvascular blood volume was higher in patients with papilloedema (3.28 v 2.54 arbitrary units, p = 0.002). The blood velocity correlated directly with haematocrit (r = 0.46, p = 0.001) and inversely with blood glucose (r = -0.49, p = 0.001). CONCLUSION: The increase in ONH microvascular blood volume in papilloedema measured by LDF is consistent with current theories of pathogenesis of papilloedema. LDF has potential as a tool to distinguish papilloedema from pseudopapilloedematous disc swellings. The relationship between blood velocity and haematocrit may relate to levels of sequestration in cerebral malaria.

Sub-foveal choroidal blood flow by LDF: measurement and application to the physiology and pathology of the choroidal circulation.

Laser Doppler flowmetry allows the measurement of relative choroidal blood flow in the sub-foveal region of the fundus (ChBF). This technique has been applied to the investigation of the regulation of ChBF in response to a variety of physiological stimuli (breathing different gas mixtures of O2 and CO2, varying the systemic and ocular blood perfusion pressures, light-dark transition and zero gravity) in normal subjects. Measurements in pathological conditions, such as diabetes, age-related macular degeneration and glaucoma indicate alterations of the response of ChBF to increased systemic blood pressure. The data provide a better understanding of the regulation of the choroidal circulation in the normal and diseased eye.

Regulation of optic nerve head blood flow in normal tension glaucoma patients.

PURPOSE: To investigate the regulation of the optic nerve blood flow (Fonh) in response to an increase of the perfusion pressure (PPm) in normal tension glaucoma (NTG) patients and in age-matched normal volunteers. METHODS: Measurements were performed in 16 eyes of NTG patients and in 10 eyes of age-matched controls. Laser Doppler flowmetry (LDF) was applied to calculate the relative flux of red blood cells at the temporal rim of the optic nerve head (ONH) in response to increases in PPm. PPm was raised through an increase in systemic blood pressure induced by isometric exercise. Before being tested, all patients had 3 weeks of washout of any local medication. RESULTS: In the NTG group, mean ophthalmic arterial blood pressure increased during isometric exercise from 73 to 89 mmHg (22%), resulting in a 29% increase of the PPm. This increase did not induce any significant change in mean Fonh. For the control group, the 28% increase of PPm also did not significantly affect Fonh. There was a trend for a greater increase in vascular resistance during isometric exercise in the NTG than in the normal control group (47% versus 25%). CONCLUSIONS: The LDF parameters, measured in the ONH, did not indicate an abnormal Fonh regulation in response to an increase of the PPm in either normal subjects or NTG patients. The maintenance of constant blood flow is achieved by an increase in local vascular resistance. Our data show a greater percent increase in vascular resistance in the NTG patients compared to the normal subjects for a similar percent increase in PPm in both groups during squatting. This suggests some alteration of the vessel tone regulatory mechanisms in NTG patients.

[Effect of chromatic flicker on circulation of the optic nerve]. / Effet du papillotement chromatique sur la circulation du nerf optique.

PURPOSE: To determine the response of human optic nerve head (ONH) blood flow (delta F) to heterochromatic equiluminant flicker modulation (eql-fl) and compare it to the response induced by pure luminance flicker (l-fl). METHODS: In 5 normal volunteers the ONH blood flow was measured by conventional laser Doppler flowmetry. Stimuli were generated by green and red light emitting diodes and delivered to the eye through a fundus camera illumination optic. Both green and red illuminances were square wave modulated in counter phase at different frequencies between 2 and 40 Hz. delta F was defined as the ratio between the ONH blood flow after 1 min stimulation and a baseline blood flow measured prior to the stimulation. RESULTS: In response to a 2 Hz eql-fl, ONH blood flow increases by 36% in average. delta F versus flicker frequency displayed the characteristics of a low-pass function with a cutoff frequency of 10 Hz for an eql-fl and a band-pass function with broad maximum around 10 Hz for the I-fl. CONCLUSIONS: delta F in human ONH can be evoked by heterochromatic equiluminant flicker modulation. The blood flow frequency response to eql-fl and I-fl are similar to the neural activity dominated by the Parvo- and Magno-cellular activity, respectively. These findings offer a new approach to study the neurovascular coupling at the ONH in both physiological and diseased conditions involving predominantly or selectively the Magno- and Parvo-pathways.

[Studies of the hemodynamics of the optic head nerve using laser Doppler flowmetry]. / Etude de la circulation de la tête du nerf optique par fluxmétrie à laser Doppler.

PURPOSE: To study hemodynamics in the optic nerve head (ONH) in animals and humans using laser Doppler flowmetry (LDF) during physiological stimuli such as flicker light stimulation, variations of the breathing gas (O(2)) concentration, and systemic mean perfusion pressure (PP(m)) variations. METHODS: LDF is based on the Doppler effect according to which the frequency of the light scattered by a moving particle (i.e., red blood cells in the vessels) is shifted in frequency compared to that of the incident light. The laser light scattered from the tissue and the red blood cells (RBCs) is collected by an optical fiber and fed to photodetector. NeXT computer analysis provides relative measurements of the Vel(ONH) which expresses the mean velocity of the RBCs, the Vol(ONH) representing the number of RBCs and the F(ONH) an expression of the flux of RBCs in the volume sampled by the laser. Modifications of the LDF parameters during physiological stimuli such as modifications of breathing gas (O(2), CO(2)), flicker light stimulation, and variations in the perfusion pressure were studied. RESULTS: The results of these studies demonstrated for the first time a dynamic coupling of blood flow to function and metabolism in the ONH, mediated by an increase in potassium and nitric oxide release. In addition the results of these studies showed that the ONH blood remains constant during variations of the perfusion pressure (PP(m)) induced by an increase in the intraocular pressure or an increase of the systemic blood pressure by isometric exercise. These results confirm an autoregulation in the ONH blood flow in animals and humans. CONCLUSION: LDF is a powerful technique for investigating changes in blood flow in the ONH of anesthetized animals and humans, induced by physiological stimuli involving the breathing of various gases, neuronal stimulation, and variations in the perfusion pressure. This highly sensitive and reproducible technique opens new avenues in the elucidation of blood flow regulation mechanisms in the ONH.

Flicker-evoked responses of human optic nerve head blood flow: luminance versus chromatic modulation.

PURPOSE: To determine the response of human optic nerve head blood flow (R:F(onh)) to heterochromatic equiluminant flicker modulation and compare it to the response induced by pure luminance flicker. METHODS: In five normal volunteers, F(onh) measured at the neuroretinal rim was monitored continuously by laser Doppler flowmetry. Stimuli were generated by green and red light emitting diodes and delivered to the fundus in Maxwellian view (field of 25(o)). Both green (G:) and red (R:) illuminances were square-wave modulated, 180(o) out of phase, with a maximum value of 10.4 for G: and 2.64 lux for R: Flicker frequency was varied from 2 Hz to 40 Hz. R:F(onh) was defined as the change in F(onh) during stimulation relative to the prestimulus F(onh). RESULTS: Defining the color ratio r as R:/(R: + G:), the R:F(onh), measured for a 15-Hz flicker, was largest at pure luminance (r = 0 and 1), declined at mixed luminance and chromatic modulations, and reached a secondary maximum at r = 0.45, the value of psychophysical equiluminance. R:F(onh) versus flicker frequency displayed the characteristics of a low-pass function for the equiluminance flicker stimulus and of a band-pass function, with a maximum at intermediate frequencies, for the luminance flicker stimulus. CONCLUSIONS: R:F(onh) in humans can be evoked by heterochromatic flicker, modulated either in luminance or chromatic equiluminant conditions. R:F(onh) may be specific for luminance and chromatic modulations, similar to neural responses dominated by the magno- and parvocellular activity, respectively. These findings offer a new approach to study the neurovascular coupling at the optic nerve head in both physiological and diseased conditions involving predominantly or selectively the magno- and parvocellular pathways.

Measurement of eye length and eye shape by optical low coherence reflectometry.

BACKGROUND: The precise and rapid measurement of eye length and eye shape is essential for investigating eye growth regulation and myopia. For this purpose, we developed an optical low coherence reflectometer (OLCR) and present preliminary measurements. METHODS: The OLCR includes a super luminescent diode (wavelength: 845 nm, coherence length: approximately 30 microm) and rotating glass cube to produce longitudinal scans at a velocity of 0.42 m/s and a repetition rate of approximately 13 scans/s. Heterodyne detection of light reflected from the anterior cornea and the posterior retina permits to measure axial eye length and eye shape (off-axis eye length). Each measurement consists of five consecutive scans. Reproducibility and precision were determined in one volunteer by measuring axial eye length five consecutive times, each time repositioning the eye. Eye shapes were determined in right eyes of four volunteers by measuring eye length every 3.3 degrees from 10 degrees nasally to 10 degrees temporally. RESULTS: Axial eye length measured repeatedly in one volunteer did not differ between or within the measurements (one-factor ANOVA). The average standard deviation was 11 microm. Eye shapes (a) varied substantially among subjects and (b) differed considerably from the corresponding shapes of spherical model eyes with identical axial eye lengths. CONCLUSION: The newly developed OLCR permits the precise and rapid measurement of eye length and eye shape. Such measurements, especially in children, may provide important information about mechanisms of eye growth regulation and the development of myopia.

Subfoveal choroidal blood flow in response to light-dark exposure.

PURPOSE: To document the response of subfoveal choroidal blood flow (ChBF) in the human eye induced by light and dark exposures and provide some insight into the mechanism underlying this response. METHODS: In a group of 12 volunteers (age, 25-60 years), ChBF was measured with a confocal laser Doppler flowmeter. Wavelength of the probing laser beam was 785 nm (90 microW at the cornea). ChBF was recorded in room light, in darkness, in room light after dark adaptation, and during strong green light exposure after exposure to room light. After dark adaptation of both eyes, ChBF was also measured in one eye while only the fellow eye was exposed to strong visible light. RESULTS: Although ChBF was stable during room light condition, it decreased significantly by 15% (P < 0.01) during dark adaptation. After 6 minutes of room light following 20 minutes of darkness, ChBF was back to baseline. Strong, diffuse, green light exposure over a field of 40 degrees, as well as the probing laser beam, had no detectable effect on ChBF. No change in ChBF was detected when the fellow eye was illuminated after both eyes had been dark adapted. CONCLUSIONS: The findings did not confirm the presence of an active process of ChBF regulation in response to light exposure in humans. They demonstrate, however, a reversible decrease in ChBF that occurs after a transition from room light to darkness, which could involve a neural mechanism.

[Effect of dark adaptation on retinal blood flow]. / Effet de l'adaptation à l'obscurité sur le flux rétinien.

PURPOSE: Laser Doppler measurements performed immediately after the transition from dark adaptation (DA) to light led to the hypothesis that retinal blood flow, Fret, is increased during DA, but the use of visible lasers had prevented measurements during DA. Our aim was to test this hypothesis by measuring Fret during and after DA. MATERIAL AND METHODS: Fret in retinal vessels at the optic disk surface was recorded quasi-continuously in one eye of 6 normal subjects (age 27 to 60 years) using a laser Doppler flowmeter in the near-infrared (810 nm). Measurements were performed during light (baseline), various periods of DA and again during light. DA lasted between 2 and 32 min. RESULTS: Average Fret for the 6 subjects did not change significantly (-2.7 +/- 8% sd, p > 0.05) during the various periods of DA, as determined from linear regressions of the flux versus time. Following the transition from DA to light, there was, in most cases, a rapid transient increase of the flux, which reached an average value of 37 +/- 10% above the pre-transition value and peaked at 30-60 sec after the transition. CONCLUSIONS: These results do not support the hypothesis that Fret in normal volunteers is increased during DA. Rather, they strongly suggest that the transient increase in flux observed after DA is induced by the transition from dark to light (FNSRS #3200-043157 et CNR, It. #95.01715.CT04).

[Effect of light on choroidal blood flow in the fovea centralis]. / Effet de la lumière sur le flux sanguin de la choroïde dans la région de la fovea centralis.

PURPOSE: To investigate whether light and dark exposures induce a response of choroidal blood flow (ChBF) in the foveal region in humans. METHODS: In a group of healthy volunteers (age 25-60 years) ChBF was measured using a new confocal laser Doppler flowmeter (probing laser at 785 nm, power at the cornea = 90 microW). ChBF was recorded at room light, in darkness, at room light following dark adaptation, and during strong light exposure following room light. RESULTS: While ChBF was stable during room light condition, it decreased significantly by 15% (p < 0.01) during darkness. After 6 min of room light following darkness, ChBF was back to baseline. Strong diffuse, green light exposure over a field of 45 degrees had no detectable effect on ChBF. In all the experiments, no significant change of blood pressure was detected. CONCLUSIONS: Our findings did not confirm the presence of an active process of ChBF regulation in response to strong light exposure in humans. They demonstrate, however, a reversible decrease in ChBF occurring after a transition from room light to darkness.

[Optical Doppler velocimetry of red blood cells at different depths in retinal vessels by varying the coherence length of the source: feasibility study]. / Vélocimétrie par effet Doppler optique des globules rouges à différentes profondeurs dans les vaisseaux rétiniens en variant la longueur de cohérence temporelle de la source: etude de faisabilité.

PURPOSE: To report on a novel approach to measure the velocity of red blood cells (RBCs) at different retinal vessel depths. METHOD: The technique is an extension of conventional laser Doppler velocimetry using light sources of various coherence lengths (CL). Light scattered by the moving RBCs interferes with that reflected from the anterior vessel wall only if the optical path difference is shorter than CL. Therefore, using low coherence light sources, localized measurements of RBCs velocity can be performed. RESULTS: Measurements of RBCs velocity at different depths in a main retinal vein (diameter: 152 microns) of a volunteer has been performed using 4 different light sources with CLs of 14 microns, 21 microns, 32 microns and > m. Measured values are in good agreement with theoretically predicted values. CONCLUSION: This new approach permits to measure RBCs velocity at different depths of retinal vessels in the human retina.

Iris blood flow response to acute decreases in ocular perfusion pressure: a laser Doppler flowmetry study in humans.

The relationship between blood flow and ocular perfusion pressure in the iris vascular bed of the human eye has not been established yet. Consequently, it is not known whether the iris circulation has some autoregulatory capacity. The aim of the present study was to investigate this relationship in the particular case where the perfusion pressure was decreased by increasing the intraocular pressure. Using laser Doppler flowmetry, mean velocity, volume and flow of blood through the iris were measured in normal subjects during acute decreases of the mean ocular perfusion pressure induced by raising the intraocular pressure with a scleral suction cup. Two experimental paradigms were applied. In the first, the suction pressure was rapidly increased from baseline, in steps of 50-100 mmHg, to a level where the intraocular pressure was above the mean ophthalmic artery blood pressure. In the second, the suction pressure was increased from baseline in four successive steps of 50 mmHg each. The suction pressure was kept constant for 2 min at each step. With the first paradigm (nine eyes), a 72% decrease in perfusion pressure induced a 63% decrease of iris blood flow due mainly to a drop in blood velocity. Immediately after suction release, blood flow increased by 62% above baseline and then returned to its baseline value within 2 min. With the second paradigm (six eyes), a 28% decrease in perfusion pressure resulted in a 30% flow reduction, which was due to significant decreases (P<0. 001) of both blood volume and velocity. Combining the results of both paradigms, we observed a significant linear correlation between iris blood flow and perfusion pressure (R =0.964, P<0.001). These results demonstrate that a decrease of the perfusion pressure due to an increase of the intraocular pressure induces a decrease of the iris blood flow. No evidence of an autoregulatory process in the iris vascular bed could be demonstrated. A reactive hyperemia was observed in response to a sudden increase in perfusion pressure occurring after a period of decreased blood flow.

Response of choroidal blood flow in the foveal region to hyperoxia and hyperoxia-hypercapnia.

PURPOSE: Arterial carbon dioxide tension and arterial oxygen tension are important determinants of retinal and cerebral blood flow. In the present study the hypothesis that changes in arterial blood gases also influence choroidal blood flow was tested. METHODS: The effect of breathing different mixtures of oxygen (O(2)) and carbon dioxide (CO(2)) on choroidal blood flow in the foveal region was investigated in healthy subjects. The study was performed in a randomized, double-masked four way cross-over design in 16 subjects. Using a compact laser Doppler flowmeter, red blood cell velocity (ChBVel), volume (ChBVol), and flow (ChBF) in the choroidal vasculature were measured during the breathing of various mixtures of O(2)and CO(2) (hyperoxia-hypercapnia): 100% O(2), 97%O(2)+3%CO(2), 95%O(2)+5%CO(2) (carbogen) and 92%O(2)+8%CO( 2). Arterial oxygen tension (pO(2)) and carbon dioxide tension (pCO(2)) were measured from arterialized blood samples from the earlobe. RESULTS: Breathing 100% O(2) had no significant effect on ChBVel (-3.7%), ChBVol (+1.7%) and ChBF (-4.3%). Addition of 3% CO(2) to O(2) also produced no significant change on these blood flow parameters. In contrast, carbogen significantly increased ChBVel (10.0 +/- 4.4%, 95% CI, p < 0.001) and ChBF (12.5 +/- 11.7%, p = 0.002). The effect of 92% O(2) + 8% CO(2) was more pronounced since it significantly increased ChBVel and ChBF by 15.5 +/- 7.5% (p < 0.001) and 16.2 +/- 11.0% (p < 0.001), respectively. None of the gas mixtures induced a significant change in ChBVol. The increase in ChBF was approximately 1.5% per 1 mmHg increase in pCO(2). CONCLUSIONS: This study demonstrates that, in healthy subjects, pCO(2) is an important determinant of foveal choroidal blood flow, whereas pO(2) has little impact on it.

Optical Doppler velocimetry at various retinal vessel depths by variation of the source coherence length.

We report on what we believe is a novel approach to measuring the velocity of red blood cells (RBC's) at different depths of retinal vessels by use of low-coherence sources. The technique, variable coherence optical Doppler velocimetry (VCODV), performs Doppler shift measurements through autodyne mixing between the light scattered by the RBC's and by the vessel front wall (reference). Only the light from RBC's moving at a depth less than half the coherence length (CL) mixes efficiently with the reference. Measurements of the Doppler shifts from RBC's with sources of four different CL's in a 152-microm vein of a volunteer confirmed the feasibility of VCODV. This approach has the potential to monitor in vivo retinal RBC velocity gradient at the vessel wall and the velocity profile within the blood vessel in the condition of symmetric blood flow profiles.

[Measuring choroid blood flow with a new confocal laser Doppler device]. / Mesure du flux sanguin choroïdien au moyen d'un nouvel instrument laser Doppler confocal.

UNLABELLED: A new instrument for the measurement of choroidal blood flow in the fovea is presented. It is based on the laser Doppler method and a confocal optical system with an indirect detection of the Doppler shifted light. METHOD: The intensity of the laser beam (785 nm) at the cornea is 90 microW. Measurements were obtained from a normal population of 21 subjects under resting conditions without dilating the pupil. RESULTS: The reproducibility of the choroidal blood flow, based on 5 measurements of 10 s each in 5 randomly selected subjects, is 9%. The minimum detectable change for a statistical significance of p < or = 0.05, based on a population of 21 subjects and 10 s measurements, is 9%. CONCLUSION: This new compact instrument appears to be suitable for the investigation of the physiology and pharmacology of choroidal blood flow and the effect of age-related macular degeneration.

[Effect of decreased ocular perfusion pressure on iris blood flow measured by laser Doppler flowmetry]. / Effet d'une diminution de la pression de perfusion oculaire sur le flux sanguin dans l'iris mesuré par fluxmétrie laser Doppler.

PURPOSE: To determine whether iris blood flow (IBF) is regulated in response to an acute decrease in mean ocular perfusion pressure (PPm = MOAP-IOP, MOAP = mean ophthalmic arterial pressure) induced by increasing the intraocular pressure (IOP). METHODS: Iris blood flow was measured using a slit lamp incorporating a laser Doppler flowmetry (LDF) module. The study was conducted on 12 normal volunteers (14 to 59 years old). IOP was raised using a scleral suction cup. In Exp. #1, the suction pressure was successively raised in steps of 50 to 100 mm Hg, each lasting about 10 sec, until IOP reached the MOAP level. In Exp. #2, the suction was raised to 200 mm Hg in 4 successive steps of 2 min duration. RESULTS: In Exp. #1, no significant change of IBF was observed for small decreases of PPm (< 23%); greater decreases of PPm resulted in a linear IBF decrease (p < 0.01). In Exp. #2, such a IBF versus PPm decrease was also observed (p < 0.001). Immediately after release of suction, a significant, transient IBF increase of 79% above baseline level was observed. CONCLUSION: These results suggest that some IBF regulation occurs for small PPm decreases (< 23%); no IBF compensatory mechanism appears to operate for further decreases of PPm (> 23%).

Autonomic denervations influence ocular dimensions and intraocular pressure in chicks.

Choroidal thickness and axial eye length in the chick undergo day/night fluctuations that can also be modulated by visual experience. In the present study, we tested the effect of parasympathetic and sympathetic denervations on both day/night and image dependent changes in ocular dimensions. We also correlated such changes with fluctuations in intraocular pressure. Parasympathectomy influenced choroidal thickness and its day/night fluctuation, but had no effect on vision dependent choroidal thickness modulation. Parasympathectomy also influenced-to a lesser extent-axial length and reduced the axial growth response to form vision deprivation. Sympathectomy had little effect on ocular dimensions, but reduced the day/night differences in intraocular pressure. We conclude that (a) the parasympathetic nervous system influences both choroidal thickness and axial length and participates in the neural control mechanism leading to form deprivation myopia and, (b) the day/night fluctuations of choroidal thickness and axial length are unlikely to be explained by fluctuations in intraocular pressure. For the regulation of choroidal thickness, we hypothesize the existence of two independent mechanisms. One involves the parasympathetic nervous system; it influences the day/night choroidal thickness fluctuation. The other uses a separate pathway and is driven by visual input.

Laser Doppler flowmetry and optic nerve head blood flow.

PURPOSE: Ischemic disorders of the optic nerve head constitute an important cause of visual loss. The optic nerve head is supplied by two main sources of blood flow: the superficial layers by the central retinal artery and the deeper layers by the posterior ciliary arteries. This study was conducted in rhesus monkey eyes to obtain a better understanding of which part of the optic nerve head circulation is measured by laser Doppler flowmetry. METHODS: By means of a fundus camera-based laser Doppler flowmetry technique to measure blood flow in the optic nerve head tissue, laser Doppler flowmetry measurements were taken at baseline and then after experimental occlusion of central retinal artery (12 eyes), posterior ciliary arteries (nine eyes), and combined occlusion of central retinal artery and posterior ciliary arteries (nine eyes). Optic nerve head, choroidal, and retinal circulations were investigated by fluorescein fundus angiography after the various arterial occlusions. RESULTS: Average laser Doppler flowmetry flow during central retinal artery occlusion alone was significantly decreased (P<.001) by 39%+/-21% (mean +/- 95% confidence interval) compared with normal baseline. Combined occlusion of central retinal artery and posterior ciliary arteries reduced laser Doppler flowmetry flow even more markedly by 57%+/-27% (P<.0005), but the difference between this flow reduction and that with central retinal artery occlusion alone was not significant (P>.20). After posterior ciliary artery occlusion alone, however, measurements showed a nonsignificant increase in laser Doppler flowmetry flow of 17%+/-37%. CONCLUSIONS: The findings of this study suggest that the standard laser Doppler flowmetry technique is predominantly sensitive to blood flow changes in the superficial layers of the optic nerve head and less sensitive to those in the prelaminar and deeper regions, and their relative proportions are not known. In this laser Doppler flowmetry technique, the weaker Doppler signal from the deep layers cannot be separated from the dominant signal from the superficial layers to exclusively study the circulation in the deep layers; the latter circulation is of interest in optic nerve head ischemic disorders, including glaucoma. Emerging new optical modalities of the laser Doppler flowmetry technique may help in selectively measuring blood flow in the deeper layers.