Georg Michelson

Principle, validity, and reliability of scanning laser Doppler flowmetry.

PurposeThe objective of this study is to present the reliability and validity of scanning laser Doppler flowmetry (SLDF) performing a high-definition topography of perfused vessels of the retina and the optic nerve head with simultaneous evaluation of blood flow. MethodsThe examination of blood flow by SLDF is based on the optical Doppler effect. The data aquisition and evaluation system is a modified laser scanning device; the wavelength of the laser source is 670 nm, with a power of 100 μW (Heidelberg Engineering, HRF). The reliability of SLDF was estimated by performing five seperate measurements in 10 eyes on 5 days. The validity of the method was tested by two experiments. First, in an experimental set-up, the capability of SLDF to measure the velocity of a moving plane in absolute units was estimated. Second, comparative measurements were performed of retinal blood flow in 16 normal eyes and in 33 glaucomatous eyes with SLDF and a commercially available single-point laser Doppler flowmeter (Oculix). ResultsWe found SLDF to produce a high reliability. The reliability coefficients rx of flow, volume, and velocity were 0.82, 0.81, and 0.83, respectively. Comparative measurements of the retinal blood flow by SLDF and a single-point laser Doppler flowmeter of corresponding retinal points showed a linear and significant relationship between flow (r = 0.83, p < 0.0001), volume (r = 0.51, p < 0.0001), and velocity (r = 0.59, p < 0.0001). In the experimental set-up, SLDF was able to quantitatively measure velocity in absolute units. ConclusionsSLDF enables the visualization of perfused vessels of the jux-tapapillary retina and the optic nerve head in high resolution by two-dimensional mapping of the optical Doppler shift and a reproducible evaluation of capillary blood flow.

Perfusion of the juxtapapillary retina and the neuroretinal rim area in primary open angle glaucoma

PurposeThe objective of this study is to evaluate capillary blood flow of the juxtapapillary retina and neuroretinal rim area in primary open angle glaucoma (POAG) by a new noninvasive method performing a high-definition topography of perfused vessels of the retina and the optic nerve head with simultaneous evaluation of blood flow. MethodsJuxtapapillary retinal and neuroretinal rim area blood flow were measured by scanning laser Doppler flowmetry (SLDF). This new technique is a combination of a laser Doppler flowmeter with a scanning laser system by which the retinal perfusion is simultaneously quantified in 16,000 sites of a retinal area of 2.7 ± 0.7 mm. In study I, retinal and optic nerve head blood flow were evaluated by SLDF in 43 patients with POAG and 43 healthy individuals. The mean age of the POAG group was 56 ± 12 years and of the control group 42 ± 15 years. In study II, age-matched normals (n = 21) were compared with glaucoma eyes with topical therapy (n = 30) and with glaucoma eyes without topical therapy (n = 16). ResultsIn study I, the eyes with POAG had an average cup/disc ratio (C/D) of 0.75 ± 0.20, with an average mean defect of the visual field of 5.3 ± 5.4 dB. The actual intraocular pressure was 17.8 ± 4.18 mm Hg in the POAG group and 15.45 ± 1.82 mm Hg in the control group. For statistical analysis, two age-matched groups of 28 normal eyes of 28 persons with 27 glaucomatous eyes of 27 patients were compared. In normals the blood flow of the neuroretinal rim area was significantly higher than that of the juxtapapillary retinal area ( + 7.73%, p < 0.01). In POAG both juxtapapillary retinal blood flow and neuroretinal rim area blood flow were significantly decreased compared to an age-matched control group: neuroretinal rim area flow —71% and juxtapapillary retina flow −49%. The decrease of neuroretinal rim area blood flow did significantly correlate with C/D. We found no correlation between reduction of juxtapapillary retinal blood flow and C/D or mean defect. Both glaucoma eyes with and glaucoma eyes without topical therapy showed significant decreased juxtapapillary blood flow compared to normals. We found no significant difference in the juxtapapillary blood flow between glaucoma eyes with and without topical therapy. ConclusionsIn POAG, optic nerve head blood flow and juxtapapillary blood flow were significantly decreased.

Automatic full field analysis of perfusion images gained by scanning laser Doppler flowmetry

BACKGROUND Scanning laser Doppler flowmetry (SLDF) enables the measurement of the laser Doppler frequency shift in retinal tissue. This process allows the quantification of retinal and optic nerve head perfusion in an area of 2.7 mm × 0.7 mm within 2 seconds and with a spatial resolution of 10 μm × 10 μm. Owing to the local heterogeneity of the retinal microcirculation itself and to heart associated pulsation the capillary retinal blood flow depends on location and time. Because of technical limitations measurements of flow are only valid in retinal points with adequate brightness and focus, and away from big vessels. To include the heart beat associated pulsation and the spatial heterogeneity of retinal blood flow into the evaluation of blood flow an algorithm was developed examining automatically the whole SLDF perfusion image. AIM To report intraobserver reliability and interobserver reliability of a new method for analysing automatically full field perfusion images. METHOD The base of blood flow calculation by the automatic full field perfusion image analyser (AFFPIA) was 16 384 intensity time curves of all pixels of the whole perfusion image gained by the SLDF. AFFPIA calculates the Doppler frequency shift and the haemodynamic variables flow, volume, and velocity of each pixel. The resulting perfusion image was processed with respect to (1) underexposed and overexposed pixels, (2) saccades, and (3) the retinal vessel tree. The rim area and the saccades were marked interactively by the operator. The capillaries and vessels of the retinal vessel tree were identified automatically by pattern analysis. Retinal vessels with a diameter greater than 30 μm, underexposed or overexposed areas, and saccades were excluded automatically. Based on the whole perfusion image total mean flow, total mean volume, total mean velocity, standard deviation, cumulative distribution curve of flow, and the capillary pulsation index were calculated automatically. Heart beat associated pulsation of capillary blood flow was estimated by plotting the mean capillary flow of each horizontal line against time. Intraobserver reliability was estimated by measuring 10 eyes of 10 subjects on five different days by one observer. Interobserver reliability of AFFPIA was evaluated by analysing 10 perfusion maps by five different operators. To find a baseline of retinal blood flow, perfusion maps of 67 eyes of normal subjects with a mean age of 40.4 (SD 15) years were evaluated by AFFPIA. RESULTS The coefficient of reliability of the intraobserver reproducibility of flow was 0.74. The coefficient of reliability of the interobserver reproducibility was 0.95. The juxtapapillary retinal capillary flow was temporally 484 (SD 125), nasally 450 (117); the rim area capillary flow was 443 (110). The mean capillary pulsation index of retinal flow was 0.56 (0.14). CONCLUSION Retinal blood flow evaluation by the AFFPIA increases significantly the interobserver reliability compared with conventional evaluation of 100 μm × 100 μm areas in SLDF images with the original Heidelberg retina flowmeter software. The intraobserver reliability of AFFPIA was in the same range as conventional evaluation.

Automated segmentation of the optic nerve head for diagnosis of glaucoma

Glaucoma is the second most common cause of blindness worldwide. Low awareness and high costs connected to glaucoma are reasons to improve methods of screening and therapy. A well-established method for diagnosis of glaucoma is the examination of the optic nerve head using scanning-laser-tomography. This system acquires and analyzes the surface topography of the optic nerve head. The analysis that leads to a diagnosis of the disease depends on prior manual outlining of the optic nerve head by an experienced ophthalmologist. Our contribution presents a method for optic nerve head segmentation and its validation. The method is based on morphological operations, Hough transform, and an anchored active contour model. The results were validated by comparing the performance of different classifiers on data from a case-control study with contours of the optic nerve head manually outlined by an experienced ophthalmologist. We achieved the following results with respect to glaucoma diagnosis: linear discriminant analysis with 27.7% estimated error rate for automated segmentation (aut) and 26.8% estimated error rate for manual segmentation (man), classification trees with 25.2% (aut) and 22.0% (man) and bootstrap aggregation with 22.2% (aut) and 13.4% (man). It could thus be shown that our approach is suitable for automated diagnosis and screening of glaucoma.

Increased Wall:Lumen Ratio of Retinal Arterioles in Male Patients With a History of a Cerebrovascular Event

Arterial hypertension is a major risk factor for stroke, and retinal vessels can be regarded as a mirror of the cerebral vasculature. Whether vascular remodeling of retinal arterioles with ageing and hypertension plays a role in cerebrovascular risk stratification has not yet been adequately addressed. In study 1, retinal arteriolar structure was assessed in 182 normotensive volunteers and 117 patients with essential hypertension. In study 2, we compared retinal arteriolar structure among 74 normotensive volunteers, 47 patients with treated essential hypertension, and 18 subjects with a history of a cerebrovascular event. Retinal arteriolar structure was assessed using scanning laser Doppler flowmetry and automatic full-field perfusion imaging analysis. In study 1, wall:lumen ratio of retinal arterioles revealed a significant correlation with age (r=0.198; P=0.001). In study 2, wall:lumen ratio was highest in patients with a history of a cerebrovascular event compared with treated hypertensive and normotensive subjects (0.46±0.08, 0.36±0.14, and 0.35±0.12; P=0.007). When the treated group with hypertension was divided into 2 subgroups according to the quality of blood pressure control, patients with poor blood pressure control showed higher wall:lumen ratio than subjects with good blood pressure control (0.40±0.13 versus 0.31±0.13; P=0.025). Thus, assessment of wall:lumen ratio of retinal arterioles emerged as an attractive tool to identify treated patients with hypertension with increased cerebrovascular risk.

Impaired Endothelial Function of the Retinal Vasculature in Hypertensive Patients

Background and Purpose— Arterial hypertension constitutes a central factor in the pathogenesis of stroke. We examined endothelial function of the retinal vasculature as a model of the cerebral circulation. Methods— Thirty-eight young subjects (19 hypertensive and 19 normotensive) were treated with the AT1-receptor blocker candesartan cilexetil and placebo, each over 7 days. Retinal capillary flow and blood flow velocity in the central retinal artery were assessed with scanning laser Doppler flowmetry and pulsed Doppler ultrasound, respectively. NG-mono-methyl-L-arginine (L-NMMA) was infused to inhibit nitric oxide (NO) synthesis. Diffuse luminance flicker was applied to stimulate NO release. Results— In normotensive subjects, L-NMMA decreased retinal capillary flow by 8.2%±13% (P < 0.05) and flickering light increased mean blood flow velocity in the central retinal artery by 19%±29% (P < 0.01). In contrast, no significant change to these provocative tests was seen in hypertensive subjects. Treatment with candesartan cilexetil restored a normal pattern of reactivity in retinal capillaries (L-NMMA: decrease in perfusion by 10%±17%, P < 0.05) and the central retinal artery (flicker: increase in mean blood flow velocity by 42%±31%, P < 0.001) in hypertensive patients. Conclusions— Endothelial function of the retinal vasculature is impaired in early essential hypertension but can be improved by AT1-receptor blockade.

Visual field defect and perfusion of the juxtapapillary retina and the neuroretinal rim area in primary open-angle glaucoma

Abstract • Background: At this time little information is available about the relationship between glaucomatous visual field defects and impaired blood flow in the optic nerve head. The purpose of this study was to examine blood flow of the juxtapapillary retina and the rim area of the optic nerve head in primary open-angle glaucoma with a borderline visual defect. • Methods: Juxtapapillary retinal and neuroretinal rim area blood flow was measured by scanning laser Doppler flowmetry (SLDF). The visual field was evaluated by static perimetry (Octopus-G1). The optic nerve head was assessed on 15° color stereo photographs. We examined 116 eyes of 91 patients with POAG with controlled IOP and 66 eyes of 44 healthy individuals. The POAG group was divided into eyes with a mean defect lower than 2 dB (POAG group I) and in eyes with a mean defect equal to or greater than 2 dB (POAG group II). The mean age of POAG group I and POAG group II was 55±11 years and 57±10 years, respectively. The mean age of the control group was 45±15 years. The eyes of POAG group I had an average C/D ratio of 0.71±0.18 with an average mean defect of the visual field of 0.97±0.68 dB; the eyes of POAG group II had an average C/D ratio of 0.80±0.17 with an average mean defect of the visual field of 8.2±6.0 dB. The intraocular pressure on the day of measurement in POAG group I was 18.2±3.7 mmHg, in POAG group II 17.6±4.0 mmHg, and in the control group 15.1±2.5 mmHg. For statistical analysis, age-matched groups of 32 normal eyes of 32 subjects (mean age 52±10 years) were compared to 18 glaucomatous eyes of 18 patients (POAG group I, mean age 55±11 years) and 59 glaucomatous eyes of 59 patients (POAG group II, mean age 55±10 years). • Results: In the eyes of POAG group I and POAG group II, both juxtapapillary retinal blood flow and neuroretinal rim area blood flow were significantly decreased compared to an age-matched control group: neuroretinal rim area “flow” POAG group I −65%, POAG group II −66%; juxtapapillary retina “flow” POAG group I −52%, POAG group II −44%. All eyes of the POAG group I (MD<2 dB) and 56 of 61 eyes of the POAG group II (MD>=2 dB) showed a retinal perfusion lower than the 90% percentile of normal blood flow. We found no correlation between reduction of juxtapapillary or papillary blood flow and mean defect in POAG eyes. • Conclusion: Glaucomatous eyes with no defects or borderline visual field defects as well as glaucomatous eyes in an advanced disease stage show significantly decreased optic nerve head and juxtapapillary retinal capillary blood flow.

Analysis of retinal arteriolar structure in never-treated patients with essential hypertension.

Objective Increased wall-to-lumen ratio of small arteries is a predictor of adverse cardiovascular prognosis. We aimed to analyze retinal arteriolar structure in never-treated patients with essential hypertension and to test whether elevated blood pressure is associated with an increased wall-to-lumen ratio of retinal arterioles. Methods The study cohort comprised 21 untreated male patients with essential hypertension (mean age 39.1 ± 5.4 years) and 29 untreated normotensive men (mean age 36.7 ± 5.9 years). Wall-to-lumen ratio of retinal arterioles was assessed in vivo using scanning laser Doppler flowmetry. Results Patients with essential hypertension had a higher wall-to-lumen ratio of retinal arterioles than normotensive individuals (0.36 ± 0.1 vs. 0.28 ± 0.1, P = 0.028). Wall cross-sectional area of retinal arterioles did not differ between the study groups. The growth index, indicating the percentage of difference in average wall cross-sectional area of retinal arterioles between both groups, was 18%. Both systolic (r = 0.360, P = 0.010) and diastolic (r = 0.536, P < 0.001) blood pressures were related to wall-to-lumen ratio of retinal arterioles. Multiple regression analysis including a variety of known cardiovascular risk factors revealed that blood pressure is independently associated with an increased wall-to-lumen ratio of retinal arterioles (systolic blood pressure: β = 0.417, P = 0.012; diastolic blood pressure: β = 0.548, P = 0.001). Conclusion The changes in arteriolar structure of retinal vessels in our study cohort revealed a similar pattern to that observed previously by other investigators in subcutaneous small arteries in essential hypertension. Blood pressure emerged as an important and independent determinant of wall-to-lumen ratio of retinal arterioles.

Flickering light increases retinal blood flow.

Purpose To examine the retinal blood flow in normal eyes before and during retinal stimulation by flickering light. Design A prospective cross-sectional study. Participants and Testing Twenty-seven eyes of 27 normal subjects with a mean age ± SD of 38 ± 15 years (study I) and 21 eyes of 21 normal subjects with a mean age ± SD of 46 ± 17 years (study II) were examined with respect to capillary retinal blood flow and central retinal artery and central retinal vein blood flow velocities during flickering light stimulation. A luminance flicker light with a frequency of 8 Hz increased the neuronal activity of retinal ganglion cells. In study I, the retinal capillary blood flow was measured before and during flickering by scanning laser Doppler flowmetry (670 nm, Heidelberg Retina Flowmeter). In study II, the blood flow velocities in the central retinal artery and central retinal vein were examined by pulsed Doppler sonography. Main Outcome Measures Change in blood flow velocities in the central retinal artery and vein and in retinal capillary blood flow after full-field flicker stimulation. Results In study I, measurements of blood flow during retinal flicker stimulation showed a significant increase in the mean value of blood flow ± SD from 317 ± 72 arbitrary units to 416 ± 103 arbitrary units. The change was on average 46 ± 19%. In study II, the systolic and end-diastolic blood flow velocities in the central retinal artery increased significantly (P < 0.0001): systolic, 9 cm/s to 15 cm/s (+62%); end-diastolic, 2.7 cm/s to 5.3 cm/s (+96%). In the central retinal vein, the systolic and end-diastolic blood flow velocities increased significantly (P < 0.0001): systolic, 4.3 cm/s to 6.7 cm/s (+56%); end-diastolic, 1.8 cm/s to 3.6 cm/s (+100%). The authors found no significant change in blood pressure and heartbeat frequency. Conclusions Visual stimulation of the retina by flickering light strongly increased the juxtapapillary retinal capillary blood flow and central retinal artery blood flow velocity in normal eyes.

Effect of breathing 100% oxygen on retinal and optic nerve head capillary blood flow in smokers and non-smokers

AIM The effect of breathing 100% oxygen on retinal and optic nerve head capillary blood flow in smokers and non-smokers was investigated using scanning laser Doppler flowmetry (SLDF) as a new non-invasive method to visualise and quantify ocular blood flow. METHOD 10 eyes of 10 young healthy non-smoking volunteers (mean age 26 (SD 3) years) and nine eyes of nine young healthy smoking volunteers (mean age 26 (4) years) were investigated. All participants were asked not to smoke or consume caffeine containing drinks for at least 4 hours before the measurements. Blood flow measurements were performed before and after 100% oxygen was applied to the subjects through a mask over a period of 5 minutes (6 litres per minute). Juxtapapillary retinal and optic nerve head blood flow were determined in arbitrary units using SLDF representing a combination of laser Doppler flowmetry and a scanning laser system allowing visualisation and quantification of the retinal and optic nerve head blood flow. Blood flow was determined in an area of 100 μm × 100 μm. The level of carboxyhaemoglobin was determined in all subjects. A Wilcoxon matched pairs signed ranks test (non-parametric) was used for statistical evaluation. RESULTS In the non-smoking group, retinal ‘flow’ was reduced by 33% (p = 0.005), optic nerve head ‘flow’ by 37% (p = 0.005). In the smoking group retinal flow was reduced by 10% (p = 0.01), optic nerve head flow by 13% (p <0.008). The difference in reactivity to oxygen breathing between smokers and non-smokers was highly significant (p <0.00001). Increased carboxyhaemoglobin levels were not found in either of the groups. A significant reduction of the mean arterial blood pressure of 6% (5%) (p <0.02) was observed in the non-smoking group after administration of oxygen. CONCLUSION These results indicate that hyperoxia leads to a decrease in capillary blood flow of the retina and optic nerve head secondary to vasoconstriction, and that smokers do not respond to oxygen breathing as non-smokers do. The findings might be based on factors such as long term effects of nicotine on the sympathetic and parasympathetic nervous system.