Retinal capillary blood flow measurement with a scanning laser ophthalmoscope. Preliminary results

Do angiographic data support a detailed classification of hypertensive fundus changes?

The eye is a target organ as well as an established prognostic indicator of arterial hypertension. Based on the ophthalmoscopically visible alterations, several classifications, the majority of them grading hypertensive fundus changes into four stages, have been suggested. Moreover, assessment of hypertensive alterations of the perivoveal microcirculation has become possible by means of fluorescein angiography. However, it has not yet been evaluated whether an angiographic equivalent for the ophthalmoscopic classifications exists. We therefore compared the perifoveal microcirculation of hypertensive patients who were staged according to the classification of Neubauer, a modification of the classification of Keith and Wagener, among each other and with that of normal subjects. According to Neubauer, who distinguishes between fundus hypertonicus (stages I-II) and hypertensive retinopathy (stages III-IV), we divided the patients (n = 143) into four groups: stage I (n = 49), stage II (n = 72), stage III (n = 16), and stage IV (n = 6). All patients underwent fluorescein angiography performed with a scanning laser ophthalmoscope. By means of digital image analysis we quantified the following parameters: (1) perifoveal intercapillary area (PIA), (2) the area of the foveal avascular zone (FAZ), and (3) capillary blood velocity (CBV). All patients with arterial hypertension demonstrated a rarefaction of the perifoveal capillary bed and a decrease of capillary blood velocity as compared with normal subjects. Significant changes of PIA (P < 0.05) and CBV (P < 0.05) were seen between mild (I-II) and severe stages (III-IV) of hypertensive retinopathy, but neither between stages I and II nor between stages III and IV. Our findings indicate significant angiographic differences between mild and severe form of hypertensive retinopathy, however, unlike in ophthalmoscopy, a differentiated division into four stages is not possible.

Scanning laser ophthalmoscopy for early diagnosis of vitreoretinal interfase syndrome

PURPOSE

To describe the angiographic signs found using scanning laser ophthalmoscopy for the early diagnosis of vitreoretinal interface syndrome. This method is useful to visualize the inner retinal layers, being more sensitive than fundus biomicroscopy.

MATERIAL

61 patients with vitreoretinal interfase syndrome were evaluated. All of them had evidence of this disease using scanning laser ophthalmoscopy but four patients were referred without diagnosis of vitreoretinal interfase syndrome. These patients showed no biomicroscopic signs and diagnosis was made with SLO.

METHODS

Confocal scanning infrared laser ophthalmoscope (Heidelberg Retinal Angiograph assembled by Heidelberg Engineering). This SLO uses an infrared diode laser source of 795 nm.

CONCLUSION

Patients included were referred with another diagnosis and with this method the correct diagnosis was made. In conclusion scanning laser ophthalmoscopy allows early diagnosis of this pathology for follow-up and treatment.

Hemodynamic changes in two patients with retinal circulatory disturbances shown by fluorescein angiography using a scanning laser ophthalmoscope

PURPOSE

To assess hemodynamic changes in two patients with severely affected retinal circulation.

METHODS

A 62-year-old man with central retinal artery occlusion and a 46-year-old woman with branch retinal vein occlusion were studied by fluorescein angiography with a scanning laser ophthalmoscope (SLO). Fluorescein angiography with SLO revealed hypofluorescent clumps of different sizes and hyperfluorescent dots in large retinal vessels. The velocities of the hypofluorescent clumps were calculated between two points on the same vessel, and movements of the hypofluorescent clumps and the hyperfluorescent dots were investigated.

RESULTS

The velocities of the hypofluorescent clumps were slow and varied in the same vessel. The velocities of the hypofluorescent clumps increased at the sites with narrow calibers. The hypofluorescent clumps occasionally changed size in the bloodstream. The hypofluorescent clumps flowed along the walls of retinal vessels. Distance between consecutive hypofluorescent clumps was wide. Some vessels filled with hypofluorescent clumps were also detected. Rolling hyperfluorescent dots were seen in fluorescent plasma.

CONCLUSIONS

The hypofluorescent clumps were concluded to be packed erythrocytes and the hyperfluorescent dots corresponded to leukocytes and platelets moving in the vessels. Fluorescein angiography with SLO is a useful method for evaluating hemodynamic changes using the hypofluorescent clumps in severely affected retinal circulation.

Capillaroscopy and the measurement of capillary pressure

Capillaries play a critical role in cardiovascular function as the point of exchange of nutrients and waste products between the tissues and circulation. Studies of capillary function in man are limited by access to the vascular bed. However, skin capillaries can readily be studied by the technique of capillaroscopy which enables the investigator to assess morphology, density and blood flow velocity. It is also possible to estimate capillary pressure by direct cannulation using glass micropipettes. This review will describe the techniques used to make these assessments and will outline some of the changes that are seen in health and disease.

Ocular haemodynamics and colour contrast sensitivity in patients with type 1 diabetes

BACKGROUND

There is evidence that altered ocular blood flow is involved in the development and progression of diabetic retinopathy. However, the nature of these perfusion abnormalities is still a matter of controversy. Ocular haemodynamics were characterised with two recently introduced methods.

METHODS

The cross sectional study was performed in 59 patients with type 1 diabetes with a diabetes duration between 12 and 17 years and an age less than 32 years and a group of 25 age matched healthy controls. Scanning laser Doppler flowmetry and laser interferometric measurement of fundus pulsation amplitude were used to assess retinal and pulsatile choroidal blood flow, respectively. In addition, colour contrast sensitivity along the tritan axis was determined.

RESULTS

Fundus pulsation amplitude, but not retinal blood flow, increased with the progression of diabetic retinopathy. Retinal blood flow was influenced by plasma glucose levels (r = 0.32), whereas fundus pulsation amplitude was associated with HbA(1c) (r = 0.30). In addition, a negative correlation between the colour contrast sensitivity along the tritan axis and retinal blood flow was observed.

CONCLUSIONS

The present study indicates that pulsatile choroidal blood flow increases with the progression of diabetic retinopathy. Increased retinal blood flow appears to be related to loss of colour sensitivity in patents with type 1 diabetes.

Perifoveal microcirculation in eyes with epiretinal membranes

BACKGROUND/AIMS

Eyes with epiretinal membranes (ERMs) often have alterations of retinal vessels. The authors studied perifoveal microcirculation in eyes with epiretinal membranes (ERMs) using scanning laser ophthalmoscope (SLO) fluorescein angiography.

METHODS

Mean capillary blood flow velocity (CFV) was measured as an index of perifoveal microcirculation by SLO fluorescein angiography in 26 eyes with ERMs (19 eyes with idiopathic epiretinal membranes, seven eyes with epiretinal membranes after retinal detachment surgery) before and 6 months after vitreous surgery, and in 23 healthy control subjects.

RESULTS

The mean CFV was significantly reduced in eyes with ERMs compared with healthy controls (p=0.012), and the postoperative mean CFV was significantly increased compared with the preoperative mean CFV (p=0.041).

CONCLUSION

Significant changes of capillary blood flow velocity in the perifoveal areas were observed between normal subjects and eyes with epiretinal membranes. This indicates that eyes with ERMs show abnormal haemodynamics in the perifoveal capillaries.

Extraction of fluorescent dot traces from a scanning laser ophthalmoscope image sequence by spatio-temporal image analysis: Gabor filter and radon transform filtering

The scanning laser ophthalmoscope (SLO) allows the tracking of fluorescent dot motion, thereby enabling the flow velocities in perimacular capillaries to be directly measured. These can serve as an important index of local retinal soundness or reflect the whole body circulation status in disorders such as diabetes. Although it is possible to perceive moving fluorescent dots with the human eye, they are so faint and unstable that it is difficult to detect them by conventional digital still-image processing methods. To solve this problem, we generated spatio-temporal images of the fluorescent dots in a capillary and applied Gabor filters tuned to the direction of the traces in order to detect them. Finally, by discriminating and integrating the output using two levels of threshold, we were able to extract their traces. Because the medium-size Gabor filter requires a considerable amount of time for two-dimensional convolution calculation, we prove that there is a certain equivalence between the Gabor filter, the radon transform, and the Hough transform. In the light of this, we propose a form of radon transform filtering that includes a radon transform Gabor filter as a very long Gabor filter. This allows a whole trace to be detected in a single step with a one-dimensional convolution, thereby shortening the processing time. In an experiment, 60% of the traces could be detected without error, which is sufficient to allow the mean flow velocity in a capillary to be measured.

Assessment of human ocular hemodynamics

Vascular abnormality and altered hemodynamics play important roles in many ophthalmic pathologies. Much of our knowledge of ocular hemodynamics was gained from invasive animal research, although a number of noninvasive methods suitable for in vivo use in humans have been developed. Data from these methods now produce a significant literature of their own. Understanding the origins of the data and appreciating their limitations can be difficult. Modern hemodynamic assessment techniques each examine a unique facet of the ocular circulation. No single facet provides a complete description of the hemodynamic state of the eye. These methods have contributed a great deal to our understanding of normal hemodynamics. More importantly, they continue to add to our understanding of altered hemodynamics found in disease. Some have found their way into limited clinical practice. The predominant ocular hemodynamic assessment techniques are reviewed with the aims of introducing the fundamental principles behind each, highlighting their inherent advantages and limitations, highlighting their contributions to understanding ocular physiology, and considering their potential to provide signs for diagnosis.

Fluorescent infrared scanning-laser ophthalmoscope for three-dimensional visualization: automatic random-eye-motion correction and deconvolution

Scanning-laser ophthalmoscope (SLO) technology has provided, among other possibilities, the potential for three-dimensional (3-D) visualization of anatomy in the posterior pole of the eye. The use of indocyanine green (ICG) as an infrared fluorescent marker of vasculature in combination with aninfrared SLO (the Heidelberg Retina Angiograph) is presented. Presently, two main factors among others discussed impede the visualization of 3-D structures in observed SLO data. Random eye motion between optical sections and (to a lesser degree) motion between raster scan lines prevent assessment of spatial orientation and connectivity of vasculature. Second, smear along the optic axis owing to the optics prevents accurate determination of vessel or lesion size and shape, especially for features spanning several optical sections. A novel, to our knowledge, deconvolution algorithm is described that automatically corrects for the poor axial (optical-sectioning) resolution of the SLO and for patient random eye motion during target fixation. Encouraging preliminary results are presented showing the usefulness of applying blind deconvolution toward improving the 3-D clarity of SLO data. Although clinical and medical research applications are broad, the specific medical sample selected shows the potential of examining microvascular 3-D morphology for diagnosis and treatment of choroidal tumors.

Prevention of diabetes-induced abnormal retinal blood flow by treatment with d-alpha-tocopherol

Hyperglycemia in diabetes mellitus has been shown to activate diacylglycerol (DAG)-protein kinase C (PKC) pathway in the vascular tissues, possibly altering vascular function. We have characterized the effects of vitamin E (d-alpha-tocopherol) on activation of PKC and DAG levels in retinal tissues of diabetic rats, and correlated its effects to retinal hemodynamics using video-based fluorescein angiography (VFA). Comparing streptozotocin-induced diabetic rats to controls, membranous PKC specific activities were increased by 71% (p < 0.05). Western blot analysis showed that the membranous PKC beta II isoform was significantly increased by 133 +/- 45% (p < 0.05). Intraperitoneal injection of d-alpha-tocopherol (40 mg/kg) every other day prevented the increases in membranous PKC specific activity and PKC beta II protein shown by immunoblots. Similar to PKC activities, total DAG levels were increased in the retina and were normalized by d-alpha-tocopherol treatment. Physiologically, abnormalities of retinal blood hemodynamics, as measured using VFA, which previously have been reported to be associated with increases of DAG and PKC levels in the diabetic rats, were prevented by d-alpha-tocopherol treatment in diabetic rats. The direct effect of d-alpha-tocopherol on total DAG and [3H]-palmitate incorporation into DAG were also examined using cultured bovine retinal endothelial cells (REC). Exposure of REC to 22 mM glucose for three days increased total DAG and [3H]-palmitate labeled DAG levels by 35 +/- 8% and 50 +/- 8%, respectively (p < 0.05). The presence of d-alpha-tocopherol (50 micrograms/ml) prevented the increase of both total DAG and [3H]-palmitate labeled DAG levels in cells exposed to 22 mM glucose. These findings suggested that the mechanism of the d-alpha-tocopherol's effect appears to be mediated by the normalization of the hyperglycemia-induced activation of the DAG-PKC pathway which leads to the normalization of abnormal retinal blood flow seen in diabetes mellitus.

Fluorescent dots in fluorescein angiography and fluorescein leukocyte angiography using a scanning laser ophthalmoscope in humans

PURPOSE

The purpose of the study is to disclose the nature of fluorescent dots and segments traditionally observed with fluorescein angiography (FA) using a scanning laser ophthalmoscope (SLO 101; Rodenstock, München, Germany). The authors developed a new method, called fluorescein leukocyte angiography (FLA), to display directly the movement of leukocytes in human retinal vessels.

METHODS

Fluorescein angiography was performed on two normal volunteers using a scanning laser ophthalmoscope and fluorescent dots and segments were observed. Fluorescein leukocyte angiography, using an injection of fluorescent buffy coat layer from which the fluorescent plasma and nonfluorescent erythrocytes have been removed externally, was performed on seven normal volunteers. Injection fluid smears were examined through a fluorescent microscope. Peripheral blood smears taken during midphase of FA and FLA also were examined. In addition, 15 early-phase FAs of central serous chorioretinitis (CSC) were studied retrospectively.

RESULTS

In the FAs of normal volunteers, fluorescent dots were detected only in perimacular capillaries at early phase. Eight of the 15 CSC FAs examined showed both fluorescent dots and segments. In the FLAs, fluorescent dots were detected in whole retinal vessels for more than 30 minutes. Fluorescent segments were observed in FA but not in FLA. Injected fluid smears from one FLA showed fluorescent leukocytes and small platelets. However, in peripheral blood smears of the FLA, leukocytes and platelets were more visible and exhibited higher contrast than those of an FA due to background plasma fluorescence. The mean velocity of 21 flowing leukocytes in perifoveal capillaries was 1.37 +/- 0.35 mm/second in 2 FAs and that of 89 flowing leukocytes was 1.41 +/- 0.29 mm/second in 7 FLAs.

CONCLUSIONS

The authors' observations suggest that fluorescent dots in scanning laser ophthalmoscope imaging are fluorescein-stained leukocytes, whereas fluorescent segments are the hyperfluorescent plasma that is located between rouleaux formations of erythrocytes. The velocity of the fluorescent dots could be measured in the perimacular capillaries by either FA or FLA; however, only FLA can display the flow of fluorescent leukocytes in large vessels.

The scanning laser ophthalmoscope

The imaging of the fundus of the eye poses two major technical challenges. First, it is necessary for both the illuminating and reflected beams to pass through the same aperture, the iris. In some commonly used instruments this leads to the use of levels of illumination close to the maximum tolerable by a patient. Second, in order to visualize the different structures present in the various layers of the fundus it is necessary to perform tomographic imaging. The scanning laser ophthalmoscope provides an answer to these particular problems. By scanning the fundus with a narrow laser beam most of the area of the iris is then available for the reflected light and so the intensity of the illuminating beam can be kept low, making it more acceptable for patients. The use of confocal imaging allows 3D images to be produced. In this short review the performance of the instrument will be discussed and its application to a number of clinical problems in ophthalmology considered. Finally there will be a brief description of other instrumentation currently under development.

Automatic extraction and measurement of leukocyte motion in microvessels using spatiotemporal image analysis

This paper describes a computer vision system for the automatic extraction and velocity measurement of moving leukocytes that adhere to microvessel walls from a sequence of images. The motion of these leukocytes can be visualized as motion along the wall contours. We use the constraint that the leukocytes move along the vessel wall contours to generate a spatiotemporal image, and the leukocyte motion is then extracted using the methods of spatiotemporal image analysis. The generated spatiotemporal image is processed by a special-purpose orientation-selective filter and a subsequent grouping process newly developed for this application. The orientation-selective filter is designed by considering the particular properties of the spatiotemporal image in this application in order to enhance only the traces of leukocytes. In the subsequent grouping process, leukocyte trace segments are selected and grouped among all the segments obtained by simple thresholding and skeletonizing operations. We show experimentally that the proposed method can stably extract leukocyte motion.

Perifoveal capillary network in patients with acute central retinal vein occlusion

PURPOSE

Reduction of visual acuity in patients with central retinal vein occlusion (CRVO) is often caused by macular edema and ischemia. The major causative factor for macular changes may be a disturbance in the macular microcirculation. The authors studied the perifoveal microcirculation in patients with central retinal vein occlusion to quantify the extent of circulatory deficiency in the macular circulation.

METHODS

Twenty-four patients (8 men, 16 women) with recently diagnosed CRVO were included in this study. The following data were quantified: mean capillary blood velocity (CBV), foveal avascular zone (FAZ), and mean perifoveal intercapillary area (PIA).

RESULTS

In patients with CRVO, the mean flow velocity was significantly reduced compared with healthy subjects (1.63 +/- 0.220 mm/sec vs. 2.89 +/- 0.41 mm/sec, P < 0.01). The FAZ and the mean PIA characterizing capillary density were significantly enlarged in CRVO (5548 +/- 1151 microm2 vs. 3872 +/- 529 microm2; P < 0.01).

CONCLUSIONS

The present study demonstrates that CRVO not only led to a decrease in capillary blood velocities, but also to an enlargement of perifoveal intercapillary areas in early stages of the disease.

Quadrant pattern ERG with SLO stimulation in normals and glaucoma patients

BACKGROUND

The pattern ERG (PERG) is one of the promising methods for investigation of glaucoma and may detect it at an early stage. The purpose of this study was to test the usefulness of the scanning laser ophthalmoscope (SLO) as a stimulator for evoking quadrant PERGs under visual control in normal and glaucoma eyes.

METHODS

The helium-neon laser of a Rodenstock SLO was used for quadrant stimulation (18 degrees x 29 degrees pattern size), while an infrared laser visualized the eye's fundus. Steady-state pattern-reversal ERGs were recorded in response to stripe patterns (8.33 Hz, 0.5 cycles/deg) in four retinal quadrants. Corresponding visual field defects were determined with the Octopus perimeter (G1, peridata). The subjects were 28 controls and 34 glaucoma patients with visual field losses and papillometric defects.

RESULTS

Amplitudes of nasal retinal areas (which include the blind spot) are smaller than those of temporal ones, and temporal lower responses are the largest. PERG amplitudes of all quadrants are significantly reduced in glaucoma (sensitivity 82%, specificity 80%). The differences between upper and lower quadrant PERGs are correlated with the differences between localized visual field defects of the same areas, (r = 0.46, P = 0.02).

CONCLUSIONS

The study demonstrates the feasibility of the SLO system for evoking localized PERG in normals and patients and shows the reduction of quadrant ERG amplitudes in glaucoma using the laser system.

Diabetic eye disease

Diabetic retinopathy accounts for most visual loss in the United States among working-age individuals. With appropriate detection, evaluation, and treatment, the risk for severe visual loss from this condition is dramatically reduced. This article details the natural history, pathophysiology, complications, grading, evaluation, and treatment for patients with diabetic retinopathy and discusses potential novel treatment modalities currently under investigation.

Measurement of retinal blood flow with fluorescein leucocyte angiography using a scanning laser ophthalmoscope in rabbits

AIMS

To measure blood flow in the rabbit retinal circulation with fluorescein leucocyte angiography using a scanning laser ophthalmoscope.

METHODS

Blood was withdrawn from the ear vein of a rabbit (New Zealand White), mixed with fluorescent dye in a test tube and centrifuged. The yellow-brown layer containing fluorescein stained leucocytes was collected and injected into the ear vein of the same rabbit while performing fluorescein angiography with a scanning laser ophthalmoscope. The image of retinal angiography displaying circulating fluorescent leucocytes was recorded on video tape. From each frame of the video tape, the consecutive positions of fluorescein stained leucocytes were digitised using an image analysis system and the velocity of blood flow was calculated.

RESULTS

Fluorescent leucocytes were clearly visualised in the retinal arteries, capillaries, and veins which allowed measurement of blood flow. The mean capillary velocity was 0.69 (SD 0.21) mm/s. The mean velocities of leucocytes measured in different sized vessels were as follows: 5.83 (2.42) mm/s in arteries over 50 microns, 3.33 (0.62) mm/s in those 35-50 microns, and 2.42 (1.08) mm/s in arteries under 35 microns, 3.08 (1.56) mm/s in veins over 50 microns, 2.79 (1.49) mm/s in those 35-50 microns, and 1.21 (0.50) mm/s in veins under 35 microns. Blood flow pulsation occurs in arteries, arterioles, veins, and venules but not capillaries.

CONCLUSION

Fluorescein leucocyte angiography can be used for simultaneous measurement of the blood flow in retinal arteries, veins, and capillaries.

Two dimensional mapping of the perfusion of the retina and optic nerve head

AIM

To present a new non-invasive method of performing a high definition topography of perfused vessels of the retina and the optic nerve head with simultaneous evaluation of blood flow.

METHOD

By a combination of a laser Doppler flowmeter with a scanning laser system the perfusion of the retina and the optic nerve head is visualised. The principles of measuring blood flow by laser Doppler flowmetry are based on the optical Doppler effect: laser light scattered by a moving particle is shifted in frequency by an amount delta f. Our data acquisition and evaluation system is a modified laser scanning tomograph. The technical data are retinal area of measurement 2.7 mm x 0.7 mm, 10 degrees field with 256 points x 64 lines, measurement accuracy 10 microns, wavelength 670 nm and 790 nm, light power 100 microW and 200 microW, data acquisition time 2.048 s. Every line is scanned 128 times by a line sampling rate of 4000 Hz. By performing a discrete fast Fourier transformation over 128 intensities of each retinal point the laser Doppler shift is calculated for each retinal point. With these data a two dimensional map with 256 x 64 points of the retinal perfusion is created. The brightness of the pixel is coded by the value of the Doppler shift. Offline capillary blood flow is estimated in arbitrary units according to the theory of laser Doppler flowmetry in every region of interest of the perfusion picture. We estimated the reliability and the validity of the method. Retinal blood flow was measured by scanning laser Doppler flowmetry (SLDF) while varying intraocular pressure by a suction cup of three healthy volunteers. Measurements of retinal blood flow performed in 47 eyes by the presented method (SLDF) were correlated with data gained by a commercially available laser Doppler flowmeter. Perfusion pictures of the superficial retinal layer and of deep prelaminar layers in the optic nerve head are presented.

RESULTS

The reliability coefficients r1 of 'flow', 'volume', and 'velocity' were 0.84, 0.85, and 0.84 respectively. We found a significant linear relation between SLDF flow and the ocular perfusion pressure (r = 0.84, p < 0.001). Comparative measurements of the retinal blood flow by SLDF and a commercially available laser Doppler flowmeter showed a linear and significant relation (flow r = 0.6, p < 0.0001, volume r = 0.4, p < 0.01). Capillaries of the retinal superficial vasculature or deep ciliary sourced capillaries of the optic nerve head became visible with a high resolution by the confocal technique dependent on the focus. Offline, the blood flow variables of areas of 100 microns x 100 microns were calculated.

CONCLUSION

SLDF enables the visualisation of perfused capillaries and vessels of the retina and the optic nerve head in high resolution by two dimensional mapping of perfusion variables which are encoded by the Doppler signal. This method achieves simultaneously qualitative and quantitative evaluation of capillary blood flow of distinct areas of the capillary meshwork.

CO2 dependence of retinal arterial and capillary blood velocity

OBJECTIVE

Blood flow to the brain is extremely sensitive to changes in PCO2. While animal studies show a similar potent PCO2 dependence in retinal and choroidal vessels, the PCO2-retinal blood flow relationship has never been adequately studied in humans.

METHODS

Video scanning laser ophthalmoscopy after fluorescein angiography was used to analyze retinal arterial and capillary blood velocity under conditions of mild hypercapnia and hypocapnia. Control conditions (end-tidal PCO2 = 38.3 +/- 0.4 mmHg) were contrasted with hyperventilation-induced hypocapnia (PCO2 = 34.0 +/- 0.4 mmHg) and hypercapnia (PCO2 = 42.3 +/- 0.5 mmHg) created by PCO2 addition to inspired gas.

RESULTS

Both larger vessel and macular capillary blood velocity was dependent upon PCO2: arteriovenous passage time fell as PCO2 rose, and both mean arterial dye velocity and capillary blood velocity rose as PCO2 rose (all p < 0.05). These changes in flow velocity occurred despite unchanged heart rate, arterial systolic and diastolic blood pressure, intraocular pressure, and calculated ocular perfusion pressure. Contrast sensitivity was also unchanged by PCO2 variation.

CONCLUSIONS

The human retinal circulation, like the whole cerebral circulation, may be strongly dependent upon PCO2 in a manner that is unrelated to perfusion pressure and apparently outside strict autoregulatory controls.

Dynamics of external ocular blood flow studied by scanning angiographic microscopy

The scanning angiographic microscope (SAM) provides a solution to the considerable technical difficulties associated with conventional episcleral fluorescein angiography. Standardised anterior segment fluorescein videoangiograms were performed using the SAM in each episcleral quadrant of the right eye in 6 normal subjects; frame-by-frame analysis proved important. Centripetal flow was seen in all 37 scleral perforating arteries investigated. Other features were the marked individual variability, much larger vertical anterior ciliary arteries, the high frequency of arteriovenous anastomoses, the complex flow patterns, the absence of a 'watershed' zone between anterior ciliary and posterior episcleral circulations, a characteristic and discontinuous distribution of 'leaky' episcleral veins, and the primacy of venous drainage into the plexus of muscular veins. Reports of retrograde blood flow in the anterior ciliary arteries in most fluorescein angiographic studies are probably incorrect, the result of unappreciated methodological problems. The SAM is an important advance on previous anterior segment fluorescein angiography techniques.

Reversal of abnormal retinal hemodynamics in diabetic rats by acarbose, an alpha-glucosidase inhibitor

Acarbose is an inhibitor of intestinal alpha-glucosidase and has been reported to decrease blood glucose concentrations and glycosuria in diabetic patients and animals. In this study we investigated whether this drug could prevent the abnormalities detected in retinal circulation of diabetic rats. Longitudinal paired studies were performed and the changes in retinal circulation were analyzed using video based fluorescein angiography (VFA) methodology in the same animal. Baseline VFA recordings were obtained from 41 rats. These rats were separated into 4 different groups: In group A (n = 12), diabetes was induced by streptozotocin (STZ) injection and the rats were fed with acarbose (40 mg/100 g powdered chow) mixed into regular rat chow; In group B (n = 10), diabetes was induced by STZ injection and the rats were fed with normal chow; In group C (n = 9), the non-diabetic rats were fed with acarbose; In group D (n = 10), the non-diabetic rats were fed with normal chow. At the end of 2 weeks, all rats again underwent VFA recordings. Blood glucose levels and body weights of rats were monitored during the experiment. The mean blood glucose concentration of Group B was raised from 98.5 +/- 8.7 to 342 +/- 30 mg/dl after STZ injection while in Group A, this change in glucose level was partially ameliorated by acarbose (from 102 +/- 15 to 247 +/- 48 mg/dl). In Group C and D, the blood glucose levels were not significantly changed during the experiment.(ABSTRACT TRUNCATED AT 250 WORDS)

Macular microcirculation in cystoid maculopathy of diabetic patients

BACKGROUND

In patients with diabetic macular oedema and central cysts ischaemia of the retina appears to be an important contributing factor in the pathogenesis of cysts. This study was performed to further elucidate the role of the inner retinal microcirculation in diabetic cystoid macular oedema (CMO).

METHODS

Video fluorescein angiography allows visualisation of the macular microvasculature and measurements of the capillary blood velocity (CBV), foveal avascular zone (FAZ), and perifoveal intercapillary area (PIA, characterising capillary density).

RESULTS

Twenty three diabetic subjects with CMO, matched diabetic patients without macular oedema (n = 23), and healthy subjects (n = 23) were included. CBV, PIA, and FAZ did not differ significantly among diabetic groups regardless of presence of cystoid changes. CBV was significantly reduced (p < 0.0001) and PIA was more than doubled in both diabetic groups (p < 0.0001) when compared with healthy subjects. Furthermore, FAZ showed a nearly doubled size in diabetic patients without macular oedema (p < 0.01) and a less pronounced enlargement (by 29%) in diabetics with CMO (p < 0.05).

CONCLUSION

The results indicate that the retinal microcirculation in diabetic patients is markedly altered when compared with healthy subjects, regardless of CMO presence. In CMO patients the microcirculatory changes are similar to those of diabetic patients without macular oedema. Thus inner retinal perfusion does not contribute to tissue ischaemia leading to cystoid formations in diabetic maculopathy.

Measurement of retinal hemodynamics with scanning laser ophthalmoscopy: reference values and variation

High resolution video fluorescein angiography using scanning laser ophthalmoscopy allows the assessment of retinal macro- and microcirculation. Data on the retinal macrocirculation were obtained from 221 healthy subjects. The data were derived from estimations of the arm-retina time, the arteriovenous passage time and mean arterial dye velocity, characterizing the passage of fluorescein to the eye, the mean arterial plasma velocity, and the arteriovenous passage through the entire vascular bed of one segment. Additionally, the transit of hypofluorescent segments in the capillary macular network were measured in 90 healthy subjects. These parameters provide a wide range of information for understanding the physiology of healthy and diseased eyes. Fundamental for all interpretations is the knowledge of the physiological variations. In the present study the inter- and intraindividual variability of retinal hemodynamics in healthy volunteers were assessed. The interindividual variation was 23.8% for the arm-retina time, 20.7% for the arteriovenous passage time, 23.7% for the mean arterial dye velocity, and 14.2% for the capillary flow velocity; the coefficient for variation, characterizing the intraindividual variation, was 26.6%, 15.6%, 16.7%, and 7.9%, respectively. The knowledge of the inter- and intraindividual variation of retinal blood flow indices allows for a priori power estimations for pathophysiologic and pharmacological studies.

Physiological perturbation of ocular and cerebral blood flow as measured by scanning laser ophthalmoscopy and color Doppler imaging

Retinal blood flow regulation in health remains poorly described. We hypothesized that retinal perfusion is controlled to provide constant O2 delivery to that tissue, and that changes in retinal blood flow in response to chemical stimuli parallel changes in carotid and retrobulbar perfusion. Accordingly, in 11 young adults with normal eye examinations, we measured retinal blood flow indices (via scanning laser ophthalmoscopy [SLO] during fluorescein angiography) and carotid, ophthalmic, and central retinal arterial blood flow indices (via Doppler imaging [CDI]) under control, hypoxic (alveolar PO2 = 55 +/- 3 mmHg) and hyperoxic (alveolar PO2 = 655 +/- 18 mmHg) conditions. The three conditions were counterbalanced in order and isocapnia was maintained in each. Retinal arterial mean dye velocity and arteriovenous passage time, as measured by SLO, were slowed by hyperoxia and accelerated by hypoxia, in rough proportion to the changes in arterial O2 content (+/- 10%; p < 0.05). In the seven subjects in which relative measurements of retinal arterial diameters were obtained, neither hypoxia nor hyperoxia significantly altered vessel diameter. At the same time, mean retinal capillary transit velocity was independent of PO2, suggesting that, in health, retinal capillaries may be recruited as PO2 falls. O2-induced changes in carotid, ophthalmic, or central retinal arterial blood flow velocities (via CDI) were not found, though a wide coefficient of variation (30% for CDI vs. 14% for SLO) may have contributed to this failure. We conclude that, under isocapnic conditions, retinal perfusion may be regulated to provide constant O2 delivery.(ABSTRACT TRUNCATED AT 250 WORDS)

Quantification of retinal capillary density and flow velocity in patients with essential hypertension

Arterial hypertension is known to be an important risk factor for cerebral and cardiovascular disease. Previous studies in rats have demonstrated that changes in both capillary density and vessel diameter may contribute to increased vascular resistance in hypertension. In vivo studies of human subjects with essential hypertension revealed a reduction in the number of arterioles in the skin and conjunctiva; no other in vivo data are available from other tissues. By means of a new imaging technique, capillary density and capillary blood flow velocity can now be assessed in the human retina. We undertook the present investigation to determine whether patients with essential hypertension and only minor clinical retinal vascular alterations have decreased retinal capillary density and altered capillary flow velocity. Seventeen hypertensive patients with only minor retinal vascular alterations and 17 healthy volunteers matched for age were selected. All study participants underwent ophthalmologic examination and fluorescein angiographic studies by means of scanning laser ophthalmoscopy. Capillary density and capillary blood flow velocity in the perifoveal network were evaluated from the angiograms. The retinal microcirculation in the perifoveal capillary network of hypertensive patients showed significant alterations. Both the capillary density and capillary flow velocities were significantly reduced compared with the control group. For the first time alterations of capillary blood flow and capillary density in a vascular network very similar to that of the brain have been demonstrated in hypertensive patients in vivo. Further studies with this technique may help identify patients at high risk for cerebrovascular diseases.

Differences between pattern-evoked electroretinograms obtained by a scanning laser ophthalmoscope and by a mechanical mirror system

Pattern electroretinograms were studied in response to stripe patterns of different spatial frequencies and intensities in the pattern-onset and reversal mode by means of a scanning laser ophthalmoscope and a mechanical mirror system (maxwellian view). The stimulus conditions in both procedures were as close as possible: 31 degrees square field, 633-nm wavelength and the same equipment for recording. The remaining differences between the two methods were the line and pixel structure in the scanning laser ophthalmoscope and the buildup speed (40 ms in the scanning laser ophthalmoscope and less than 1 ms in the mirror system). This difference was reflected in the response peak times of the pattern electroretinogram, which were up to 10 ms in the scanning laser ophthalmoscope measurement. The pattern-related onset response was smaller with the scanning laser ophthalmoscope, indicating a strong sensitivity to the slow build-up speed, while the offset and reversal responses showed no differences in amplitudes. All differences were found only with the use of high luminances. The results suggest that responses evoked with patterns displayed on a television screen could be faster and larger if stimulators faster than the conventional television standard were used. Because of the smallness of onset amplitudes, the pattern-reversal mode might be better for stimulation in a clinical study when the scanning laser ophthalmoscope is used.

Capillary blood flow velocity measurements in cystoid macular edema with the scanning laser ophthalmoscope

Using a scanning laser ophthalmoscope, we calculated the velocity of retinal blood flow in a juxta-foveolar capillary during the course of cystoid macular edema after partial central retinal vein occlusion in a 53-year-old woman. The mean velocity of the fluorescent dots in the macular capillary of the right eye with cystoid macular edema was 1.59 +/- 0.08 mm/sec at the initial examination. Despite the systemic administration of indomethacin (75 mg/day for three weeks), best-corrected visual acuity decreased from 20/30 to 20/70, and the velocity became 0.82 +/- 0.13 mm/sec. Prednisolone (30 mg/day orally for one week) improved the cystoid macular edema, and the velocity was 0.96 +/- 0.06 mm/sec 12 days after initiation of the drug. The velocity gradually improved; one year later it was 1.65 +/- 0.17 mm/sec and visual acuity was 20/22. Velocity in the left eye, which did not have cystoid macular edema, was 2.16 +/- 0.16 mm/sec. Thus, scanning laser ophthalmoscopy proved useful for measuring the velocity of retinal blood flow.

Stereoscopic photography with a scanning laser ophthalmoscope

Digital fundus angiography has advantages over conventional photography. However, digital angiography as it is normally performed, provides no stereoscopic information. Because stereoscopic data can be crucial in the evaluation of conditions such as age-related macular degeneration, we developed methods to record stereoscopic information during video angiography with a scanning laser ophthalmoscope and to produce high-quality static stereoscopic images. Stereoscopic information was collected by moving the scanning laser ophthalmoscope side-to-side and with a modified Allen separator. Stereoscopic images were displayed as stereoscopic pairs on 35-mm film, stereoscopic pairs from a digital printer, and as stereoscopic images directly on a conventional video monitor. This system to capture and display stereoscopic information is convenient, uses readily available technology, and can be adapted to any digital angiography system.

Retinal hemodynamics using scanning laser ophthalmoscopy and hemorheology in chronic open-angle glaucoma

PURPOSE

Recent studies suggest that elevated intraocular pressure is not the only causative factor for the development of visual field loss and optic nerve damage in glaucomatous eyes. The authors determine whether retinal hemodynamics or blood fluidity are alternated in eyes of patients with open-angle glaucoma compared with those of age- and sex-matched healthy subjects.

METHOD

High-quality video fluorescein angiograms were obtained from single eyes of 51 patients with chronic open-angle glaucoma. From these angiograms, the arm-retina time, mean dye velocity, and arteriovenous passage time were quantified. The data from patients were compared with those of an age- and sex-matched group of healthy subjects.

RESULTS

In patients with chronic open-angle glaucoma, an 11% reduction of the mean dye velocity (P < 0.05) and a 41% prolongation of the arteriovenous passage time (P < 0.01) was observed relative to the values obtained among the control subjects. Among hematocrit values, plasma viscosity, and erythrocyte aggregation, only plasma viscosity showed a significant increase (4%; P < 0.01) in patients with chronic open-angle glaucoma.

CONCLUSION

These results indicate that a pronounced circulatory deficit exists within the retinal vasculature of glaucomatous eyes, which may coexist with, but cannot be fully attributed to, an increase in plasma viscosity.

Three-dimensional structure of human retinal vessels studied by vascular casting

A new casting technique for studying the three-dimensional structure of the human retinal vascular system on post mortem tissue was developed. The technique was designed to provide an overview presentation of the retinal vasculature such that localized areas with vascular pathology could be identified for further analysis of three-dimensional structure. The paper presents qualitative characteristics of three-dimensional structure in the normal human retinal vascular bed, to provide a basis for future studies on vascular impairment in retinal disease.

Evaluation of idiopathic epiretinal membranes by a scanning laser ophthalmoscope

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Segmentation of fluorescence in the retinal microcirculation--is it a valid indicator of blood cell flow?

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Cellular automata: retinal cells, circulation and patterns

Cellular automata modelling is a useful mathematical technique for simulating complex biological systems. An area to be studied is broken into a lattice of adjacent cells depicted by picture elements on a computer screen. The initial tissue pattern evolves on the computer screen, directed by a rule that considers the state of each cell and its neighbours in the lattice. Simulations of wound repair, cell proliferation, retinal circulation and pigment aggregation serve to illustrate the potential value of cellular automata modelling in ophthalmic research and practice.