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Methods for visualization and diagnosis of glaucoma
Last reviewed: 25.06.2018
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It has been established that the goal of the treatment of glaucoma is to prevent the further development of symptomatic vision loss with the maximum reduction of side effects or complications after surgical interventions. In the context of pathophysiology, the reduction of intraocular pressure to a level at which the axons of ganglion cells of the retina are not affected.
Currently, the "golden standard" for determining the functional state of axons of ganglion cells (their stress) is an automated static monochromatic study of the visual fields. This information is used to diagnose and evaluate the effectiveness of treatment (progression of the process with cell damage or its absence). The study has limitations that depend on the extent of axon loss, which should be determined before the study, in which changes are identified, diagnosed and compared to establish progression.
Retina thickness analyzer
The retina thickness analyzer (ATS) (Talia Technology, MevaseretZion, Israel) calculates the thickness of the retina in the macula and measures two-dimensional and three-dimensional images.
How does the retinal thickness analyzer work?
In mapping the thickness of the retina with a retinal thickness analyzer, a green 540 nm HeNe laser beam is used to produce a retinal image. The distance between the intersection of the laser with the vitreoretinal surface and the surface between the retina and its pigment epithelium is directly proportional to the thickness of the retina. Do nine scans with nine separate fixation targets. When comparing these scans, cover the zone in the central 20 ° (in the measurement - 6 to 6 mm) of the fundus.
In contrast to OCT and SLP, which measure START or HRT and OCT, where the contour of the optic nerve disk is measured, the thickness of the retina in the macula is determined with the retinal thickness analyzer. Since the highest concentration of retinal ganglion cells is in the macula and the layer of ganglion cells is much thicker than their axons (which make up the SNB), the thickness of the retina in the macula can be a good indicator of the development of glaucoma.
When a retinal thickness analyzer is used
The retinal thickness analyzer is useful in detecting glaucoma and monitoring its progression.
Restrictions
For the analysis of thickness of the retina, a pupil measuring 5 mm is required. The use of this method is limited in patients with multiple floating opacities or significant opacities of the eye. Due to the use of short-wave radiation in the ATS, this device is more sensitive to nuclear dense cataracts than the OCT, confocal scanning laser ophthalmoscopy (HRT) or SLP. To convert the obtained values into absolute values of the thickness of the retina, corrections must be made for the error of refraction and the axial length of the eye.
Blood flow in glaucoma
The increase in intraocular pressure was associated with the progression of visual field disturbances in patients with primary open-angle glaucoma for a long time. However, despite the reduction in intraocular pressure to the target level, in many patients the field of vision continues to narrow, which indicates the impact of other factors.
From epidemiological studies it follows that there is a link between arterial pressure and risk factors for the development of glaucoma. In our studies, it was found that to compensate and reduce blood pressure in patients with glaucoma alone, autoregulatory mechanisms are not enough. In addition, the results of studies confirm that in some patients with normotensive glaucoma observed reversible vasospasm.
As research progressed, it became clearer that blood flow was an important factor in the study of the vascular etiology of glaucoma and its treatment. It was revealed that abnormal blood flow exists in the retina, optic nerve, retrobulbar vessels and choreoid in glaucoma. Since there is currently no single available method that could accurately examine all these areas, a multi-instrumental approach is used to better understand the blood circulation of the entire eye.
[7], [8], [9], [10], [11], [12], [13], [14], [15]
Scanning laser ophthalmoscopic angiography
Scanning laser ophthalmoscopic angiography is based on fluorescent angiography - one of the first modern measurement technologies for collecting empirical data on the retina. Scanning laser ophthalmoscopic angiography has overcome many of the drawbacks of conventional photographic or video angiography techniques by replacing the incandescent light source with an argon laser of low power to achieve better penetrating power through the lens and clouding the cornea. The frequency of laser radiation is chosen in accordance with the properties of the injected dye, fluorescein or indocyanine green. When the dye reaches the eye, the reflected light exits the pupil on the detector, which measures the intensity of light in real time. As a result, a video signal is created that passes through the video timer and is sent to the video recording device. Then the video is analyzed in an autonomous mode with obtaining such indicators as the time of arterio-venous passage and the average speed of the dye.
Fluorescent scanning laser scanning laser ophthalmoscopic ophthalmoscopic angiography with angiography of indocyanine green
Goal
Assessment of hemodynamics of the retina, especially the time of arterio-venous passage.
Description
Fluorescein dye is used in combination with laser radiation of weakly penetrating frequency for better visualization of retinal vessels. High contrast allows you to see individual vessels of the retina in the upper and lower parts of the retina. At a light intensity of 5x5 pixels, as the fluorescein dye reaches the tissues, areas with nearby arteries and veins are identified. The time of arterio-venous passage corresponds to the time difference at the transition of the dye from the arteries to the veins.
Evaluation of the choroidal hemodynamics, especially the comparison of the optic nerve and macular perfusion.
Description
The indocyanine green dye is used in conjunction with laser radiation of deeply penetrating frequency for better visualization of choroid vasculature. Choose 2 zones next to the optic disc and 4 zones around the macula, each 25x25 pixels. In the analysis of the dilution zone, the brightness of these 6 zones is measured and the time required to achieve the preset brightness levels (10 and 63%) is determined. Next, 6 zones are compared to each other to determine their relative brightness. Since there is no need to adjust due to differences in optics, lens opacities or movement, and all data are collected through the same optical system, where all 6 zones are removed simultaneously, relative comparisons are possible.
Color Doppler Mapping
Goal
Assessment of the state of retrobulbar vessels, especially the eye artery, the central artery of the retina and the posterior ciliary arteries.
Description
Color Doppler mapping is an ultrasound method that combines an image in a gray B-scan scale with a superimposed color image of the blood flow obtained at Doppler-shifted frequencies and pulse Doppler blood flow velocity measurements. To perform all functions, one multifunctional sensor is used. Typically from 5 to 7.5 MHz. Vessels are chosen, and deviations in returning sound waves are used to perform blood flow velocity measurements based on the Doppler equalization principle. These blood flow velocities are depicted as a graph with respect to time, and the peak with a depression is defined as the peak systolic velocity and the final diastolic velocity. The Purscelot resistance index is then calculated to assess the descending vascular resistance.
Pulse eye blood flow
Goal
Assessment of the choroidal blood flow to the systole when measuring the intraocular pressure in real time.
Description
In the device for measurement of pulsatile ocular blood flow, a modified pneumotonomer is used, connected with a microcomputer to measure the intraocular pressure approximately 200 times per second. The tonometer is applied to the cornea for a few seconds. By amplitude of the pulse wave of intraocular pressure, the change in eye volume is calculated. It is believed that pulsation of intraocular pressure - systolic eye blood flow. It is assumed that this is the primary choroidal bloodstream, since it accounts for approximately 80% of the volume of the circulation of the eye. It was revealed that in patients with glaucoma, in comparison with healthy people, pulsatile ocular blood flow was significantly reduced.
Laser Doppler Velosimetry
Goal
Assessment of the maximum velocity of blood flow in large vessels of the retina.
Description
Laser Doppler Velosimetry is a precursor of retinal laser Doppler and Heidelberg retinal flowmetry. In this device low-power laser radiation is aimed at large retinal vessels of the fundus, analyze the Doppler shifts observed in the scattered light of moving blood cells. The average velocity of the blood cells is obtained from the maximum rate, which is then used to calculate the flow parameters.
Retinal Laser Doppler Flowmetry
Goal
Evaluation of blood flow in retinal microvessels.
Description
Retinal laser Doppler flowmetry is an intermediate stage between laser doppler Velosimetry and Heidelberg retinal flowmetry. The laser beam is directed away from the visible vessels to assess blood flow in the microvessels. Due to the random location of the capillaries, only an approximate estimate of the blood flow velocity can be made. The volumetric blood flow velocity is calculated using the Doppler shift frequencies (denote the velocity of the blood cells) with the signal amplitude of each frequency (denotes the ratio of blood cells at each rate).
Heidelberg retinal flowmetry
Goal
Assessment of perfusion in peripapillary capillaries and capillaries of the optic disc.
Description
The Heidelberg retinal flow meter has surpassed the capabilities of laser Doppler cycling and retinal laser Doppler flowmetry. In the Heidelberg retinal flow meter for scanning the fundus, infrared laser radiation with a wavelength of 785 nm is used. This frequency was chosen due to the ability of oxygenated and deoxygenated red blood cells to reflect this radiation with equal intensity. The device scans the fundus and reproduces the physical map of the blood flow of the retina without any difference in the arterial and venous blood.The analysis of the computer program from the manufacturer when changing localization parameters, even minute, gives a large number of options for reading the results. Via pointwise assay developed Glaucoma Research and Diagnostic Center, examined cards large flow area, with a better description. To describe the "shape" of the distribution of blood flow of the retina, Keys and perfused avascular zone designed histogram individual flow values.
Spectral retinal oximetry
Goal
Assessment of the partial pressure of oxygen in the retina and optic nerve head.
Description
To determine the partial pressure of retinal oxygen and the optic nerve head, the spectral oximeter of the retina uses different spectrophotometric properties of oxygenated and deoxygenated hemoglobin. A bright flash of white light reaches the retina, and reflected light returns to the digital camera through the image distributor 1: 4. The image distributor creates four equal illuminated images, which are then filtered into four different wavelengths. Then, the brightness of each pixel is converted to optical density. After removing the camera interference and calibrating the images into the optical density, an oxygenation map is calculated.
The isosbestic image is filtered according to the frequency with which the oxygenated and deoxygenated hemoglobin is identically reflected. The oxygen-sensitive image is filtered according to the frequency at which the oxygenated oxygen is reflected to a maximum, and compared with the reflection of deoxygenated hemoglobin. To create a map reflecting the oxygen content in terms of the optical density coefficient, the isosbestic image is separated by an oxygen-sensitive image. In this image, in more light areas, more oxygen is contained, and the raw pixel values represent the level of oxygenation.