Currently, in the practice of an ophthalmologist, new devices and technologies have begun to appear that can assess structural changes in the optic nerve head (optic nerve disc). Confocal scanning laser ophthalmoscopes (KSLO), optical coherence tomographs (OCT) and scanning laser polarimeters (SLP) have been created. OCT works on the principle of an ultrasound B-scan and evaluates the optical section of the retina. KSLO evaluates the surface of the structure under study and allows you to evaluate the topography of the entire optic nerve disc. This method has proven its diagnostic value and is recommended for use in glaucoma offices, diabetological centers and ophthalmological hospitals. SLP allows you to obtain quantitative data on the state of the retinal nerve fiber layer (tissue that is primarily affected in glaucoma). The work of the SLP is based on the delay (shift) of the phase of the infrared ray due to birefringence as it passes through the layer of retinal nerve fibers. A unique feature of the latest generation of DES is Variable Corneal Compensation (VCC). With this function, the system measures the birefringence of the cornea and then optically cancels this refraction while imaging the retinal nerve fiber layer (RNFL).
This technique has been tested abroad. According to the authors, SLP is more sensitive in early detection of glaucomatous changes even in comparison with automated static perimetry performed by such highly sensitive techniques as white – white and blue – yellow. This technique has also proved to be more sensitive than automated static perimetry when monitoring patients in dynamics.
The analysis of the image of the surface of the optic nerve head and the peripapillary retina obtained during the examination is performed automatically by comparing the data obtained within the calculated circle with the normative database contained in the memory of the device. The calculated circle is located around the optic nerve disc, has a diameter of 3 mm and a width of 0.4 mm. These sizes correspond to the average emmetropic eye.
The scanning results are presented in the form of three images of the investigated part of the fundus within 20°x20°. The color image (Fig. 1-A) allows you to make an initial assessment of the image quality and the possibility of using it for further analysis. The area of the optic nerve head and retina under study is represented by color diagrams and graphs showing the thickness of the nerve fiber layer (RNFL) in different segments. The smallest thickness is colored dark blue, which indicates a lower delay of the infrared ray, the largest is bright red (long delay) (Fig. 1-B). Normally, the image is bright yellow and red in the upper and lower sectors, green and blue in the nasal and temporal sectors.
Figure 1-B shows the deviation from the normal circuit, i.e. comparing the patient’s RNFL thickness with the values contained in the regulatory database. On a black-and-white image of the fundus, colored squares show the degree of deviation of the results of this study.
For each eye studied, a TSNIT graph is presented, where the shaded area denotes the normal range. The values obtained from a specific patient are indicated by a dark line (Fig. 1-D). The analysis of the graph is based only on the data within the calculated circle. The plotting of the graph begins from the temporal side, and then the results of the upper, nasal, lower and again temporal positions are entered.
The results of polarimetric laser scanning are summarized in the table (Fig. 2). Reflected are the values of the average thickness of the layer of retinal nerve fibers within the calculated circumference (TSNIT Average); the average value in the area of the upper (Superior Average) and lower (Inferior Average) sectors within 120°; standard deviation from the mean (TSNIT Standard Deviation); correlation of values of symmetric points for OD and OS (Inter – Eye Symmetry); Nerve Fiber Condition Indicator (NFI), indicating the likelihood of glaucoma. The NFI value can range from 0 to 100. Moreover, the larger the numerical value of the result obtained, the higher the likelihood of glaucomatous damage to the optic nerve.
In modern ophthalmology, there are a number of diagnostic methods aimed at early detection of glaucoma and allowing monitoring of visual functions in such patients.
Structural changes in the optic nerve head and retina in glaucoma are expressed by the violation of all visual functions. At the same time, the most informative and accessible at present is the study of the visual field. Early signs of glaucoma development are focal scotomas in the central visual field, which, as the process progresses, merge into an arc paracentral scotoma (in the Bjerrum zone), separate or associated with a “blind” spot. In a number of patients, changes in the visual field can be manifested by a general decrease in the light sensitivity of the retina. Central and/or peripheral nasal rungs characteristic of glaucoma are often associated with paracentral scotomas.
At the present stage of development of medical technology, computer perimetry makes it possible to detect the described changes in the visual field in the early stage of glaucoma. The most advanced computer perimeters currently are the Humphrey and Octopus field of view analyzer. They allow screening and threshold studies of the central and peripheral visual fields using static and kinetic perimetry.
In our country, the method of color static campimetry has been used to detect early glaucoma for more than 10 years.
Since 2003, information has appeared about the use of the automated measuring and computing complex (IVK) “Campi”, developed by LLC “ASoftXXI” in cooperation with the academic group of the academician of the Russian Academy of Medical Sciences prof. A.P. Nesterov and the Department of Eye Diseases of the Medical Faculty of the Russian State Medical University. Measuring and computing complex “Campi” is designed to study the central and individual sections of the peripheral field of view using a personal computer monitor as a measuring device. IVK can be used to study the visual field in patients with diseases of the optic nerve and retina in hospitals and at home, as well as to create and maintain a single database for all patients who have ever been examined. Depending on the tasks set, it is possible to carry out several options for examinations for each eye. In most cases, the threshold research strategy is taken as a basis. The proposed technique has proven its effectiveness in monitoring patients with glaucoma.
In the eye clinic at the Department of Eye Diseases of the Faculty of Medicine, the scanning laser system GDx VCC was tested.
Objective of the study: to evaluate the results obtained using polarimetric laser scanning of the optic nerve head and peripapillary retina; to determine the purpose of the scanning laser polarimeter GDx VCC in a number of diagnostic methods aimed at detecting glaucoma; to compare the diagnostic value of polarimetric laser scanning, perimetry using the G2 Octopus program and computer campimetry (Threshold – 120 and Threshold – 83) performed on the automated measuring and computing complex “Campi”.
Materials and Methods The study involved 33 people (56 eyes) aged 37 to 75 years (mean age 59 years). Among the surveyed, the main group consisted of patients with glaucoma in the initial (19 eyes), advanced (12 eyes) and advanced (11 eyes) stages and the control group (14 eyes) who did not suffer from ophthalmic diseases. All patients underwent standard ophthalmological examination, scanning laser polarimetry GDx VCC, dG2 perimetry (Octopus 101) and campimetry according to the Threshold-120 and Threshold-83 programs (IVC “Campi”).
Polarimetric laser scanning allows you to estimate the average thickness of the retinal nerve fiber layer within the calculated circumference (TSNIT Average); in the area of the upper (Superior Average) and lower (Inferior Average) sectors within 120 ° (Table 1; histogram 1).
The results of the study show that as the glaucoma process progresses, the thickness of the layer of nerve fibers decreases. Differences in all studied values in groups of II and III stages of glaucoma compared with control (p <0.5%, p <0.1%, respectively) are significant. The difference in mean values in the control group and with stage I glaucoma is insignificant.
Thus, polarimetric laser scanning is informative in the diagnosis of advanced and advanced stages of glaucoma. However, one cannot exclude the fact that with an increase in the number of observations, the information content of the SLP method in the diagnosis of initial glaucoma will increase.
A comparative analysis of the study of the layer of nerve fibers by scanning laser polarimetry, computer perimetry using the G2 program (Octopus 101) and computer campimetry – Threshold-120 and Threshold-83) (IVK “Campi”) (Fig. 3). The research results are presented in table 2.
Comparative analysis of the data showed that in the diagnosis of the initial and advanced stages of glaucoma, the methods of computerized perimetry G2 (Octopus 101) and campimetry Threshold – 120 (IVK “Campi”) have a higher sensitivity.
Conclusions
- Scanning laser polarimetry allows you to obtain an accurate mathematical calculation of the thickness of the layer of nerve fibers in the peripapillary region within the calculated circle.
- The most informative data are the data of the scanning laser polarimeter GDx VCC in advanced and advanced stages of glaucoma.
- Scanning laser polarimetry is less sensitive for detecting initial glaucoma. In the early diagnosis of glaucoma, SLP is recommended to be used as an additional method with a mandatory study of the central visual field by perimetry and / or campimetry methods.
The original article was published on the website of RMJ (Russian Medical Journal)