Psychophysical methods of studying intraocular pressure in glaucoma

In a broad sense, psychophysiological testing refers to the subjective assessment of visual function. In a clinical sense, for a patient with glaucoma, the term refers to perimetry to assess the peripheral vision of the eye. Given the earlier onset of peripheral vision impairment in glaucoma compared to central vision, visual field assessment is useful for both diagnostic and therapeutic purposes. It is important to note that the use of the term peripheral vision does not always imply the far periphery. In fact, most visual field defects in glaucoma occur paracentrally (within 24° of the fixation point). The term peripheral vision should be understood to mean everything except central fixation (i.e., more than 5-10° from the center).

The information presented is intended to demonstrate representative models of the visual fields in glaucoma and does not provide a comprehensive discussion of perimetry. There is literature devoted exclusively to a more detailed description of perimetry, as well as atlases of perimetric data.

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Diagnostics

Automated monochromatic visual field testing as part of the initial evaluation of a patient suspected of having glaucoma is important in the diagnosis of glaucomatous optic nerve damage. Visual field abnormalities are important for localizing lesions along the entire optic tract from the retina to the occipital lobes of the brain. Glaucomatous visual field defects are usually associated with optic nerve damage.

It is very important to note that so-called optic nerve field defects (i.e. defects resulting from damage to the optic nerve) are not diagnostic of glaucoma by themselves. They should be considered in conjunction with the characteristic appearance of the optic nerve and the anamnesis. Intraocular pressure values, gonioscopy results, and anterior segment examination data can help to determine the specific type of glaucoma. All optic neuropathies (anterior ischemic optic neuropathies, compressive optic neuropathies, etc.) lead to the formation of optic nerve field defects.

It is also important to note that the absence of optic nerve field defects does not exclude the diagnosis of glaucoma. Although automated achromatic static visual field testing was established as the “gold standard” for assessing optic nerve function in 2002, the sensitivity of this method for detecting ganglion cell loss is still limited. Clinical and experimental data indicate that the earliest visual field defects detected by this method correspond to a loss of approximately 40% of ganglion cells.

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Introduction

Automated achromatic static visual field testing in conjunction with serial optic nerve assessment remains the "gold standard" of glaucoma monitoring. To protect the optic nerve from the damaging effects of elevated ophthalmostatus, scientists try to achieve a target intraocular pressure. The target intraocular pressure is an empirical concept, since its level must be determined independently. Automated achromatic static visual field testing and serial optic nerve assessment are the ways to determine whether the empirically achieved pressure level is effective in protecting the optic nerve.

Description

Perimetry is needed to determine the limit of vision at a certain location in the visual field. The limit of vision is defined as the minimum level of light perceived at a given location in the visual field (retinal sensitivity). The limit of vision differs from the lowest level of light energy that stimulates the photoreceptor cells of the retina. Perimetry is based on the patient's subjective feeling of what he or she can see. Thus, the limit of vision is "psychophysical testing" - a certain level of cognitive and intraretinal perception.

The highest limit of vision is characteristic of the central visual fovea, which is the center of the visual field. As we move to the periphery, sensitivity decreases. A three-dimensional model of this phenomenon is often called the "hill of vision." The visual field for one eye is 60° upward, 60° nasally, 75° downward, and 100° temporally.

There are two main methods of perimetry: static and kinetic. Historically, various forms of kinetic perimetry were developed first, generally they are performed manually. A visual stimulus of known size and brightness is moved from the periphery, beyond the limits of vision, to the center. At a certain point, it passes the point when the subject begins to perceive it. This is the limit of vision in this place. The study continues with stimuli of different sizes and brightness, creating a topographic map of the "island of vision". Goldmann attempted to create a map of the entire field of vision.

Static visual field testing involves presenting visual stimuli of varying sizes and brightness at fixed points. Although there are many different methods for determining the limit of vision, most follow a basic principle. The examiner begins the perimetry by presenting stimuli of high brightness, then presents stimuli of lower brightness at certain intervals until the patient no longer sees them. The test is then usually repeated, presenting stimuli of gradually increasing brightness at shorter intervals until the patient again no longer perceives the stimulus. The resulting brightness of light is the limit of vision in that area of the visual field. In general, static visual field testing is automated; white stimuli are presented on a white background, hence the name of the method - automated achromatic static visual field testing. There are many devices that perform this test, including the Humphrey (Allergan; Irvine, CA), Octopus, and Dicon. In our work, we prefer the Humphrey device.

Many research algorithms have been developed, such as the full vision limit, FASTPAC, STATPAC, Swedish Interactive Vision Limit Algorithm (SITA), etc. They differ in duration and slightly in terms of the depth of the visual field defect.

Common visual field defects found in patients with glaucoma

In glaucoma, the defects are located in the optic nerve and focally in the cribriform plate. When examining the visual fields, their defects have relatively specific manifestations, which is associated with the anatomy of the retinal nerve fiber layer. This layer consists of axons of ganglion cells and projects through the optic nerve to the lateral geniculate nucleus.

The axons of ganglion cells nasally to the optic disc pass directly into the disc; lesions of the optic nerve affecting fibers from this region produce a temporal wedge defect. The axons of ganglion cells temporal to the optic nerve curve into it. The line across the central optic fossa and the optic nerve is called the horizontal suture. Ganglion cells above this suture curve superiorly and send fibers into the supratemporal region of the optic nerve. The fibers of ganglion cells temporal to the optic nerve and below the horizontal suture have the opposite direction.

Lesions of the optic nerve that affect fibers from the region located temporally to the nerve simultaneously produce nasal steps and arcuate defects. Nasal steps are so named not only because of their nasal localization, but also because such defects are located in the region of the horizontal meridian. The horizontal suture is the anatomical basis of these defects. Arcuate defects are so named because of their appearance. Nasal steps and arcuate defects are much more common than temporal wedge-shaped defects. As glaucoma progresses, multiple defects can be found in the same eye.