![]() ![]() If the threshold at each retinal point was evaluated in 1 dB steps, it would take a lot of time, and the process will tire out the patient easily. Then the intensity is increased by 2 dB till it is seen, and then the intensity is reduced again by 1 dB till it is not seen. Īlternatively, the intensity of a seen stimulus is reduced by 4dB till it is not visible. Then the intensity is increased by 1 dB till again it is visualized. Once it is visible, the intensity is reduced by 2 dB steps till it is again not visible. If the initial stimulus is not seen, then the intensity of the stimulus is increased by 4 dB steps until it is seen. There can be two ways to use this method. The threshold is measured by using the staircase method "4-2-1". Threshold: It is the intensity of the light stimulus, which, when presented at a particular location, "n" number of times is detected by the corresponding retinal point at least 50% of the time. A 40 dB reading represents one apostilb on the Humphrey visual Analyzer, whereas it represents 0.4 apostilbs on the Octopus perimetry. A 0 dB reading represents 10000 apostilbs on the Humphrey visual field analyzer, whereas it represents 4000 apostilbs on the Octopus perimetry. The decibel value is relative, so it varies from machine to machine. So a zero decibel stimulus will be visible to a point on the retina with the lowest sensitivity and vice versa. Zero decibels (dB) represent the brightest light stimulus, and 50 dB represents the dimmest stimulus. ![]() It has a direct correlation to the sensitivity of the retina. Decibel is the logarithmic representation of the intensity of the light stimulus. Therefore, the sensitivity to light is measured in decibels (0-50 in standard automated perimetry). Also, the human eye responds to a wide range of apostilbs. However, apostilb and retinal sensitivity are inversely proportional to each other. The intensity of the light stimulus is measured in apostilb. The sensitivity of the retina of a healthy individual is highest at the fovea and then reduces towards the periphery. Measurement of visual field or perimetry: The measurement of the retina's sensitivity to light is shown at a given location in the visual field printout. The blind spot is located 10 to 20 degrees temporal to the point of fixation. Mariotte was the first one to report that the physiologic blind spot corresponds to the location of the optic disc. Binocular visual fields extend temporally to 200 degrees with a central overlap of 120 degrees. The temporal extension is up to 100 degrees, and the inferior extent is up to 80 degrees. This consists of the central 30 degrees field and extends to 60 degrees superiorly and medially. The extent of a uniocular visual field was given by Thomas Young. The sensitivity decreases as we move to the periphery of the retina. The apex of the hill of vision is the area with the highest retinal sensitivity and represents the fovea. Harry Traquair defined it as "an island of vision surrounded by a sea of blindness," also known as the hill of vision, considering its three-dimensional aspect. Visual field: The visual field is the extent of an area visible to an individual during steady fixation of the eye in any one gaze or direction. Tate were the first ones to automate the process of perimetry. Jannik Bjerrum used a tangent screen to map the details of the central 30° diameter of the visual field. In subsequent years, Hans Goldmann developed a bowl perimeter that provided a uniform background illumination and a moving optical projection system that could superimpose bright stimuli on the background. The field defect corresponds to the arcuate-like arrangement of the axons emerging from the optic disc. Landesberg was the first to describe the arcuate defect of glaucoma. However, he attributed the field defects at that time to amblyopia due to a lack of knowledge about glaucoma. Successively, Ulmus was the first to publish the first illustration of visual fields. Albrecht von Graefe was the first one to publish visual field defects that are characteristic of glaucoma as known today. This article discusses the process of perimetry and the interpretation of the visual field printouts (Humphrey visual field Analyzer). The history of visual field analysis dates back 2000 years, when Hippocrates first reported a case of hemianopsia. It helps in diagnosing glaucoma and assessing the progression of glaucoma. Perimetry is an important diagnostic tool to identify and measure the extent of defects in the visual field. ![]()
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