If you do not have a test to assess central visual field, you can make it yourself. If you use the same place of a room with the same illumination each time, the visual field measurements are comparable over time.
For the measurement on white paper you can arrange a low luminance level or a high luminance level or both. Record carefully the lamp, its distance from the test and position; i.e. in which direction the lamp is directed. If you make the measurement on a black, non-glossy paper you will have a third kind of visual field. These three visual fields give you a lot of information on the function of the central visual field at different luminance levels. Combined with the visual acuity and contrast sensitivity values that you can measure at 80 cd/m2 and 3 cd/m2 with the ESV1500 lightbox, you have a good battery of tests for detection of high contrast black-and-white stimuli in the central visual field. Add grating acuity, a quantitative colour vision test, and motion perception tests and you have more information for functional assessment than most university hospitals use.
You can measure the size of the visual field with the LEA Flicker Wand and VIV test, which can be used e.g. to measure the size of a ring scotoma in the mid-peripheral visual field of a child who has retinitis pigmentosa (RP) like changes in the retina. In such a case it is also important to learn more about the central visual field, especially in countries where RP causes patchy loss of central visual field quite early.
Figure 1. The test idea for measurement in photopic luminance conditions. The location of the edge of the blind spot has been marked with a thin line. In the pdf, the copy that is used in the measurements does not have the lines in the central part of the test because the lines would disturb the detection of the test dot.
Markings on the floor:
Patient’s ocular motor functions, nystagmus:
Figure 2. Recording of location of a small scotoma on the 0 meridian close to the point of fixation and the edge of the blind spot.
B. Measurement of the size of scotomas, Figure 3:
Figure 3. Recording the scotoma with its less damaged area around the absolute scotoma. The measurement was made with the test where the lines are not visible in the centre. The result is here drawn on the paper with the meridian lines to show the location of the scotoma.
Now you have measured the “relative scotoma” around the “absolute scotoma”. The absolute scotoma may not be so total that a bright dot of light would not be detected inside it but it is absolute to black details like parts of letters. However, because the eyes move and the brain interpolates over empty areas of the visual field, the child does not see the area with no information in it. It may cause distortion of lines or distortion in the text with some letters higher than other or disappearance of parts of words, which causes reading errors.
If the scotoma is on the right side of the fixation of the visual field it affects reading in countries where the text is read from left to right. If the scotoma is on the left of the fixation it disturbs reading when the direction of reading it from right to left. Knowing the form of the scotoma you can advice the patient to tilt the text so that the scotoma moves below the line to be read.
A case report:
The student had earlier had good binocular central vision with a preferred right eye, no strabismus and some peripheral retinal changes typical to retinitis pigmentosa, which in that Arab country often causes loss of central vision.
During a control visit the student had slightly eccentric fixation of both eyes to the right and down during binocular viewing. The right eye had 2 lines lower visual acuity value than the left eye. In the binocular situation visual acuity was that of the right eye. The child preferred using the eye with the lower visual acuity. There was a fine nystagmus in the regular room light.
When we arranged the luminance level, nystagmus disappeared at 5 to 6 cd/m2, which is at the edge of photopic and mesopic luminance level (the colours were barely perceptible still). This revealed that there was a relative loss of cone cells so that their activity did not prevent the rod cell activity from entering retinal function. At the usual room light level, the rod cells were overstimulated and disturbed the image. At the lower luminance level they functioned normally and, thus, could function at the same time as the cone cells (as in twilight vision). This important observation is generally not known. Children with retinitis pigmentosa-like changes are thought to see best in daylight.
The first finding occurred at the beginning of the test when the student was asked to look with the right eye at the point where the lines would cross if they had been drawn through the middle. The student said “but then the cross disappears”. This told us that the scotoma extended to the very middle of the right visual field, so the 3 mm dot was placed in the middle and disappeared there. However, if it was moved a little to the left, its edge became visible. It could be moved a little bit more to the right before the edge became visible. The edge of the scotoma was the same with the smaller dot; i.e. sharp.
When the right eye was covered and the student was asked to look at the point where the lines would cross, the student told us that the X was clearly visible. If the student fixated on either side of the X, it disappeared. Fixation above or below X showed a little bit more extension of the tiny central field. The scotomas in the left eye were larger than in the right eye and, especially on the left side of fixation, caused a “hole” in the central reading field at the site of the eccentric fixation of the right eye (Figure 4). Therefore, the right eye had better quality of field for reading.
The results of the visual field measurements explained why the eye with the lower visual acuity was preferred. It had a better central field for reading than the left eye (Figure 4).
Figure 4. Small scotomas in the centre of the visual field in both eyes drawn on the recording sheet with the lines so that comparison of the two central field defects is easier. In the right eye the scotomas are in the foveal area and to the right of fixation. The scotoma of the left eye caused an empty space in the central reading field left to the fixation mark, which did not disturb the right eye’s image. When reading Arabic, changes in the central field, left to the fixation area, disturb planning of saccades and make characters to disappear.
Figure 5. The stimulus dots are seen in the correct size on the pdf . Preferably print the sticks on a thick non-glossy paper. The small dots are for assessment of nearly normal central visual fields; the large dot for peripheral visual field and for assessment of severely damaged central visual field. Printable black version.
The size of the scotomas is likely to be larger than what can be recorded with this simple technique. However, this technique can provide information that makes it easier to describe the reading situation to parents, teachers, and therapists. The loss of central visual field advances continuously and thus children with this type of retinitis pigmentosa change their reading technique often and, where possible, soon prefer to use a CCTV, which makes the text so large that small scotomas disappear.
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