The Amsler Grid is a square-shaped grid used to detect or monitor metamorphopsia or scotoma involving the central visual field in various disorders of the macula and optic nerve head. Specifically, it is used as an inexpensive home monitoring device for early detection of metamorphopsia due to wet age-related macular degeneration. Also, it may be used as a macular function test before cataract surgery.
The grid gets its name from Marc Amsler (1891-1968), a Swiss ophthalmologist, who described the chart in 1947. He was a student of Jules Gonin (1870-1935). He might have gotten the idea of the grid from Edmund Landolt (1846-1926), who devised a similar small card with a grid pattern to be kept in the center of the perimeter. Jacob Hermann Knapp (1832-1911) described scotoma and metamorphopsia in traumatic maculopathy with choroidal rupture using horizontal lines in 1869. Richard Forster (1825-1902) demonstrated metamorphopsia using a square grid in 1874.
The location of the fovea is approximately two disc diameters (3 mm on average) temporal to the temporal margin of the optic disc. The physiological blind spot on the visual field corresponds anatomically with the optic disc. In the visual field, the blind spot is located usually at 15 degrees temporal to the point of fixation (anatomically corresponding to fovea). The shape of the blind spot is vertically oval measuring 7.5 degrees vertically and 5.5 degrees horizontally. Another anatomically important fact is that the density of cones at the center of the fovea is maximum, extrapolated to around 150000 to 180000 cells/mm, which reduces rapidly to 6000 cones/mm at 1.5 mm from the fovea, which leads to the best visual acuity at the fovea.
The Amsler grid is valid in the following scenarios:
There is no contraindication to using Amsler grid. There should be a reasonable vision level to discern the lines of the grid. However, the sensitivity to detect wAMD changes may be low, and the Amsler grid does not replace the need for ophthalmic examination. As the grid evaluates only 10 degrees temporal to fixation, the physiological blind spot is not appreciated in Amsler grid. Glaucomatous visual field defects may not be detected in Amsler grid until the disease is advanced and field defect reaches close to fixation.
The grid measures 10 cm × 10 cm. When used at 33 cm from the eye, the grid can check for changes in a 20° field. This field spans 10° on each side (above, below, and on the left and right side) of the fixation.
There are 7 charts of the Amsler Grid each with a dimension of 10 cm × 10 cm.
The grid is kept at 33 cm from the eye. The illumination of the chart should be good. Light exposure directly over the eye (in the form of slit lamp examination, indirect ophthalmoscopy, and others) should be avoided immediately before using the grid to avoid a photostress effect. The patient wears near correction if any. Pupils should not be dilated. The patient is asked to close one eye. With the open eye through the near glasses, the patient is instructed to look at the central dot of the grid. In patients with poor fixation, the eye should focus at the center of the grid so that all the 4 corners are simultaneously visible. While keeping the eye fixated over the center of the grid, the patient has to appreciate is the lines are parallel or seem distorted (metamorphopsia). Also, he/she should note if some of the small squares are missing or blurry. The patient should mark the grid with the area of abnormality so that it can be compared later for any progression, stabilization, or improvement. If the patient can not see the corners, diseases like glaucoma and retinitis pigmentosa should be ruled out. Amsler grid should be checked at least once weekly.
Alternative techniques to document metamorphopsia include:
Other techniques/instruments which may be used to document scotoma include-
Amsler grid is an important simple tool to detect or monitor macular diseases.
Metamorphopsia- Macropsia is denoted by the widening of the small squares compared to the nearby small squares. The parallel lines seem to be curved away from each other at this area. In the area of micropsia, the parallel lines seem to be drawn towards each other.
Scotoma- Merriam Webster dictionary defines scotoma as 'a spot in the visual field in which vision is absent or deficient.' A scotoma is surrounded by an area of the normal field of vision. A scotoma may be absolute (absent visual recognition of that area in spite of increasing the stimulus intensity) or relative (the stimulus presented at the scotoma becomes visible if stimulus intensity is increased). In Amsler grid, the relative scotoma appears as a veil partially obscuring the smaller squares and may be of different shapes. Such scotoma may be better appreciated if the illumination of the grid is reduced. Scotomas may be positive (something like spot or haze or smoke obscures central vision) or negative (the patient is unaware of scotoma unless tested).
Amsler grid home monitoring has various limitations including
The modifications of the Amsler chart include:
The nurses or optometrists who are involved in patient care should be taught how to handle the instrument properly without any damage. Also they should know how to give proper instructions to the patient so that the final result will not be ambiguous. It is also important to instruct the nurse not to apply any mydriatic in the eye prior to the test.
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