Overview of Amsler Grid
The Amsler Grid is similar to a checkerboard used by eye doctors to identify early signs of eye conditions, such as age-related macular degeneration (AMD) which affects sight. The grid is also helpful for individuals to keep an eye on their vision at home. The grid is particularly useful for catching changes in vision that may not have obvious symptoms at first.
Named after Marc Amsler, a Swiss eye doctor who introduced it in 1947, it’s a simple, cost-effective tool that can detect vision problems caused by disorders of the macula and optic nerve head. The macula is a part of the eye responsible for central vision, and the optic nerve transmits visual information from the eye to the brain.
The Amsler Grid is also sometimes used to assess the function of the macula before cataract surgery. Cataracts occur when the lens in your eye, which is normally clear, becomes cloudy, impairing vision.
In the past, similar measuring tools were used to describe scotoma (blind spots in your vision) and metamorphopsia (when straight lines appear wavy, which is usually due to problems with the macula), going as far back as 1869. The Amsler Grid has carried on this valuable method of eye-health monitoring.
Anatomy and Physiology of Amsler Grid
The fovea, a spot in the eye where vision is sharpest, is found about two disc diameters (or about 3 millimeters) to the side of the optic disc, the spot where the optic nerve leaves the eye. The blind spot in our vision, a point where we can’t see anything, aligns with the optic disc. In our field of view, this blind spot generally appears about 15 degrees off from where we are directly looking. The shape of this blind spot is an upside-down egg, measuring 7.5 degrees in height and 5.5 degrees in width. The center of the fovea has the highest density of cone cells (the cells responsible for detecting color and fine detail), which can go up to 150,000 to 180,000 cells per square millimeter. This density drops significantly to just 6,000 cones per square millimeter only 1.5 millimeters away from the fovea.
Metamorphopsia is a condition where your vision is distorted – that means objects may look twisted or incorrectly sized. There are a few different factors that might cause this distortion:
One possibility is that the photoreceptors, cells in the eye which sense light to create an image, are not aligned properly and misrepresent where an image is located.
Another cause could be a disorder called epiretinal membrane, where a film forms over the retina causing metamorphopsia. This happens due to the thickening of the Inner Nuclear Layer (INL), a layer in the retina that plays vital role in image formation. This abnormal thickening may disrupt the functions of the cells in this layer, making the photoreceptors less sensitive and hence causing metamorphopsia. Furthermore, if the Ganglion Cell Layer (GCL), another layer of the retina, thickens along with the INL, it could distort the retina and degrade the quality of vision.
Lastly, other processes in the brain that interpret vision might be involved in metamorphopsia. This includes the brain’s processing of visual information, and the characteristics of the object or scene being viewed may play a role.
Why do People Need Amsler Grid
The Amsler grid is a tool that can help doctors detect some vision problems caused by damage to the macula (the central part of the retina) or the optic nerve. Here are a few scenarios where it’s used:
* Wet age-related macular degeneration (wAMD): This is a condition where abnormal blood vessels grow under the retina. These vessels can leak fluid and blood, which causes blurred or distorted vision and blind spots.
* Central serous chorioretinopathy (CSCR): This is a condition where fluid builds up under the retina. This can distort vision and create a kind of a blind spot in the middle of your field of vision. The shape of this spot can be round or oval, depending on the shape of the area where the retina is lifted.
* Epiretinal membrane and other diseases affecting the vitreous (the gel-like substance filling the eye) and the retina: These can cause metamorphopsia, a vision distortion where straight lines appear wavy or curved.
* Acute macular neuroretinopathy: This condition causes damage to the macula, the central part of the retina. It’s known for symptoms like rapid vision loss and seeing unusual shapes in the central visual field (like flower petal-like blind spots). These symptoms become more noticeable in certain types of retinal imaging like near-infrared (NIR) or multicolor imaging.
* Cystoid macular edema: This condition causes swelling in the macula, the central part of the retina. This can make objects appear smaller (micropsia), among other visual changes. and can occur due to conditions such as diabetic maculopathy, retinal vein occlusion, inflammation of the middle layer of the eye (intermediate uveitis), and other diseases.
The Amsler grid can also reveal changes in vision in conditions affecting the optic nerve like non-arteritic anterior ischemic optic neuropathy, where it might show partial vision loss. It can be used in cases of pituitary tumors where it may demonstrate a vision loss that affects both eyes and occur on the outer halves of the field of vision (bitemporal hemianopia). Usage of the Amsler grid is currently not recommended for detecting vision changes caused by retinal damage from the medication hydroxychloroquine.
When a Person Should Avoid Amsler Grid
There are no specific reasons to avoid the use of an Amsler grid, a tool used to monitor your vision at home. However, you need to have a certain level of eyesight to see the lines on the grid. While it’s a useful tool, it doesn’t have a high sensitivity for picking up changes in wet age-related macular degeneration (wAMD), a condition that affects your central vision.
Additionally, the Amsler grid doesn’t replace the need for an eye exam. This is because it only looks at a specific part of your field of vision, meaning it doesn’t cover the natural blind spot in your vision.
Furthermore, visual field defects caused by Glaucoma, which is a condition that damages your optic nerve, might not show up on the Amsler grid until they’re pretty advanced and reach towards the center of your field of vision.
Equipment used for Amsler Grid
The Amsler Grid is a tool that your doctor uses to check your eyes. It’s a square that measures 10 cm by 10 cm, and it’s used at a distance of 33 cm from your eye. This helps your doctor to see how your eyes are working in a field of 20 degrees, which means they check how your eyes work when looking straight ahead and about 10 degrees to each side and up and down.
There are seven different versions of the Amsler Grid, all the same size of 10 cm by 10 cm. Let’s explain each one:
1. Chart 1: This grid has white lines against a black background. There are 20 little squares along each side of the grid, which makes each little square measure 5mm on a side. Each square lets your doctor check one degree of your field of vision when the grid is 33 cm from your eye.
2. Chart 2: This grid has four white diagonal lines added on top of the first chart. This helps doctors check an eye that may have a place where you can’t see (central scotoma).
3. Chart 3: This one is like the first chart, but has red lines instead of white ones, against a black background. This particular chart helps doctors to spot subtle issues with your ability to see red. These issues can happen with certain conditions like tumors or damage from toxins.
4. Chart 4: This one has a black background with no lines; instead, it features a big white dot in the center and many small random white dots elsewhere. This helps doctors tell apart different types of vision problems, as there are no lines to distort the image.
5. Chart 5: This one displays a square with 21 white lines spread 5mm apart on a black background. It has a white dot at the center for you to focus on. This is specifically used to find any issues along any certain direction, particularly for people who have trouble reading.
6. Chart 6: This one is altered from chart 5 having black lines on a white background. The lines in the center are closer together than in chart 5. This helps doctors check for small vision issues near where you are looking.
7. Chart 7: This chart is a version of the first chart but it has a central area of smaller squares which are only half-degree big. This helps doctors find any small spots of vision problems or distortions near where you’re looking.
How is Amsler Grid performed
The process being described involves examining someone’s eyes using a chart called an Amsler grid, which helps identify any irregularities in their vision. This commonly used grid is placed about 33 cm (approximately a foot) from the patient’s eye. It is critical to ensure that the room has balanced lighting, and bright lights directly shining into the patient’s eye should be avoided beforehand. This is to prevent any temporary changes in vision due to those bright lights, which can affect the test results.
If the patient needs glasses for reading, they should wear them during this test. Their pupils should be in their natural state, and not expanded by any medications. The patient will look at the grid with one eye at a time. They will be instructed to focus on a central dot. If it’s hard for them to keep their gaze fixed, they should make sure that they can see all four corners of the grid at the same time. This ensures that their entire visual field is being assessed.
While looking at this central dot, the patient needs to notice whether all the lines are straight or if they appear distorted. They should also look out for any missing or blurry squares. Any abnormalities should be marked down so they can be compared later and checked for changes. If someone can’t see the corners of the grid, this could signify diseases like glaucoma or retinitis pigmentosa, which affect the peripheral vision. To keep track of changes in vision, it’s a good practice to repeat this eye exercise once a week.
There are also alternative tests to detect distortions in visions, such as the M chart, Preferential Hyperacuity Perimeter or Foresee Home, PreView PHP, Shape Discrimination Hyperacuity, Differential perimetry, and Microperimetry. These devices are typically used to detect slight changes in how straight lines appear to the viewer or imagined differences in position between two points that are super close together, which can signal damage to the eye’s macular (the part of the eye responsible for sharp, central vision). This damage often occurs due to diseases like age-related macular degeneration or diabetic eye disease. But these devices should be used carefully in patients with cognitive disorders or those having difficulty in taking tests.
These eye examinations provide crucial data to eye specialists so they can monitor, diagnose, and treat vision problems early in their progression. Patients can also be more engaged in their eye health by understanding how these tests work and the importance of taking them regularly.
What Else Should I Know About Amsler Grid?
The Amsler grid is a simple tool that can help identify or keep track of diseases affecting the macula of the eye. The macula is a part of the eye that helps us see detail. You might encounter two main issues on the Amsler grid: Metamorphopsia-Macropsia and Scotoma.
Metamorphopsia-Macropsia is when the small squares on the grid look wider compared to the others. The lines that are supposed to be straight might appear as if they’re bending outwards or towards each other. This represents a change in your perception of size or shape.
A scotoma is a blind spot or an area of impaired vision surrounded by normal vision. In other words, it’s like having a ‘spot’ in your field of vision where you can’t see properly or at all. On the Amsler grid, a relative scotoma appears like a veil partially covering the smaller squares, and it may come in different shapes. Scotomas can be either positive, where you see spot or haze blocking your central vision, or negative, where you’re not aware of the blind spot unless you’re tested specifically for it.
However, using the Amsler grid at home does have its limits. For example, it doesn’t measure how well you’re focusing on it, it doesn’t quantify metamorphopsia (distortion of vision), and it’s not interactive. Moreover, it may not be as effective in detecting and mapping scotomas, and you will need decent near-sighted vision to accurately see the grid.
Please note that your results may vary when using the Amsler grid repeatedly. This is due to the ‘perceptual completion phenomenon’, which is when your brain ‘fills in’ the missing information when part of an image lands on a blind spot in your visual field. It’s why we normally don’t notice our own blind spot caused by the optic nerve of the eye.
There have been modifications to the Amsler chart to make it more effective. For instance, they’ve tried lowering the brightness of the white grid on a black background, which may help highlight scotomas. Using red grids has also been found to better highlight changes.
A different kind of Amsler chart developed by Shinoda and colleagues has a grid of black lines on a white background. It asks the patient to trace the lines as they see them, and the length of these distorted lines drawn by the patient is then measured. Other modified versions of the Amsler grid include ones with sine waves replacing lines, and those showing cushion or barrel-like distortion.
