Corneal astigmatism is the most common refractive error and unfortunately the most misdiagnosed.
Some specialists simplify the diagnosis to nearsightedness or farsightedness, leaving astigmatism uncorrected and forcing the concept of spherical equivalence to the maximum. This concept applies only in certain cases.
Causes of diagnostic errors:
In a simple refractive error we have 3 variables for each eye: the spherical value of the diopters, the interpupillary distance and the fitting height, in a compound astigmatism we have 5 variables, the 3 from the spheres plus their cylindrical values and axes. This will complicate things for a patient with astigmatism, these 10 variables can be wrong from the consultation (different testing methods) or when fitting the glasses. For this reason, in some cases, specialists choose to simplify the diagnosis by spherical equivalence to astigmatism with medium or low values.
The consequences of these mistakes:
Whenever the eyeballs do not work properly (the image does not form on the retina due to a dioptric need or improper optical correction), the ocular lenses will constantly adjust, trying to compensate for the lack or surplus of the diopter (secondary function of the ocular lenses). This is the main cause of eye discomfort (in addition to eye strain in the near environment without breaks) and various symptomatic conditions resulting from these causes: dizziness, headache, dry eye and itching, redness of the eyeball, etc.
For a correct approximation, the specialist must use the axes from degree to degree (or maximum every 5 degrees) and the cylindrical power of 0.12 or the classic 0.25.
Being a lifelong static refractive error, it is very easy to check our specialist by comparing the cylindrical values and the axes on the previous recipes.
These values must be the same for any specialist, on all recipes and throughout life!
If these values differ from one specialist to another, then one of them is wrong, or in the meantime you had a corneal trauma!
For the people that are wearing glasses, we check if the glasses have the necessary cylindrical correction and for the people that are not wearing glasses, we check if they have astigmatism.
The test is performed with each eye separately (astigmatism can occur monocularly) and then with both eyes(optional).
To check if we have an uncorrected astigmatism, we can use the following test (full-screen):
We sit at a 50cm -2m distance (depending on the size of the display: mobile / desktop) and cover one eye.
We try to notice a difference between the lines that have an angle of 90 degrees between them (horizontal versus vertical or oblique but with an angle of 90 degrees between them).
We repeat the test for the other eye and note which lines are more accentuated in each eye (in case there are 90 degrees differences between the lines).
If you look at the test with one eye and rotate the head or display used, you will notice that both clear and blurred lines will move in the same direction as your movement.
If the values of astigmatism are below 0.75 diopters (0.25 – 0.50) then the differences between the lines will be difficult to notice (up to about 25 years old). The higher the values of the cylinders, the more the differences will increase and will be visible to you at almost any age.
If you are wearing glasses and notice differences between the lines with 90 degrees between them, then you have an uncorrected astigmatism on the glasses you wear (possible misdiagnosis), if you are not wearing glasses and you notice differences between the lines with 90 degrees between them, then you have an uncorrected astigmatism (visit a specialist).
Note the axes in which the lines are most accentuated in each eye, in that meridian you need a cylindrical optical correction. For a correct check, ask the specialist to show you the receipt issued by the dioptron to check if the axes found by you are the ones you need (the dioptron must work correctly and be set on negative cylindrical values), or simply check the recipe axes.
If the recipe is written with positive cylindrical values, then the axes found in the test above will have a difference of 90 degrees from those written on the recipe.
Checking visual acuity and any dioptric needs – we check whether or not we need optical correction (glasses). The test is performed monocularly and binocularly (with each individual eye and with both eyes).
Checking glasses – we check if the diopters we have on permanent (or distant) glasses still correspond to current visual needs (whether or not it is necessary to reach a specialized consultation). The test is performed monocularly and binocularly. In the near future and testing near glasses.