Spherical equivalent in myopia with astigmatism: Gain insights into what it means, how to calculate it, and how it can help when checking your child’s vision status.
Are you a parent of a child living with myopia? If so, you might have come across terms like "spherical equivalent" during discussions with eye care professionals. Grasping this concept is important to ensure effective management of myopia for your child, especially when they are dealing with both myopia and astigmatism.
In this article, we will explore what is spherical equivalent in myopia and why it matters when it comes to myopia care.
The plus and minus system in eyeglass prescriptions
Before we delve into the details of spherical equivalents in myopia, let's address an issue that parents may face when understanding their kid’s myopia. Myopia, or nearsightedness, is defined as a condition in which the spherical equivalent refractive error of an eye is equal to or less than -0.50 D when ocular accommodation is relaxed.1 The problem lies in the fact that sometimes the plus and minus systems for vision correction prescription suggest different values for the same eye.
For example, a plus system might indicate a refractive error of less than -0.5 D, but when converted to the minus system, it could be more than -0.5 D. This disparity may lead to uncertainty in determining the status of your child’s vision or if your child should be considered as having myopia or not.
What is the spherical equivalent in myopia?
The spherical equivalent is an estimate of your eyes' refractive error, and it's calculated independently for each eye. It combines the spherical component (which deals with nearsightedness or farsightedness) and the cylindrical component (astigmatism) of the refractive error into a single value. This simplification is particularly useful in clinical research, as it allows for easy comparison and analysis of data. In simpler terms, the spherical equivalent is like finding a single number that represents your child's overall vision prescription, taking into account both nearsightedness and astigmatism.
Moreover, the utility of the spherical equivalent goes beyond the realm of research. Eye care professionals can employ it in cases involving individuals with mild astigmatism when conventional glasses or contact lens prescriptions don't work well, or when the prescription for correcting astigmatism is not available.
How to calculate the spherical equivalent
Calculating the spherical equivalent is straightforward. You can use the following formula:
Spherical Equivalent (SE) = Sphere Power (SPH) + 0.5 * Cylinder Power (CYL)
Sphere Power (SPH) is the measurement in diopters that corrects nearsightedness (myopia) or farsightedness (hyperopia).
Cylinder Power (CYL) is the measurement in diopters that corrects astigmatism.
The reason we use one-half (by multiplying it with 0.5) of the cylinder power is because it represents the disparity in refractive power between the two principal meridians of the eye. To simplify, think of it as reshaping a football-shaped surface, where one half is steeper than the other, into a completely spherical surface, much like a basketball. It may not be a perfect analogy, but it provides a close approximation. Additionally, we eliminate the axis value from the equation.
For instance, if your child's complete refraction indicates a right eye prescription of SPH +2.50 D, CYL -0.50 D with Axis 90°, and a left eye prescription of SPH +1.50 D, CYL +1.00 D with Axis 90°, the spherical equivalent would be calculated as follows:
For the right eye (RE): SE(RE) = +2.50 + (0.5 * -0.50) = +2.25 D
For the left eye (LE): SE(LE) = +1.50 + (0.5 * +1.00) = +2.00 D
The role of spherical equivalents in lens prescriptions
Spherical equivalents in myopia become relevant when optometrists and ophthalmologists deal with patients who may not tolerate lenses with a cylindrical correction. While spherical-corrected lenses may not offer the highest visual acuity, they can provide improved vision without the challenges associated with toric lenses. Healthcare professionals will consider when to use this method for prescribing lenses, depending on the patient's need.
Understanding myopia with the spherical equivalent
Now, you might be wondering how all this ties back to myopia. The spherical equivalent serves as a valuable tool in vision-related research and publications, as it simplifies complex refractive data, standardizes reporting, and enables comparisons across studies.
For instance, consider the definition of myopia, which states that an eye is myopic if its spherical equivalent refractive error is equal to or less than -0.50 D, for example -1.0 D, when ocular accommodation is relaxed.1 Regardless of whether you're using the plus or minus system, you'll arrive at the same value.
So, if your child has a spherical power (SPH) of -0.25 D and cylindrical power (CYL) of -1.0 D or SPH of -1.25 D and CYL of +1.0 D, both will have a spherical equivalent of -0.75 D, which, by definition, is myopia.
For the first scenario with a minus system -- (SPH -0.25 D and CYL -1.0 D):
· SE = -0.25 D+ (0.5 * -1.0 D) = -0.75 D
For the second scenario with a plus system (SPH -1.25D and CYL +1.0D):
· SE = -1.25 D+ (0.5 * +1.0 D) = -0.75 D
However, it's important to note that the spherical equivalent isn't perfect. Two eyes with very different refractive errors can have the same spherical equivalent, in other words, they can be corrected using the eyeglasses. In such cases, your healthcare professional will make the necessary interpretation to determine your child’s status.
Understanding the spherical equivalent in myopia is crucial for parents of children with myopia. It simplifies complex refractive data, making it easier for healthcare professionals to manage your child's eye health. So the next time you receive an eyeglass prescription, you'll have a better grasp of what it means, and you can confidently navigate the world of myopia with your child's best interests in mind.
And remember, when it comes to eye health, we're all just trying to see things more clearly – in every sense of the word!
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1. Németh, János et al. “Update and guidance on management of myopia. European Society of Ophthalmology in cooperation with International Myopia Institute.” European journal of ophthalmology vol. 31,3 (2021): 853-883. doi:10.1177/1120672121998960.