Understand the differences in prescription numbers and terms between glasses and contact lenses, as well as optometrists and ophthalmologists.
Have you ever found your child's eye prescription to be confusing? The array of numbers and abbreviations can often feel like a foreign language, leaving you perplexed and uncertain about their significance. As parents of children with myopia, it is important to comprehend and interpret your child's prescription in order to ensure their visual well-being.
In this article, we will explore and demystify the terminology commonly used in eye prescriptions. By the end, you will gain a clearer understanding of what these numbers and terms signify for your child's vision.
How to read my childâs eye prescription?
Understanding your child's eye prescription can be overwhelming, especially if you're not familiar with the terminology. Here, we'll break down the commonly seen components of an eye prescription and explain their roles in correcting myopia.1
OD, OS, and OU:Â On your child's prescription, you may find abbreviations such as OD and OS. OD stands for "oculus dexter," which means the right eye, while OS stands for "oculus sinister," representing the left eye. If you see a column labeled OU, it means "oculus uterque," referring to both eyes.
SPH (Sphere):Â The term you'll often encounter on your child's eye prescription is SPH, which stands for âsphereâ or "spherical error." The SPH value indicates the lens power required to correct myopia or hyperopia. A negative value (-) indicates myopia, while a positive value (+) suggests hyperopia. If the prescription is for contact lenses, the abbreviation PWR (power) is commonly used too.
Note: If cylinder correction is not required, a prescription may note âSPH,â âDS,â or âSphericalâ in the CYL column.
CYL (Cylinder) and AXIS:Â If your child has been diagnosed with astigmatism, their prescription may include the terms CYL and Axis. The CYL value represents the amount of lens power needed to correct astigmatism. If there's no number associated with CYL, it means there is no astigmatism to correct. The Axis value, measured in degrees, denotes the orientation of the astigmatism correction and helps the optician align the corrective lenses properly on your child's eyes.
(Learn more about the link between astigmatism and myopia progression in children)Â
PD (Pupillary Distance):Â Pupillary Distance, abbreviated as PD, measures the distance between the two centers of your child's pupils in millimeters, and it is an important value for proper lens fitting. For reference, the average kidâs PD is between 43-58 mm and the average adultâs PD is between 54-74 mm. And it can be measured manually or with a specialized device called a pupillometer.Â
Having an accurate PD measurement on the vision prescription ensures that your child will be looking through the ideal spot in their lenses. If the pupillary distance is not accurate, the prescription power of their lenses will be off-center from the eyes, and they wonât be able to see clearly.
ADD:Â This value indicates the lens power adjustment needed to bring the focal point closer. It is measured in positive (+) diopters, indicating the positive value added to the spherical error. Many myopia-control contact lenses or glasses incorporate an ADD value into their design to induce peripheral myopic defocus. Therefore, it may not be explicitly stated in the prescription itself. However, in exceptional cases where bifocal or progressive additional spectacle lenses have been prescribed, you might find the ADD value specified on their eyeglasses prescription.
Prism and Base: Prism corrections are used in eyeglasses for individuals experiencing double vision, which could be caused by conditions such as strabismus. Prism assists in aligning the two images so that only one image is perceived. The amount of prism correction is measured in prism diopters (â), and it is accompanied by directions known as the base, such as UP (BU), DOWN (BD), IN (BI), and OUT (BO).
D (Diopters): Diopters are a unit of measurement used to quantify the refractive power of lenses. When you see the abbreviation "D" on your child's eye prescription, it refers to diopters. This measurement is crucial in determining the optical correction needed to address refractive errors.
How to read contact lens prescriptions?
A contact lens prescription and an eyeglass prescription appear different because they serve different purposes. The mechanics of how they function and the necessary information for a proper fit differ significantly. Moreover, eyeglasses are positioned slightly away from the eyes, whereas contact lenses rest directly on the cornea. Consequently, the required prescription correction power can vary between the two. To comprehend a contact lens prescription, it's helpful to familiarize yourself with some additional terms.2
BC (Base Curve): This value refers to the curvature of the contact lens. It is measured in millimeters (mm). Please note that most multifocal contact lenses come in a single base curve, so your childâs eye prescription may not have the number explicitly stated. However, some single-vision lenses offer several different BC values to choose from, which typically range from about 8.0 to 9.5mm.Â
DIA (Diameter):Â This term refers to the overall width of the contact lens, measured in millimeters. It is crucial for the lens to fit comfortably over your child's cornea. Typically, the diameter of contact lenses ranges between 12.5 to 16 millimeters. Most contact lenses come in a single diameter and may not have a number listed.
Brand:Â The brand of the contact lens is stated for fitting reasons. If you decide to switch brands, it's possible that your child may need a different prescription. Think of contact lens prescriptions as clothes sizes measurement; there are slight variations in fit across different manufacturers, especially the base curve measure. Therefore, it's important to specify the brand when getting a prescription for contact lenses.
How do optometrists and ophthalmologists differ and prescription writing approaches?
When you visit different eye doctors, especially if one of them is an optometrist and the other one an ophthalmologist, you may also notice variations in your prescription numbers. This might leave you wondering if one of the doctors made a mistake. However, the difference may stem from the distinct prescription writing approaches.
Ophthalmologists typically write astigmatism prescriptions using the positive (+) cylinder notion, while optometrists write prescriptions with the negative (-) cylinder notion. This difference in cylinder notation can lead to variations mentioned. However, it's important to note that despite these differences, the vision correction you receive will remain the same.
The historical and practical reasons behind these two approaches are quite fascinating. Around 1800âs, cylinder were noted with plus (+) since cylinder that that time was on the front of lenses with spheres on the back. However, in the 1900s, it was discovered that optically, it was better to have the cylinder on the back of the lenses, and using minus notation for cylinder was more appropriate.3
Fast forward to today, with two systems, when prescribing glasses, optometrists prefer using negative (-) cylinder notation due to the practicality of lens cutting by the eyeglass lab. However, for surgical planning purposes, plus (+) cylinder notation may be more helpful. For example, incisions at the plus cylinder axis can help reduce astigmatism. This distinction might explain the differences in the preferred prescription writing approaches between optometrists and ophthalmologists.3
How to convert an eye prescription between the different notations?
It's important to understand that even though prescriptions can be written in two different ways, they still mean the same. To convert a prescription from one notation to another, a simple three-step process is followed:4
1. Â Â Â Add the sphere and cylinder values to obtain the new sphere value.
2. Â Â Â Swap the sign of the cylinder while keeping its power the same.
3. Â Â Â Add or subtract 90 degrees from the axis value.
Let's consider a simple example to illustrate this conversion process. If the original prescription for the right eye reads SPH -2.50 D, CYL -1.00 D, Axis 5 degrees (with-the-rule astigmatism), it is equivalent to SPH -3.50 D, CYL +1.00 D, Axis 95 degrees.
1. Â Â Â Add sphere and cylinder values: -2.50 D + -1.00 D = -3.50 D
2. Â Â Â Swap the sign of the cylinder: +1.00 D
3. Â Â Â Add or subtract 90 degrees from the axis value: 5 degrees â 90 degrees = 95 degrees
Key takeaways
As parents, understanding how to read your child's eye prescription is important. It allows you to stay informed about the status of your childâs eyesight and play an active role in managing their vision health.
Furthermore, it's important to recognize the distinctions between how glasses and contact lenses function, as well as the unique information required for proper fitting, which can result in prescription differences between the two.
Additionally, it's essential to be aware of the two prescription writing approaches commonly used. Optometrists often employ a negative (-) cylinder notion, while ophthalmologists may use positive (+) cylinder notions. However, despite these variances, converting between the two notations is a simple three-step process.
This understanding can help you navigate discussions with different eye care professionals and ensure accurate communication regarding your child's vision needs.
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1. American Academy of Ophthalmology. How to Read an Eyeglasses Prescription. Retrieved June 1, 2023, from https://www.aao.org/eye-health/glasses-contacts/how-to-read-eyeglasses-prescription
2. American Academy of Ophthalmology. 2020â2021 BCSC Basic and Clinical Science: Course. Chapter 5: Contact Lenses https://www.aao.org/education/bcscsnippetdetail.aspx?id=07859993-1551-43bf-af7c-3139ef307ee5Â
3. Dunphy, C. (2018, September 18). Why do Optometrists write Rx's in negative cylinder while Ophthalmologists use positive cylinder? Optician Now. Retrieved June 1, 2023, from https://opticiannow.com/
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4. Optical Academy. Transpose RX: How to Transpose an Eyeglass Prescription. Retrieved June 1, 2023, from https://optical-academy.com/blog/transpose-rx-how-to-transpose-an-eyeglass-prescription/
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