How do I know if I have astigmatism from my prescription?
Astigmatism is the most common refractive error of the eye. It affects the eye's ability to focus light correctly on the retina, leading to blurred or distorted vision. If you are curious about astigmatism, look at the numbers on your eyeglass prescription. Typically, astigmatism is indicated by the presence of specific values in the "CYL" (cylinder) and "AXIS" fields of your prescription.
Examining the details of your eyeglass or contact lens prescription can help you determine if astigmatism is present and to what extent it affects your vision. Astigmatism is measured in a unit called a diopter, and the diopter value indicates the severity of the astigmatism.
Brief overview of astigmatism
Astigmatism occurs when the eye's cornea or lens is irregular, causing light to focus on multiple points within the eye rather than a single point on the retina. Eyeglasses, contact lenses, or refractive surgery can be prescribed to correct how light is focused on the retina. This irregular curvature results in blurred or distorted vision at all distances. Unlike nearsightedness (myopia) or farsightedness (hyperopia), which affect vision at a specific range, astigmatism can impact overall clarity and fine detail, making it difficult to see objects up close and far away.
Symptoms of astigmatism include eye strain, headaches, and difficulty seeing at night. It is often due to the glare or halo visible around car lights or streetlights. It can be present from birth, change over time, and may be combined with either myopia or hyperopia.
Diagnosing astigmatism involves a comprehensive eye exam where an eye doctor measures how light passes through the eye. A key part of this examination uses a phoropter to determine one’s refraction. A refraction is part of an eye exam where you are asked, “Which is better, one or two?”
Importance of the right interpretation of eye prescription
It is important to correctly interpret your eyeglass prescription for optimal vision correction and overall eye health. Accurate interpretation ensures that your prescription lenses address your specific refractive errors (astigmatism, nearsightedness, farsightedness) and optimize your vision. Misinterpretation can lead to incorrectly made lenses, resulting in vision problems, eye strain, and headaches. Understanding the key components allows the eyeglass wearer to communicate with their optician and order the best eyewear for their visual needs. More information on understanding your eyeglass prescription can be found on our blog.
Signs of Astigmatism in Prescription
Identifying astigmatism in an eyeglass prescription involves looking for specific notations that indicate its presence and severity. A prescription's key signs of astigmatism are the cylinder (CYL) value and the axis value. For instance, your prescription might read: SPH -4.00, CYL -1.00, Axis 90. This means there is a spherical correction of 4 diopters for nearsightedness and an additional -1.00 diopters of cylinder correction for astigmatism oriented at 90 degrees.
Cylinder (CYL) Value: This value indicates the degree of astigmatism. It measures the amount of lens power needed to correct the curvature difference between the eye's different meridians. A non-zero CYL value (e.g., -1.25 or -2.50) signifies the presence of astigmatism. Astigmatism is measured in 0.25D increments. The higher the diopter value, the greater the astigmatism. Astigmatism in the range of 0.25 to 1.00D is considered mild, 1.25D to 2.50D is considered moderate, and over 2.75D is a significant amount. If your astigmatism has changed significantly from your previous prescription, it is normal to take your eyes and brain time to adapt to the new prescription, sometimes weeks.
Axis Value: This value specifies the orientation of the astigmatism correction in degrees, ranging from 1 to 180. The axis indicates where the curvature difference is located and appropriately aligns the corrective lens. For example, an axis of 90 degrees means the astigmatism correction is oriented vertically, while 180 degrees is oriented horizontally.
What Do Astigmatism Measurements Mean?
The eyeglass prescription includes three main components: the sphere (SPH), cylinder (CYL), and axis values. The sphere value indicates the degree of nearsightedness (myopia) or farsightedness (hyperopia) present, which may be present with astigmatism. In other words, how many diopters it takes to move the light from in front or behind the eye to the retina?
The cylinder value measures explicitly the amount of astigmatism, the extent to which the curvature of the cornea or lens deviates from a perfect sphere. You can have some degree of astigmatism, but it may not be enough to cause vision difficulties. As a general rule, any astigmatism over 0.75D is likely to affect your vision. If you have 0.50D or less, you will unlikely notice any or little effect on your vision.
The axis value is expressed in degrees from 1 to 180, the orientation of astigmatism. Think of it like numbers on a clock. Since astigmatism involves irregular eye curvature, the axis value specifies the direction for correction. For example, an axis of 90 degrees would mean the astigmatism correction is oriented vertically, while an axis of 180 degrees aligns horizontally. The precise orientation is key because it determines how the corrective lenses are designed to redirect the light correctly onto the retina, providing you with clear vision. The orientation of the axis also affects your vision. Individuals with 0.75D at axis 090 will have more blurry vision than those with 0.75D at axis 180.
Common Symptoms of Astigmatism
Common symptoms of astigmatism include blurred or distorted vision both near and far. People with astigmatism often experience eye strain and discomfort, especially after prolonged reading or screen use. Frequent headaches are another symptom, usually resulting from the effort to focus. Individuals with astigmatism may have difficulty seeing at night, making driving more difficult. Additionally, squinting to see more clearly is a common sign of astigmatism because it temporarily improves focus by slightly altering the shape of the eye.
Additional Diagnostic Tests
In addition to looking at the eyeglass prescription and performing the refraction, several diagnostic tests are used by eye doctors to determine the presence and extent of astigmatism.
- Visual Acuity Test: The Snellen test measures how well you can see at various distances and involves reading letters on a chart at 20 ft in the distance and 40cm up close, which can be used to identify blurred vision due to astigmatism.
- Keratometry: This test measures the curvature of the cornea using a keratometer. The device focuses a circle of light on the cornea and measures the reflection. Irregular reflections can indicate the presence of severe or irregular astigmatism.
- Corneal Topography:The equipment provides a detailed map of the cornea's surface curvature. The device works by projecting light patterns onto the cornea and analyzing the reflected light. It is commonly used to fit specialty contact lenses or for refractive surgery.
- Autorefractor/Autokeratometer: These automated devices initially measure your eye’s refractive error and corneal curvature. They are often used to quickly assess the presence and degree of astigmatism before the doctor performs the refraction.
Conclusion
In conclusion, if your eyeglass prescription has a cylinder and axis value, you have some degree of astigmatism correction. Understanding these notations allows you to be better informed about the specific correction needed for your vision and is helpful when ordering new eyeglasses. Regular eye exams and accurate prescriptions are essential for managing astigmatism and ensuring you have clear, comfortable vision.
