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Astigmatism is one of the most commonly tested topics on the CPO and CPOA exams, and for good reason -- it appears in nearly every area of paraoptometric practice. Understanding astigmatism means understanding how to read prescriptions, operate a keratometer, verify lenses on a lensometer, explain eyewear options to patients, and recognize when corneal irregularities may indicate disease.
In simple terms, astigmatism means the eye does not have a single, uniform curvature. Instead of being shaped like a basketball (a perfect sphere with equal curvature in every direction), the astigmatic cornea or lens is shaped more like a football -- steeper in one meridian and flatter in the perpendicular meridian. This creates two different focal points instead of one, resulting in blurred or distorted vision at all distances.
Almost everyone has some degree of astigmatism. Very small amounts (less than 0.50 D) are usually visually insignificant and may not be corrected. But when astigmatism reaches a level that affects acuity or causes symptoms, it must be addressed with cylinder correction in glasses, toric contact lenses, or refractive surgery.
Astigmatism can originate from the cornea, the crystalline lens, or both. Knowing the source matters because it affects how we measure astigmatism and what we can learn from comparing different tests.
Clinical Connection
Comparing keratometry readings (corneal astigmatism) with the refraction cylinder (total astigmatism) is a valuable clinical skill. When the two differ significantly, lenticular astigmatism is present. This comparison is also important in contact lens fitting -- a rigid gas permeable (RGP) lens corrects corneal astigmatism by creating a new optical surface but does not correct lenticular astigmatism. If most of the patient's astigmatism is lenticular, an RGP lens alone will not fully correct it.
The orientation of astigmatism describes which meridian of the cornea is steepest. This classification appears frequently on certification exams and has practical implications for contact lens fitting and understanding age-related changes.
The steepest corneal meridian is vertical (at or near 90 degrees). The cornea is shaped somewhat like a football lying on its side -- steeper top-to-bottom than side-to-side. This is the most common type in children and young adults, likely due to upper eyelid pressure that steepens the vertical meridian.
The steepest corneal meridian is horizontal (at or near 180 degrees). The cornea is steeper side-to-side -- like a football standing on its end. This type becomes more common with age as the upper eyelid becomes lax, reducing the vertical pressure on the cornea, allowing the horizontal meridian to become relatively steeper.
The steepest meridian is between 30-60 degrees or 120-150 degrees -- neither vertical nor horizontal. Oblique astigmatism is less common and tends to cause more noticeable visual distortion because the human visual system is less tolerant of blur at oblique orientations. Patients with oblique astigmatism may report that images appear tilted or slanted.
This distinction is clinically important and commonly tested. The type of astigmatism determines how it can be corrected.
Unlike myopia (blurry at distance) or hyperopia (blurry at near), astigmatism causes blur at all distances because neither focal point falls precisely on the retina. The specific symptoms depend on the amount and type of astigmatism.
The hallmark symptom. Unlike myopia (where near is clear) or hyperopia (where distance may be clear in young patients), uncorrected astigmatism blurs both distant and near objects. Patients may describe vision as fuzzy, smeared, or not quite sharp.
Rather than simple blur, patients with astigmatism often see a ghost or shadow of images -- letters may appear doubled or have a directional smear. This is because the two focal lines created by the astigmatism produce overlapping but offset images.
Particularly with oblique astigmatism, patients may perceive that straight lines appear tilted or that objects lean in one direction. This can be especially noticeable when new glasses are first dispensed -- the patient may feel the floor is tilted or that door frames are not straight.
Headaches, eye fatigue, and discomfort with prolonged visual tasks are common, especially with low-to-moderate uncorrected astigmatism. The visual system constantly tries to find the best compromise focus between the two focal points, leading to fatigue. Squinting and head tilting are compensatory behaviors to look through a narrower aperture or align the visual axis.
Astigmatism is represented by two values in the prescription: the cylinder (CYL) and the axis. Together with the sphere, these three components define the complete correction. Understanding how they work together is essential for every paraoptometric.
Example Prescription: -2.50 -1.25 x 180
Understanding Power in Each Meridian
In the example above: At axis 180, the power is -2.50 D (sphere only). At 90 degrees (perpendicular to axis), the power is -2.50 + (-1.25) = -3.75 D (sphere plus cylinder). On the lensometer, you would find the less minus reading at -2.50 and the more minus reading at -3.75, with the difference (-1.25) representing the cylinder.
Correcting astigmatism requires a lens that has power in one meridian but not in the perpendicular meridian -- a cylinder lens. This additional power compensates for the difference in curvature between the eye's two principal meridians, merging the two focal points into one on the retina.
The standard correction: a combination of sphere and cylinder ground into the lens. The axis must be aligned precisely -- even a few degrees of rotation significantly degrades the correction, especially with higher cylinder values. This is why proper fitting, correct PD measurement, and ensuring the frame sits level on the face all matter for astigmatic patients.
Toric soft contacts have built-in cylinder correction and use stabilization methods (prism ballast, thin zones, or truncation) to prevent rotation on the eye. If the lens rotates, the axis shifts and the correction becomes inaccurate. Paraoptometrics may assess lens rotation during follow-up visits -- each degree of rotation off-axis causes a proportional decrease in astigmatism correction effectiveness.
RGP lenses correct corneal astigmatism by replacing the irregular corneal surface with the smooth, spherical front surface of the rigid lens. The tear film fills the gap between the lens and the cornea, neutralizing the corneal astigmatism. This makes RGPs excellent for irregular astigmatism (keratoconus, corneal scars) where spectacles and soft toric lenses cannot provide adequate correction.
LASIK, PRK, and other procedures can correct astigmatism by selectively reshaping the cornea to equalize its curvature in both meridians. Wavefront-guided or topography-guided treatments are particularly effective for astigmatism because they map the specific corneal irregularities and customize the ablation pattern accordingly.
Keratometry is one of the most important pretesting procedures for astigmatism, and paraoptometrics are expected to understand how K readings relate to the astigmatic correction. The keratometer measures corneal curvature in two principal meridians and reports the results in diopters and degrees.
Example K Readings: 44.00 D @ 90 / 42.50 D @ 180
Interpretation: The vertical meridian (90 degrees) is steeper at 44.00 D, and the horizontal meridian (180 degrees) is flatter at 42.50 D.
Corneal astigmatism: 44.00 - 42.50 = 1.50 D
Type: With-the-rule (steepest meridian is vertical)
Expected minus cylinder axis: Near 180 degrees (the flatter meridian)
Keratometry Mires and Astigmatism
On a manual keratometer, astigmatism is visible when the mires (the reflected circles or plus signs) do not overlap when the instrument is at the flat meridian setting. You must rotate the instrument to find both the steep and flat meridians and record their powers and axes separately. Distorted or irregular mires that cannot be properly aligned suggest irregular astigmatism -- this finding should be documented and reported to the doctor as it may indicate keratoconus or other corneal pathology.
Comparing keratometry findings with the refractive cylinder is a skill that appears on certification exams and is used daily in clinical practice. The key relationships to understand are:
K astigmatism roughly equals refractive cylinder
When the cylinder in the refraction closely matches the difference in K readings, most of the astigmatism is corneal. This is the most common scenario.
Refractive cylinder greater than K astigmatism
Additional lenticular astigmatism is present. The lens is contributing to the total astigmatism beyond what the cornea accounts for.
Refractive cylinder less than K astigmatism
The lenticular astigmatism is partially neutralizing the corneal astigmatism. The lens astigmatism is oriented in the opposite direction, reducing the total amount.
Axes do not match between K readings and refraction
When the axis of K astigmatism and refractive astigmatism differ significantly, lenticular astigmatism is at a different orientation than corneal astigmatism, creating a more complex optical situation.
Study Tip
Astigmatism questions on the CPO and CPOA exams can be among the most challenging because they require you to connect concepts across multiple domains: optics (how cylinder lenses work), instruments (keratometry), prescriptions (reading sphere/cylinder/axis), and dispensing (toric contacts, axis alignment). Practice connecting K readings to expected prescription components -- this integrative thinking is what the exam tests.
How the three core prescription components work together.
Master corneal curvature measurement and K reading interpretation.
Specialty contact lens designs for astigmatism and presbyopia.
Browse all CPO and CPOA study topics in one place.
Astigmatism means the eye has two different curvatures in two perpendicular meridians, somewhat like the surface of a football (more curved in one direction) rather than a basketball (equally curved in all directions). Because of these two different curvatures, light entering the eye focuses at two different points instead of one, causing blurry or distorted vision at all distances. It is corrected with a cylinder lens that has power in only one meridian, equalizing the focus so both focal points merge onto the retina.
Astigmatism appears in two parts of the prescription: the cylinder (CYL) value and the axis. The cylinder indicates the amount of astigmatism correction in diopters (e.g., -1.25), and the axis (a number from 1 to 180) indicates the orientation of the correction. For example, -2.50 -1.25 x 090 means 2.50 diopters of sphere correction with 1.25 diopters of cylinder correction at axis 90 degrees. The axis specifies the meridian that does NOT receive cylinder power; the cylinder power acts perpendicular to this axis.
With-the-rule (WTR) astigmatism means the steepest corneal meridian is vertical (around 90 degrees), so the correcting minus cylinder axis is at or near 180 degrees. This is the most common type in younger patients. Against-the-rule (ATR) astigmatism means the steepest meridian is horizontal (around 180 degrees), so the correcting minus cylinder axis is at or near 90 degrees. ATR astigmatism becomes more common with age as the upper eyelid pressure on the cornea decreases and the horizontal meridian steepens. Oblique astigmatism has the steep meridian between 30-60 or 120-150 degrees.
Most astigmatism is corneal -- the front surface of the eye has an oval shape rather than being perfectly round. This is usually a normal anatomical variation present from birth. Lenticular astigmatism comes from the crystalline lens having unequal curvatures. Some astigmatism is acquired through injury, surgery (particularly after cataract surgery), or conditions like keratoconus that progressively distort the cornea. Keratometry measures corneal astigmatism directly, while the refraction captures total astigmatism from all sources.
Yes. The amount and type of astigmatism commonly shifts with age. Young patients typically have with-the-rule astigmatism, which gradually shifts toward against-the-rule as they age due to changes in eyelid tension on the cornea. The magnitude may also change. Additionally, conditions like keratoconus cause progressive irregular astigmatism in younger patients. This is why regular eye exams are important -- astigmatism changes may require updated prescriptions. Significant or sudden changes in astigmatism, especially if irregular, should be flagged for the doctor as they may indicate corneal disease.
Paraoptometrics can detect and measure astigmatism through several pretesting procedures. Autorefraction provides an objective measurement of astigmatism including cylinder power and axis. Keratometry specifically measures corneal curvature in two principal meridians -- the difference between these K readings indicates the amount of corneal astigmatism. Corneal topography provides a detailed map of the entire corneal surface, revealing both regular and irregular astigmatism. During visual acuity testing, astigmatic patients may misread letters that differ in orientation (confusing O with D or H with N) because certain orientations are clearer than others.