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.
Types by Location: Corneal vs. Lenticular
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.
Corneal Astigmatism
- Source: Unequal curvature of the anterior corneal surface
- Frequency: Most common type -- accounts for the majority of astigmatism
- Measurement: Directly measured by keratometry or corneal topography
- K reading example: 44.00 D @ 90 / 42.50 D @ 180 shows 1.50 D of corneal astigmatism
- Clinical note: Essential for contact lens fitting, especially toric and RGP lenses
Lenticular Astigmatism
- Source: Unequal curvature or tilt of the crystalline lens
- Frequency: Less common as the primary source; often small amounts
- Measurement: Cannot be measured directly -- inferred from the difference between total (refractive) and corneal astigmatism
- Key relationship: Refractive astigmatism = corneal + lenticular astigmatism
- Clinical note: If K readings show 1.50 D of corneal astigmatism but refraction shows 2.00 D total, the remaining 0.50 D is lenticular
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.
Types by Orientation: With-the-Rule, Against-the-Rule, and Oblique
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.
With-the-Rule (WTR) Astigmatism
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.
- Steepest K reading near 90 degrees
- Minus cylinder axis at or near 180 degrees (x 180)
- Plus cylinder axis at or near 90 degrees (x 090)
Against-the-Rule (ATR) Astigmatism
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.
- Steepest K reading near 180 degrees
- Minus cylinder axis at or near 90 degrees (x 090)
- Plus cylinder axis at or near 180 degrees (x 180)
Oblique Astigmatism
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.
- Steepest K reading at an oblique angle (30-60 or 120-150 degrees)
- Minus cylinder axis at an oblique angle
- Often more symptomatic per diopter than WTR or ATR
Regular vs. Irregular Astigmatism
This distinction is clinically important and commonly tested. The type of astigmatism determines how it can be corrected.
Regular Astigmatism
- Definition: The two principal meridians are perpendicular to each other (90 degrees apart)
- Correction: Fully correctable with cylinder lenses in glasses or toric contacts
- Cause: Normal anatomical variation in corneal or lenticular shape
- Keratometry: Produces clear, regular mires that can be focused
- Prevalence: The vast majority of astigmatism is regular
Irregular Astigmatism
- Definition: Principal meridians are not perpendicular, or curvature varies unpredictably across the cornea
- Correction: NOT fully correctable with standard cylinder lenses; may need RGP contacts or scleral lenses
- Causes: Keratoconus, corneal scars, corneal dystrophies, post-surgical irregularities
- Keratometry: Produces distorted, irregular mires that cannot be properly aligned
- Red flag: Increasing irregular astigmatism in a young patient suggests keratoconus
Signs and Symptoms of Astigmatism
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.
Blurred Vision at All Distances
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.
Ghosting and Shadowing
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.
Tilted or Slanted 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.
Asthenopia (Eye Strain)
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.
How Astigmatism Appears on a Prescription
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.
Practice CPO & CPOA questions on astigmatism
Test your understanding of astigmatism types, prescriptions, and correction with adaptive practice questions and detailed AI explanations.
How Astigmatism Is Corrected
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.
Spectacle Lenses (Sphero-Cylinder)
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 Contact Lenses
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.
Rigid Gas Permeable (RGP) Lenses
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.
Refractive Surgery
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: Measuring Corneal Astigmatism
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.
The Relationship Between K Readings and the Refraction
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.