Astigmatism Severity Scale

What is Astigmatism?

Astigmatism is a common type of refractive error. It occurs when either the cornea (the eye’s front surface) or the natural lens of the eye is irregularly shaped. 

Typically, the lens and cornea are smoothly curved. This allows light to bend evenly and properly focus on the retina, the light-sensitive surface at the back of the eye.

For people with astigmatism, the cornea or lens is shaped more like a football than a soccer ball. This irregular shape affects the eye’s ability to focus light properly, leading to blurry vision.

If your cornea is shaped more like a football than a soccer ball, your eye won’t be able to focus light rays at a single point. If the eye’s lens curvature changes, it can worsen astigmatism.

Types of Refractive Errors

People with astigmatism also usually have other refractive errors. There are four types of refractive errors, including astigmatism. They include:

Myopia (Nearsightedness)

Myopia occurs when your eye focuses light in front of the retina. People with myopia have trouble seeing objects that are far away.

Hyperopia (Farsightedness)

Hyperopia occurs when your eye focuses light behind the retina. This causes blurred vision when looking at objects nearby.

Presbyopia

Many people who have never had to wear glasses need to start wearing glasses to read or see objects up close as they age. This is known as presbyopia or age-related farsightedness. 

Astigmatism

This refractive error occurs when your eye focuses light at different points in front of and behind the retina. Astigmatism can cause blurry vision when viewing objects both far away and close up.

What Causes Astigmatism?

Many people are born with astigmatism. It can also develop later in life due to natural eye changes or from one of the following conditions: 

• Eye injury
• Eye surgery
• Eye diseases like keratoconus

How Does Astigmatism Affect Visual Impairment? 

People who have mild astigmatism may not notice any symptoms or visual impairment. 

Astigmatism symptoms generally include:

• Blurry vision
• Trouble seeing objects both close up and far away
• Fluctuating vision
• Double vision
• Eye strain
• Squinting
• Headaches
• Difficulty with night vision
• Seeing glares and halos at night
• Seeing shadowy images

Does Astigmatism Worsen with Age?

Astigmatism tends to worsen over time. This is due to age-related changes in the muscles of your eyelids. These muscles play a role in maintaining the shape of your cornea.

Is 1.0 Astigmatism Bad?

An astigmatism measurement of 1.0 is considered mild. It may not cause symptoms, or it may only cause mild vision problems. 

However, not all astigmatism is the same. Astigmatism can vary in severity. 

What Do Astigmatism Numbers Mean?

Your eye doctor will measure your astigmatism in diopters. 

For context, a perfectly round eye with no astigmatism has 0 diopters. Most people have between 0.5 and 0.75 diopters of astigmatism.⁵

People who have 1.5 or more diopters of astigmatism usually need prescription glasses or contact lenses to see clearly.

Where to Find Astigmatism Measurements on a Glasses Prescription

There are three numbers on any eyeglass prescription, and the last two (CYL and Axis) refer to astigmatism:

• Spherical (sphere). Indicates whether you are nearsighted (a minus sign) or farsighted (a plus sign). The higher the number, the stronger the lens power needed.
• Cylinder (CYL). Measures the severity of astigmatism in diopters.
• Axis. Measures where astigmatism is located in degrees from 0 to 180.

Astigmatism Severity Scale

How much astigmatism you have determines how your vision is affected. To find your level of astigmatism, look at the cylinder (CYL) number on your glasses prescription.

The astigmatism severity scale ranges from mild (less than 1.00 diopter) to extreme (more than 3.00 diopters).

Mild Astigmatism: < 1.00 diopter

Mild astigmatism is considered normal. Most people have it and don’t need corrective lenses or eye surgery to see clearly. You may not notice symptoms with mild astigmatism.

Moderate Astigmatism: 1.00 to 2.00 diopters

Moderate astigmatism is more serious. It generally requires corrective lenses or laser eye surgery for clear vision. 

While you can get by without wearing glasses or contacts, you’ll probably notice the symptoms of moderate astigmatism. Glasses and contact lenses can improve vision problems due to moderate astigmatism.

Severe Astigmatism: 2.00 to 3.00 diopter

Severe astigmatism can cause serious symptoms that affect daily functioning. Blurred vision from severe astigmatism can cause headaches that impact your health.

Extreme Astigmatism: > 3.00 diopter

Extreme astigmatism will require treatment to easily see objects near and far. If you do not treat extreme astigmatism, you won’t be able to see clearly at all.

What Level of Astigmatism Requires Glasses? 

Wearing glasses or other astigmatism treatment is typically only necessary if you have moderate astigmatism (1.00 to 2.00 diopters) or worse.

Some eye doctors prescribe glasses for people who have a 0.75 diopter. This is because many people still notice vision improvements.

It’s important to schedule regular eye exams with your eye doctor because astigmatism can change over time. If you wear glasses, you must ensure your prescription stays correct.

5 Types of Astigmatism

Astigmatism can be broken down into five categories. Here’s what you should know about each:

1. Simple Myopic Astigmatism

Simple myopic astigmatism is when the light hits two focal points: one before the eye’s retina and one on the eye’s retina.

2. Simple Hyperopic Astigmatism

Simple hyperopic astigmatism happens when the light comes to two focal points: one directly on the eye’s retina and one behind the eye’s retina.

3. Compound Myopic Astigmatism

Compound myopic astigmatism happens when the light hits two focal points at two different locations in front of the eye’s retina.

4. Compound Hyperopic Astigmatism

Compound hyperopic astigmatism is when the light comes to two different virtual locations behind the eye’s retina.

5. Mixed Astigmatism

Mixed astigmatism happens when light rays hit two focal points: one before the eye’s retina and one behind the eye’s retina.

How Is Astigmatism Diagnosed?

An optometrist can diagnose astigmatism with a comprehensive eye exam. They’ll measure how your eye focuses light and determine the lens power needed to correct astigmatism.

Your eye exam may include:

• Eye chart. Reading letters on an eye chart tests your visual acuity (vision sharpness) at different distances.
• Phoropter. This is a large device that looks like binoculars. It contains many lenses of various powers. Your doctor will ask which lenses provide the clearest vision.
• Autorefractor. This device measures your astigmatism or other refractive error by shining a light into your eye.
• Keratometer or corneal topography. This measures the curvature of your cornea.

Can LASIK Correct Astigmatism?

Yes, LASIK (laser in situ keratomileusis) and PRK (photorefractive keratectomy) can fix mild to moderate astigmatism. Both types of laser surgery permanently fix the shape of your cornea so you don’t need to wear eyewear for clear vision.

LASIK removes the tissue from the cornea’s inner layer with an excimer laser; PRK removes it from the superficial and inner layers of the cornea.

What Happens During LASIK?

In LASIK surgery, the surgeon will use a mechanical microkeratome (blade) or a femtosecond laser to cut a flap in your cornea. They’ll leave a hinge at one end of the flap. They’ll fold it back to reveal the stroma (middle section of the cornea).

Then, they'll use pulses from a computer-controlled laser to vaporize part of the stroma and replace the corneal flap.

The LASIK procedure only takes about 10 minutes per eye — and the laser itself only takes about 20 to 50 seconds to correct your vision, depending on how much correction you need.

Other Ways to Treat Astigmatism

Most people with astigmatism wear prescription glasses or contact lenses to improve their vision. 

Depending on your needs and preferences, astigmatism treatment may include:

Eyeglasses

Glasses are a simple, cost-effective way to treat astigmatism. Your optometrist will prescribe a specific cylindrical lens prescription that corrects astigmatism.

Depending on your prescription, you may need one of the following types of eyeglasses:

• Single-vision lenses. These are the most common type of eyeglass lenses. They correct vision at one particular distance, either near or far. 
• Bifocal and trifocal lenses. These multifocal lenses correct vision at two or three distances, respectively. 
• Progressive lenses. These lenses are similar to bifocals and trifocals, but they don't have distinct lines separating each section. 

Contact Lenses

Many contact lenses are specifically designed for treating astigmatism.

Some people prefer contacts over glasses because they can provide clearer vision and a broader field of view. However, contacts require regular cleaning and care to protect eye health. 

Orthokeratology

Orthokeratology (ortho-k) involves wearing special hard contact lenses that reshape your cornea. You wear the contact lenses for a certain period, such as overnight, then remove them.

Some people with moderate levels of astigmatism can avoid wearing corrective lenses with ortho-k treatment. However, vision may return to its previous state if you stop treatment.

Refractive Surgery

LASIK and PRK are two common laser refractive surgeries used to correct astigmatism. They reshape the cornea. 

Details about LASIK for astigmatism are included in the above section.

Summary

Astigmatism is an eye condition that causes vision changes due to an irregularly shaped cornea or lens. It can be genetic, occur in childhood, or worsen with age. Different types of astigmatism vary in severity.

The astigmatism severity scale ranges from mild to extreme. Mild astigmatism usually doesn’t require correction. For those with moderate or severe astigmatism, contact lenses, eyeglasses, or eye surgery like LASIK may be needed.