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A Guide to Refractive Errors

A Guide to Refractive Errors

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Whether it's nearsightedness, astigmatism, or farsightedness, they all cause vision problems making it more difficult to see clearly. These are known as refractive errors and are caused when the shape of the eye prevents light from correctly focusing on the retina. 

Over 150 million Americans suffer from some type of refractive error; however, a large number of those Americans are completely unaware that they could be seeing better. This is why it's critical for eye care professionals to stress the importance of regular eye exams to their patients. 

At Keeler, we offer a range of products to help with diagnosing refractive errors, including retinoscopes. Let's take a closer look at the four primary types of refractive errors.

Myopic Refractive Errors

Today, an estimated 40% of the United States population suffer from nearsightedness, or “myopia,” and this number is expected to increase according to the American Optometric Association. The American Academy of Ophthalmology estimates a staggering 50% of the world's population, almost 5 billion, will be myopic by the year 2050. 

Simply put, myopia is a condition in which there is an inability to see distant objects clearly. It's caused when refracted light is focused in front of the retina instead of onto the retina. Myopia typically occurs if the individual's cornea has too much curvature or if the eyeball has more of an elongated shape. 

The Degrees of Myopia 

All refractive errors are measured in diopters (D), which are optical units used to describe lens strength. Lens powers that correct myopia are preceded by a (-) minus sign and are typically in .25 D increments. The severity of myopia is categorized as:

  • Mild nearsightedness: -.25 to -3.00 D
  • Moderate nearsightedness: -3.25 to -5.00 D or -6.00 D
  • High nearsightedness: greater than -5.00 D or -6.00 D

While mild nearsightedness usually doesn't pose risk for eye health, moderate and high myopia can be associated with dangerous, vision-threatening side effects, such as:

Hyperopic Refractive Errors

Today, almost 14.2 million Americans aged 40 and older are hyperopic, or suffer from farsightedness. Hyperopia occurs when distant objects are easier to see than objects nearby. When this refractive error is severe enough, vision can be blurry at any distance. 

Instead of light hitting directly onto the retina, hyperopia is the result of light being bent behind the retina. Farsightedness occurs when there isn't sufficient curvature in the cornea or if the eyeball is too short. While hyperopia tends to affect older individuals, it can occur at any age. 

The Degrees of Hyperopia

Just like myopia, hyperopia is measured in diopters. While myopic prescriptions have negative measurements and numbers, hyperopia prescriptions will have positive (+) numbers, such as:

  • Low or mild hyperopia: +2.00 D or below
  • Moderate hyperopia: +2.25 D up to +5.00 D  
  • High hyperopia: Is a refractive error over +5.00 D

Astigmatic Refractive Errors 

Astigmatism is a very common refractive error that occurs in an estimated one out of three people. In the most simple sense, astigmatism causes distorted or blurred vision because of an abnormally shaped cornea — less than perfectly round. With this type of refractive error, the eye fails to equally focus light on the retina, which leads to distorted or blurred vision on nearby and far-away objects. 

Astigmatism can be present at the time of an individual's birth or can gradually develop in life. This common eye refractive error typically will occur with farsightedness or nearsightedness. 

The Degrees of Astigmatism 

  • Normal vision: less than 0.6 D of astigmatism
  • Low astigmatism: 0.6 D to 2 D
  • Moderate astigmatism: 2 D to 4 D
  • High or significant astigmatism: above 4 D

Three Types of Astigmatic Refractive Errors

There are three different types of astigmatism:

  • Myopic astigmatism is where one or both meridians of the eye are nearsighted. If both eyes have myopic refractive errors, they will have varying degrees.
  • Hyperopic astigmatism is where one or both principal meridians have a farsighted refractive error. If both meridians are farsighted, they will have differing degrees of hyperopia.
  • Mixed astigmatism is where one principal meridian is farsighted, and the other is nearsighted.

Astigmatic refractive errors may also be classified as irregular or regular:

  • In regular astigmatism, the principal meridians are perpendicular.
  • In irregular astigmatism, the principal meridians aren't perpendicular.

Presbyopic Refractive Errors

Researchers estimated that almost two billion people around the world have a presbyopic refractive error. As an inescapable part of aging, presbyopia is the condition in which older and middle-aged adults have difficulty seeing things up close, especially while reading. 

This is caused by the eye becoming more rigid and losing the ability to flex as easily. Presbyopia can be combined with astigmatism, hyperopia, or myopia. Individuals will develop presbyopia, even if they've never had a vision problem previously. Even those who have nearsightedness will notice their vision blur when wearing their usual contact lenses or eyeglasses. 

Five Types of Presbyopia 

There are five different types of presbyopia:

  • Incipient presbyopia is the earliest stage and explains when the individual may have mild difficulty reading tiny print.
  • Functional presbyopia involves the individual noticing increased problems with seeing nearby objects.
  • Absolute presbyopia explains when the individual's eyes fail to focus on up-close objects at all.
  • Premature presbyopia is used to explain when the refractive error occurs prior to the age of 40.
  • Nocturnal presbyopia explains when focusing on nearby objects is difficult at night or in low light conditions.

Questions? Contact Us Today!

For more than 100 years, Keeler has been a leader in the optometric industry, offering cutting-edge diagnostic equipment and solutions. We offer a vast range of different ophthalmic and optometry supplies and equipment, including: 

We regularly partner with different high-tech ophthalmic solution providers, general medical instrument manufacturers, veterinary diagnostic specialists, and more to provide specialized OEM manufacturing.

About the Author Eugene VanArsdale

Eugene is the Director of Marketing Communications at Keeler Instruments. He has been with Keeler since 1982 and is co-holder of two patents for the company. Eugene has a true passion for the eye care industry and has dedicated himself to understanding the ins and outs of the optometric and ophthalmic equipment market.