Why Keratoconus Causes Nearsightedness

Errors in the way light is refracted through the lens or cornea of the eye can result in distortions in vision. Nearsightedness occurs when light is focused or refracted in front of the retina rather than on it. Around 42 percent of children in North America (aged 6-19) are nearsighted, according to an article in BMC Ophthalmology in 2020. While nearsightedness (termed myopia) is due to abnormal refraction consequent to lens shape in most people, keratoconus-afflicted people experience myopia due to a refraction error caused by an abnormally-shaped cornea (though an abnormal lens shape may also contribute to the myopia).

How Cornea Shape Causes Myopia

The normally round-shaped cornea is elongated into a cone-shape in keratoconus (per the American Academy of Ophthalmology). Light enters the eye first through the cornea, passes through the pupil, and and then continues through the lens of the eye. After “hitting” the retina, the nerve-stimulating, inverted image is essentially conducted through the nerves of the retina via the optic nerve to the brain. As described by Johns Hopkins Medicine, the brain then translates the image into what is viewed by the person.

Not only does the abnormally-shaped cornea cause myopia in a person with keratoconus, but it can cause other vision-impacting symptoms such as sensitivity to sunlight and glare. Meanwhile, myopia resulting from keratoconus tends to be progressive until about 40 years of age.

The usual way that either children or adults are treated for myopia is corrective lenses (such as eyeglasses or contact lenses). However, keratoconus-afflicted people do not have the same breadth of choices for corrective lenses as people without an abnormal cornea.

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Corneal Fibers – How Keratoconus Can Affect Their Strength

An article in Biophysical Reviews notes that collagen – the main, dry, structural component of the cornea – can aid the cornea in maintaining its shape (and collagen is composed of a multitude of fibers arranged in a particular structure). In keratoconus, the fibers in the cone-shaped cornea can become weakened – causing worsening of the cone-shape, blurred vision, and corneal pain.

Not everyone living with keratoconus will develop weakened corneal fibers to the point of corneal pain, but a procedure called corneal cross-linking is sometimes performed to improve keratoconus symptoms in keratoconus-afflicted people with corneal pain.

Astigmatism in Keratoconus-Afflicted People – Its Ramifications

Besides myopia, people afflicted with keratoconus often experience a vision distortion termed astigmatism. The condition of astigmatism causes objects – regardless of whether near or far – to appear blurry, and this is also due to a refraction error. However, the cone-shaped cornea causes astigmatism, so most keratoconus-afflicted youth diagnosed with myopia will develop astigmatism if not already existent.

The problem for people experiencing both nearsightedness and astigmatism is that traditional “soft” contact lenses only correct for myopia. Therefore, people who have an astigmatism are typically prescribed “hard” contact lenses (which are Rigid Gas Permeable [RGP] lenses) rather than “soft” contact lenses.

Eye Pain in Keratoconus-Afflicted People

A weakened cornea is more susceptible to an abrasion (scratch) and/or infection, and thereby become painful. Three other reasons for the eye pain experienced by a significant percent of people living with keratoconus are:

  • Development of “dry eye” syndrome caused by a weakened (thinned) cornea, increasing the risk for corneal abrasions and infection.
  • Pressure on the cornea by small-diameter contact lenses (such as standard “soft” and “hard” lenses) resting on the cornea, and resulting in cornea pain.
  • Eye strain due to frequent night-driving in a person with light-sensitivity to the glare of car headlights, and who does not wear corrective lenses that decrease such glare.

Cornea pain can preclude wearing any type of small contact lens for vision correction for more than a short period of time. Scleral contact lenses that rest on the eye’s sclera (white of the eye) rather than the cornea may be preferable in someone experiencing cornea pain.

The professionals at the Precision Keratoconus Center are available to assist you if your child (or you) has been diagnosed with keratoconus.

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