Refractive Errors

What Are Refractive Errors?

The cornea and lens bend or refract light rays so they can be focused on the retina, the nerve layer that lines the back of the eye. The retina receives the picture formed by these light rays and sends the image to the brain through the optic nerve. A refractive error means that the shape of the eye does not allow the light to be properly refracted making images blurry.

An intraocular lens (IOL) is a clear, artificial lens surgically implanted inside the eye to replace the natural lens when it is clouded by a refractive error or a cataract. An IOL acts like the eye’s natural lens by focusing light that enters the eye through the cornea and pupil onto the retina. Similar to prescription glasses or contact lenses, the IOL is chosen to match the patient’s visual needs.

Types of refractive errors include:

• Myopia (near-sightedness)
• Hyperopia (far-sightedness)
• Astigmatism
• Presbyopia
• Monovision
  
  

Myopia (near-sightedness)

Myopia (near-sightedness) occurs when light rays are focused in front of the retina instead of directly on the retina. Myopia is a vision problem experienced by approximately one-third of the population. When the eyeball is too long from front to back, the image of a distant object focuses in front of the retina, instead of directly on it. As a result, the distant object appears blurred. The more myopic the eye, the closer an object must be before it is in sharp focus. Near-sighted people have difficulty seeing objects at a distance, such as highway signs, but usually can see up-close for tasks such as reading or sewing.

Some people with myopia can use their natural near-sightedness to read without glasses at an age when other people must wear reading glasses. However, if they have refractive surgery to correct myopia, they may be able to see distant objects without glasses, but will probably need to wear glasses to read sometime after age 40, due to presbyopia.

Signs or symptoms:

• Headaches
• Eyestrain
• Squinting
• Fatigue while driving or playing sports

Patients who experience these symptoms while wearing glasses or contact lenses may need a comprehensive eye examination, as well as a new prescription.

Causes: Near-sightedness runs in families and usually appears in childhood. This vision problem may stabilize at a certain point, although sometimes it worsens with age. This is known as “myopic creep.”

Treatments: Non-surgical treatment options for myopia include glasses and contact lenses. Surgical treatment options include clear lens extraction, LASEK, LASIK and phakic IOL. While there are numerous surgical options available, not all individuals are good candidates for specific procedures. Patients should review these options in depth with their physicians prior to making any final decisions.

Hyperopia (far-sightedness)

Hyperopia or farsightedness occurs when light rays are not bent enough to focus on the retina. Hyperopia is a common vision problem, affecting about one-fourth of the population. If the eye is too short from front to back, light rays reach the retina before they converge (focus). People with hyperopia can sometimes see distant objects very well, but may have difficulty seeing objects that are close.

Young eyes can sometimes compensate for this refractive error —- depending on age and the degree of hyperopia present. But with aging, the human lens loses this ability and a hyperopic person eventually may have difficulty seeing objects at a distance, as well as those that are nearby. In fact, by age 40, even those with little or no refractive error will begin to experience difficulty focusing on close objects.

Signs or symptoms:

• Headaches
• Eyestrain
• Squinting
• Fatigue while performing work at close range

Patients who experience these symptoms while wearing glasses or contact lenses may need a comprehensive eye examination, as well as a new prescription.

Causes: Most children are born with hyperopia, but most of them “outgrow” it as the eyeball lengthens with normal growth. Sometimes people confuse hyperopia with astigmatism; both cause difficulty in seeing close objects, but have different causes.

Treatments: Non-surgical treatment options include glasses and contact lenses. Surgical treatment options include clear lens extraction, CK, PRK, LASIK and phakic IOL. While there are numerous surgical options available, not all individuals are good candidates for specific procedures. Patients should review these options in depth with their physicians prior to making any final decisions.

Astigmatism

Astigmatism occurs when light rays are focused at more than one point on the retina. Astigmatism is the most common vision problem. It occurs when the corneal surface is not ideally rounded but is curved more along one axis than the other —- that is, when the eye is shaped more like the side of a football than a basketball. Light entering the eye does not focus symmetrically on the retina. The result is astigmatism, which blurs both near and distance vision. This refractive error may occur in patients who are either myopic (near-sighted) or hyperopic (far-sighted). There are various types of astigmatism included regular, mixed and irregular astigmatism.

Signs or symptoms:

Patients with only a small amount of astigmatism may not notice it or may have slightly blurred vision.

• Headaches
• Eyestrain
• Distorted or blurred vision

Eyeglasses or contact lens prescriptions with three parts indicate some amount of astigmatism. A prescription with three parts looks like this: -2.75 -1.25 × 180. The first part indicates the main spherical correction, while parts two and three show the extent and location of your astigmatism.

Treatments: Currently, excimer lasers in the US are approved for treatment of regular and mixed astigmatism. Recent advances in technology now allow for the therapeutic treatment of induced irregular astigmatism. In the future, this application may be expanded to include all types of irregular astigmatism. Patients who believe they have astigmatism should discuss this subject in further detail with their physician to gain a better understanding of the mechanisms in their case.

Presbyopia (age-related difficulty with near vision)

With increasing age, the lens inside of the eyes loses the ability to focus on nearby objects. The problem usually starts at around age 40 and can be corrected with bifocals or reading glasses. Some people with myopia can use their natural near-sightedness to read without glasses at an age when other people must wear reading glasses. However, if they have refractive surgery to correct myopia, they will be able to see distant objects without glasses, but probably will need to wear glasses to read sometime after age 40 due to presbyopia.

Monovision

Monovision is a method of distance vision correction to account for presbyopia. In monovision, refractive surgery is used to adjust one eye for “near” vision and the other eye for “distance” vision. Contact lenses or glasses may be required for best distance or night vision activities, including driving. This option is not suitable for everyone and a trial period of monovision using contact lenses may help decide if it is right for the patient.