What is Refractive Error?
Worldwide, refractive error is the leading cause of correctable vision impairment. Estimates indicate that more than 2.3 billion people in the world suffer from poor vision due to refractive error; of those 670 million people are considered visually impaired because they do not have access to corrective lens treatment. Refractive errors, if left uncorrected, result in an impaired quality of life for millions.
Refractive errors are the result of a mismatch between the eye’s focal power and axial length. Refractive error can change throughout life. Most newborns start out with hyperopia and against-the-rule astigmatism and shift towards with-the-rule astigmatism by 4 years of age. Adults tend to be less hyperopic but have a broad variance of refractive error. There is an increase in myopia with age over time.
- Uncorrected refractive error is the leading cause of correctable visual impairment.
- There are 3 types of refractive error: myopia, hyperopia, and astigmatism
- Refractive error is due to a combination of genetic and environmental factors.
Understanding Refractive Error
Myopia, hyperopia, and astigmatism are the 3 types of refractive error. Presbyopia is not traditionally considered a refractive error by researchers though it is diagnosed and treated similar to other refractive errors. Prebyopia is due to aging.
Myopia, also known as near-sightedness, is a very common condition that typically begins in childhood. Severe forms of myopia (pathologic myopia) are associated with an increased risk of retinal detachments, glaucoma, and cataracts. Myopia affects all populations but is reaching epidemic proportions in East Asia.
Myopia occurs when the axial length of the eye is longer than normal. When the light enters the myopic eye, it falls in front of the retina resulting in blurry distance vision.
Children typically have a small amount of hyperopia in infancy and childhood. It is extremely important that all children undergo an eye exam. Uncorrected amounts of hyperopia can lead to reading and learning difficulties and an incorrect diagnosis like ADHD and dyslexia.
Hyperopia occurs when the length of the eye is short. When the light enters the hyperopic eye, it falls behind the retina. Depending upon the amount of hyperopia, the patient may be able to focus the light onto the retina resulting in good visual acuity. If the amount of hyperopia is too great, the individual may experience blurry vision at distance and at near, eye fatigue, headaches, and eye strain. Hyperopia can also result in strabismus (crossed eyes).
Astigmatism is a common refractive error and accounts for 13% of the refractive errors of the human eye. Ocular astigmatism can occur because of unequal curvature along the two principal meridians of the anterior cornea, posterior cornea, unequal curvatures of the front and back surfaces of the crystalline lens or unequal refractive indices across the crystalline lens (internal astigmatism).
Astigmatism is often described as the eye being shaped like a football or egg. When the light hits the oblong shape of the eye, it results in a blur or halo effect around letters or numbers. Uncorrected astigmatism can lead to headaches, eye fatigue, and blurry vision.
Presbyopia is not traditionally considered a refractive error. Presbyopia is the loss of near vision with age due to changes in the crystalline lens inside the eye. Individuals over the age of 40 experience presbyopia when the lens begins to harden and is unable to bend and flex to focus on near images.
Risk Factors for Refractive Error
Refractive error varies across geographic, racial, age, and ethnic boundaries. Myopia is caused by both genetic and environmental risk factors. Myopia occurs in more than 50% of the population in many industrialized countries and is expected to increase with digital device use. The prevalence of myopia is higher in individuals whose parents are myopic, suggesting that genetic factors are involved. Population studies suggest that myopia development is associated with education, and the amount of time spent doing near work.
A greater risk of astigmatism is associated with Hispanic, African American, and Asian race, and myopic and hyperopic refractive errors. It is unknown why astigmatism develops although theories include genetics, extraocular muscle tension, visual feedback, and eyelid pressure.
Age is the primary risk factor for presbyopia.
Refractive Error Symptoms
- Decreased vision
Diagnosing Refractive Error
A comprehensive eye exam including a refraction is used to determine refractive error. Refraction is the test when you view the letters and the doctor asks you which appears better, choice one or choice two. The refraction process has not changed in over 200 years.
Refractive Error Treatment
An individual is unable to cure refractive error naturally. A refractive error can be diagnosed, measured, and corrected with the aid of optical correction such as spectacles and contact lenses or by refractive surgical procedures.