What are refractive errors?
Refractive errors include aniseikonia, anisometropia, aphakia, astigmatism, hyperopia (farsightedness), myopia (nearsightedness), and presbyopia. During the refraction process, the cornea and lens bend light to focus it on the retina. When light hits the retina in the correct place, it produces good vision. In people with refractive errors, the light is not bent correctly and hits the retina in the wrong place. This results in imperfect vision.
These eye conditions are very common and most people have one or more of them. Most refractive errors can be corrected with eyeglasses or contact lenses. Ocular surgery, including laser assisted procedures, have becoming a popular option to correct refractive errors.
What is myopia (nearsightedness)?
In nearsighted or myopic eyes, the cornea is curved too much, or the eye is too long, and light gets focused in front of the retina. As a result, people with myopia can clearly see close objects, but distant objects appear blurry.
What is hyperopia (farsightedness)?
In farsighted eyes, the eye is too short, and the cornea is not curved enough. Images are focused on a point behind the retina. This makes close-up objects appear blurry, while faraway objects are seen clearly.
What is astigmatism?
Astigmatism is a condition in which the cornea is curved unevenly. A normal cornea is round, with even curves from side to side and top to bottom. With astigmatism, the cornea is shaped more like the back of a spoon, curved more in one direction than in another. This causes light to have more than one focal point and focus on two separate areas of the retina. As a result, people with astigmatism have blurry and distorted vision for both faraway and close-up objects.
Astigmatism rarely happens on its own. It is usually happens with myopia or hyperopia. Two-thirds of Americans with myopia also have astigmatism.
What is presbyopia?
Presbyopia is a condition that causes problems with focusing on objects up close. This usually happens with age. Around 40 to 50 years old, a person with presbyopia might realize he or she has trouble reading words at close distances. The letters of the phonebook might be “too small” or he or she has to hold the newspaper farther away to see it clearly. While people with presbyopia have problems with close-up vision, their ability to see far away objects stays the same.
What is aniseikonia?
Aniseikonia is a condition that causes each eye to perceive the size of objects differently. Aniseikonia is considered clinically significant when the difference of the images between the eyes is greater than 0.75%. Around 20-30% of the general spectacle wearing population may have a measurable amount of aniseikonia, however only 5-6% are clinically significant. There is no prevalence for gender, ethnicity or age. Symptoms can include headaches, double vision, disorientation or dizziness, tearing, eye pain or fatigue, sensitivity to light, visual acuity issues, trouble reading, as well as nausea. Aniseikonia can occur naturally or as an unintended consequence of ocular surgery.
Retinal Aniseikonia can occur when retinal damage causes distortion in size perception, (e.g. macular degeneration). Neurological Issues such as a lesion on the occipital lobe of the brain can also cause aniseikonia. Different visual tests, which focus on the perception of binocular space, and the direct comparison of image size perceived by each eye, are administered to diagnose aniseikonia. Spectacle and medical contact lenses are the current treatment for aniseikonia.
What is anisometropia?
Anisometropia is a condition in which each eye has a significant difference in refractive power, meaning that each eye has different ability to focus on objects at a distance. Most people have some difference in the focusing ability of each eye, but it is rarely noticeable in small amounts. Anisometropia occurs when the difference between eyes is intolerable. Types of Anisometropia, include: simple anisometropia, which occurs when only one eye has a refractive error, and the other eye can focus normally; iso anisometropia (compound anisometropia), which occurs when there is a significant focusing strength difference between two eyes in which both are myopic (nearsighted); and antimetropia (mixed anisometropia), which occurs when one eye is hyperopic (far-sighted) and the other is myopic (nearsighted). Common symptoms include diplopia, or “double vision,” headaches, light sensitivity, nausea, disorientation, fatigue or blurred vision.
Because each eye has a different focusing strength, both eyes are not able to focus on the same point in space together, resulting in problems with binocular vision. Because of this, there is a tendency to automatically favor one eye over the other. As a result, the less-favored eye will become weaker can result in amblyopia (“lazy eye”). This can be especially problematic in younger patients and why pediatric eye exams are recommended. Anisometropia affects approximately 6% of children between the 6 and 18 years-of-age, and there is a greater than 6% prevalence in older adults.
Anisometropia can result from ocular trauma, is more prevalent in myopic patients and often results from the eyes being different in shape or curvature. Tests to diagnose anisometropia, include a visual acuity test of the eyes’ ability to see various objects of different sizes clearly and retinoscopic measures of reflected light projected onto the retina to determine the refractive error of each eye. Immediate treatment is recommended when anisometropia is diagnosed, and can include:corrective contact lenses or spectacles with different prescription strengths for each eye; “patching” or covering one eye to strengthen the less-favored eye which may have become weakened by nonuse; or laser surgery to correct refractive error, making the eyes more similar in focusing power.
What is aphakia?
Aphakia is a condition where the eye is missing an important focusing structure called the crystalline lens (often shortened to “lens”). The lens accounts for approximately one-third of the eye's focusing power, and when it is missing, vision is drastically affected. The lens is located directly behind the iris. The Iris is the colored part of the eye that relaxes and constricts to control pupil size, therefore, controlling the amount of light that enters the eye. There are surgical, congenital and traumatic causes and risk factors for aphakia. Surgically, aphakia is caused by the removal of the lens when it has become clouded by cataracts.
Congenital factors also can cause some people to be born without a functioning lens. Injury to the eye which displaces the lens is another cause for aphakia.The most common symptom of aphakia is hypermetropia, or what is commonly known as “far-sightedness,” in which people are able to see distant objects better than near objects. When the lens is missing in aphakia, the eye becomes unable to focus on objects at different distances. Iridodonesis, or an iris instability, is another symptom of aphakia. Contact lenses are the preferred treatment, over spectacles, with different prescription strengths for each eye.