Humans rely heavily on sight to accomplish nearly every activity of daily living. Eye abnormalities that cause mild to severe vision problems commonly affect children and adults, interfering with school, work and recreational activities. The Centers for Disease Control and Prevention reports that an estimated 14 million adults and children older than age 12 in the United States have visual impairment.
Refractive Errors
To see images clearly, the eye bends light rays to focus them precisely on the retina--the image-perceiving nerve tissue at the back of the eye chamber. Refractive errors occur due to imperfect bending of light rays, causing blurred image perception, explains the University of Michigan Kellogg Eye Center.
Different types of refractive errors occur. A person with myopia, or nearsightedness, sees close objects clearly while distant images appear blurred. Hyperopia, or farsightedness, is the opposite condition, with near images appearing blurred and distant objects seen clearly. Presbyopia is an age-related condition in which the lens of the eye loses its elasticity, causing difficulty when focusing on near images.
Astigmatism occurs when the clear eye surface, or the cornea, bears an irregular curvature that distorts all images, regardless of distance. Astigmatism may occur in addition to myopia, hyperopia and presbyopia.
Corrective lenses can remedy most refractive errors. Refractive surgery to correct some types of refractive errors is an option for people who have no contraindications to the procedure.
Strabismus
Strabismus is an eye disorder characterized by failure of the eyes to move in unison due to misalignment. Children with strabismus typically appear cross-eyed. Approximately 3 percent of children have strabismus, reports "The Merck Manual for Healthcare Professionals." Most cases are due to eye muscle imbalance or hyperopia. Rarely, tumors, nerve disorders or congenital cataracts underly this condition.
One or both eyes may be affected with strabismus. Treatment usually begins with corrective lenses, eye exercises and eye patching. If these measures fail to induce eye alignment, surgery is generally performed to align the eyes and balance the eye muscles.
Amblyopia
Amblyopia, commonly referred to as lazy eye, is a condition wherein vision does not develop properly in one eye. The brain increasingly relies on images from the stronger eye while "ignoring" input from the weaker eye. The University of Michigan Kellogg Eye Center notes that strabismus is the leading cause of amblyopia. Three-dimensional vision and depth perception require equal input and brain processing of images from both eyes. If left untreated during childhood, when the capacity for binocular vision is perfected, amblyopia can cause permanent visual impairment.
Amblyopia correction requires forcing use of the weaker eye. Patching the stronger eye to necessitate use of the weaker eye is a mainstay of treatment. Drops or lenses that blur vision in the stronger eye may also be used, with the same rationale.
References
- Centers for Disease Control and Prevention: Vision Health Initiative: Fast Facts
- University of Michigan Kellogg Eye Center: Refractive Errors
- Merck Manual for Healthcare Professionals: Strabismus: Eye Defects and Conditions in Children
- University of Michigan Kellogg Eye Center: Strabismus
- University of Michigan Kellogg Eye Center: Amblyopia
