Glasses, Why Some People Need Them
Very few people have perfect eyesight. More than half of us need glasses to correct a vision problem some of the time, if not all of the time.
More than half of all people in the United States use some type of lens (glasses or contacts) to correct their vision. More than 12 million school-age children, or one in four, has a vision problem. One in twenty preschoolers needs vision correction. Prevent Blindness America recommends eye exams at the following intervals: birth, six months, pre-school, if there is a sign of an eye problem, and regularly throughout the school years to ensure good vision in both eyes.
If not found early, vision problems in children can lead loss of vision, learning difficulties, and delays in development. Screening is a means for targeting vision problems early in life so that they can be diagnosed and treated appropriately. Screenings are typically conducted in primary care, school, and community settings. Screening is not for diagnosis or to determine treatment, but to provide information.
Eye Care Professionals
There are three main types of eye care professionals. Only an eye doctor can conduct an eye exam, diagnose an eye problem and correct it with glasses or other treatment.
Ophthalmologist
An ophthalmologist is a physician (doctor of medicine or doctor of osteopathy) who specializes in the comprehensive care of the eyes and visual system in the prevention of eye injury and disease. The ophthalmologist has completed four or more years of college premedical education, four or more years of medical school, one year of internship, three or more years of specialized medical and surgical training, and clinical experience in eye care.
The ophthalmologist is a physician who is qualified by lengthy medical education, training, and experience to diagnose, treat, and manage all eye and visual system problems and is licensed by a state regulatory board to practice medicine and surgery. The ophthalmologist is the medically trained specialist who can delivery total eye care: primary, secondary, and tertiary care services and diagnose general diseases of the body.
Optometrist
Doctors of optometry, optometrists, are independent primary health care providers who specialize in the examination, diagnosis, treatment and management of diseases and disorders of the visual system, the eye and associated structures, as well as the diagnosis of related systemic conditions. The optometrist has completed pre-professional education at a college or university, four years at a college of optometry, and in some cases, a residency. Doctors of optometry are specifically trained and state licensed to provide primary eye care services. These services include comprehensive eye health and vision examinations; diagnosis and treatment of eye diseases and vision disorders; the prescribing of glasses, contact lenses, low vision rehabilitation, vision therapy, drugs and medications; and the counseling of patients regarding their vision needs as related to their occupation, avocations and lifestyles.
Optician
Opticians are professionals in the field of designing, finishing, fitting, and dispensing of eyeglasses and contact lenses, based on an eye doctor's prescription. The optician also may dispense colored and specialty lenses for particular needs, as well as, low-vision aids and artificial eyes.
Visual Acuity
Vision may be tested in a number of ways. Visual acuity testing is the primary measure of the visual system. Visual acuity is the keenness of perception or the ability to discern fine visual differences. Visual acuity testing is performed to determine the integrity of the eye's neural elements, the accuracy of retinal focus, and the interpretive faculty of the brain. Visual acuity is recorded as a fraction, i.e. 20/20. The larger the bottom number the worse the vision. The top number represents the distance from the person to the chart. The bottom number indicates the smallest line a subject could read correctly from the chart. The LEA and the ETDRS charts are most typically used to measure acuity.
LEA & ETDRS Charts courtesy of National Eye Institute
The term 20/20 vision describes how a person sees at a distance. With 20/20 vision an individual sees clearly at 20 feet what the person with average vision sees at 20 feet. If someone is described as having 20/200 vision, they must be as close as 20 feet to see what a person with normal vision clearly sees at 200 feet.
Many individuals with less than normal vision can achieve 20/20 vision through the use of contact lenses or glasses. Twenty/twenty vision does not necessarily indicate perfect vision. Additional factors such as near vision, peripheral vision, eye muscle coordination, depth perception, and color distinction are included in the determination of perfect vision.
If young children are unable to identify their letters, a LEA chart may be used for acuity testing. LEA symbols consist of a house, apple, circle, and square; these symbols should be presented to the child before formal vision testing to ensure that they can be correctly identified. Flash cards containing the symbols are displayed for the child. At least 3 of the 4 figures should be identified for each size or distance. The visual acuity is determined by the smallest symbols that the child is able to correctly identify at 10 feet. A matching activity is available with the LEA test that may be helpful in testing very young children.
Lea Symbols
A Little History
The first eyeglasses were two magnifying glasses connected together at the end and hung over the nose. They date back to the late 13th century. The invention of the printing press in 1440 increased the number of reading materials available and thus the need for eyeglasses for many. By the mid-1500s eyeglasses had become a trend and were worn even by those who could not read. In 1508 Leonardo daVinci developed the concept of contact lenses, but they were not produced until 1887. These lenses, however, were made of glass and were extremely uncomfortable and impractical. Practical lenses appeared in 1938.
Recently some people are opting for laser surgery for the permanent correction of their vision. The surgery, however, is not for everyone; it does have risks.
Lenses
Lenses are classified by shape as either converging or diverging and as spherical or cylindrical. Each type is used to correct different vision problems. These two terms, converging and diverging, refer to the effect a lens has on parallel beams of light. Spherical surfaces are the most common and used for basic refractive errors such as myopia (nearsightedness), hyperopia (farsightedness), and presbyopia (aging eye). Refractive error is a defect in the optics of the eye that results in a lack of precise focus of the light rays on the retina, causing a blurred image. Light rays entering the eye cannot be brought to a single focus. Instead they may focus in front of, in back of, or irregularly on the retina.
Eye anatomy leading to refractive errors.
Some vision problems are more common than others. The predominant eye problems seen in children include nearsightedness, crossed eyes, and lazy eye. Nearsightedness is the most common as it affects more than 25 percent of the population.
Common Refractive Errors
Myopia (Nearsightedness)
Myopic or nearsighted people generally can see near objects clearly, but distant objects are out of focus. This is the result of a refractive error of the eye in which the image of a distant object is formed in front of the retina and cannot be seen distinctly; near objects are seen more clearly than distant objects.
- Myopia is thought to be primarily hereditary in nature.
- Myopia, the most common refractive error, affects more than 25% of the population.
- Myopia usually becomes evident in children between the ages of 8 and 12 and worsens until early adulthood.
- Myopia is rare in infants and toddlers.
- Myopia that starts in infancy can be more severe than in school-age children.
Hyperopia (Farsightedness)
Hyperopia occurs when the eyeball is too short. The reduced length means the point of focus lies beyond the back wall of the eye, and light rays are not yet in focus when they arrive at the retina. Hyperopic or farsighted people generally can see distant objects clearly, but near objects are out of focus. In more severe cases of hyperopia, even distant objects can be blurred.
- Hyperopia usually exists in infancy – as the child grows, so does the size of the eye. Most children lose much of their hyperopia by the time they are teenagers.
- Despite have hyperopia, most children can see well at all distances because the accommodation provided by the lens is enough to counteract minor refractive errors.
Astigmatism
The curvature of the cornea and/or the lens prevents light rays from focusing on a single point on the retina, resulting in a blurred image. Visual acuity is poor for near and far objects.
- Astigmatism is an overall inability of the eye to focus clearly at any distance, usually because of uneven curvatures of the cornea. Essentially, the cornea is oval, having a surface shaped more like a football or the back of a spoon, rather than being rounded like a basketball.
- Virtually all corneas have at least a mild degree of astigmatism. For many, the resulting distortion is not discernable. But as the curvature of the cornea becomes more uneven, image distortion increases.
- Astigmatism often is inherited
Common Eye Problems
Strabismus (Cross-eye)
Strabismus refers to eyes that are not straight or properly aligned. As a result of eye muscles not working together, one eye may turn in (crossed eye), turn out (wall eye), turn up, or turn down. The deviation, or eye turn, may be constant or come and go. In some instances, it alternates eyes – first one eye turns and then the other. In very young children, their often is an appearance of false or pseudostrabismus caused by a wide spacing of skin between the eyes that covers more of the white of the eyes than in adults. Pseudostrabismus is a common source of vision screening over-referral that can be prevented by a correctly performed vision screening.
It is critical for strabismus to be diagnosed and corrected at an early age because children with uncorrected strabismus may go on to develop amblyopia, a loss of vision in an eye that has not been used. In young children, strabismus may vary not only from one day to the next, but during the course of a single day. The condition usually will worsen if the child is ill, upset, or tired.
The preschool years are critical in the development of a child's eyes. Parents/caregivers and health care professionals always should be vigilant of any misalignment in one or both eyes. In addition to hindering the development of useful vision, strabismus may affect a child's personality. Children with strabismus may become embarrassed by their problem, feeling that they look different.
- Strabismus affects approximately 3 to 5% of children in the U.S. Half of those with strabismus are born with the condition or usually develop it within the first six months of life.
- Some of the most common causes of strabismus are birth injuries, hereditary, faulty muscle attachments, need for glasses, and illness.
- Strabismus sometimes can be found in conjunction with cerebral palsy, prematurity, and neurodevelopmental conditions. Three out of four children with cerebral palsy have strabismus.
Amblyopia (Lazy Eye)
amblyopia vision and normal vision
Photo source: National Eye Institute
Amblyopia is reduced vision in an eye that has not received adequate use during early childhood. An estimated 2 to 5% of the general population suffers from this visual impairment. If not treated early enough, an amblyopic eye may never develop good vision and may become functionally blind. A condition that causes amblyopia and is left untreated until about the age of 6 most often will result in some permanent visual impairment. However, it is important that the treatment of amblyopia be pursued until at least age 10. The critical age for treatment to prevent permanent vision impairment varies from individual to individual. The earlier treatment is started, the more likely it will be easy and successful.
- Strabismus, ptosis, cataracts, and refractive errors can lead to amblyopia.
- Amblyopia is detected by finding a difference in vision between eyes.
- Amblyopia treatment involves two steps. First, correct the underlying vision problem if the amblyopia is caused by a refractive problem. Second, correct the amblyopia by retraining the brain.
Causes of Amblyopia
Amblyopia may be caused by several conditions.
Strabismus
When one eye turns while the other is in straight gaze, a double image is sent to the brain. The brain solves the confusion by ignoring the message from the turned eye, which weakens from lack of use. However, early diagnosis and treatment can restore sight. As with visual acuity problems, generally, the earlier the treatment, the better the opportunity to prevent permanent vision loss. If the strabismus requires a surgical correction, the amblyopia must be corrected first.
Anisometropia
Both eyes may be nearsighted or farsighted, but to differing degrees. Alternately, one eye may be farsighted and the other nearsighted. When there is a marked difference in refractive error between the eyes, the brain sees differing images from the two eyes and eventually ignores the eye with the poorest image.
Other factors
Other factors causing a difference in image quality between the eyes, such as cataracts or drooping eyelids (ptosis), can cause amblyopia. The brain suppresses the image of poorer quality, causing permanent vision loss in the affected eye, unless treated. Any condition that causes the brain to receive images of unequal quality from the two eyes can lead to amblyopia.
Color Vision Deficiency (Colorblindness)
Children with so-called "color blindness" are not blind to color but have difficulty identifying certain colors.
Blindness
Blindness
Photo source: National Eye Institute
The leading cause of childhood blindness worldwide is a deficiency in vitamin A, also referred to as xerophthalmia. The most common causes of blindness in young children in the United States are congenital cataracts, retinopathy of prematurity (ROP), and other complications caused by premature births. Prenatal cataract is a leading cause of legal blindness of children under age 5. However, when appropriately managed, there has actually been a decrease in prevalence.
Advances in medicine are allowing medical professionals to save smaller and more premature infants. The lower the birth weight of a premature infant the higher the risk for and impact of abnormality.
Premature infants are born pre-term; a full-term pregnancy is 38-42 weeks. During the last 12 weeks of the pregnancy the baby's eyes develop rapidly, with the retina completing its growth a few weeks to a month after birth. In the eye of the premature infant, if the blood vessels may not have reached the edge of the retina, then the retina may not get enough oxygen and nutrients, and abnormal blood vessels will develop. These abnormal vessels are fragile and weak and can bleed, leading to retinal scarring. As scars shrink, the retina is pulled and detaches from the back of the eye. ROP does not occur in all premature infants, but when it does, it usually develops in both eyes.
Treating Eye Problems
Treatment to correct for eye problems includes:
- Glasses to compensate for refractive error, correct a focusing problem, or overcome an eye turn
- Medications (eye drops or ointments) to treat infections, glaucoma, and occasionally strabismus or amblyopia
- Patching one eye is common in treating amblyopia
- Surgery to remove the lens if it has a cataract, reduce the pressure of glaucoma, halt vision loss due to diabetic retinopathy, or adjust an eye muscle if strabismus exists.
