By Eyecare NGOverview | Types | Sign & Symptoms | Detection & Diagnosis | Treatment
Refractive errors are optical defects that cause light rays not to focus properly on the retina. This causes blurring of vision.
Refractive errors in children are usually a result of abnormalities in the length of the eyeball and/or in the curvature of the cornea or lens. They are the most common eye problems in children.
About 20% of children have refractive errors. Some of these children compensate for these errors using the natural focusing ability of the lens and gradually outgrow them with age. In others, the refractive errors are so severe that they interfere with learning and development. Therefore, it is important that children with refractive errors are monitored and given treatment, if necessary.
Types and causes of refractive errors
Types of refractive errors include myopia (short-sightedness), hyperopia (long-sightedness) and astigmatism.
Myopia or short-sightedness:
In myopia, the light rays entering the eye focus in front of the retina. This occurs either because the eyeball is too long or the cornea is too curved. Children who have myopia find it difficult seeing things clearly at far. They may bring objects closer to the eyes or move closer to screens and displays to see clearly. Myopia can be corrected with concave or minus lenses.
Hyperopia or long-sightedness:
In hyperopia, light rays entering the eye focus behind the retina. This occurs either because the eyeball is too short or the cornea is too flat. Children with hyperopia may experience blurring of vision or eye strain while looking at near objects. They may avoid reading or complain of headaches and brow ache. Hyperopia is usually corrected with convex or plus lenses. These lenses help converge light rays as they enter the eye so that rays which would normally focus behind the retina are brought forward to focus on the retina.
In astigmatism, light rays fail to come to a single focus on the retina. Rather, they form multiple foci points in a line (line of focus). This happens when the cornea has an unequal curvature resulting in unequal refraction on the different meridians of the cornea.
Children with astigmatism may experience blurring of vision while looking at both near and far objects. Objects may also appear distorted or tilted. Children may try to overcome mild cases of astigmatism by squinting and this can cause headaches, brow aches, and eye strain. Astigmatism can be corrected with cylindrical lenses.
General signs and symptoms
Children with refractive errors may:
- Hold reading materials too close to their eyes
- Move close to the television or board or prefer to sit in front of the class
- Squint or frown when reading or copying from the board
- Abandon, avoid or dislike reading or near task
- Lose focus or attention while performing a task
- Complain of headaches and brow aches when reading or writing.
- Skip lines or miss words while reading or copying from the board
- Have difficulty differentiating some letters like “C” “G” and “O”, “E” and “F”, “N”, “M” and “H”
- Have watery eyes
- Rub the eyes
- Use their fingers to guide eyes while reading
- Tilt the head
- Close one eye when looking at an object
Detection and Diagnosis
Early detection and treatment of refractive errors are critical to the prevention of complications such as amblyopia (lazy eye) as well as learning and development difficulties that may occur due to uncorrected refractive errors.
The most effective way for the detection and diagnosis of refractive errors is the comprehensive eye examination. However, vision screening is commonly done as a quick and cost-effective way of identifying children who may have refractive errors or other common eye health and vision problems that are not obvious.
Note that vision screening is generally not effective for detecting more subtle problems that may threaten children’s eyesight.
Vision screening can be done as a first step to identifying refractive errors and other common eye conditions that could impede children’s development or impact on their ability to learn or perform at school. It is not a replacement for an eye examination. Therefore, children who obviously have eye problems do not need vision screening. They should be referred directly to an eye doctor.
Vision screening is quick, cost-effective and gives every child an opportunity to have their eyes checked in a non-clinical setting. Children who fail vision screening test should be referred to an eye doctor for examination and treatment (if necessary).
In schools, teachers or school nurses can be trained to do vision screening. To test for the possible presence of a refractive error, a visual acuity chart is placed at a distance of 6 metres (20 feet) away from the reading position. The child is then asked to identify the letters on the chart. If visual acuity is reduced, the child should be referred to an eye doctor for refraction and other eye examination.
Refraction usually has two parts – objective and subjective. In objective refraction feedback from the child is not required to obtain a result; but in subjective refraction, feedback from the child is required to arrive at a result.
In most children, especially preschoolers, subjective refraction may not be useful in determining the type and degree of refractive error. Eye doctors most times rely on an objective means such as retinoscopy and autorefractometer, to measure refractive error in infants, toddlers, preschoolers and even some school-aged children who may not comply with instructions during subjective refraction.
Sometimes, the usual objective refraction is not enough. A child may continue to experience eye symptoms even after undergoing an eye exam and receiving a prescription. The reason may be that the lens in the child’s eye over-focuses during the examination and compensates greatly for the refractive error making it difficult to determine the right prescription through normal objective refraction. In this case, the eye doctor may consider a cycloplegic refraction.
A cycloplegic refraction is one done to obtain the exact refractive error of the eye without the accommodative ability of the lens. To do this, a cycloplegic drug (a drug that temporarily paralyzes the muscles that control the lens) is put into the eye and once the lens is relaxed, refraction is done.
Cycloplegic refraction is very useful for children with hyperopia (long-sightedness) who may compensate for the error by squinting. Though a useful test, it has temporary side effects such as blurring of vision for hours, inability to read small prints and increased sensitivity to light. These symptoms disappear as soon as the effect of the drug on the muscles wears off.
Correction of refractive errors in children differs from the correction given to adults. A number of factors are taken into consideration before a decision to correct or not to correct is made. Factors considered include the age of the child, the degree of error, its impact on the daily activities of the child as well as his/her overall learning and development.
- An infant with myopia may not need treatment unless the condition is severe enough to cause complications like amblyopia (lazy eye) and developmental problems.
- If a child has low degree hyperopia and is not having any symptoms like headaches; he may not be prescribed glasses but monitored. This is because children tend to be long-sighted and usually outgrow this with age.
- If there is a significant degree of hyperopia and astigmatism, a partial correction may be prescribed to encourage the child to wear the glasses. The prescription may be increased gradually over time.
- Children with squint and amblyopia are usually prescribed full correction and the prescription is reviewed periodically.
- A shortsighted child may be prescribed full correction for distance. Depending on the near prescription, he may be instructed to take off the glasses while performing near tasks such as reading. He can also be prescribed bifocal glasses if different prescriptions are required to perform distance and near tas
Good vision is essential for learning and development. Uncorrected refractive error can affect a child’s development and performance at school. Parents and teachers should pay close attention to children. Children should be observed for unusual changes and behaviours to identify early signs of eye health and vision problems. Children who show signs and symptoms of vision problems should be referred to an eye doctor for examination and treatment.
Children who have been prescribed corrective lenses should be encouraged to wear them both at home and at school.