Most of us have seen bifocals. We understand that as a person ages their eyes lose the ability to accommodate (adjust their focus to view close objects). This is the time of life when we begin to wear reading glasses or some form of multifocal, if we need correction for distance and near viewing. When a young child’s prescription calls for a bifocal it is usually for a very different reason and therefore the glasses must be designed and dispensed for the special requirements of the pediatric client.
When a doctor prescribes a bifocal for a child it us often because a child has strabismus. Bifocals are also used for amblyopia and anisometropia. Strabismus is when both eyes do not align simultaneously. One or both eyes will look in an opposing direction. The brain cannot fuse the signals coming from the divergent gazes into one image and will select one eye from which to accept signals and suppress the signals coming from the other. This creates amblyopia or lazy eye. Anisometropia is when there is a significant difference in the refractive power of each eye. Sometimes one eye is hyperopic (far sighted) and the other is myopic (near sighted) sending different image sizes to the brain. Again, the brain has difficulty making one image from these unequal sizes and will chose one eye to use and suppress the signals from the other resulting in lazy eye. The prescribing doctor will prescribe a moderately strong plus lens. Most children are slightly near sighted and adapt to the added plus of the prescription. The stronger power is used to draw the eye to the optical center of the lens. Our eyes are naturally drawn to the optical center which is the point in a lens where the light passes straight through without being deviated by the prismatic effect of the refractive power in the lens. You can imagine it as the lenses having a magnetic effect on the eyes. Both eyes will now be sending signals to the brain that will be used to create one image. The eye that had been suppressed is active and the muscles controlling eye movement are being strengthened and trained. So why the bifocal?
Children usually do not have difficulty accommodating for near viewing. However sometimes one or both eyes will turn inward when looking at near objects (esotropia) and the doctor will prescribe a bifocal to help keep the eyes aligned when reading or other engaged in other close range activities. Because the child does not perceive a focusing problem they are not naturally attracted to looking through the bifocal as an adult is. It is necessary to design the glasses so that the child has no choice but to look through the bifocal. A lined bifocal is used for younger children instead of a no-line progressive bifocal. The reading power in the progressive no-line is much lower in the lens than in the lined bifocal and the child would rarely look that far down. The lined bifocal is placed high, usually bisecting the pupil or placed just below it. This way anytime the child looks down their gaze will be through the bifocal. Many children will unconsciously try to avoid any part of the lens altogether so it is very important that the frame fit the child well and be adjusted properly. The glasses are likely slip some and this must be taken into consideration. With properly designed and fitted glasses the child’s visual system will be kept active at this crucial developmental time. This greatly enhances the child’s ability to learn and develop motor skills.
These types of visual problems are not always easy to detect. School screenings or looking at an eye chart in the pediatrician’s office are not enough to detect them. With an estimated 5% of all children having some type of strabismus it is important to have your child’s eyes examined by an eye doctor as early as possible, certainly before entering school.