Why do some people need glasses?
Why do some people need glasses?
Eyes are like cameras, photographic cameras or video cams. They are special optic capable to focus parallel light rays at a plane on the retina called macula to have clear vision. This ideal state is known as emmetropia.
Ametropias on the other hand are not ideal conditions where an eye that is not capable of focusing light rays on the retina and this results in blurry vision. These are eyes are in need of optical to correction to focus properly.
Ametropias are also known as refractive errors.
- Hyperopia or farsightedness happens when an eye is too short and the image is focused on a plane behind the retina. These patients experience blurred images at near. Some children can adapt to this condition by a mechanism called accommodation and this is particulary important in condition like esotropia ( convergent strabismus) and amblyopia ( lazy eye). Most children are born hyperopic and it usually resolves during childhood if it is not high. Symptons of hyperopia are headaches, blurred near vision , sensitivity to light and crossed eye sensation. Treatment is usually optical correction with glasses or contact lenses and should be prescribed according to the age and symptoms of the patient. Patients with strabismus should be given full prescription as part of the treatment for their muscular imbalance.
- Myopia or nearsightedness occurs with an eye that is too long and light rays are focused on a plane in front of the retina. Myopia can be progressive because the eye can get longer as the patient grows. Most cases are detected by 9 years of age. Symptoms associated with myopia are blurred distance vision and headaches. These patients usually squint to focus at distance resulting in headaches. A myopia that progresses as much as 4 diopters a year is called pathological myopia and can be associated to problems like retinal detachment. These patients should have routine follow ups with a retina specialist. Prescription must be given according to the patient ´s age and symptoms by an expert
- Astigmatism is a little more complex to understand. This condition is related to the shape of the cornea, the transparent frontal part of the eye. Ideally the cornea is round and symmetrical like a sphere but in astigmatism is shape more like a football or an egg resulting in a lens that cannot focus light rays properly. These patients may experience headache, head tilting and turning, tired eyes or asthenopia and blurred vision both distance and near. Optical correction is needed with prescribed cylindrical glasses
- Presbyopia is a decrease in the capacity of the crystalline lens to perform the accommodation needed for focusing objects that are near. This lens functions like the zoom of camera, if the object is near the lens increases its power to focus. The most important sign is difficulty reading and adjustment for near vision distances, headache and tired eyes. It presents itself around 40 years of age.
There is also a special condition called anisometropia . This is a state where there is a difference between the refractive errors in both eyes. This means that one eye can be emmetropic( perfectly focused) and the other eye can be extremely myopic and astigmatic or one eye hyperopic and the other highly myopic. You get the idea. This condition is extremely important because it can lead to profound amblyopia ( lazy eye) and it can only be diagnosed by a complete examination by an expert specially in childhood. Because in the majority of patients one eye is seeing well and the other one is not, is very hard to detect it because we are always seeing with both eyes open and the better eye is doing all the work while the other one is becoming lazy. Most children are diagnosed because they have developed secondary strabismus or in the office of an ophthalmologist during a routine examination. Patients with family history of anisometropia should be evaluated at an early age.
Treatment for refractive errors can be achived by prescription glasses , contact lenses or refractive surgery. The therapy should be decided by an expert after a complete exam.
Dr. Ricardo Toro
Pediatric Ophthalmology and Strabismus UBC
International Member of the American Association for Pediatric Ophthalmology and Strabismus AAPOS