The terms refraction error and refractive error are the same and refer to an issue where light cannot be accurately focused on the retina because of the shape of a person’s eye. There are multiple forms of refractive error, but the more commonly known problems include far-sightedness, near-sightedness, presbyopia, and astigmatism. Those who suffer with near-sightedness experience blurred objects when far away, meanwhile, those with presbyopia or far-sightedness are the opposite – objects become blurred up close. However, astigmatism can result in objects appearing blurry at any distance, or stretched out. Additionally, symptoms one my encounter includes headaches, double vision, or eye strains.
The shape directly affects the persons quality of vision. For instance, near-sightedness is caused form the eyeball’s length being longer than it should be, while far-sightedness is the result of eyeballs being shorter than it should be. Presbyopia occurs from aging lens preventing it to from sufficiently changing shapes. Astigmatism happens when the cornea develops improper shapes. Study’s have shown certain refractive errors could be genetic, therefore, children of affected parents are at higher risks. An eye examination is required for a proper diagnosis.
There are several approaches for correcting refractive errors, depending on the severity of the problem. The treatments range from contact lenses or eye glasses to surgery. The simplest approach is eye glasses, and often the cheapest route, followed by contact lenses which offer a larger field of view, but have risk of infections due to finger to eye contact. Finally, for severe cases surgery could be required, which permanently changes the cornea’s shape.
It is estimated that as many as 2 billion suffer from these type of vision problems. The numbers vary greatly between regions as well, with an estimated 80% of Asians being impacted, while only 25% of Europeans are affected. The leading disorder is near-sightedness, with 15% to 49% of adults being affected and between 1.2% to 42% in children.
Kids and the elderly are more affected by near-sightedness, while the majority of people with presbyopia are over 35 years old. In 2013, it was estimated that 10 out of 100 people went without correcting their refractive errors, or around 660 million. From those, nearly 10 million was blinded by their conditions, being a leading cause for vision loss in addition to vitamin A deficiency, macular degeneration and cataracts.
When the eye does not have defects in vision it is referred to as being emmetropic, which means it has the ability of focusing rays of light from distant objects to the retina with no help. In this situation, distance can be described as further than 20 feet away, because the light from objects at this distance is mainly parallel rays in reference to the limited perception of humans.
Meanwhile, when the eye has defects in vision when viewing objects from a distance, it is referred to as being ametropic. Therefore, the person lacks the ability of focusing rays of light from distance objects to the retina, or requires assistance to be able to.
There are several forms of ametropia, including hyperopia, myopia, and astigmatism and are commonly defined as being spherical or cylindrical errors.
These happen as the eyes’ optical power is too small, or becomes too large to properly focus the light to the retina. Those with these issues commonly experience blurred vision.
As optics become too weak in comparison with the eyeball’s length, this is known also known as hyperopia. The crystalline lens or cornea lacks appropriate curvatures (refractive hyperopia, if it’s short (axial hyperopia) or too long these can be fixed using convex lenses to direct the light into the cornea.
This happens when the opitcs become too strong in comparison with the eyeball’s length, this is also known as myopia. The crystalline lens or cornea developed too much curvature, or not enough length. The use of concave lens’ can correct these conditions by displacing rays of light prior to hitting the cornea.
This causes a person to visualize things less clearly and/or with lines. Additional light must be refracted within one meridian compared to the other, the use of cylindrical lenses works for this.
As the lens loses flexibility over time, people can begin having issues with viewing things up close, such as reading. Often times these conditions can be mitigated by the use of progressive lenses, bifocals or reading glasses.
However, when the eye’s optical power becomes too great or weak throughout a single meridian, it can result in astigmatism. As the curvature of the cornea can become more cylindrical. This is referred to as the cylinder axis.
Genetic Risk Factors
There has been study’s suggesting genetics have a role in vision conditions. This puts people at higher risk who have a family history of these symptoms. For instance, myopia could be a heritable connective tissue disorder associated with a number of diagnoses, including Stickler syndrome (type-1 and type-2), Marfan syndrome, and Knoblock syndrome. Furthermore, there have been reported myopia cases connected with X-linked disorders as a result of mutations.