Almost half of the visual impairment in the world can be attributed to uncorrected refractive errors, and myopia constitutes a significant proportion of this problem.
Myopia occurs when the focused image falls anterior to the retinal photoreceptor layer of the eye. The prevalence of myopia is increasing, especially in Asian countries; therefore, both understanding the pathophysiology and timely diagnosis of this condition are pivotal in tackling this problem.
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Models of myopia development
The exact etiology and progression of myopia have been investigated for many decades. For much of this time, it has been proposed that myopia is related only to time spent on reading and close work with minimal reading distance.
Current models agree that prolonged near work leads to myopia as a result of the blurred retinal image that arises during near focus. Such retinal blur subsequently initiates a biochemical process that leads to biochemical and structural changes in the sclera and choroid, leading to axial elongation.
Infants are typically born hyperopic or far-sighted, and their eyes typically grow with them to where they can see clearly. The absence of formed vision leads to uncontrolled growth of an eye with constant searching for a focal point, bypassing emmetropia, and developing axial myopia.
Additional myopiogenic stimuli such as protracted reading or frequent exposure to activities that require a considerable amount of near work may lead to mild myopia later in life.
When children have a familial or ethnic proneness to myopia, the emmetropisation process usually continues, but they become mildly myopic early in life. Exposure to myopiogenic stimuli, such as that which can occur during extensive near work that produces defocused and blur images on the retina, resumes the process of myopisation. This, in turn, can result in axial elongation and moderate myopia in late adolescence.
Nearsightedness can occur as an isolated finding or as a manifestation of specific genetic syndromes. Furthermore, there is a significant body of evidence that supports the significant role that genetic factors have in the development of nonsyndromic high myopia. Many independent studies confirm a positive correlation between parental myopia and myopia in their offspring, indicating a hereditary element in myopia susceptibility.
A plethora of genetic syndromes has characteristic systemic findings with myopia as a consistent clinical feature. For example, Stickler syndrome represents an autosomal dominant connective tissue disorder where ocular, facial, and skeletal abnormalities can be noted. Marfan syndrome is another well-described autosomal dominant disorder with diverse clinical features such as myopia, lens dislocation, tall build, pneumothorax, and increased aortic wall distensibility.
What is Myopia (Short sightedness)?
Still, determining the precise role of genetic factors in the development of nonsyndromic myopia has been hindered by the high prevalence of the disease, its clinical spectrum, and genetic heterogeneity. Nonsyndromic high myopia likely results from alterations of multiple genetic factors. Comparatively, the existence of a genetic contribution is mainly based on evidence of familial aggregation and twin studies.
Recent mapping studies are the best way to identify implicated genes for high myopia. An X-linked recessive high myopia was linked to the first high-grade myopia-1 locus (MYP1) on chromosome Xq28. SLITRK6 mutations are known to cause myopia and deafness in humans and mice. One of the genes implicated in the development of myopia in several studies was RASGRF1.
Establishing a diagnosis
A diagnosis of myopia is made by using several procedures that measure how eyes focus the light. The power of optical lenses that are needed to correct the reduced vision is also established during that process. A classic vision test is most often pursued, during which the patient is asked to read letters on a chart placed at the other end of the room. This test measures visual acuity, which is expressed as a fraction.
If a vision test indicates that nearsightedness is the problem, different devices are used to figure out what is causing it. A retinoscope is employed to shine light into the eyes, so the light reflection off the retinas can be observed. Another device known as a phoropter contains a series of lenses. Flipping these lenses back and forth helps to establish the precise prescription that will correct the patient's vision.
Current clinical practice in detecting pathological myopia is heavily dependent on the manual screening efforts of the examiner; therefore, a complete eye exam can take up to 60 minutes. Novel techniques such as autorefraction and photoscreening are introduced to overcome some of the difficulties faced when screening young children.
The development of retinal imaging algorithms and computer-aided diagnosis systems to automatically spot pathological myopia from retinal fundus images towards screening is recently taking a great interest in the scientific community. With the large amount of potential data that can be obtained, the challenge remains on how to combine such data in a cohesive fashion to make the best use of their individual advantages.