Almost half of visual impairment in the world can be attributed to uncorrected refractive errors, and myopia constitutes a significant proportion of this problem. This condition occurs when focused image falls anterior to the retinal photoreceptor layer of the eye. The prevalence of myopia is increasing, especially in Asian countries, thus both understanding the pathophysiology and timely diagnosis represent are pivotal in tackling this problem.
Models of myopia development
The exact etiology and progression of myopia have been investigated for many decades. Even though it has been proposed that myopia is related only to time spent on reading and close work with reading distance, current models agree that prolonged near work leads to myopia occurs due to the blurred retinal image that occurs during near focus. Such retinal blur subsequently initiates a biochemical process which 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 in turn emmetropia and developing axial myopia. Additional myopiogenic stimuli such as protracted reading or frequent exposure to activities that require considerable amount of near work may lead to mild myopia later in life.
When children have familial or ethnic proneness to myopia, the emmetropisation process usually continues, but they become mildly myopic early in life. Exposure to myopiogenic stimuli (i.e. extensive near work, which produces defocused and blur images on the retina) resumes the process of myopisation, with resulting 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 genetic factors have a significant role 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 have 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 well-described autosomal dominant disorder with diverse clinical features such as myopia, lens dislocation, tall build, pneumothorax and increased aortic wall distensibility
Still, determining the precise role of genetic factors in the development of nonsyndromic myopia has been hindered by the high prevalence of disease, clinical spectrum of this condition and genetic heterogeneity. Nonsyndromic high myopia likely results from alterations of multiple genetic factors, and 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 the eyes focus the light, but the power of any optical lenses needed to correct the reduced vision are also established during that process. A classic vision test is most often pursued where 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 phoropter contains a series of lenses; flipping them back and forth helps to establish the precise prescription that will correct the vision.
Current clinical practice in detecting pathological myopia is heavily dependent on the manual screening and efforts of the examiner, therefore a complete eye exam can take up to 60 minutes. Novel techniques such as autorefraction and photoscreening attempt 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 of the scientific community. With the large amount of potential data that can be obtained, the challenge remains how to combine such data in a cohesive fashion to make the best use of their individual advantages.