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An Emmetropic Eye

Paper type: Essay
Pages: 4 (989 words)
Categories: Eye
Downloads: 11
Views: 105

An emmetropic eye is one in which the eye focuses parallel light rays on the retina at the back on the eye (Benjamin and Borish 2006). An eye which is unable to focus parallel light rays on the retina is said to have refractive error. Refractive error can be instigated by the axial length eye or the refracting powers of an individual’s eye. Refractive error can be measured objectively during retinoscopy as well as a subjectively using a Snellen or reading chart.

There are variations in types of refractive error an individual may have, which include myopia, hyperopia and astigmatism, which is usually found alongside myopia or hyperopia. Uncorrected refractive error is the leading cause of global visual impairment (Naidoo et al. 2016). Myopia is the most common type of refractive error in the world. In addition to it being the most common it also increasing the risk of the population to further sight threatening ocular pathologies like myopic macular degeneration and retinal detachment (Verhoeven et al.

2013)

Anatomically we know how refractive error is caused, however the focus of this is essay is to look at what causes myopia. According to Borish and Benjamin (2006), the argument between hereditary and environmental development of myopia remains unsolved after nearly 400 years. Mutti et al (2002) concluded heredity being the most important factor when observing the correlation between parental myopia and juvenile myopia. They found with both parents being myopic, of those 32.9% of children were myopic compared to 18.2% of children with only one myopic parent. In the case of neither parent being myopic only 6.3% of children had myopia (Mutti et al. 2002).

Ethnicity is another feature of biological causes of myopia. A study conducted of multi-ethnic British children found a considerably higher prevalence of myopia in British South Asian children than that of black African Caribbean and white European children (Rudnicka et al. 2010). The difference in the myopic prevalence between ethnicities is attributed to the difference in the ocular anatomy with black African Caribbean and in particular, South Asians having increased axial lengths versus white European children (Rudnicka et al. 2010). With aging, the accommodative power of the crystalline lens decreases due to loss of elasticity of the lens causing a hyperopic shift leading to presbyopia. Cataract is an age-related opacification of the eye. In the presence of a nuclear cataract, the hyperopic shift is paused, and a slight myopic shift occurs (Samarawickrama et al. 2007). This myopic shift can be resolved with cataract surgery.

Despite the significant genetic influences of myopia, this alone is unable to explain the swift increase of myopic prevalence over the last two generations (Shah et al. 2017). Therefore, other factors must be taken into account when determining cause. One noteworthy factor to consider, is what environmental influences contribute to myopia. Influences such as lifestyle, hobbies, geographical location, education and socioeconomic status could all be contemplated. Several studies indicate a link between education level and myopia (Verhoeven et al. 2013, Shah et al. 2017, Cumberland et al. 2015). All levels of education require an involvement in near work and as an individual spends longer time in education, e.g. up to university, their exposure to near work increases and is prolonged. The risk of developing myopia increases four-fold in an individual with a university education versus those educated at primary school level (Verhoeven et al. 2013). The prolonged exposure may lead to excessive eye growth and the development of myopia. Another factor to contemplate is the association between near work and the time spent outdoors. Studies conducted suggest an increase in near work negatively impacts the time spent outdoors (Shah et al. 2017). Therefore, by increasing the time spent outdoors, this can greatly reduce the risk of developing myopia particularly in children aged 3 to 8.5 years (Shah et al. 2017).

Although not as significant, trauma is a factor which can cause a myopic shift. Blunt force trauma resulting from a motor accident and a firework accident were found to be a cause of transient myopia (Ikeda et al. 2002). They concluded the blunt force to the face produced ciliary body oedema resulting in thickening of the crystalline lens and shallowing of the anterior chamber caused by the lens-iris diaphragm to shift anteriorly. Similarly, the alteration in the anatomy of the eye can cause a myopic shift from subsequent pharmaceutical drug use (Lan and Hsieh 2018). Weiler (2015) and Lan and Hsieh (2018) reported sudden onset acute angle-closure glaucoma and a myopic shift after the administration of Topiramate and Zonisamide, used to treat epileptic seizures and migraines. However, after withdrawing the medication and additional treatment if needed, vision returned to normal.

There are multiple factors contributing to the cause of myopia however there is no specific leading cause. There are limitations to my research conducted therefore further investigation would be needed to validate this. It is clear from the research I have conducted biology plays a key role in influencing the development of myopia. REF. There is a strong link signifying an offspring is more likely to develop myopia if their parent is myopic (Mutti et al. 2002). Additionally, the ethnicity of an individual influence’s myopic development specifically in south Asian and black African Caribbean children where the axial length of the eye is longer (Rudnicka et al. 2010). Cumberland et al. (2015) reported the myopic percentage amongst participants were 26.8% male and 26.9% female. In the same study, it also found myopia in females in the age group 40-59 years old was more likely, than males. However, between 60-69 years old, myopia was least likely in females. This leads to suggest myopia is not gender favoured. When looking at the environmental causes, an increase of time spent in education can result in excessive eye growth causing myopia (Verhoeven et al. 2013). As well as, a lack of time spent outdoors, trauma and pharmaceutical drugs all contribute to the development of myopia.

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An Emmetropic Eye. (2019, Dec 12). Retrieved from https://studymoose.com/an-emmetropic-eye-is-one-in-which-the-eye-focuses-parallel-light-example-essay

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