What Is Refractive Error?

Vision is a pretty amazing thing – except for when it isn’t. Over 75 per cent of Americans are estimated to wear optical correction in the form of spectacles or contact lenses. And it’s thought that even more should be wearing some form of optical correction but instead wear some form of denial to address refractive error.

Refractive error occurs when the eye cannot properly focus which, of course, results in blurred vision. The 4 most common types of refractive error are myopia, presbyopia, hyperopia, and astigmatism.

Myopia (Short-sightedness)
Myopia is also known as near- or short-sightedness and chances are high that you know someone with it. The prevalence of myopia in the US sits at over 40 per cent, with higher rates in Southeast Asian communities due to genetic factors. Myopia occurs when the length of the eyeball is too long for the focusing power of the eye – this means that light coming into the eye falls short of where it needs to be to form a clear image. The use of concave lenses – minus-powered lenses – manipulates the light rays such that they come to a focal point further into the eyeball, ideally focusing on the retina to form a clear image. Myopic patients have compromised vision when looking into the long distance but will often have decent vision closer up.

Although it sounds harmless, a myopic eyeball comes with many increased risks of subsequent eye diseases, including cataract, glaucoma, retinal tears and detachments, and myopic maculopathy. There are currently strategies employed by eye care providers to slow down the progression of myopia in children, where it is known that short-sightedness can advance aggressively. Myopia control techniques include multifocal contact lenses, pharmaceutical eye drops, and hard contact lenses for overnight wear.

Presbyopia (Far-sightedness)
Once upon a time there was a woman named Karen. When Karen was in her 20s she had no trouble reading at all. In her 30s, Karen still had no issue focusing on the words in her book. One day, Karen turned 45 and couldn’t focus on those words as well anymore. This type of farsightedness is referred to as Presbyopia.

Inside the eye is a crystalline lens, which starts off pretty flexible when we’re born. This lens can actively change its shape to bring forward the focusing point of light entering into the eye (but the converse is not true in being able to push back the focal point of light any further than when the lens is already fully relaxed). From the day of birth, however, the lens begins to slowly lose its flexibility as lens fibers continue to form around the nucleus. Around the mid-40s is when presbyopia usually hits, a depressing reminder that age marches on no matter how many Botox injections. In the early stages of presbyopia, which is in fact derived from the Greek for “old eye”, one may be able to get away with simply holding one’s reading material a little further away but unfortunately eventually one realizes one’s arms are just not long enough. Cue reading glasses.

Presbyopia can occur simultaneously in the presence of any of the other refractive errors mentioned here. In some cases, a myopic patient may be able to achieve good reading vision by simply removing their distance spectacles and allowing their natural short-sightedness to shine through.

Hyperopia (Long-sightedness)
This is the most confusing of all refractive errors to explain because the effects of hyperopia, or long-sightedness, varies with age even if the script remains the same. Hyperopia, also called far-sightedness or hypermetropia, is due to the length of the eyeball being too short for the focusing power of the eye. Sounds just like the opposite of myopia, doesn’t it? Sounds just like a hyperopic patient should have great vision at long distance but not be able to see well at near, doesn’t it? Wrong, my friend. So wrong.

The confusing nature of hyperopia is due to the aging lens aforementioned in the discussion about presbyopia. A young child with a very flexible crystalline lens (and smooth, elastic skin to go with) is able to easily and automatically change the shape of their lens to bring forward the focusing point of light and allow a clear image in a hyperopic eyeball at both distance and near viewing, a process known as accommodation. Depending on the degree of hyperopia and whether there are any other vision dysfunctions present, the effort of constantly accommodating to see clearly may manifest in eyestrain or headaches, especially when looking at objects up close due to the increased accommodative demand, but a young patient will still be able to achieve clear vision at both distance and near. Conversely, an older patient with a less flexible crystalline lens, such as the aforementioned Karen, will not be able to compensate so well over a hyperopic script and experience blurry vision even at long distance.

Far-sightedness is corrected with plus-powered convex optical lenses, which bring forward to the focal point of light so that it falls clearly onto the retina of the shortened hyperopic eyeball.

Astigmatism (Seeing distorted images)
Potentially the most commonly mispronounced refractive error – if you had ever gone around thinking you had “a stigma” in your eye, you wouldn’t be the only one – astigmatism is also considered the most common refractive error. In adults across the US, astigmatism prevalence sits around 45 per cent. It often occurs in combination with either myopia or hyperopia (and if you’re sailing past the mid-40s, you can add presbyopia to that list, too).

Astigmatism arises from two different anatomical locations. The most common source is an irregularity of the cornea, the clear front surface of the eye. The other area that can cause astigmatism is that pesky crystalline lens inside the eyeball. In simple terms, astigmatism is when one or both of these round surfaces have an irregular curvature, which causes light to focus at two different points at the back of the eye instead of one. For example, a cornea with astigmatism is shaped more like a football with one axis more curved than the other, rather than a soccer ball where the ball is curved by the same degree no matter which way you turn it. Vision from uncorrected astigmatism may not just be blurry but can also appear distorted – slanted to one side or smeared out. Non-pathological astigmatism is easily corrected with the use of spectacle lenses and in many cases, contact lenses. An eye disease known as keratoconus can often cause very high and irregular degrees of astigmatism; these cases are more difficult to correct with conventional methods.

It’s important to remember that not all vision impairment is correctable with eye glasses or contact lenses, such as vision loss from eye disease. But luckily the vast majority of cases of suboptimal vision is able to be improved with the clever and discriminate application of optics to correct what you now understand as refractive error.

 

 

References
US optical overview and outlook. https://www.thevisioncouncil.org/sites/default/files/Q415-Topline-Overview-Presentation-Stats-with-Notes-FINAL.PDF
Nearly 10 million adults found to be severely nearsighted in the United States. https://www.aao.org/newsroom/news-releases/detail/ten-million-severely-nearsighted-united-states
Global and regional prevalence of refractive errors: systemic review and meta-analysis. https://www.sciencedirect.com/science/article/pii/S2452232517300227
 

 


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