Vision Disorders – and Their Correction
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- Vision Disorders – and Their Correction
The eye lets you “see” by performing two functions: receiving light, and focusing it on the retina, which converts the light into electrical impulses and transmits them on to the brain. Both the cornea and the lens are involved in focusing that light on the retina. If the light does not hit the retina in focus, the a “refractive error” – or “ametropia” occurs. In Europe, almost 25% of the population show a refractive error.
When cornea and lens bend the rays of light in a way so that it is focused on the retina, then the eye is without optical error. A person with normal vision is able to see objects clearly both at near and at far. This vision is termed “emmetropia”.
If the light rays that enter the eye are focused in a point in front of the retina, then the eye is “myopic” and the person is described as being “short-sighted”. Objects in the distance are blurred, and only light coming from nearby objects is projected onto the retina sharply. Myopia typically occurs because the eye is too long for its optics, but it may also be due to a steeply curved cornea.
If the light rays that enter the eye are focused in a point in front of the retina, then the eye is “myopic” and the person is described as being “short-sighted”. Objects in the distance are blurred, and only light coming from nearby objects is projected onto the retina sharply. Myopia typically occurs because the eye is too long for its optics, but it may also be due to a steeply curved cornea.
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The opposite of myopia is called hyperopia (farsightedness). Rather than being focused on the retina, rays of light are instead focused on a point behind it. In this case, the eye is too short for its optical system (or the curvature of the cornea is too flat). However, many people can correct for their farsightedness through lens accommodation – they can consciously change the shape of their lens to focus light sharply on the retina to view near objects. While this works fairly well at young age, it will lead to eye strain and headaches by the age of 45: the ability of the lens to accommodate diminishes, and the ability to see nearby objects in focus is lost.
To see in focus, the image of the outside world needs to be projected onto the retina. But in people with normal astigmatism, the cornea has a shape that nakes it looke like an egg, rather than a ball. The focal point is scattered; this means that the image isn’t completely in focus on the retina; some parts are in focus, other parts aren’t.
If you’ve ever used a traditional SLR camera, you know that you can manually focus the lens by twisting it, which changes the distance of the lens to the photographic film or digital sensor. It’s the same with your eye’s lens. The lens is held in a transparent bag that is suspended from ciliary muscles – which can contract and relax to change the shape of the lens, achieving focus.
If the SLR camera’s focusing mechanism starts to rust, it gets harder and harder to move it and focus; eventually it will stop moving and it gets fixed to one focal length: only objects at one distance will be in focus; everything else is blurred. It’s similar with the eye; as you age, the lens gets stiffer and stiffer; the ability of the ciliary muscles to change its shape gets lower and lower, until (like the rusted SLR camera lens focusing mechanism) the lens is rigid, fixed to a single distance and everything else is blurred.
The classic method of vision correction has been to use spectacles. But there are a number of other methods that can be used to correct visual defects.
They offer an excellent way of correcting refractive errors, and are usually well tolerated to begin with. However, over time, many people become intolerant of contact lenses. Wearing them becomes uncomfortable and the risk of infection increases.
Photorefractive keratectomy (PRK) involves the use of an excimer laser to reshape the cornea to correct refractive errors like myopia, hyperopia and astigmatism.
This is a completely non-contact method of performing PRK, in which the excimer laser is used to remove the corneal epithelium before reshaping the corneal stroma. The laser platform we use offers “SmartSurface” PRK which generates a smooth cornea which has optical advantages over previous generation lasers.
Laser in situ keratomileusis. The cornea is cut to generate a flap, which is folded back and an excimer laser reshapes the underlying cornea. The modern variant is femto-LASIK, in which the incision required to make the flap for the procedure made by a second laser (a femtosecond laser) instead of a mechanical blade called a microkeratome.
