Correcting visual disturbances
The eye lets you “see” by performing two functions: receiving light, and focusing it on the retina, which converts the light it receives 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 doesn’t land on the retina sharply in focus, that’s described as “refractive error” – or “ametropia”. In European countries, almost one person in four has a refractive error.
When the eye (cornea and eye lens) bends (refracts) rays of light in such a way that it is sharply focused on the retina, then eye is without optical defect. A person with normal vision is able to see clearly and clearly objects both near and far. This vision is termed “emmetropia”.
If the light rays that enter the eye are focused at a point in front of the retina, that 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 are refracted on to the retina sharply. Myopia typically arises because the eye is too long for optics of the eye, but it can also be caused by a steeply curved cornea.
The opposite of myopia is hyperopia (in other words, 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 change the shape of their lens to focus light sharply on the retina to view near objects. (However, this can lead to eye strain and headaches). But by the age of around 45 years, the ability of the lens to accommodate diminishes, and the ability to see nearby objects clearly is lost.
To see clearly, the image of the outside world that’s projected needs to be sharply projected on the retina. But in people with astigmatism, their cornea has an irregular shape (e.g. rather than being mostly spherical in shape, it’s more like an egg, or one region is flatter than the other). The focal point is scattered; this means that the image isn’t completely sharp 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 held in a transparent bag that’s 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.
Correction of visual disturbances
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.
Contact lenses: They offer a good 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.
PRK: Photorefractive keratectomy (PRK) involves the use of an excimer laser to reshape the cornea to correct refractive errors like myopia, hyperopia and astigmatism.
TransPRK: 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.
LASIK: 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.