Refractive errors are one of the most common causes of poor vision worldwide, so it is important to understand what it means.
The eye is actually a very complex system which functions like a camera, ensuring that the light entering the eye is focused on the retina (which is the light sensitive part of the eye). When this light is focused directly on the retina, our vision is clear. When this focusing is not proper, the image formed is not clear, and our vision becomes fuzzy. This disease is called refractive error. Your doctor will usually prescribe corrective glasses or contact lenses, or even laser surgery (like Lasik) for vision correction.
How does the eye focus images perfectly?
When light travels from one medium to another, it bends, and this phenomenon is called refraction of light. The eye has several structures that work synchronously to ensure that light entering the eye undergoes proper refraction, focusing it directly on the retina. Because of this phenomenon, we can see the objects are various distances clearly. The structures of the eye that ensure clear vision include
Cornea:
The cornea is the transparent front part of the eye. It is convex (or dome shaped), and therefore acts like a converging lens, and focuses light rays entering the eye on the retina. The tear film is also important in ensuring a clear and sharp image, and dry eyes can cause a vision blur.
Lens:
The natural lens of the eye is responsible for focusing objects at various distances by a phenomenon called accommodation, which involves the lens changing its shape and curvature. This is because the lens is transparent and flexible. The lens is located directly behind the iris and so, the pupil can regulate the amount of light entering the back of the eye, like the aperture of a camera. It also helps to focus light and images on the retina. With age, the lens loses its transparency, which means a gradual and progressive loss of vision, and this disease is called cataract. The loss of the flexibility of the lens means that we need glasses for near work, especially reading, this is called presbyopia.
Retina:
The retina is the light-sensitive portion of the eye, like the film in a camera. The image is captured on the retina, and then is transmitted via the optic nerve to the brain. So, it is the brain that actually “sees” the world.
What are the common refractive errors?
When the eye does not focus the images sharply on the retina, it results in hazy vision. This defect of vision is called a refractive error, and can be due to two reasons, or a combination of both:
- The combined focusing power of the lens and cornea is not optimal
- The length of the eyeball is not optimal
- There is an imbalance between the two above mentioned parameters, which means that the image formed on the retina is not sharp.
The common errors of refraction include:
Myopia or Nearsightedness
Those who have myopia can see near objects clearly, but distant objects are blurred. In myopia, the rays of light entering the eye are focused in front of the retina. This can happen in two situations: either the focusing power of the cornea and lens is very high (more convex than normal), or when the eyeball is too long. Myopia is corrected by concave or minus lenses which may be prescribed as either eye glasses or contact lenses.
Hypermetropia, hyperopia or far sightedness
In this refractive error, patients can see distant objects more clearly than near objects. This is because light rays entering the eye get focused behind the retina. Again, it happens for one of the reasons: the focusing power of the cornea and lens is very low (flat cornea), or because the eyeball is shorter in length than normal. Hypermetropia is corrected by convex or plus lenses which may be prescribed as spectacles or contact lenses.
Astigmatism
Astigmatism is a refractive error in which the image is blurred because of an abnormal or irregular shape of cornea or lens. Normally, the surface of the cornea is rounded (convex), but in astigmatism, it is shaped more like an American football, which means that the focusing power of the eye is different in different meridians. Astigmatism is corrected by spectacles with cylindrical lenses, which may have a plus or minus power.
What are the symptoms of refractive errors?
The most common symptoms of refractive errors include:
- Blurred or hazy vision (for distance, near or both)
- Tiredness and fatigue of eyes, especially after prolonged near work
- Sore, irritated and painful eyes
- Eye strain, especially towards the end of the day
- Squinting or misalignment of eyes, more common in children
- Headaches, especially frontal headaches which are relieved by rest and sleep
- Lazy eye
Amblyopia or Lazy Eye
When a refractive error is uncorrected for a long time (or when the stimulus for vision does not reach the brain), especially in children, the connections in the brain for vision remain immature. In such cases, even when corrective lenses or glasses are provided, the vision does not improve much. This decreased vision in an eye which does not have any structural reason for the loss of vision is called lazy eye or amblyopia. This is why regular eye examinations for children are absolutely essential.
What is the treatment for refractive errors?
Refractive errors are one of the most common causes of poor vision worldwide. Especially since children cannot express themselves, they run a risk of developing lazy eye. This is why ophthalmologists around the world recommend regular eye checks for children and adults alike.
The treatment for refractive errors has changed dramatically since the advent of ophthalmic lasers which have provided millions of patients better sight without glasses.
The accepted modalities of treatment of refractive errors include:
Optical treatment:
Surgical treatment:
- LASIK including variants like SBK, FemtoLASIK, Contoura LASIK and SMILE
- Surface ablations like LASEK, PRK etc.
- Implantable Collamer Lenses, ICLs or Phakic IOLs
- Refractive lens exchange or clear lens extraction along with IOL implantation.
