What is refractive error?
The function of the eye is to see clearly the objects around us. The inability of the eye to accurately focus the rays of light coming from distance on the retina is called refractive error. This condition may be either because the eye is too short or long in length, or because the cornea or lens does not have the required refractive power. There are three types of refractive errors:

Myopia (near-sight): this is the condition in which the eye is too long and the light is focused in front of the retina. Distant objects are blurred but the near objects are seen clearly. The eye has too much optical power and to correct it the optical power is reduced by either minus glasses or contact lenses, or by surgery.

Hypermetropia (long-sight): this is the condition in which the eye is too short and the light is focused behind the retina. The eye has less optical power than is needed. When young the eye can use the lens within the eye to compensate, but reading glasses are needed at a relatively early age. Later, distance glasses (plus) are needed as well, such that glasses for distance and near are required.

Astigmatism: this is the condition where the eye does not focus the light evenly, usually due to the cornea of the eye being more curved in one direction than the other. It may occur on its own or may be associated with myopia or hypermetropia.

What is Presbyopia?
Presbyopia is the normal aging process, where the lens progressively loses its capacity to increase its power for near vision (loss of accommodation). The distance vision may be normal, but the near vision becomes blurred with age greater than about 45 years. This is corrected by wearing reading glasses (plus) for the near work. This condition may occur in itself or may be present alongwith pre-existing myopia, hypermetropia or astigmatism.

What are the ways of treating refractive errors?
The refractive errors may be treated by either of the following ways:

• Glasses: this is the simplest and most popular way of correcting refractive errors
• Contact lens: this option is cosmetically much better though the maintenance and cleaning of contact lenses may be cumbersome. There is also a slight risk of infection to the cornea, if the contact lenses are not cleaned properly before use.
• Refractive surgeries: these are increasingly becoming popular with the advent of more predictable laser treatments.

What are the various refractive surgeries?
The refractive power of the eye can be changed by any of the three approaches:

• Changing the curvature of the cornea: this is the most popular mode of refractive surgery.
• Removing the natural lens and replacing it with an artificial lens of adequate power: it is similar to a routine phacoemulsification surgery, except that it is done in a clear lens and not a lens with cataract. As it is an invasive procedure and it also increases the chances of retinal detachment in eyes with high myopia, it is not recommended nowadays.
• Putting an additional artificial lens within the eye on top of the existing natural lens: this technique is still not very popular as it is also invasive and may increase the chances of cataract formation.

What are the ways of changing the curvature of cornea?
The commonly used methods for refractive surgery are:

• PhotoRefractive Keratotomy (PRK) : It is being used less commonly nowadays.
• Laser-Assisted In-Situ Keratomileusis (LASIK) :This is the most popular form of Laser treatment.
• LASEK (or Epi-LASIK) :This newer form of treatment may be suitable in some selected patients with very high power.

What is Excimer laser?
Excimer laser is a far Ultra Violet (UV) light energy of wavelength 193 microns. It is invisible to human eye. This laser breaks the chemical bonds within the molecules (photoablation) with minimal thermal damage to the surrounding tissues. Thus it is very precise and can remodel the cornea with an accuracy of more than a thousandth of a millimeter.

How does Excimer laser correct the refractive error?
The effect of excimer laser on the cornea is very similar to grinding of a glass lens to change its refractive power. In eyes with refractive errors, excimer laser because of its high level of precision, can change the shape of the cornea to change its refractive power to the desired state and thus correcting the refractive error.

In myopia, the central part of the cornea is made flatter, and thus decreasing the refractive power of the cornea and of the eye, and thus correcting the refractive error.

Similarly in hypermetropia, the laser removes a ring of tissue from the peripheral part of the cornea and thus makes the central part steeper and corrects the refractive error.

As we know, in astigmatism, the cornea is more curved in one direction. Excimer laser can correct astigmatism also by selectively ablating the cornea in the required direction.

What is LASIK?
LASIK involves putting the PRK treatment not on the surface of the cornea, but under a protective corneal flap. A very thin (about 0.16 mm) and precise flap is raised by a special instrument known as microkeratome. The result is a corneal flap attached at one edge, the hinge. The surgeon folds the flap to expose the inner stromal layer of the cornea. The excimer laser treatment is applied on this stromal bed to remodel it. After this the flap is repositioned to its original position and it does not require any suture. Since the corneal epithelium has only been minimally disturbed, there is only mild discomfort after the procedure.

What is Customised LASIK?
This is a special form of LASIK in which the treatment parameters are customised for the particular patient, based not only on the refractive error, but also on the corneal map of the eye and other findings detected by special tests. This procedure tries to correct aberrations, maintains normal shape of the cornea and gives better night vision.

Who is a suitable candidate for LASIK laser surgery?
The person must be 18 years or older with a stable power. A contact lens user must discontinue the use of contact lenses at least 2 weeks before the procedure. A detailed eye checkup is done to look for suitability for the procedure. Before the Laser is done the eyes are checked with special machines to determine the exact power, the corneal mapping is done and corneal thickness is measured. In patients with high minus power, a special retinal checkup is done for detecting any possible weak areas in the retina, which may need to be treated before the LASIK is performed. The LASIK laser is performed only after ruling out any contraindication and confirming the suitability of the procedure.

What happens during the Laser surgery?
The LASIK surgery is done as an outpatient procedure and does not require any admission. It is painless and is done after putting the anesthetic drops and does not require any injections. The laser procedure takes approximately 15-20 minutes for both eyes. After the procedure, the patient can go back home after 20-30 minutes.

What are the complications of LASIK?
LASIK is a very safe procedure with a majority of patients achieving very good results. However, since it is a surgical procedure, it does carry some chances of complications as well, which would be discussed with you before the surgery. The overall rate of significant complications in LASIK is only of the order of 1-2%. Some of these complications may be:

• Undercorrection or Overcorrection
• Glare and difficulty in night driving
• Flap complications, perforation
• Infection
• Scarring of the cornea

What are the precautions to be followed after LASIK, and when can one resume work?
After LASIK surgery one needs to avoid using cosmetics in and around the eye for a week or two. One should also avoid wetting or rubbing the eyes for some period. Use the medications regularly and report immediately in case of any discomfort, redness, injury or any other problem. There is no restriction in reading, watching TV, going for walks etc. In majority of cases, routine office or simple household work may be resumed in a day or two.

How are the results of refractive surgery?
Before undergoing any refractive surgery procedure, one must realize that none of these procedures can guarantee you perfect vision without glasses in all the cases. These surgeries are based on the average calculations from a large population, but as different individuals may respond differently to surgery, there might be some variation in the outcome results obtained. The aim is to decrease your dependence on glasses/contact lenses. The reliability of the procedure is quite good in mild to moderate levels of refractive errors, with most of people being able to carry out their daily activities without glasses. But for high degrees of refractive errors, the variation is more and some people may still need glasses, though of much lesser power than before, to see clearly.

If my refractive error is not corrected fully, can I go in for a repeat procedure?
Yes. If there is a significant residual refractive error, then your surgeon might advise you to undergo a repeat surgery at an appropriate time. In LASIK the same flap can be raised again and the laser treatment applied as before.

Can I get my reading glasses (presbyopia) removed by surgery?
No. As we know, presbyopia is the inability of natural lens to change its power according to the need of the person. None of these procedures described can help in this. There are a few techniques being developed to correct this problem, but they are in the stage of investigation and as of today, are not advisable.

What is the minimum age before which refractive surgeries are not done?
None of the refractive surgeries are performed till the power of the eye becomes stable. This generally occurs after 18 years of age, when the eyeball stops growing. Even after this age, the refractive error should be stable over a period of few months before a surgery is contemplated.

I am a contact lens wearer. What additional precaution must I take before going for refractive surgery?
Since the contact lens can change the curvature of cornea, it is advised to discontinue use of contact lenses at least 2 weeks before the refractive surgery.

What are the situations in which refractive surgery is not advisable?
Refractive surgery may not be considered appropriate in the following conditions, as there may be additional risk or poor outcome may be involved:

I have a high degree of myopia. I was told that because of high myopia, my retina is 'weak' and I am at a risk of getting retinal detachment. Will correction of myopia by LASIK lead to decrease in risk of retinal detachment also?
No. You should remember that the treatment involves only reshaping of cornea, so that the light gets focused properly on your retina. It does not correct the 'weakness' of your retina and the risk of retinal detachment remains. Even after refractive surgery you should continue taking the special precautions and regular checkups with your ophthalmologist.

HIGH MYOPIA

People who have minus number glasses more than 6 diopter in power are said to have high or pathologic myopia. The glasses number of such patients may even be as high as 15 to 20 diopters. The eyeball in such cases is enlarged leading to thinned out coats of the eyeball so the central area may be very weak (chorioretinal degeneration) leading to poor vision. The retina in these eyes is weak in the periphery also and usually has some degeneration, atrophic holes, or even retinal tears. These retinal holes or tears may sometimes lead to a serious condition of retinal detachment, leading to sudden loss of vision, and may require major surgery urgently to settle the retina.

So a regular retinal checkup is very essential in these patients to look for the holes and to treat them with laser or else they might lead to a retinal detachment, which needs surgery. Also, whenever the patient with high number develops symptoms like flashes or floaters, he/she must immediately get the eyes examined to look for retinal hole, tear or retinal detachment.

High myopes are also predisposed to develop abnormal new vessels in the central area (macula) called Choroidal Neovascular Membrane (CNVM), which leads to sudden marked fall in vision due to hemorrhage or fluid leakage. The treatment modalities being tried out for myopic CNVM are anti-VEGF agents like Lucentis, Avastin and Macugen, photodynamic therapy (PDT) with verteporfin, Transpupillary Thermo Therapy (TTT) laser and surgical options like macular translocation.