Laser Vision Correction (LASIK)

Q. What kind of laser is used for removing eye numbers?

A. It is known as an Excimer laser. There are many methods of using the same machine to achieve the same result. The commonest is LASIK (do not bother to remember the full form of this acronym – laser assisted keratomileusis in situ). In this method, a partial thickness hinged flap (approximately 160 microns thick) is raised (somewhat like opening a Pepsi/Coke/Thums Up can – I have no shares in any of the 3 companies!) Laser is then applied to the bed of cornea, removing minute amounts of tissue at each laser shot. The number of shots varies with the patient’s spectacle number and is pre-determined by the built in computer in the machine. Of course, the eye surgeon has fed in the correct data into the computer before hand! When the deed is done, the surgeon gently reposits (puts back) the hinged flap, which falls perfectly into place. Just a couple of minutes later, the flap is well fused with the rest of the cornea and will not dislodge even under moderate stress (patients are sincerely advised not to test the veracity of this statement by vigorously rubbing their eyes 15 minutes post procedure). The surgeon then goes on to perform LASIK on the second eye.

Q. Is this safe?

A. Is driving or walking on the roads safe? One interesting statistic conjured up by someone in the west stated that LASIK is less prone to complications compared to driving or walking on the roads. Jokes aside, more than 95% of patients will have virtually no complications after LASIK, when done after careful patient selection and using standard techniques. Of the remaining 5%, the complications in most cases are minor, reversible or curable with treatment and not sight threatening. Less than a fraction of a fraction of a percent of the patients will have sight -threatening complications.

Q. Give me details of all the common complications.

A. You must remember that the complications I am now going to talk about affect less than 5% of LASIK patients. The commonest complications are under-correction and over-correction. This is because all LASIK machines use complex computer calculations based on the average of thousands of patient data to calculate how much of the corneal stroma to shave to achieve the desired result. However your eye may not behave like the average of these thousands of eyes, i.e. you may heal differently from the average, leading to an under-correction or over-correction. Usually the amount of residual refractive error is 0.5 or less, which is quite compatible with normal vision and nothing further needs to be done. In case you are left with +/- 1.0 or greater, this can be got rid of by repeating the laser treatment – at no cost to you, if done within the first year of treatment. This ‘touch-up’ treatment is called an ‘enhancement procedure’ by we LASIK specialists! Luckily, in almost all cases, over or under correction occurs within the first year, after which the vision remains stable.

Q. What are the other ‘not-so-common’ complications?

A. Besides under and over correction, which can easily be corrected by enhancement procedures, a patient can develop ‘dry eye’ i.e. a lessening of his normal tear production, needing use of lubricant eye drops for sometimes up to a couple of months. This problem usually resolves over time. In Western countries, where there is a tendency to have large pupils, which dilate more than ours in semi-darkness, there is a small percentage who will develop glare, difficulty in night-driving and haloes around light. These are rarely seen in dark skinned populations. Since the lifting of the flap is a surgical procedure, there is the theoretical chance of infection if adequate sterilization precautions are not taken. Most Lasik surgeons prescribe topical antibiotics for a week to two weeks after the procedure as a preventive measure.

Q. I heard that Indian corneas are thinner than their Western counterparts. Is this true?

A. There have been a few studies documenting this fact. However, every surgeon takes care to leave a residual bed (the thickness of cornea left behind after shaving off the required amount) of at least 250-300 microns. This is internationally accepted as safe. Approximately 10 microns of tissue removal corrects one diopter of spherical or cylindrical number. For example, if you have 5 diopter of minus number and your corneal thickness (measured with an instrument called a pachymeter) is 500 microns; the corneal flap created is 160microns; I zap off around 50 microns, to correct your number; your ‘residual bed’ thickness is now 290 microns, which is perfectly safe. However, if you had a spectacle number of minus 10 diopters, it would need 100 microns of zapping, leaving a residual bed of 240 microns, which is not ideal. On the other hand, if your original corneal thickness was 550microns, this would not be a problem. Therefore, rather than talk about Western eyes and Indian eyes, suitability of a patient for LASIK depends on what his corneal thickness is and what amount of correction you aim to achieve. Even in those with a high original spectacle number and a relatively thinner cornea, one can inform the patient in advance that in the interest of safety, we will get rid of only part of your spectacle number. Many patients are quite comfortable with wearing thinner glasses after the procedure rather than risk making the cornea too thin.

Q. What is “ Wave-front Guided” LASIK?

A. Besides spherical and cylindrical numbers, our eyes also have what are technically referred to as ‘higher-order aberrations’. Our refractive media are the cornea and the lens of the eye, which together bend the incoming rays of light so that they focus on the light sensitive area of the retina. These usually induce mild distortions of the light; undetectable till the new ‘aberrometers’ came into existence. These distortions are also corrected by the modern LASIK machines that are ‘wave-front’ guided, giving enhanced quality of vision. However, this usually entails zapping off of slightly more tissue than standard LASIK, hence they are not suitable for all cases. It is worthwhile asking your LSIK surgeon if you are a suitable candidate for the procedure. He will perform the aberrometry test, determine if you do have significant higher order aberrations and also determine if your residual corneal bed at the end of the procedure will be thick enough and then tell you about the extra charges you need to pay for this procedure!

Q. There is a new procedure called ‘LASEK’. What is this?

A. In those in who the corneas are too thin to remove the full spectacle number, the corneal flap instead of being 160 microns in thickness, is reduced to only around 50microns in thickness. This means that the surgeon has another 110 microns of cornea available for zapping. However, creating such a thin flap (only epithelium is peeled off) has slightly enhanced flap related complications.

Q. How soon can I resume my normal activities after LASIK?

A. Usually there is a mild amount of soreness, watering and discomfort on the first day. One can resume office after 24-48 hours. There is no restriction on any visual activity such as TV, computers or reading following the procedure. Most surgeons will prescribe some antibiotics and lubricant drops for a couple of weeks post-operatively along with some mild steroid or non-steroidal anti-inflammatory drops. The first post-operative check-up should be within 24 hours after the procedure; the second one may be after a week. After this, each surgeon has his own timetable of frequency of post-operative visits.

Q. I am 55 years old and require glasses only for reading. Can I get rid of my glasses?

A. Yes. If you are prepared to have ‘monovision’, i.e. one eye corrected for near and the other for distance, you can opt to have LASIK in one eye, for near correction. There is another procedure called ‘conductive keratoplasty’ which uses radio-frequency waves at 8 spots in the periphery of the cornea to correct the near spectacle number in one eye. A few LASIK surgeons are doing presbyopic correction by LASIK for both eyes, but at the time of writing (mid 2006) is not yet a standard procedure. If you are adventurous enough and motivated enough, you can also opt to have ‘refractive lens exchange’ done on both eyes. In this procedure, your natural lens is surgically removed even before it has had a chance to turn cataractous and replaced by an artificial multi-focal lens in both eyes, therefore making you independent of glasses both for distance and near. However, this is a surgical procedure, with the theoretical possibility of operative complications such as haemorrhage, infection and loss of sight.