When we think of correcting vision, we think of Laser or Lasik. Lasik is the ideal procedure for vision correction. Day by day, as the technology is advancing, so are hopes and expectations from Lasik. But even with the latest techniques of LASIK, there are always situations where Lasik cannot be done. Limitations of Lasik can be described as follows:
1: Limitation of age:
- Minimum 18 years:By 18 years, the number tends to become stable in most of the people. However, in a human system, there can be no hard and fast rule. This means that in some people the number may become stable even by 16 years, while in some it may not be stable even at 22 years. Even if the number is stable by 16 years, Lasik should preferably not be done at this stage, because the future cannot be predicted, and if the number increases later, it can only lead to regrets. As a general observation, it is seen that low numbers like -1.0 to -3.0 tend to become stable even before 18 years of age. Numbers from 3.0 to 6.0 may become stable by 18 to 20 years. Number from 6.0 to 9.0 may become stable by 22 years, while numbers beyond 9 may or may not be absolutely stable even beyond 22 years.
- Maximum age:There is no limit to maximum age. We have done Lasik in people even at the age of 60 years. It is a question of when one decides in life that enough is enough, and that one would like to see the world without these ‘optical crutches’. In our personal experience of 20 years of Lasik, we have seen that Indians usually accept glasses as part of their identity by the age of 40. Rarely have I seen people asking for Lasik beyond this age. However, in the last few years, I have noticed a definite social change. Mothers and father of children, who have got Lasik done successfully, are now asking whether they too can get rid of their glasses at this stage. So, I would now put the Indian mind set at 50, and the western country mind set at 60.
- Ideal age:There is always an age band when people feel that this is the ideal age for Lasik. We feel that this band is within the age of 20 to 40 years. During this period, there is a strong desire to look good without glasses. Matrimony is a major reason in India, while other reasons like lifestyle freedom, sports, personality enhancement and self confidence booster play an important role.
2: Limitation of pregnancy or breast feeding:
Lasik is not advisable during pregnancy or breast feeding. During this period, the hormonal levels in the female body are high, and there is an associated relaxing of tissues and increase in spectacle power. Even though this increase in spectacle power may happen in only 10 % of the ladies, still this number is pretty high to caution against Lasik during this period. In those females where the number increases during pregnancy, it is observed that the change is usually between 0.5 to 1.0 D spectacle power. It is also observed, that if the spectacle power increases in first pregnancy, there may be a similar increase in the subsequent pregnancy.
3: Limitations of corneal thickness:
Cornea should be adequately thick to perform Lasik. Too thin a cornea becomes is a limiting factor. A normal Indian cornea has a central thickness of 530 microns or 0.53 mm. In this thickness, one can safely correct a number of -8.0 by SBK (thin flap Lasik) or the Bladeless Femto lasik. A cornea which is less than 495 microns is considered thin, while a thickness of less than 450 microns is a ‘no go’. The natural assumption from these figures is that if the cornea is thicker than 530 microns, then more spectacle number can be corrected. This is not correct as treatment of more than -8.0 leads to excessive flattening of the cornea and resultant disturbances in vision called spherical aberration. It is therefore recommended that the ideal upper limit is -8.0, while the acceptable upper limit is -10.0. Beyond this number, Lasik should be avoided, and a better way would be to go in for an ICL. The upper limit of correction by Lasik for a plus number i.e. hypermetropia is +5.0.
4: Limitations of corneal curvature:
Too flat a cornea or too steep a cornea can also be a limiting factor.
- Flat Cornea:A normal human cornea has a curvature of 44.0 D. A cornea with a curvature of 41.0 D or less is considered a flat cornea. Since Lasik involves lifting up of a cornea flap, in a very flat cornea, the size of the flap lifted may be too small. This means that the area of cornea available for laser treatment may have to be reduced, which may result in visual problems like halo’s and glare around lights at night. Femto Second Laser made flaps do better even in flat cornea.
- Steep Cornea:A cornea which is more than 48.5 D is considered a steep cornea. In such steep corneas, lifting the flap may result in button holing of the flap, or an irregular thickness of the flap. This happens in only a very small percentage of steep corneas. In case of button holing of the flap, the flap is re-positioned bask without doing the laser treatment, and allowed to heal. The flap may be re-lifted after 3 months, preferably with a Femto Second Laser, as the latter gives more dependable flaps in extremes of corneal curvatures.
5: Limitations in corneal pathology:
Keratoconus is a corneal condition where the corneal tissue is inherently weak. Due to the normal eye pressure within the eye ball, the cornea starts bulging out. This makes the cornea steep and irregular. Rays of light entering the eye through such corneas get irregularly scattered thereby resulting in loss of quality of focus and decrease in vision.
Lasik involves ablating the cornea to change its curvature. Ablating the cornea makes the cornea thin. Acceptable level of thinning does not weaken a healthy cornea. However, in an inherently weak cornea, the Lasik induced thinning may weaken the cornea to an extent that it will start to bulge causing optical distortions and decrease of vision. This abnormal bulging of the cornea may then be stopped by ‘C3R’ or what is called Collagen Cross Linking.
The importance of cornea screening is to detect sub-clinical or hidden Keratoconus, and reject these cases, so that the rest of the eyes undergoing Lasik treatment are safe. With modern methods of corneal scanning like Pentacam, it is possible to detect most of these hidden Keratoconus cases, thereby making lasik very safe. However, very rarely, one may still have to face a tricky situation, and Collagen Cross Linking may then have to be done to arrest the progress of the disease.