Thursday, November 29, 2007

1.6 Contacts or LASIK?

For those with school myopia, there are always alternatives to spectacles. You know the old sales pitch: you see better/more with the contacts; with LASIK, you can see the digital clock clearly in the middle of the night... It maybe worthwhile to go into contact lenses and refractive surgery as indeed these are not your grandfather's limited choices anymore.

Contact lens is a marvel of modern technology. It is small, from 9-15mm in diameter, and light-weighted (less than 0.02mg) that can be worn comfortably for clear vision. However, it is still a challenge to corneal physiology that is based on aerobic glucose metabolism. Biochemistry 101 will have told you that in aerobic glycolysis, 1 mole of glucose produces 38 moles of ATP, while in anaerobic glycolysis, only 2 (plus lactate). A large part of contact lens R&D is devoted to the search for materials that allow high oxygen transmission. And Voilà! The most recent generation of contacts combine water and silicone (the silicone hygrogel lenses) now permit 5-6 times more oxygen than the older yet still popular HEMA-based soft lenses.

Of course, not all kinks have been ironed out. Contact lens is simply a device. Within the framework of contact lens wear, we still need to ensure that the cornea remains reasonably healthy, totally free from immunological and microbiological issues. Sometimes, the problems are unexpected. A case in point:

In the summer of 2005, the Health Department of Hong Kong was alerted to cases of Fusarium solani fungal keratitis. It is a devastating infection that often leads to the destruction of the cornea. The Ministry of Health of Singapore then also reported that between March, 2005 and May, 2006, 66 cases (68 eyes) were diagnosed, 65 wore soft contact lenses and 62 used Bausch & Lomb ReNu (42 on ReNu MoistureLoc) solution. The first case in the US was reported on March 3, 2006, and subsequently, 164 cases in all were identified. Of which, 124 cases reported using ReNu MoistureLoc. On April 13, this solution was removed from the US market, and on May 15, from the World market. Exhaustive investigation, however, failed to uncover any deficiencies in the manufacturing process. The "Fusarium epidemic" on the other hand, was over. The suspicion is that some polymer components in the solution, when dried on the surface of the contact lens case or the tip of the solution bottle, may have harbored this fungus.

Let's not forget hard contacts, now highly gas-permeable that allow 100 times more oxygen than the soft lenses. And relevant to the myopia discussion is the use of GP lenses to retard myopia progression. Thus far, the evidence points to a limited effect similar to that in multifocal clinical study mentioned in "Children with bifocals" (Nov 28, 2007). And there is always an element of orthokeratology.

A little more on orthokeratology: it is popular in some parts of the world/US. This procedure takes advantage of the deformability of the cornea. By sleeping with gas-permeable hard contacts with the back surface curvature flatter than the cornea, the corneas will stay flattened for a few hours sometimes longer during daytime. Clear vision is thus achieved, seemingly without optical aids. This ortho-K effect is not permanent, though. And wearing contacts during sleep has some inherent risks. The eyes may become inflamed/infected and the lenses may adhere to the corneas necessitating an emergency visit to the doctor's office. All part of the deal.

If you wear contacts, soft or hard, ortho-K or otherwise, do follow the doctor's instructions. Of course, it is also possible to chuck the contacts all together and go straight for refractive surgery.

For myopia surgery, it is a matter of how to flatten the corneal curvature to remove power from the optical system of the eye. This was first done by the now out-of-favor RK - with radial cuts on the cornea but sparing the pupillary area. After healing, the central corneal curvature flattens. Using the Excimer laser to blast off tissue and reshape the cornea is the standard these days. We now have PRK and LASIK. LASIK (and the like) has one extra step: lifting the corneal epithelium before applying the laser. The epithelium is re-attached after laser so the healing is faster and far less painful than PRK.

A pre-op consultation will first make sure your corneas are healthy, refractive error is stable, corneal contours are regular, and most important, the corneas are thick enough. With the advent of wave-front technology, it is possible to maximally improve your vision. If high myopia with thin corneas, the alternative is to implant an intraocular lens (i.e., a phakic IOL).

As in all surgery, refractive surgery carries certain risks; although none involves irreversible vision loss - none reported in the US thus far. And the complications can be severe dry eye, halo or glare at night, or over-/under-correction. These become less frequent as both laser technology (now wavefront-guided) and surgical procedures (e.g., new ways of creating "flaps") have vastly improved.

Contacts or LASIK? For young and healthy eyes, either one is good. For middle-aged people, first think about how close you are to wearing reading glasses. It is now only a matter of choice; doctors can take care of the rest, usually.

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