Aspheric benefits

Traditionally, LASIK for myopic eyes has involved flattening the central part of the cornea with postoperative side effects potentially including glare, halos around bright light sources or loss of contrast sensitivity, especially in dim lighting. These symptoms are more likely in high myopes and patients with large pupils. "The new technology, which takes into account the corneal curvature and asphericity, helps to avoid spherical aberration," says Dr Chan.

"Spherical aberration is a problem of optical distortion that you get after LASIK," says Chan, explaining that in a normal, natural eye there is very little spherical aberration from birth to death. The aspheric shape of a cornea is ideal for good vision, but any kind of cornea-sculpting technique - including PRK, Epi-LASIK, LASIK and LASEK - involves removal of corneal tissue (from the centre in the case of myopes), which in turn alters the pattern of the cornea and induces spherical aberration. "After the first few months, many patients adapt to the different vision but in some cases, especially patients with high degrees of myopia and large natural pupils, the side effects are really an issue," he says.

In a randomized and controlled clinical study undertaken in four centers - SNEC, The Eye Institute at Tan Tock Seng Hospital in Singapore, Rutnin Eye Hospital in Bangkok, Thailand, and Asian Eye Institute in Manila, the Philippines - the Zyoptix Aspheric was reported to better preserve the aspheric shape of the cornea and reduce spherical aberrations by an average of 76%.

Between September and December 2005, the four centres performed a comparative study of Aspheric LASIK and standard LASIK in 170 eyes of 85 patients. The study reported that 97% of the patients who underwent Zyoptix Aspheric surgery attained 20/20 vision or better in normal daylight conditions. But, most notably, 47% of those who got aspheric LASIK correction could see better in low-light conditions than their best preoperative corrected vision (glasses or contact lenses) as compared with 27% of patients who underwent standard LASIK. The study also indicates that changes in corneal curvature were reduced by 60%.

Dr Chan hastens to note that the new treatment does not replace standard LASIK or negate the importance of tissue-saving algorithms. "If the myopia is only 2 to 3 diopters (D), the pupils are not large, the corneas are thick and the patient does not have a professional requirement, then standard LASIK works perfectly," says Dr Chan. "But if the myopia is 5 to 6 D, which is typical in Singaporean eyes, then because of the associated risk of problems in night vision, I opt for aspheric LASIK." If the myopia is extremely high, that is, more than 10 D, Dr Chan prefers tissue-saving. Zyoptix Aspheric removes about 5 to 10% more tissue compared with conventional LASIK, so if saving cornea is more important, then tissue-saving takes precedence, Dr Chan says.

In largely ethnic Chinese Singapore, where an estimated 40% of the population is said to be myopic, Dr Chan's LASIK practice now constitutes 80% Aspheric LASIK. In India, 30% of the adult population is estimated to be myopic.

The cost of treatment could be of concern in developing countries, but Dr Chan believes the added cost is a minor increment that is negated by the significantly better post-op results. "A surgeon should speak about it. After all we are in the quest for best vision," he advises.

This technology is currently being rolled out in Europe and is being offered by refractive surgeons in Germany, France, Italy, Sweden, Finland, Norway, Denmark, Spain, Portugal and Belgium.