The changing face of refractive Lasik surgery

The changing face of refractive Lasik surgery

Several promising techniques are already available

Ametropia, when the eye needs corrective lenses to bring an image into focus, is common and has been so for centuries. The Roman emperor Nero observed gladiator fights through an emerald to correct his ametropia. The presence of 1 dioptre (D) of refractive error reduces Snellen visual acuity to less than 6/12that is, below the level needed to drive. More than half of all eyes have a refractive error of greater than 1D and 15% have an error greater than 2D. Although the vast majority of patients will continue to wear spectacles or contact lenses, the popularity of refractive surgery to correct ametropia continues to grow. So too does the variety of surgical techniques available to correct ametropia.
The factors exerting the greatest influence on refractive power are corneal curvature, the power of the lens, and axial length. The surgeon can thus act on the cornea or the natural lens with or without introducing artificial lenses. Assessing the best procedure for a particular refractive error is difficult because of the speed of change within this field. Most data come only from case series, and individual surgeons may also be limited by lack of experience in particular refractive techniques or by not having access to required equipment.
Myopia has been corrected for many years by radial keratotomy (radial corneal incisions), though this has few proponents in Britain. The advent of the excimer laser, with its ability to ablate tissue to submicron accuracy, has largely superceded radial keratotomy. The excimer laser can be used either on the surface, after removing corneal epithelium before treatment (photorefractive keratectomy), or within the stroma, after cutting a corneal flap (laser in situ keratomileusis or LASIK). In the United States over 70% of procedures performed are laser in situ keratomileusis, whereas in Britain most procedures are still photorefractive keratectomy.
The popularity of laser in situ keratomileusis is due to its "wow effect": visual recovery is almost instantaneous and painless. Patients are thrilled. After photorefractive keratectomy there may be considerable pain (unless a bandage contact lens is used), and the final result is not achieved for several months. However, the long term results of laser in situ keratomileusis in randomised trials seem to be no different from those of photorefractive keratectomy.There is a significant difference in price between the two procedures (twofold in some centres), with laser in situ keratomileusis also requiring considerably more training.
Myopia up to -5D is amenable to photorefractive keratectomy, and myopia of over -7D is best treated by laser in situ keratomileusis, with both procedures being acceptable between -5 and -7D. Worry about late ectasia (bulging of the cornea) is growing, and the maximum myopia treated with laser in situ keratomileusis is reducing (to maximum of -12D). There is considerable overlap between the degree of myopia treated by these two procedures based on surgeons' and patients' preference and cost. Case series suggest that complications with photorefractive keratectomy increase with the size of refraction and hence magnitude of treatment (haze and regression), whereas case series and a review of the literature on laser in situ keratomileusis show a 3-5% complication rate (flap related complications) regardless of treatment size. Vision of 6/12 or better is achieved in 94.4% of eyes with photorefractive keratectomy and in 49.2-83.2% of eyes with laser in situ keratomileusis. The degree of preoperative myopia is different in the two groups, however, so the difference in results reflects the higher degrees of error generally treated with laser in situ keratomileusis. The predictability reduces with increasing magnitude of correction and with additional treatment of astigmatism.
Other treatments for myopia are intracorneal rings and phakic intraocular lenses. The excitement about intracorneal rings for low to moderate myopia has waned as the surgery is difficult and also has a high risk of induced astigmatism, though initial results were comparable to those of other modes of surgery. Their main advantage is their reversibility. Phakic intraocular lenses (intraocular lenses inserted in the presence of the natural lens) start as low as -5D but are generally above -15D up to -30D (they are also available for long sightedness or hyperopia up to +10D). The advent of these lenses is very exciting for people with high myopia. Results from large series are scarce. There is also concern about the lens's effect on the natural lens or the cornea, depending on the type used. All intraocular surgery can end with an intraocular infectionwhich can be devastating; case series after other forms of intraocular surgery show infection rates of up to 0.3%.
Extreme myopia is probably best treated by a combination of phakic introcular lenses and laser in situ keratomileusis. Otherwise compromises are necessary because of the thickness of the lens and small optical zone. The alternative is to remove the natural lens and implant an artificial lens as in cataract surgery (also suitable for high hyperopia). This does, however, lead to loss of accommodation and is best suited for patients who need reading glasses; one case series showed that 42.3% of patients achieved vision of 6/12 unaided. Another series showed that this procedure is also suitable for patients with high hyperopia.
Hyperopia can be treated to +4D with photorefractive keratectomy and to +6D with laser in situ keratomileusis. The problem with hyperopia is that a wide treatment zone seems necessary for stability of treatment. However, the treatments do not seem to be as effective or as accurate as myopic corrections except in low corrections.
The future is difficult to predict. Solid state lasers will reduce the cost of equipment and improve the stability of laser energy output. Corneal inlay lenses and intrastromal picosecond lasers are under trial. Presbyopic treatments such as scleral implants seem unstable or variably effective at present but demand remains high. Even without these new developments, however, the ability to reduce refractive errors with reasonable accuracy is already here.