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The advent of the excimer laser, with its ability to ablate tissue to submicron accuracy, has largely superceded radial keratotomy in the treatment of myopia. 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.
Refractive Surgery Techniques
PRK, or photorefractive keratectomy, is a laser procedure for the treatment of myopia (nearsightedness), hyperopia (farsightedness) and astigmatism with the excimer laser. A computer generated, cold laser beam is used to precisely remove and sculpt corneal tissue at the microscopic level. The PRK procedure itself usually takes only 1 to 2 minutes to perform. The eye is anesthetized with topical drops, a lid retainer is placed to hold the eyelids back, PRK is performed, and then a bandage contact lens is placed on the eye. There is usually no discomfort with the treatment itself, but there may be mild to moderate discomfort for 1 to 2 days following the PRK, because of the healing taking place on the outer layer of the cornea. This is usually controlled with topical or oral medications.
LASIK, or Laser in-situ Keratomileusis, is a procedure for nearsightedness (myopia), farsightedness (hyperopia) and astigmatism. It involves the making of a thin circular flap with an instrument called a microkeratome. This is a precise instrument that creates a thin, corneal flap with a hinge on one side of the cornea. Once the flap is made and folded back, PRK is then performed in the deeper layers of the cornea. The flap is then carefully placed back in its original position without the need for sutures. The procedure is usually painless. Patients typically have minimal to no postoperative discomfort and have a low risk of developing corneal scarring/haze. The treatment allows for fast rehabilitation with rapid return of clear vision. Studies have shown that 97% of patients see 20/40 or better on the first post-operative day. The main risk is related to the creation of the corneal flap, but in expert hands, the risk is only 1-2%.
Laser Epithelial Keratomileusis (LASEK) is a simple technique based on the detachment of the epithelial flap by an alcohol solution prior to the application of laser energy, which resculpts the cornea. This hybrid procedure is an advanced form of PRK. With the dilute solution of alcohol and a lifting motion with a surgical spatula, the epithelial flap is gently detached, gathered and folded away from the laser ablation area. At this point the treatment procedes as a traditional PRK. At the conclusion, the epithelial flap is repositioned over the cornea. The advantages of LASEK are that it may reduce post-surgical discomfort, lead to faster visual rehabilitation and reduce the incidence of corneal haze.
Myopia Treatments and Eligibility
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.
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. 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.
Other Conditions and their Treatment
Laser technology is also useful in treating Glaucoma. The most common glaucoma laser surgeries are:
Laser Peripheral Iridotomy (LPI): Often used in people with narrow-angle glaucoma. Narrow-angle glaucoma occurs when the angle between the iris and the cornea in the eye is too small. This causes the iris to block fluid drainage, which makes the inner eye pressure increase. LPI makes a small hole in the iris, allowing it to fall back from the fluid channel and helping the fluid drain.
Argon Laser Trabeculoplasty (ALT): Used in people with primary open angle glaucoma (POAG). The laser beam opens the fluid channels of the eye, helping the drainage system to work better. In many cases, medication will still be needed. It has successfully lowered the eye pressure in up to 75% of patients treated.
Selective Laser Trabeculoplasty (SLT) Uses a combination of frequencies.
Nd:YAG Laser Cyclophoto-coagulation (YAG CP): Used in people with severe glaucoma damage that is not being managed by standard glaucoma surgery.
Macular edema, a complication arising from branch vein occlusion, can be treated by macular grid laser photocoagulation, which improves retinal oxygenation causing auto-regulatory vasoconstriction - the macular arterioles constrict 20% and the venules constrict 14%. Another BVO treatment is laser-induced chorioretinal anastomosis, where bypass of the normal retinal venous drainage channels is attempted by creating a communication between the obstructed vessel and the choroid.
Laser treatment (photocoagulation) has also been found to be effective in halting or reversing new vessel growth in Diabetic Retinopathy. Overall, the incidence of blindness over two years can be reduced by 66% for eyes with new vessels on the optic nerve and by 37% for eyes with new vessels in other locations. The laser treatment commonly consists of applying many (1000-2000) laser burns to the peripheral retina (see drawing). Although mildly uncomfortable, the treatment usually can be done without the need for local anesthesia. The treatment is often divided into several sessions. In addition, when the main problem is not growth of abnormal new blood vessels, but rather leakage from previously normal capillaries causing waterlogging of the central retina, reading vision may be stabilized in some cases by using laser to help stop the leakage and dry out the retina.
The future is difficult to predict. Solid state lasers will reduce the cost of equipment and improve the stability of laser energy output. Intrastromal picosecond lasers are under trial. Even without these new developments, however, the ability to correct and treat eye conditions with reasonable accuracy is already here.