Excimer Laser Surgery

EXCIMER LASER SURGERY

The Excimer laser is a surgical instrument used to treat refractive errors by reshaping the cornea. It operates in the far ultraviolet region and it is used to remould or shape the surface of the cornea.

The interaction with corneal tissue results in ablation of the surface of the cornea by a process of photochemical decomposition of molecules without thermal damage. Computer controlled modulation of the laser beam is able to alter the profile or shape of the cornea leaving a smooth surface. These lasers are incorporated in suitable computer controlled medical devices which ophthalmic surgeons use to alter the power of the cornea and correct refractive errors such as myopia, astigmatism and hypermetropia, singly or in combination.

Experimental ophthalmic use of the excimer laser began in 1982 and clinical studies in patients commenced in 1989. Results of these studies indicated that excimer laser treatment is an effective surgical method of correcting certain categories of refractive error with low and acceptable rates of complication in these circumstances. Keratectomies performed with excimer laser can be categorised into either phototherapeutic keratectomy (PTK) and photorefractive keratectomy (PRK/LASIK).

PHOTOTHERAPEUTIC KERACTECTOMY (PTK)

PTK is a technique for removal of surface irregularities or superficial stromal scars. The superficial layers of the cornea are ablated, sometimes with the use of a surface smoothing substance leaving behind a smooth clear corneal surface with an epithelial defect. The corneal epithelium heals over in three to five days during which time there may be significant pain. The possibility of an infection occurring during the healing phase exists, but is considered very unlikely. Although there are other surgical approaches to these corneal problems the excimer laser provides an alternative treatment, which in many cases is less invasive.

PHOTOREFRACTIVE KERACTECTOMY (PRK)

PRK or surface laser is a similar procedure, but in this instance the superficial ablation of the central cornea is precisely controlled to alter the corneal curvature in a specific manner to correct pre-existing refractive errors. Clinical experience and publications over the last six years indicate that the results of the procedure are most satisfactory in patients with lesser degrees of myopia. The procedure is also capable of correcting astigmatism as well as myopia in a procedure which is less often termed PARK (photo astigmatic refractive keratotomy).

It is not unusual, however, for residual astigmatism to be present following laser surgery especially in cases with high levels of astigmatism.

With higher degrees of myopia the predictability of the procedure is reduced, and the incidence of haze or scarring increases. Nevertheless, with appropriate patient counselling, excimer laser PRK for myopia is considered an acceptable therapeutic modality. Experience with the correction of hypermetropia with excimer laser is relatively limited. Although the results of PRK with low degrees of hyperopia are acceptable, the results of treatment of higher levels of hypermetropia indicate that the problems with predictability and regression render this treatment currently unacceptable.

LASER IN SITU KERATOMILEUSIS (LASIK)

Laser in situ Keratomileusis or LASIK, developed in 1991, describes the use of the Excimer laser to ablate and reshape the corneal stroma following elevation of a corneal flap with a microkeratome. The flap is then repositioned and the corneal shape is therefore modified without disturbing the surface layer. The amount of post-operative discomfort is less and the visual recovery more rapid, but the major advantage is the greater predictability and lower incidence of postoperative haze and regression that the technique offers for patients with higher levels of myopia and hypermetropia. The technique allows retreatment of residual refractive error with greater facility than PRK.

There is, however, a small but significant additional risk of surgical complications with LASIK which are not associated with PRK.

The long term effect of removing corneal tissue is unknown but there is no current data to suggest that this will cause a later complication.

SUMMARY

The Royal Australian and New Zealand College of Ophthalmologists considers that PTK is an acceptable surgical procedure for the correction of some superficial corneal irregularities, or superficial stromal scars, and that PRK and LASIK are acceptable surgical procedures for the correction of refractive errors. As for any surgical procedure, appropriate counselling of patients as to expected outcome and complications is required, and the relative merits of each method need to be considered by the surgeon and the patient in selecting the most appropriate technique for an individual patient.