Expert opinion: Assessing laser refractive surgery

NICE now takes a more positive view of laser eye surgery, says Mr Stephen Tuft.

The NICE guidance on photorefractive (laser surgery) for the correction of refractive errors which was published in March 2006 replaces the previous guidance on laser in situ keratomileusis (LASIK) issued in March 2004, which expressed reservations about the long-term safety of the technique.

However, many of the ophthalmologists who perform refractive surgery were unhappy with the 2004 document because it was not based on a systematic review of the literature. They did not believe that the guidance provided an up-to-date or balanced assessment of the risks of the procedure.

The new guidelines present a more positive assessment of LASIK surgery.

Refractive error

Refractive error of the eye occurs when light from an image cannot be brought naturally to a focus on the retina. If the image is in front of the retina the eye is myopic (short-sighted) and if the image is focused beyond the retina the eye is hyperopic (long-sighted). The principal cause for refractive error is disparity between the eye's axial length and the power of the focusing system of the eye, the lens and the cornea.

In myopia, the eye is relatively elongated and the retina is placed beyond the focal point for incoming light. It is not feasible to surgically shorten the eye, but the power of the major refractive component of the eye, the anterior surface of the cornea, can be accurately modified and the focusing power dramatically changed.

Photoablation

During laser refractive surgery a UV laser is used to re-shape the optical zone of the cornea. Each pulse of the laser removes a thin layer of corneal tissue by a process called photoablation. By controlling the shape and position of the laser beam, the radius of curvature can be adjusted and thus the refractive power of the cornea changed.

However, as the laser removes tissue the cornea is inevitably thinned as a result, and the higher the refractive error corrected the greater the resultant thinning. For example, a treatment for eight diopters of short sight will thin the central cornea by about 100 microns compared to the average central corneal thickness of 520 microns.

Because the corneal epithelium rapidly regenerates, the treatment must be applied to the fibrous stromal layer beneath the epithelium to have a permanent effect.

What is LASIK?

The different treatment options to achieve this can be broadly classified as either surface treatment - in which the front surface of the cornea is treated directly after the epithelium has been removed - or flap treatment in which the cornea is re-shaped beneath a superficial flap has been mechanically cut into the cornea. This procedure is called LASIK.

The thickness of the flap cut for LASIK is typically 130 to 180 microns.

The flap is held in position by suction, but can be dislodged by an abrasion in the early postoperative period. The thickness of the flap is not thought to contribute mechanically to the strength of the cornea after it is replaced and the strength of the cornea is thus probably less after LASIK than after surface treatments.

Various permutations of surface treatment have been developed; if the epithelium is removed and discarded, the procedure is termed photorefractive keratectomy (PRK), but if the epithelium is removed as a sheet and replaced the procedure is termed LASEK. Both PRK and LASEK are being reviewed by NICE.

Improved technique

The limit to the amount of refractive error that can be corrected by laser is principally determined by a recommendation that a residual corneal thickness of 250 microns is required to prevent subsequent forward bowing of the thinned cornea (ectasia), and also by the observation that there is image degradation if the cornea is re-shaped excessively.

In practice this means that the technique is not usually appropriate for correction of more than nine diopters of short sight or six diopters of long sight.

For patients who fall beyond the treatment range other techniques such as intraocular lens implantation or lens removal (clear lens 'cataract' surgery) are available.

The other major alternatives to laser surgery for the correction of refractive error are glasses and contact lenses. The risk of a severe corneal infection from wearing a soft contact lens (1:5,000) is similar to the risk of corneal infection following LASIK.

Patients considering having elective laser refractive surgery need to have a careful assessment of their suitability for treatment, their likely outcome in terms of accuracy, and the risks they will be exposed to if they proceed with treatment.

Although treatment will dramatically reduce refractive error, the outcome is rarely perfect and the majority of eyes will still have a small residual error, although most patients do not feel the need to use a correction for this. A small regression of effect occurs in some eyes, but in most the refractive correction following LASIK is permanent, although it does not eliminate the eventual need for reading glasses.

Guidance

The recent NICE guidance provides a valuable and independent resource for patients considering refractive surgery.

Laser refractive surgery is safe for use in appropriately selected patients.

Although there is no evidence to date of the emergence of new long-term complications, NICE recommends that follow-up studies continue to monitor for potential adverse outcomes.

- Mr Tuft is consultant ophthalmologist at Moorfields Eye Hospital, London

NICE RECOMMENDATIONS

  • Laser surgery for the correction of refractive errors is not appropriate for correction of more than nine diopters of short sight or six diopters of long sight.
  • Patients considering having elective laser refractive surgery need to have a careful assessment before going ahead with treatment.
  • Although treatment will reduce refractive error, the outcome is rarely perfect and the majority of eyes will still have a small residual error.
  • More follow-up studies should be carried out to continue monitoring for potential adverse outcomes.

REFERENCES

- Shortt A J, Allan B D. Photorefractive keratectomy (PRK) versus laser-assisted in situ keratomileusis (LASIK) for myopia.Cochrane Database Syst Rev. 2006 Apr 19;(2):CD005135.

- Sakimoto T, Rosenblatt M I, Azar D T. Laser eye surgery for refractive errors. Lancet. 2006 Apr 29; 367(9,520):1,432-47. Review.

- www.nice.org.uk. NICE guidance on photorefractive (laser) surgery for the correction of refractive error. March 2006.

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