Excimer laser PRK/PARK vision surgery for low to moderate myopia and myopic astigmatism respectively, is another established surgical technique which has proved to be very safe and effective. Studies have shown that the majority of patients with myopia of between -1.0 to -6.0 diopters enjoy a very satisfactory outcome (20/20 vision or better) following PRK vision surgery. About 80% do not require eye glasses or corrective lenses.

Where PARK is performed, the best outcomes are achieved for myopic eyes with associated astigmatism not exceeding 1.5 diopters. As higher corrections are attempted, the outcome becomes less predictable although satisfactory results are often achieved for moderate myopia and astigmatism.

H-PRK is the most recent form of surface-based laser vision surgery. It is generally used for low degrees of of hyperopia (farsightedness) as the procedure is still in its infancy. However, early results look promising.

The three PRK procedures outlined above are all performed in a very similar manner. Tissue immediately below the thin layer of corneal surface cells (epithelium) is rubbed away to reshape the cornea during a painless procedure. The sole difference between them is the pattern created on the cornea. LASIK vision surgery, by contrast, slices a thin, hinged flap from the top of the cornea and is used for higher corrections.

The benefits of PRK vision surgery.

* PRK does not involve the creation of a hinged corneal flap which is a very exacting procedure. As a result, PRK is associated with fewer complications than LASIK surgery.

* The complications, however, in LASIK can be serious and lead to permanent loss of best corrected vision if the corneal flap is incompletely reseated or damaged in any way.

* Following PRK laser surgery minor symptoms are quite common in the first few months, but soon disappear altogether. However, problems like halo and distorted vision (now less common with the introduction of larger diameter treatment zones), or severe haze and scarring, which can result in permanently affected vision, only affect around 1-2% of patients.

* Anisometropia (a difference in focus between the two eyes after one eye only has been treated) is more marked for patients who undergo LASIK surgery for higher levels of myopia. The difference in focus might be quite marked and make driving unsafe.

Those patients with moderate myopia who cannot wear contact lenses might well consider PRK instead. Their symptoms of anisometropia may well be less pronounced between the treatment of the first and second eyes.

The drawbacks of PRK vision surgery.

* A PRK procedure generally requires a lengthy period between the first and second treatments of between 6 and 12 months. In LASIK, on the other hand, stabilization occurs earlier than after PRK and retreatment can typically go ahead between 1 and 3 months after the primary treatment.

* As the period between primary and retreatment is a lengthy one in PRK, so several months need to elapse before 20/20 results are achieved, and then eye glasses may have to be worn.

* While the actual laser treatment is usually pain-free, most patients will experience mild to moderately severe eye pain lasting up to 36 hours. Pain killers are prescribed for what is, in effect, a large abrasion on the cornea. LASIK is relatively pain-free.

What to expect from PRK vision surgery.

Before going ahead with a PRK procedure, your eyes will be assessed by an eye surgeon which will include computerized corneal topography capable of revealing otherwise undetectable irregularities in the cornea.

Such a detailed mapping of the cornea will allow the surgeon to remove tissue to a pattern pre-determined by the patient's own refraction.

Before this corneal tissue is removed, the epithelium (thin layer of surface cells) is rubbed off, a process which takes just one to two minutes. The corneal ablation is even quicker depending upon the degree of correction being treated.

After treatment, topical drops and ointment are applied to the eye. You will also wear a pad for 6-24 hours post-surgery. Eye drops will keep the tear film lubricated and quite possibly you will need to wear glasses, and best vision could take six months to achieve.

How to choose a PRK eye surgeon.

We finish with the most important question of all: choosing your eye surgeon. Personal recommendation is usually the best method, followed by an interview of likely candidates.

Be well prepared with your questions which should include accurate information about your likely outcome; the cost of treatment (and any retreatment); what the cost of treatment comprises (are clinic reviews and medication included?) and, of course, the surgeon's qualifications and track record. Oh, and read the fine print.