Laser Eye Surgery - an acceptable alternative to glasses and contact lenses?

From a talk given to the Manchester Society of Engineers in the University of Manchester Institute of Science and Technology (UMIST) on 9th May 2000 and  printed in their journal.

Also printed with variations in PULSE medical magazine for GP's on March 6th 1999

Recent high profile people having laser eye surgery rather than wear contact lenses or glasses, such as Richard Branson, Tiger Woods and Nicole Kidman, have made people realise that perhaps this is a technology that is coming of age. Many tens of thousands of people in the UK have had PRK (Photorefractive Keratectomy), which is the most popular modality in the UK and now LASIK (Laser Assisted Keratomileusis), a mixture of laser and microsurgery, offers a quick and pain-free recovery. The ophthalmologist who first thought of this surgery, Prof. Steven Trokel in New York, did his first degree in engineering!

HOW IT WORKS

This technology utilises an ultra-violet "excimer" laser of 193 nm to alter the shape of the cornea. This is an argon-fluoride laser with a very high photon energy of 6.5 eV. This cuts off about 0.25 microns per pulse. In the case of myopia (short sight), the cornea is flattened and in hyperopia (long-sight) it is steepened. The process might be thought of as "carving a contact lens" onto the eye. The cornea is about 550 microns (0.55 mm) thick and, in an average myope of 3 Dioptres (D), the laser takes off about 40 microns. The cornea is not weakened physically and the operation is not visible to the naked eye (and indeed it is often not possible to detect even using an operating microscope). The laser hardly raises the temperature of the cornea and hence does not cause any scarring due to collagen thermal damage.

ACCURACY                                                                                               

No-one yet knows how to measure what the laser is doing in "real time" i.e. during the operation. People are not as identical as the inert materials of circuit boards, for which this type of laser has been used for many years in industry, and the laser may cut off a bit more or less from the cornea than predicted. People’s healing characteristics also vary. Hence, the bigger the prescription, the bigger the spread of results. As a rough guide, most low myopes (less than -6D) achieve within ½ D of aim and most higher myopes (-6 to -10D) achieve within 1D. Although it is less accurate for the higher myopes, the patients are often even more pleased, as they are effectively blind without glasses or contact lenses. (Try putting on a couple of +3.5 reading glasses, one on top of the other, to see what a -7D myope is like!). About 25% of the adult Caucasian population are myopic and 90% of these are -6D or less. PRK/LASEK  is suitable for up to about 10D of myopia, 4D of astigmatism and 3D of hyperopia. 

RISKS

No operation has zero risk, including PRK, despite the "street cred" of lasers. PRK is elective surgery on a healthy eye, so the criteria are more strict than operating on a diseased organ. No one, to my best knowledge, has been blinded by PRK and at least 50,000 have now been treated in the UK alone.

The commonest risk is loss of sharpness of vision that is not correctable with glasses. About 5% will lose 1 line of vision on the Snellen chart (the commonest eye chart) and about 1 in 1000 will lose 2 lines. This is mostly due to micro irregularities of the surface that cannot be corrected optically by the regular surface of glasses. Most patients will not notice the loss of one line of vision but will notice 2 lines of loss. Patients can also have a "touch-up" for residual refractive errors and most clinics do not charge extra for this. About 5% of the higher myopes will have such an "enhancement".

Patients have to understand that they could have some ghastly problem such as a corneal abscess and might end up needing a corneal graft. Such situations may be exceedingly rare, but if it happens to you then statistics are no consolation! The risk of such a disaster is probably about the same as when wearing a soft lens, which many patients will have used for some years.

How about LASIK?

LASIK (Laser in situ keratomileusis) is the other major alternative in laser eye surgery. This combines PRK with an older surgical procedure known as keratomileusis. In this, a powered microkeratome (a fancy sort of bacon slicer!) is attached to the cornea with a suction ring and a partial flap of about 160 microns is created with a "hinge" at one side. The excimer laser is then fired in the same way as in surface PRK and the flap is replaced. It has hence been called the "flap and zap" operation! The flap re-attaches initially by osmotic pressure and no sutures are needed. LASIK is a better surgical experience for the patient than PRK because, as the corneal epithelium is left almost intact, there is little pain and a faster visual rehabilitation. A LASIK patient at day one post-op will see what a PRK patient will see at 1-2 weeks. This has been called the "wow" factor of the surgery. Most refractive surgery in the USA is now LASIK. There are many detailed internet sites (Links) which will give you more on the surgical details. I personally use a Hansatome microkeratome and usually a Nidek EC5000 laser

  or a Technolas 217

Recently I have been using the Lasersight laser and microkeratomes from Moria

What are the risks?

LASIK is clearly a "proper" surgical procedure and has more to go wrong then PRK. Perhaps the most important piece of advice for the patient contemplating LASIK is to choose the surgeon well! LASIK costs more than PRK, being about £800-1500 per eye as opposed to £700 -850

How does a person decide whether to opt for PRK or LASIK?

The end results of PRK and LASIK are the same. As they both use the same laser, the accuracy levels and numbers of people who lose sharpness of vision is also about the same. LASIK "gets there" faster and with less pain than PRK whereas PRK is safer. Which procedure to chose depends on each patient’s attitude to risk versus convenience. Neither procedure should be used for myopia greater than -10 Dioptres as the optical zones carved on the cornea are too small for low light vision. One eventually just "runs out of cornea"! If the cornea is thicker than average then one can do more treatment and, correspondingly, if it is thinner then one can do less. The corneal thickness is measured using an ultrasound probe and only takes a few seconds to do. PRK is the procedure that most low myopes chose and LASIK is better for the high myopes because of the speed of visual recovery. There is an "overlap" area of those between -5 to -7 Dioptres where the pros and cons are about even. (90% of myopes are -6D or less).

SUMMARY

PRK and LASIK are now beyond the experimental stage and into the developmental stage. Having treated my brother (-6.5D)and my GP (-2.25 D), who are both delighted, I think that it is a reasonable alternative to glasses or contact lenses, particularly for the lower myope.

The Royal College of Ophthalmologists (Tel: 0207-935-0702, Fax: 0207-935-9398) publishes an excellent leaflet for prospective patients which costs £5. This is also on line at the College Web Site.

© S J Doyle July 2007

If you want help to decide whether to go for LASIK or LASEK (PRK Epiflap) then click here