Overview

Introduction

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 has come of age. Many tens of thousands of people in the UK have had laser eye surgery, especially  LASIK (Laser Assisted Intrastromal Keratomileusis), a mixture of laser and microsurgery, that 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, although  recent developments are arriving in this area. 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 0.25 D of aim and most higher myopes (-6 to -8D) achieve within 0.50D. 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. Eyes are  is suitable up to about 9D of myopia, 5D of astigmatism and 5D of hyperopia.

Risks

No operation has zero risk, including PRK/LASEK, despite the "street cred" of lasers. This 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 it and at least 50,000 have now been treated in the UK alone.

People worry that they will lose some sharpness of vision. It is rare these days for anyone to loose vision  About 1 in 1000 will lose one line on the Snellen chart (the commonest eye chart) and it is a rare event to 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 commonest form of laser eye surgery. This combines surface laser (PRK - PhotoRefracative Keratectomy)  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 week. This has been called the "wow" factor of the surgery.