What is the need for this technique?
The need arises from the desire to have good vision without using glasses or contact lenses.
Laser eye surgery works by carving a refractive lens permanently onto the cornea using an excimer laser (wavelength 193nm).
It was first performed in the late 1980s to correct myopia. Since 2000, it has been successfully used to correct hypermetropia (long sight) and astigmatism. The correction is for distance vision, not reading.
To focus an image on the retina requires the focusing power of the eye - a combination of natural lens (approximately 20 dioptres) and cornea (40 dioptres) - to match its length (approximately 22mm) and surface curvature.
Imbalance causes refractive errors. These are remarkably common, especially in urban populations. Myopia prevalence is up to 80 per cent in Taiwan and in the US it affects one third of the population. Aetiology is partly genetic and also thought to be due to environmental factors, such as near vision use. Some errors are also caused by surgery or trauma.
Which patients may benefit and what refractive issues need to be discussed?
There are two groups who derive most benefit from laser eye surgery.
The first is young adults (>21 years) with a stable glasses prescription. Using the current generation of lasers, myopia up to 10 dioptres, hypermetropia and astigmatism up to 5 dioptres can be corrected with predictability and safety. Dissatisfaction occurs if patients have unrealistic expectations that a full correction of their refractive error is guaranteed and they will never need glasses again.
Pre-operative counselling should cover risks and refractive changes from ageing so that the full implications of laser surgery are understood.
As the eye ages, the lens gradually loses ability to change shape to focus for near tasks so that by age 50 years most non-myopes will need glasses to read (presbyopia).
Myopes are naturally focused for near sight and will continue to read unaided throughout their lives. If laser eye surgery fully corrects myopia, presbyopia will occur and require use of reading glasses approaching 50 years of age.
The second group comprises older presbyopic adults. Fumbling for reading glasses to discover they are in the car is a feature of maturity that most adults would happily forgo.
Excimer lasers are for distance vision although they can be used to set the focus of one eye for near, a technique called monovision widely used by optometrists.
The newest excimers use sophisticated nomograms to increase the depth of focus provided by monovision. A new generation of femtosecond lasers designed specifically for presbyopia is on the immediate horizon.
What are the procedures and when are they used?
This is a field of rapid change. Currently there are two principal techniques: either sculpting the refractive shape onto the superficial corneal stroma (surface technique) or cutting a flap through the cornea with a femtosecond laser to allow excimer sculpting deeper in the stroma (flap-based technique).
The advantages of flap-based surgery (LASIK: laser in situ keratomileusis) are that recovery is rapid (one to two days) with little pain as corneal epithelium is minimally disturbed and deep stromal wound healing causes less regression of the laser effect. However, the cornea may be weakened over the long term causing a form of iatrogenic keratoconus.
Surface techniques (PRK: photoablative refractive keratectomy, LASEK: laser epithelial keratomileusis) have longer recovery with pain for several days, until regrowth of corneal epithelium occurs, and may have more corneal healing reaction with haze (scarring) and regression of the refractive effect.
Surface techniques are simpler and cheaper than flap-based. They are better for corneas too thin for making a flap but are less convenient and so less popular with patients.
When is surgery performed?
Laser refractive surgery is performed after fully informed consent. It is only available on the NHS if there is disabling refractive imbalance between the eyes not amenable to other treatment. The technique is also used for non-refractive reasons for smoothing or removal of corneal scars.
What are the potential problems and risks?
The most common problem is a residual need for glasses, which is often cured by a repeat procedure. Another side-effect is dry eyes, which may last for months and be relieved by artificial tear supplements.
Regression, unstable refraction, glare when driving at night (less frequent with the newest lasers) and keratoconus are possible. There are rare serious complications, such as corneal infection.
The calculation of intraocular lens power for later cataract surgery is less accurate and young adults must be given their pre-laser keratometry readings (power and characteristics of corneal shape) to make later calculations when needed.
How long do patients take to recover?
After flap-based techniques recovery is fast: the eye will be comfortable and vision sharp after a day or less. Topical steroids and antibiotics are used for the first weeks after the surgery.
Surface techniques are far slower. To aid initial comfort a soft contact lens is fitted immediately after the procedure.
Pain may be disabling until corneal epithelium resurfaces the eye (two to three days) and vision is blurred for two to four weeks.
What outcomes can we expect?
Outcomes are outstanding, fuelling the extraordinary uptake of this technology and ever-increasing public expectation of near-miraculous vision correction. For example, a recent review found that 64.8 per cent of patients with low- to-moderate myopia were within +/-0.5 dioptres of zero refractive error after LASIK.1
What is the cost?
The cost varies and is typically between £1,000-2,000 per eye.
Where is it available?
All major UK cities have private refractive surgery centres, grouped into consultant-led clinics and high street chains.
Usually NHS ophthalmic units have access to this technology when needed, either their own or provided by outsourcing to the high street.
- Ms Vafidis and Mr Levy are consultant ophthalmic surgeons at the Central Middlesex Hospital, London
Reference
1. Dirani M, Couper T, Yau J et al. Long-term refractive outcomes and stability after excimer laser surgery for myopia. J Cataract Refract Surg 2010; 36(10): 1709-17.
Resources
Sutton GL, Kim P. Laser in situ keratomileusis in 2010 - a review. Clin Experiment Ophthalmol 2010; 38(2): 192-210.
NICE. Photorefractive (laser) surgery for the correction of refractive error - guidance. IPG 164. London, NICE, 2006.
