Refractive / Laser Eye Surgery
Refractive Errors
Images are focused on the retina by the cornea (the clear part of the front of the eye) and the natural lens (suspended within the eye just behind the pupil). About half the Irish population (50%) requires glasses or contact lenses to focus a clear image of a distant object on the retina (Figure 1).
Glasses and contact lenses either add or subtract focusing power to help form a clear image on the retina where the natural focusing power of the eye is incorrect. Defects in natural focusing power are called refractive errors. Refractive errors are measured in units of lens power (“dioptres” or D) and represented for each eye in your spectacle or contact lens prescription by a number prefixed by a sign (e.g. +2.00D or –6.50D). The sign indicates whether glasses required are for long (+) or short (-) sight.
Short sight (myopia) and long sight (hypermetropia) are often accompanied by an element of uneven focusing power (astigmatism). In this case the cornea is more rugby ball or egg-shaped than football shaped. This is represented in your glasses prescription by a second number and an angle (e.g. –4.00D at 80°) indicating the focusing power and orientation of the lens required to correct the astigmatic component of your refractive error.
The younger eye is able to increase focusing power, or accommodate, to see near objects clearly. This flexibility of focus is provided by flexibility in the shape of the natural lens. As we get older, the natural lens becomes less flexible (presbyopia), and the ability to accommodate diminishes. This is why normal sighted people need reading glasses from their mid forties on. The final component of your glasses prescription describes the reading addition, or the difference between your prescriptions for distance and reading glasses. Typically, this varies from +1.00D in your mid forties to a maximum level of +3.00D by your early sixties. (Figure 2)
Refractive Surgery
Refractive surgery is not normally performed until the glasses prescription has been stable (no change greater than 0.50D) for 1-2 years. Spectacle prescriptions typically stabilise in the late teens or early twenties for myopic patients (-ve prescriptions), and later for hypermetropic patients (+ve prescriptions) who are able to compensate by accommodation (as in focusing for near vision) whilst younger, and often only become spectacle dependent in mid life.
Refractive surgery techniques are available to reduce or eliminate the need for spectacles in most patients with a stable spectacle prescription. For younger patients, good distance vision in both eyes is usually the aim. For patients in the reading glasses age group, a compromise (monovision) in which better distance vision is targeted in one eye and better near vision in the other is commonly employed to minimise spectacle dependence over a range of activities.
Lower refractive errors (in the range +3.50 to -8.00D) are usually corrected using laser techniques (LASIK, LASEK or PRK) to reshape the cornea. Higher errors are corrected using lens implant based methods (Refractive lens exchange/RLE or Intraocular collamer lens/ICL implantation). Astigmatism and age are also influential in determining the most appropriate technique. Many patients over 60 are better suited to lens exchange (RLE) than laser refractive surgery, particularly if the early signs of cataract (lens opacity) are present.
Refractive Surgery Procedures
LASIK (Laser in situ keratomileusis) - range +3.5D to -8D with up to 6D astigmatism.
Principle = wavefront guided excimer laser corneal reshaping beneath a protective corneal flap which is replaced at the end of surgery (Figure 3).
In LASIK, a femtosecond laser (the Intralase FS60) is used to create a standard size hinged flap of surface corneal tissue. The tissues beneath are then reshaped using a computer-controlled excimer laser before the protective flap is replaced. In essence, the cornea is reshaped without surface damage. Because the surface tissue damage is minimised in LASIK, visual recovery is rapid and virtually pain free.
- Flap information - a thin flap of surface corneal tissue is formed using a blade-free laser
- Wavefront guided laser reshaping - the flap is flipped over to allow laser reshaping of the cornea without damage to the surface tissues
- Flap replacement - the surface tissue flap is repositioned on the reshaped cornea, and normally adheres quickly without stitches
Intralase LASIK – (all laser / blade-free LASIK)
Prior to 2007, automated mechanical microkeratomes were used to make the flap (like a mini ham slicer used in delicatessens) until bladeless surgery was introduced using femtosecond lasers. Since 2007, surgeons in the Blackrock Clinic use the Intralase femtosecond laser to create the corneal flap under which the corneal stroma is then reshaped with the Visx excimer laser. Bladeless refractive surgery has safety advantages over previous techniques and has resulted in far fewer flap-related complications. More information about the intralase approach can be found at www.intralase.com
LASEK / PRK (Surface laser refractive surgery) - range -1D up to -8.0D with up to 6D astigmatism.
Principle = wavefront guided excimer laser corneal reshaping of the corneal surface. A skin layer is removed prior to surgery and regenerates in the week after. Recovery time is longer for surface laser treatments (LASEK and PRK) but they have safety advantages for patients with thinner corneas. For patients found to have a relatively thin cornea at their initial consultation, surface laser treatments such as PRK, or variations such as LASEK, are often the safest options.
In LASEK, an anti-scarring drug, mitomycin C, is often applied to the corneal surface to reduce haze (loss of corneal clarity) and regression (corneal shape changes during healing which can dilute the effect of treatment). A bandage contact lens is then applied to protect the operated area for a few days whilst the skin layer heals. Eye drops are used to minimize discomfort, but approximately 5-7 days, in which the eyes are relatively sore, light sensitive and watery is normal. Vision returns over the next few weeks whilst the new skin layer remodels. Although the recovery is slower after LASEK than after LASIK, final visual results are normally equally good 2 months after surgery. (Figure 4).
RLE (Refractive Lens Exchange) - range: myopia/hypermetropia at any level can be treated with RLE.
Principle = replacement of the natural lens with an intraocular lens (IOL) delivered through a self-sealing micro incision which does not affect eye wall strength. Incisional techniques or specialized toric IOLs can be used in tandem with RLE to reduce astigmatism, and multifocal IOLs can be implanted to reduce spectacle dependence for near vision. RLE is identical to modern cataract surgery. The new lens is implanted within the capsule of the natural lens which shrink wraps the implant and stabilises it in the natural position. The natural lens becomes more misty with age, and patients over 60 years old are often more suited to RLE than laser refractive surgery (Figure 5). Complications of this operation are the same as those for cataract surgery.
Wavefront guided laser refractive surgery
Laser refractive surgery is based on microscopic reshaping of the cornea to correct myopia, hypermetropia, and astigmatism. The excimer laser removes tissue with submicron precision and has been used to reshape the cornea in refractive surgery for the last 20 years.
Wavefront Scanning
Wavefront scanning aims to take account of optical aberrations in the eye above and beyond myopia, hypermetropia, or astigmatism. These higher order aberrations are best understood by thinking of your spectacle prescription as an average of the focusing requirements for light rays entering the eye through different points on the eye surface (Figure 7).
If your refraction is, for example, -3.00D, this may be an average of -2.80D for rays entering through the centre of the cornea and -3.20D for rays entering around the edge. A wavefront scan produces a map of the focusing requirement at each point. Information from this scan can be programmed into the excimer laser with the aim of reshaping the cornea to produce sharper vision than glasses. Current wavefront guided laser treatments do not routinely produce better vision than glasses and contact lenses. But they have two benefits over non-wavefront guided treatments:
Wavefront guided treatments are less likely to result in surgically induced increases in higher order aberrations (ie the corneal shape produced by a wavefront guided treatment is closer to the ideal natural shape of the eye).
Wavefront guided surgery is less likely to cause night vision problems or degradations of other elements of visual performance.
The wavefront scanning and excimer laser refractive surgical system used at the Blackrock Clinic is the latest generation AMO CustomVue System (www.VISX.com).
Refractive Surgery Consultation
Your suitability for refractive surgery is determined in a refractive surgical consultation. The consultation includes a multi-staged examination of your eyes for which you should allow 1 – 2 hours (some waiting between key stages is inevitable). First, you will see a specialist nurse for scanning of your eyes. You will then see Mr O’Brien to review information from these tests, examine your eyes, and discuss your procedure choice.
Preparation
Before attending the consultation, you should leave your contact lenses out for:
Gas permeable hard lenses - 2 weeks
Soft lenses - 1 week
Continuous contact lens wear can produce temporary changes in the shape of your cornea. It is important to leave contact lenses out prior to the consultation, as specified above, to ensure that your corneal scanning and refraction tests are accurate.
Wavefront LASIK or LASEK may not be suitable if you have:
Only one good eye
If you have a common condition called amblyopia (a lazy eye), or for any other reason you have one eye in which the vision cannot be improved to a normal level with either glasses or contact lenses, any risk to the vision in your good eye is harder to justify. This issue requires particularly careful consideration if you are reliant upon good vision in one eye only for work or driving.
Ocular surface disease
If you have dry eyes, active blepharitis, or allergic eye disease requiring regular drug treatment, corneal healing after laser refractive surgery may be adversely affected.
Corneal disease or abnormality
You will be carefully screened to ensure that you do not have a condition called keratoconus prior to laser treatment. Mild forms of this corneal shape abnormality are relatively common, and can predispose to a poor result from laser refractive surgery.
Glaucoma
Altering the shape of the cornea can affect the accuracy of intraocular pressure measurements and therefore the treatment of glaucoma. But corrections to pressure measurement after laser refractive surgery can be applied where necessary.
Cataracts
Cataract extraction presents an opportunity for refractive correction without laser surgery. An artificial lens is normally implanted in the eye during cataract surgery. The power of this lens can be chosen to correct any pre-existing refractive error. Patients over 50 years old commonly have early stage cataracts.
Time off work
Treatments are normally performed on a FRIDAY afternoon with initial postoperative review the following morning (SATURDAY) for LASIK and 4-6 days later for LASEK (Surface treatment). Office based work is usually no problem from the day after LASIK. For outdoor occupations, professional drivers and patients having LASEK, work can normally be resumed 7 days following treatment.
Surgery in both eyes
Most patients elect to proceed with LASIK in both eyes. LASIK in the second eye can either be done on the same day as the first eye (simultaneous bilateral LASIK) or at any convenient time thereafter (sequential bilateral LASIK). Clinical trials comparing simultaneous and sequential LASIK have shown no difference in safety and outcomes. Having both eyes corrected on the same day has considerable practical advantages, and is the preferred option for the vast majority of patients. Bandage contact lenses and routine drop treatments reduce postoperative discomfort to manageable levels for after surface treatments, and bilateral treatment is now the preferred option for most patients undergoing LASEK.
Monovision – an option normally only considered for patients over 40
Electing to leave one eye relatively myopic (typically a 1.00D to 1.50D under correction) to assist near vision (monovision) is the strategy of maximum spectacle independence for patients in the reading glasses age group (43 years and over). Glasses will still often be required to read fine print comfortably but monovision may delay the onset of spectacle dependence for reading until the mid fifties or later. Even then, spectacle dependence is decreased for common near vision tasks such as computer work, reading a menu or a price label. Monovision does not work for everyone, and may increase spectacle dependence for some distance vision tasks (typically night driving), but the compromise for distance vision is relatively small. If you are in the age group for reading glasses (presbyopic), the pros and cons of monovision will be discussed with you by Mr. O’Brien at your refractive surgical consultation. It must be remembered that monovision is a compromise. If the eye corrected for distance is covered then the other eye will have subjectively reduced vision (as it will be left under-corrected purposely to allow unaided reading).
Outcomes and Risks of refractive laser eye surgery
Over 97% of wavefront LASIK patients in the range treated at the Blackrock Clinic are able to see the driving standard or better without glasses after one treatment. Results for surface treatments (e.g. LASEK or PRK) are similar.
Enhancement treatment (retreatment) is highly effective in correcting significant residual refractive errors (>1.00D). Retreatment is carried out in approximately 5-10% of cases. There is no additional charge for retreatments carried out within a year of primary treatment (the need for any retreatment can normally be determined within 6 months of surgery).
Realistic expectations are important. Laser refractive surgery is highly effective in reducing spectacle dependence. At minimum, you should expect to be able to play sport and socialize comfortably without glasses after treatment; but some patients may still prefer to wear thin glasses for selected distance vision tasks.
Wavefront laser refractive surgery of any type does not routinely deliver above normal vision. Most patients have low-level residual refractive errors after surgery. These refractive errors are at a similar level to those normally found in non-spectacle wearers, and do not usually require retreatment.
Vision should otherwise remain stable and good permanently after treatment. Your risk for developing other eye problems should not be altered. Natural variations in the distance spectacle prescription can occur at any age. These are usually small, but can be large enough to warrant further treatment in a small percentage of patients (less than 2% in the first 5 years after treatment).
The benefits of refractive surgery are primarily functional. It is not a cosmetic procedure. Freedom from glasses and contact lenses will enable you to enjoy sport and physical exercise, and to pursue a healthy active lifestyle.
Laser eye surgery is real surgery and is performed in an operating theatre in the Blackrock Clinic Hospital. Despite the risks of surgery, large studies have recently reported that this form of surgery has the highest patient satisfaction ratings of any surgery throughout the world. The overall complication rate for LASIK is about 5% (figures for LASEK are similar). Most problems are minor, and almost all complications can be corrected safely without affecting the final visual result. Additional consultations and treatments may be required. There is no charge for any additional treatment required for the correction of complications.
The risks associated with laser eye surgery include the following:
Vision loss
Less than 1 in 1000 patients is left with significant scarring or corneal irregularity requiring a corneal graft or hard contact lens for visual rehabilitation. This level of risk is similar to the cumulative risk of corneal scarring resulting from infection in 5-10 years of contact lens wear.
Large studies indicate a 0.5% decrease of 2 lines of visual acuity. The incidence of severe vision loss is very low and is usually due to infection. A recent study (Ophthalmology 2010) has shown that the risk of corneal infection after refractive surgery is 1 in 2800 cases. Antibiotic eyedrops are used to reduce your risk.
Kerectasia
Most complications occur within 1 month of LASIK or LASEK. The risk of long-term loss of corneal shape stability (keratectasia) after LASIK has recently been estimated at less than 1/1000. Current patient selection strategies further reduce this risk. The risk for keratectasia after LASEK is negligible.
Unexpected refractive outcome
Not everyone responds identically to the surgery, leading to under or overcorrection with possible need for enhancement. Enhancement is required in 5-10% of cases and is usually performed 3-12 months after the initial surgery. Younger age and higher prescription glasses are risk factors.
Possible need for glasses
Some patients are not suitable for retreatment and may therefore require glasses if they have regression. It can be difficult to fit contact lenses after laser eye surgery in some circumstances due to the flattened central cornea.
Flap complications
This occurs in less than 1% of cases and includes a free flap, buttonhole or incomplete flap and may require abandoning the procedure and rescheduling surgery six months later. This risk has been greatly reduced by using the laser (Intralase) to cut the flap.
Flap striae of wrinkles occur rarely but may necessitate refloating the flap. Rarely there may be noninfectious inflammation under the flap called diffuse lamellar keratitis (1%), which commonly responds well to topical steroids.
Epithelial Ingrowth
Epithelial ingrowth occurs when epithelial cells are implanted or migrate under the stromal flap after LASIK. Small degrees (<1 mm) of peripheral epithelial ingrowth are not uncommon. Larger areas of ingrowth occur approximately 1% of the time after primary LASIK and up to several times that amount after enhancements where the flap was lifted. It is generally detected within 2 months of surgery and may disappear, remain the same, or continue to worsen. Epithelial ingrowth can cause several problems including flap necrosis, irregular astigmatism, and scarring, all of which may result in decreased vision. There are a variety of methods to manage epithelial ingrowth including observation, lifting of the flap lift and rubbing out the epithelial cells.
Glare and halos
Not common with larger treatment zones and newer generation lasers. If this occurs it is frequently a temporary phenomenon and settles by week 4.
Transient light sensitivity
Following laser the eyes may become abnormally light sensitive (1% of cases). Topical eyedrops can help alleviate this symptom. Sunglass wear is advised in the early post-operative phase (1-2 weeks).
Presbyopia
This is the inherent need for reading glasses as the ability of the eye to focus up close is progressively lost as one gets older. In general, people need reading glasses aged 45-50 years. This surgery does not eliminate the need for reading glasses.
Dry eye
Almost everyone has dry eyes following LASIK surgery, which is only temporary in the majority. All patients are prescribed lubricant eyedrops for 1-2 months following surgery and if dryness persists temporary punctual plugs are utilised. If one does not have significantly dry eyes before surgery it is highly unlikely that postoperative dryness will persist.
Double vision
The preoperative screening exam will usually identify if you are at risk of double vision.
Subconjunctival haemorrhage
This is of no visual significance and resolves in 2-3 weeks.