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Laser eye surgery: an update on the latest procedures

Sarah Brewer
MA MB BChir
Medical Journalist
E:drsarahbrewer@cs.com

Keratomileusis - the surgical correction of visual refractive errors by changing the shape of the deep layer of the cornea - was developed as long ago as 1949 by Professor Jose Barraquer in Bagota, Columbia. A number of procedures have since evolved from his initial work, and millions of people worldwide have benefited from the technique to correct myopia (shortsightedness), hyperopia (longsightedness) and astigmatism.
The advent of the excimer laser to reshape the cornea was a great step forward in refractive surgery, and laser treatment to correct vision is now one of the fastest-growing areas of medicine. Laser energy can be used to reshape the surface of the cornea (known as photorefractive keratectomy, or PRK) or can be delivered deeper to reshape the middle layer of the cornea by the means of lamellar surgery (known as Laser Assisted In-Situ Keratomileusis, or LASIK).
During LASIK, a corneal flap is initially created using a microkeratome before the lamellar surgery using an ultraviolet laser. Preserving the flap of corneal epithelium, which is repositioned after surgery, improves post-operative comfort significantly compared with standard PRK, in which an epithelial ulcer the size of a halfpenny takes several days to heal. Visual recovery is also more rapid with LASIK, and the reduced wound healing response is associated with less visual regression for high corrections, and a lower rate of complications such as haziness.(1) LASIK is now the most commonly used technique, although PRK is still available for mild-to-moderate corrections, especially in people with unusually thin corneas, recurrent erosions, or whose lifestyle increases the risk of postoperative trauma, which can cause loosening of the epithelial flap formed during LASIK. Recently, a modification of PRK, known as laser subepithelial keratomileusis (LASEK) was developed in which an epithelial flap is first fashioned then replaced after the superficial laser ablation. As with LASIK, this reduces postoperative discomfort, speeds visual recovery and produces less haze.(1,2)
A number of modifications to the LASIK technique have also been developed. These include the introduction of Bausch & Lomb's Zyoptix 100 system, which, although based on the standard LASIK procedure, gives increased accuracy and speed. Treatment is individually tailored to each eye using iris recognition technology similar to that used for high-security identification. This ensures the correct eye is treated each time. The new laser also operates twice as fast as its predecessors, so treatment time is shortened by half.
Another advance is the use of wavefront technology, in which a beam of light is reflected off the retina to identify abnormalities in the way light penetrates through the lens and cornea. This allows the prescription to be individually adjusted if light penetrates less readily in some areas, creating aberrations in vision. This latest technology has evolved out of space research to correct atmospheric aberrations in pictures beamed back to earth. The aim of this customised corneal ablation is to achieve supernormal vision in terms of acuity and contrast - often referred to as "eagle-eyed" sight.
It is also possible to opt for Monovision, in which one eye is purposefully left slightly nearsighted for reading, while the dominant eye is adjusted for better long- distance vision.

Effectiveness
Laser eye surgery can generally treat myopia up to -13 dioptres (D), astigmatism up to 6D, and hypermetropia up to +5D, depending on the thickness of the cornea and the size of the pupil.
A meta-analysis evaluating the effectiveness of LASIK and PRK in treating shortsightedness concluded that LASIK and PRK were of similar effectiveness in correcting myopia from -1.5 to -15 D in a greater than 6-month follow-up.(3)
A study comparing the effects on contrast sensitivity following wavefront-guided (23 eyes) versus standard LASIK (22 eyes) measured contrast sensitivity before and 1 month after surgery. Uncorrected visual acuity of 20/20 or better was achieved for 72% of eyes treated with wavefront-guided LASIK and by 70% of those treated with standard LASIK. The ability of wavefront-guided LASIK to correct optical aberrations resulted in significantly improved contrast sensitivity 1 month following surgery, with 88% of contrast sensitivity measurements improving in those receiving wavefront-guided LASIK compared with the standard LASIK group (40% of contrast sensitivity measurements improved).(4)

Initial consultation
A patient considering laser eye surgery will usually receive an initial, free eye examination with an optometrist to assess their suitability for the procedure. They are advised not to wear soft contact lenses for 1 week before consultation or treatment, and not to wear hard or gas-permeable lenses for at least 4 weeks beforehand. The initial assessment includes refraction, slit lens examination, fundoscopy, topography to assess the shape of the cornea, and pachymetry to measure the thickness of the cornea using ultrasound.
The size of the pupil is also assessed in dim light using a pupillometer, as those with a larger pupil diameter may experience more postoperative glare at night and have more quality-of-vision symptoms in the early postoperative period, although this effect generally disappears by 6 months.(5) Tear film production is also measured to assess the risk of postoperative dry eye. Overall, around 30% of people assessed are found not to be suitable, either because their refractive error is too great, the cornea is too thin or the pupil diameter is too great.
Most surgeons will write to the patient's GP at this stage, requesting details of past medical history or contraindications.
Absolute contraindications include: cataract, keratoconus, unstable prescription and active connective tissue disorder. Although most surgeons will not treat people aged under 18 years, LASIK has been used to treat severe shortsightedness in children. In one study, surgery was performed on six children aged 2-12 years, of whom five achieved improved refraction and visual acuity.(6)
Relative contraindications include: severe dry eyes, pregnancy, diabetes and psychiatric illness - patients must have realistic expectations of what is possible.

Day of the procedure
The patient is advised to come to the clinic with a relative or friend who can take them home. The patient should not to wear any perfume, aftershave or make-up, as even small concentrations of volatile agents such as alcohol can affect some laser equipment. The refractive error is usually checked again just before the procedure, and a repeat slit lamp examination may be carried out as a final check. The patient should also be reminded of possible outcomes and side-effects, as well as having an opportunity to ask any outstanding questions before signing their consent form.
Local anaesthetic drops are inserted, and patients are offered light sedation (10mg diazepam or 1mg lorazepam), although only around 10% of people take this up.
The eyelids are held apart with an eye-guard, and a suction ring is centred around the cornea to hold the eye still and increase eye pressure. A flap of tissue 160 microns thick is then cut from the corneal epithelium using a microtome. The flap is vertical (hinge at the top) and is reflected out of the way and the suction ring removed. The patient is asked to look at a blinking target light, and the ultraviolet laser is used to accurately reshape the stromal layer of the cornea according to their prescription. Laser reshaping takes from a few seconds to less than 2 minutes, and the corneal flap is then replaced - it will naturally stick back to the front of the eye without the need for stitches. Antibiotic drops and topical steroids are then inserted .
Patients are advised to wear a plastic eye shield at night for the first month to prevent rubbing their eyes during sleep, which might cause wrinkling of the corneal flap. It is also sensible to wear plain glasses when outdoors, to help stop foreign bodies entering the eye.

Follow-up
Patients are seen 30 minutes after the procedure, before going home, on the next morning, 1 week afterwards, then 1 month later, 3 months later and finally 1 year after the treatment. They are then advised to have routine eye checks with an optometrist as normal.

Complications
Numerous clinical trials have shown that LASIK is safe and effective when performed by experienced surgeons. The risk of complications during the procedure itself (eg, irregular flap or flap cut through) is low at 1-2% and usually just requires postponement of the procedure to a later date.
Postoperatively, the most common side-effect is dry eyes, experienced by 1 in 3 people. As tear film production is assessed preoperatively, those likely to be affected are usually screened out or are willing to accept that it may occur. Treatment involves using artificial tears for up to 12 months, plus frequent blinking.
The second most common complication is night glare due to normal postoperative swelling. This usually resolves over 4-6 weeks. The risk of persistent "halo" or glare around bright lights is now low with the new-generation lasers used and with screening to exclude those with larger pupils (dilation of pupils in dim light allows light through untreated areas of the cornea).
Risk of complications at the three-month follow-up of LASIK patients varies from 2.6 to 6% in various studies: for example, corneal oedema (0.6%), corneal scarring (0.1%), persistent epithelial defect (0.5%), significant glare (0.2%), persistent discomfort (0.5%), interface epithelium (0.6%), cap thinning (0.1%) and interface debris (3.2%, although this usually causes no visual problems).
In one study of 795 people (1,488 eyes treated) who underwent LASIK for shortsightedness, complaints of night vision problems decreased considerably from 25.6% at 1 month to 4.7% 12 months after the procedure. Those with initial shortsightedness of greater than 5 dioptres were 2.8 times more likely to have problems.(7)
A few cases of infectious keratitis due to fungal infection have been reported in the literature.(8) Only 1 in 10,000 people undergoing LASIK experiences a serious ocular infection that affects vision. Mild conjunctivitis or dry eyes may present to the GP and should be treated in the usual way.
Slight under- or overcorrection can occur, especially in people with high refractive errors. Most can be re-treated to achieve a better result.
A recent survey of 604 people treated for myopia found that satisfaction with the procedure was high. Although halos were reported by 30%, glare by 27% and starbursts by 25% of subjects, 97% said they would recommend LASIK to a friend.(9)

References

  1. Ambrosio R Jr, Wilson S. LASIK vs LASEK vs PRK: advantages and ­indications. Semin Ophthalmol 2003;18(1):2-10.
  2. Feit R et al. LASEK results. Ophthalmol Clin North Am 2003;16(1):127-35.
  3. Yang XJ, Yan HT, Nakahori Y. Evaluation of the effectiveness of laser in situ keratomileusis and ­photorefractive keratectomy for myopia: a meta-analysis. J Med Invest 2003;50(3-4):180-6.
  4. Kaiserman I, et al.Contrast sensitivity after wave front-guided LASIK. Ophthalmology 2004;111(3):454-7.
  5. Schallhorn SC, et al. Pupil size and quality of vision after LASIK. Ophthalmology 2003;110(8):1606-14.
  6. O'Keefe M, Nolan L. LASIK surgery in children. Br J Ophthalmol 2004;88(1):19-21.
  7. Pop M, Payette Y. Risk factors for night vision complaints after LASIK for myopia. Ophthalmology 2004;111(1):3-10.
  8. Alfonso JF. Acremonium fungal infection in 4 patients after laser in situ keratomileusis. J Cataract Refract Surg 2004;30(1):262-7.
  9. Bailey MD, et al. Patient satisfaction and visual symptoms after laser in situ keratomileusis. Ophthalmology 2003;110(7):1371-8.