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Introduction to Refractive Surgery - Bruce Allan MD FRCS
Refractive Errors We see by forming an image on the retina - a carpet of light sensitive cells, each acting like a pixel on a computer screen, that lines the inside of the back of the eye. Images are focussed 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 UK population (50%) requires spectacles or contact lenses to focus a clear image of a distant object on the retina.
Spectacles and contact lenses either add or subtract focussing power to help form a clear image on the retina where the natural focussing power of the eye is incorrect. Defects in natural focussing 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. +1.00D or –12.50D). The sign indicates whether the spectacle correction required is for long (+) or short (-) sight.
Refractive Surgery Refractive surgery is not normally performed until the spectacle prescription has been stable (no change greater than 0.50D) for 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 focussing for near vision) whilst younger, and often only become spectacle dependent in mid life.
Refractive Surgery Procedures LASIK (Laser in situ keratomileusis) - range +4D to -10D 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. This protective flap is most commonly formed using a femtosecond laser (intralasik). Because the surface tissue damage is minimised in LASIK, visual recovery is rapid and virtually pain free.
Surface laser treatments – range +4D to –10D 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 (Epi-LASIK, LASEK and PRK) but they have safety advantages for some patients with thinner corneas. Although recovery time is slower, final results for LASIK and surface treatments are similar.
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.
ICL (Intraocular Contact Lens) implantation - range: up to -17D myopia; up to +10D hypermetropia. Principle = implantation of a soft flexible artificial lens which is seated just in front of the natural lens and behind the iris. Preservation of the flexible natural lens helps to avoid reading glasses and toric ICLs can be used to correct astigmatism. This approach is particularly suited to younger patients who are out of range for laser refractive surgery.
(Each of these procedures is described in detail in procedure specific information) 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 Mr Allan’s team for scanning and refraction tests. You will then see Mr Allan to review information from these tests, examine your eyes, and discuss your procedure choice. Having read the relevant patient information packs (these are normally sent out to you when you initially enquire about refractive surgery in Moorfields) it is useful to make a list of any particular questions you may have and bring this with you to the consultation. Key stages are:
Preparation Before attending the consultation, you should:
If you are a contact lens wearer, prior to the consultation you should leave your 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.
Refractive surgery is normally postponed until the spectacle prescription is stable. Small variations in your spectacle prescription are normal, but if there is a change of more than 0.5D over the last 2 years, we would normally wait 12 months before proceeding with surgery to ensure that there is no progressive change. Travelling to Moorfields Nearest Tube – OLD STREET on the NORTHERN LINE Telephone If you wish to arrange a preliminary consultation, please telephone 020 7566 2156 or email Angelique.Singh@Moorfields.nhs.uk |
