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- Procedures / Sight correction
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 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 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 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. +1.00D or –12.50D). The sign indicates whether the spectacle correction required is for long (+) or short (-) sight.
Short sight (myopia) and long sight (hypermetropia) are often accompanied by an element of uneven focusing power (astigmatism). Imagine that the cornea is more rugby ball shaped than football shaped. The extent of the difference in focusing power between the smallest and largest radius of curvature for this uneven (toric) surface would be the amount of astigmatism.
This is represented in your spectacle 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 normally sighted people need reading glasses from their mid forties on. The final component of your spectacle 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 late fifties.
Sample spectacle prescription
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). Hypermetropic patients 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 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 spread of focus 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 +4.00 to -10.00D) are usually corrected using laser techniques (LASIK or PRK) to reshape the cornea. Higher errors are corrected using lens implant based methods (RLE or 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 early signs of cataract (lens opacity) are present. The summary table below is designed as a rough guide; but the age and refractive range cut offs given for each technique are not absolute, and the risks and benefits of appropriate alternative approaches to correcting your refractive error will be discussed with you at your consultation.
-1.00 to -6.00Moderate
-6.00 to -10.00High
+1.00 to +4.00High
21 - 50 yearsLASIK/PRKLASIK/SMILEICLLASIKICL/RLE
50 - 60 yearsLASIK/PRKLASIK/SMILERLE/ICLLASIKRLE
over 60 yearsRLE/LASIK/PRKRLE/SMILERLERLE/LASIKRLE
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:
- Corneal scanning and refraction check
- Wavefront scanning
- Ocular examination and review with Mr Allan
Before attending the consultation, you should:
- Leave your contact lenses out (1 week for soft and 2 weeks for rigid contact lenses)
- 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 to ensure that your corneal scanning and refraction tests are accurate.
- Bring a record of your spectacle prescriptions over the last two years
- Refractive surgery is normally postponed until the spectacle prescription is stable. Small variations in your spectacle prescription are normal, but if there has been a change of more than 0.5D over the last 2 years, we would normally wait 12 months before proceeding with surgery after a repeat check to ensure stability.
Travelling to Moorfields
Nearest Tube – OLD STREET on the NORTHERN LINE
Examination involves the use of drops to dilate the pupil. Pupil dilation causes temporary visual blurring. You should not drive to the consultation. If you are being driven, limited NCP car parking is available near the hospital.
If you wish to arrange a preliminary consultation, please telephone 020 7566 2156 or 07484 081815 (or from outside the UK +44 20 7566 2156 or +44 7484 081815) or email firstname.lastname@example.org