LASIK for hypermetropia

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LASIK Surgery

LASIK is a recently developed operation, which can correct both long and short sightedness and also astigmatism. An excimer laser is used to sculpt the cornea to a new curvature that will correct the optical defect.

LASIK surgery is carried out with a local anaesthetic (eye drops). A small metal suction ring is placed on the surface of the eye around the cornea. This stabilises a cutting device (microkeratome) that shaves 160 microns of tissue from the surface of the cornea to create a very thin flap. The microkeratome and metal suction ring are then removed from the eye and the tissue flap is temporarily folded to one side. The exposed area of cornea is sculpted by an excimer laser and steepened centrally. The laser treatment takes 3 or 4 minutes to complete, after which the flap of corneal tissue is folded back and a plastic shield protects the eye.

Photo-refractive Keratectomy (PRK)

PRK was one of the first methods of treatment introduced using the excimer laser to correct optical defects of the eye. The thin layer of cells (epithelium) that covers the surface of the cornea is gently scrubbed off by the surgeon and the laser treatment is applied directly onto the surface of the cornea. At the end of the operation the normal covering of epithelial cells is missing from the treated area, and it is necessary for the surrounding cells to grow back over the central part of the cornea. Following PRK the eye is very painful for the first day or two. Although the cells will cover the central area in a few days, it takes several months for the cell layer to stabilise over the treated area, and there is often a degree of haze or scarring of the cornea in the treated area.

Advantages of LASIK
LASIK is a modification of PRK in which a tissue flap covers the laser-treated area. There is no raw area of cornea exposed at the end of the operation, and so little or no pain after the surgery, and the eye settles down much more quickly. The optical recovery is faster and the vision generally stabilises rapidly. It is rare to encounter scarring in the visual axis after LASIK.

Disadvantages of LASIK
LASIK surgery is technically more complex compared to PRK and requires additional instrumentation. This makes the procedure more expensive, and there are additional risks associated with the flap.

Problems with Refractive Surgery
The amount of corneal tissue that can safely be removed by the laser is limited by the thickness of the cornea. To minimise the amount of tissue that needs to be removed by the laser, the optical correction is restricted to a small central area of the cornea. It can be seen from the diagram that the optically corrected area is generally smaller than that achieved by a contact lens. Provided that the treated area is directly over the pupil then the optical performance of the eye is satisfactory. If the pupil is very large (e.g. when driving at night) then the light rays may pass through both optically corrected and uncorrected parts of the cornea and this can lead to glare and poor vision. If the optical area treated is not perfectly centred on the pupil the quality of vision may also be reduced. If the strength of the optical correction achieved by the treatment is inaccurate, it is possible to carry out further treatment by lifting the flap and applying additional laser to the bed. A small optical correction requires less laser treatment than a larger correction. The greater the refractive error that is being corrected the greater the chance of a less than perfect outcome. At present the laser cannot correct above +10 dioptres of hypermetropia.

Amblyopia (Lazy eye)
Occasionally in people with severe long sight who are considering having LASIK it is found that the quality of vision obtained with glasses is less than 6/6. This is because the visual development of the eye has never reached its full potential. In such cases the vision following LASIK treatment will be no better than that which was obtained with glasses before the treatment.

As one get older the ability of the lens of the eye to focus on close objects declines. Around the age of 40, someone who has not previously needed spectacles finds that they require reading glasses. Similarly someone in this age group whose hypermetropia is successfully corrected so that their distance vision is perfect, will still require reading glasses.

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