Surgical Correction of Refractive Errors

The purpose of the globe is to receive light from the outside world and transmit it to the brain for processing. There are two aspects to this function. In the first instance, the light rays have to be correctly focused onto the back of the eye. Then, this information has to be converted to electrochemical signals by the cells within the retina and transmitted to the brain.

Refractive errors arise as a result of anatomical variations within the eye that prevent the image being perfectly focused onto the retina (please see our dedicated article on refraction for further detail). Such variations can occur within the cornea and the lens. Refraction is also affected by the depth of the anterior chamber of the eye and the length of the globe from front to back (axial length). Where these errors arise, lenses have traditionally been used to modify the refraction of rays of light entering the eye. Increasingly, surgical techniques are being developed in order to carry out this function and so obliviate or minimise the need for extraocular corrective lenses.

Refractive surgery has been performed for about 100 years² but it is only in the past few decades that it has really expanded and techniques have been refined - mainly due to the emergence of lasers and their use in this type of surgery. The basis of surgical correction lies in the notion that modifying the shape of the cornea or inserting an artificial lens can correct a refractive error. Generally, this type of surgery is aimed at correcting myopia, hypermetropia and astigmatism. It also has an increasing role in the correction of presbyopia.

Keratotomies

• Description³ - these are radial or arcuate incisions made in the cornea in order to modify its shape. Arcuate keratotomies may be used in combination with compression sutures.
• Use - radial keratotomy (RK): low and moderate degrees of myopia; arcuate keratotomy (AK): moderate and high astigmatism, often following penetrating keratoplasty (corneal transplant).
• Complications - these procedures cause irreversible structural weakening of the cornea and may be associated with subsequent problems relating to this. There may be over or under correction of the refractive error.
• Outcome - RK has largely been superseded by excimer laser surgery (see below) which is more accurate and which can address a greater degree of myopia. AK outcomes are generally good but this depends somewhat on the presence of other factors such as whether it is being carried out following a transplant procedure.

Implant procedures

Intrastromal corneal ring (ICR)

• Description - semi-circular pieces of plastic can be inserted into the corneal stroma (the thickest, middle part of the cornea), in order to modify its shape. The entrance wounds are sutured off and these sutures have to come out in due course. It is a reversible procedure.
• Use - mild myopia and occasionally, certain corneal degenerative conditions such as keratoconus.
• Complications - this is generally a safe procedure but complications can include perforation of the cornea, extrusion of the ring or a breakdown of the epithelial/stromal interface.⁴ Infection, abnormal wound healing and irregular astigmatism have also been reported.⁵
• Outcome - this is generally good but if the resultant visual correction is not satisfactory, this is a reversible procedure and the cornea usually returns to its original shape within a few weeks.²

Intraocular lenses

• Description - artificial lenses can be implanted inside the eye - either in the anterior or posterior chambers or indeed within the natural lens capsule if the lens is removed. This is usually in addition to the natural lens (phakic intraocular lens) which remains in situ but occasionally, this is done at the time of a cataract extraction (aphakic intraocular lens). Accommodative or multifocal intraocular lenses can be also be used but have varying refractive success. A refractive lens exchange (or 'clear lens extraction') refers to the situation where a healthy crystalline lens (as opposed to a cataract) is removed and replaced by a corrective lens.
• Use - myopia, hypermetropia and astigmatism. These procedures tend to be used for patients whose degree of refractive error exceeds the safe limit for correction with excimer laser surgery.
• Complications³ - these vary with the type of lens and which part of the eye this is inserted in but complications include uveitis, cataract formation (where a phakic lens is inserted) and pupillary distortion (where the lens is inserted into the anterior chamber). Some lenses are associated with a small risk of retinal detachment and where the lens is inserted for astigmatism, there is a possibility that the lens orientation changes which, for this particular problem, gives rise to further refractive errors. The complication rate tends to be higher for intraocular lens insertion than it is for excimer laser surgery.²
• Outcome - good when complications are not encountered.⁶

Overview

These are most commonly used for correction of refractive errors (as described below) but non-refractive excimer laser surgery can also be used for the removal of corneal scars and smoothing of its surface. Generally, these procedures are increasingly used owing to their precision and the fact that the globe is not penetrated, so greatly reducing the risk of intraocular damage or infection. This type of surgery is not usually available on the NHS but it may be carried out in some NHS hospitals. It costs ~ £1000 - £1500 per eye.

Qualifying for surgery⁷

The patient must be over 21 with a stable prescription over 2-3 years and have otherwise healthy eyes. Extremes of refractive errors are usually not considered as the procedure may yield unpredictable results and be associated with higher complication rates. Patients are counselled prior to surgery with regards to the results that can realistically be hoped to achieve in their case. Pregnant or breast-feeding women as well as patients with certain medical conditions (e.g. diabetes, rheumatoid arthritis, systemic lupus erythematosus) will not be able to go ahead with surgery as well as those on certain medications such as steroids.

Operative considerations

Refractive surgery can be done for errors ranging from ~10.0 dioptres of myopia to 6.0 dioptres of myopia and up to 4 cylinders of astigmatism.⁸ Other factors that will be taken into consideration by the surgeon when considering a patient for refractive surgery is the angle kappa (which measures how close the centre of the pupil is to the central visual axis - the two are not always perfectly aligned) and the pupillary size.

The procedure

These are day-case procedures done under local anaesthetic (a mild sedative may be used), carried out in specialist centres. As photorefractive keratectomy (PRK) is often painful, usually one eye only is done at a time but laser in-situ keratomileusis (LASIK) can be a unilateral or bilateral procedure.

Complications: general problems common to all procedures⁸

Under or over correction. It is important that patients going for this type of surgery understand that not everybody can achieve a perfect refractive outcome. Thus, an uncorrected visual acuity of 6/6 may be the aim for some but 6/12 would be an excellent outcome for others. Over or under correction relates to what the aim was for that individual patient and one of the problems with this type of surgery is unrealistic patient expectations. Other problems include astigmatism, anisometropia, presbyopia, glare or halo effects, dry eye symptoms and reduced contrast sensitivity.

Photorefractive keratectomy (PRK)

• Description - a laser is used directly on the surface of the cornea to sculpt it into a predetermined shape.
• Use - wide range of myopia and astigmatism. It may also be used for low degrees of hypermetropia. Laser epithelial keratomileusis (LASEK) is a modified form of PRK whereby there is a chemical loosening of the corneal epithelium which is then replaced on the corneal surface according to the refractive requirement.⁸ PRK may be the preferred type of procedure in patients who have a thin cornea and in whom creation of a corneal flap (see LASIK below) would be dangerous.
• Problems⁸ - immediately post-operatively, it is common to experience pain, photophobia and tearing. This usually settles with a bandage contact lens and NSAID eyedrops.⁵ Commonly, there is a residual corneal haze which can cause night glare. Uncommonly, there can be scarring, abnormal epithelial healing and irregular astigmatism. Infection and acute corneal necrosis are also described.³
• Outcome - visual acuity only really settles after about 3 months but the final outcome is good: between ~55 and 85% of patients achieve an uncorrected vision of 6/6.⁵

Laser in-situ keratomileusis (LASIK)

• Description - this involves cutting a flap of cornea and on peeling this back, applying the laser to the corneal stroma (middle layer of the cornea). The epithelial flap is variously vaporised depending on the refractive requirement and the residue is replaced. This technique has been widely adopted since the mid-1990s and is now the most commonly performed procedure for the surgical correction of refraction.
• Use - it is more versatile than PRK, being able to address a wider range of refractive errors, particularly in hypermetropes where PRK is limited.
• Problems⁸ - there are a number of intraoperative complications which can arise relating to the flap formation and post-operative problems include wrinkling, distortion or dislocation of the flap, keratitis, epithelial ingrowth (underneath the flap), keratitis and anterior segment ischaemia.³ Optic neuropathy can also occur.
• Outcome - this is very good with ~70-85% of patients achieving an uncorrected visual acuity of 6/6.⁵ It is also associated with a high rate of patient satisfaction. LASIK possibly has the upper edge over PRK in that visual recovery is faster and less likely to result in loss of best spectacle-corrected visual acuity.⁹

Laser thermal keratoplasty (LTK)

• Description - a laser is aimed around the cornea, creating symmetric spots that cause stromal shrinkage and therefore modification of corneal shape. The amount applied determines the refractive outcome.
• Use - low hypermetropia and in some cases, presbyopia.
• Complications - this is generally a safe procedure. The most common complications include development of astigmatism and need for further treatment as the cornea tends to return to its original shape over time.
• Outcome - good: visual improvement is instantaneous.²

Monovision²

• Description - this emerging approach is not yet widely in use. It is based on the principle of using one eye (usually the non-dominant one) to see close up and the other (usually the dominant one) for distant viewing. One or both eyes may need to be treated to achieve this.
• Use - presbyopia.
• Complications - difficulty in adjusting to the different focusing abilities of each eye: about 25% of patients cannot tolerate this.⁷ A pre-operative trial with contact lenses is often used in order to see if this adjustment does eventually occur.
• Outcome - good when tolerated but this tolerance may take up to eight weeks to develop.