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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 on to 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 on to the retina (see separate article Refraction and Refractive Errors 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 eradicate or minimise the need for extraocular corrective lenses.
Overview of surgical correction
Refractive surgery has been performed for about 100 years 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.
- Description - 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- Outcome - RK has largely been superseded by excimer laser surgery (see 'Excimer laser procedures', below). This is more accurate and can address a greater degree of myopia. AK outcomes are generally good but this depends somewhat on the presence of other factors, such as a recent corneal transplant procedure.
Intrastromal corneal ring (ICR)
- Description - crescent-shaped rings of polymethylmethacrylate (PMMA) are inserted in the periphery 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 (up to 2%) or a breakdown of the epithelial/stromal interface. Infection, abnormal wound healing and irregular astigmatism have also been reported.
- Outcome - this is generally good (especially for keratoconus patients) 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. The National Institute for Health and Clinical Excellence (NICE) has concluded that the benefits of this type of surgery are somewhat limited and unpredictable and therefore this method os treatment should not be considered in the absence of other ocular pathology (eg keratoconus).
- Description - artificial lenses can be implanted inside the eye - either in the anterior or posterior chamber or, indeed, within the natural lens capsule if the lens is removed. This may be added to the natural lens which remains in situ (phakic intraocular lens) or 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. The procedure is carried out under local anaesthetic.
- Use - myopia, hypermetropia and astigmatism. These procedures tend to be used for patients for whom wearing spectacles is difficult (eg disability or professional requirements) and whose degree of refractive error exceeds the safe limit for correction with excimer laser surgery (see below).
- Complications - these vary with the type of lens used and which part of the eye this is inserted in. Short-term complications include corneal damage, 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 gives rise to further refractive errors. The complication rate tends to be higher for intraocular lens insertion than it is for excimer laser surgery. There are no long-term data regarding the safety of these lenses.
- Outcome - good when complications are not encountered. An intraocular lens exchange is possible where the visual outcome is not satisfactory but there remains the risk of the other surgical complications outlined above.
Excimer laser procedures
These are most commonly used for correction of refractive errors 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. Costs vary, depending on the type of procedure, but allow ~£1,000-£1,500 per eye (commercially quoted prices often relate simply to the procedure and omit the necessary work-up required beforehand).
Refractive surgery can be done for errors ranging from approximately -10.0 dioptres of myopia to +4.0 dioptres of hypermetropia and up to 4 cylinders of astigmatism. The exact refractive error considered depends on the type of surgery used. Other factors that will be taken into consideration by the surgeon when considering a patient for refractive surgery are 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 patient must have a stable prescription (<0.5D change over one year) 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. Other absolute contra-indications to laser refractive eye surgery include keratoconus, presence of infection, intraocular inflammation and uncontrolled glaucoma. The list of relative contra-indications is longer and includes surface problems (dry eye, blepharitis, severe atopy, neurotrophic cornea, a history of herpes simplex), cataract, glaucoma, nystagmus, and ocular trauma. A history of ocular surgery is generally considered to be a relative contra-indication but NICE has recently issued guidelines suggesting that laser correction of refractive error following non-refractive ophthalmic surgery (eg cataract surgery or corneal transplantation) is possible in the hands of experienced corneal surgeons. (Conversely, those who have had laser surgery may experience complications in non-refractive surgery and so should make their history known and be managed by more senior surgeons.)
The patient must be aged over 21. Pregnant or breast-feeding women as well as patients with certain medical conditions (eg diabetes, rheumatoid arthritis, systemic lupus erythematosus) will not be able to go ahead with surgery. This also applies to those on certain medications, such as steroids and hormone replacement therapy.
Patients will be carefully assessed prior to listing for surgery to ensure that they are suitable. This will include a detailed history, refraction (usually a more detailed type than is performed by an optometrist, known as wavefront aberrometry), keratometry (measuring the curvature of the cornea), corneal thickness assessment, slit lamp examination and dilated fundoscopy. Prospective patients should not wear hard contact lenses for four weeks or soft contact lenses for two weeks prior to this assessment.
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. Following the procedure, patients are prescribed a course of prophylactic antibiotics, will be asked not to drive home and will have been advised to wear sunglasses until the (normal) mild photophobia resolves. Recovery can take between days and a few weeks, depending on which procedure is done and whether both eyes were done or not.
Outcomes: general points common to all procedures
The outcome measured in laser refractive surgery is the unaided visual acuity, the best corrected visual acuity (with spectacles) and the deviation from the planned postoperative refraction. The two are not necessarily corrected equally: an individual can experience some loss of their best corrected visual acuity but experience a proportionally much improved unaided visual acuity. It is also worth noting that a preoperative and postoperative vision of 6/6 may actually be subjectively different to the patient owing to a change in the contrast sensitivity. This may be an important point for those with high visual demands and will be discussed with them prior to any procedure.
Re-treatment is reported in up to 10% of operated eyes as a result of under- or over-correction (see 'Complications', below). If this is required, the surgeon will wait at least three months, once the cornea has healed and the refraction has stabilised.
Complications: general problems common to all procedures
It is worth noting that the healing response in these procedures is not predictable and therefore subsequent scarring patterns and precise refractive outcome may vary. So, the principle complication is under- or over-correction. Patients are counselled prior to surgery with regards to the results that can realistically be hoped to be achieved in their case. 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:
- Glare or halo effects
- Reduced contrast sensitivity
- Dry eyes
More serious complications are rare (in the order of 0.2% or less) but can include:
- Flap-related complications
- Corneal ectasia
A recent study has demonstrated that wearers of contact lenses may be at increased risk of postoperative complications on account of contamination of contact lens cases (see separate Contact Lenses (Types and Care) article for details about care of contact lenses). Patients will have been told to inform the team should they experience loss of vision, increasing redness and anything more than mild pain. This is a fast-evolving area of treatment with frequent new modifications to techniques; long-term safety data remain scarce at present but all practice is audited and a clearer picture should emerge over the next few years.
Photorefractive keratectomy (PRK)
- Description - a laser is used directly on the surface of the cornea to sculpt it into a predetermined shape.
- Use - myopia up to -6D, hypermetropia to +3D and astigmatism. 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. There is no clear evidence favouring one procedure over the other in hypermetropes. However, PRK may be the preferred type of procedure in patients who have a thin cornea and in whom creation of a corneal flap (see 'Laser in situ keratomileusis', below) would be dangerous. It also has an advantage in those where there might be issues of wound stability (eg those in the military or participating in contact sports).
- Problems - immediately postoperatively, it is common to experience pain, photophobia and tearing. This usually settles with a bandage contact lens and non-steroidal anti-inflammatory eye drops. 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. Long-term complications include problems in accurately measuring the intraocular pressure and in calculating the intraocular lens power prior to refractive surgery.
- Outcome - visual acuity only really settles after about three months but the final outcome is good: between ~55% and 85% of patients achieve an uncorrected vision of 6/6.
Laser in situ keratomileusis (LASIK)
- Description - this involves cutting a flap of cornea, peeling this back and 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 (-12D to +4D), particularly in hypermetropes where PRK is limited.
- Problems - there are a number of intraoperative complications which can arise relating to the flap formation, and postoperative problems include wrinkling, distortion or dislocation of the flap, keratitis, epithelial ingrowth (underneath the flap), keratitis and anterior segment ischaemia. Other complications include corneal haze, corneal scarring and keractasia.
- Outcome - this is very good with ~70-85% of patients achieving an uncorrected visual acuity of 6/6. 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, less painful and less likely to result in loss of best spectacle-corrected visual acuity.
Laser thermal keratoplasty (LTK)
- Description - a laser is aimed around the cornea, creating symmetrical 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.
- 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. A preoperative 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.
Further reading and references
; Royal College of Ophthalmologists (May 2009)
American Academy of Ophthalmology - Basic and Clinical Science Course Section 3: Clinical Optics (2005-2006)
Clinical Ophthalmology: A Systematic Approach
; Preliminary results of intrastromal corneal ring segment implantation to treat J Cataract Refract Surg. 2011 Jun37(6):1125-32.
; NICE Interventional Procedure Guidance, July 2007
; Optical coherence tomography to assess intrastromal corneal ring segment depth in keratoconic eyes. J Cataract Refract Surg. 2006 Nov32(11):1860-5.
; Overview of Refractive Surgery. Am Fam Physician [online], 2001 64: 1183-1190, 1193-1194
; Long-term Study of Artisan Phakic Intraocular Lens Implantation for the Correction of Moderate to High Myopia Ten-Year Follow-up Results. Ophthalmology. 2007 Jan 31.
; Intraocular lens exchange surgery in dissatisfied patients with refractive intraocular lenses. J Cataract Refract Surg. 2009 Oct35(10):1706-10.
; Royal College of Ophthalmologists
; Laser refractive eye surgery. BMJ. 2011 Apr 20342:d2345. doi: 10.1136/bmj.d2345.
; NICE Interventional Procedure Guidance, March 2006
Denniston AKO, Murray PI; Oxford Handbook of Ophthalmology, Oxford University Press, 2009
Moorfields Manual of Ophthalmology
; NICE Interventional Procedure Guidance, March 2011
; Contamination of Contact Lens Storage Cases of Asymptomatic Refractive Surgery J Refract Surg. 2011 May 20:1-7. doi: 10.3928/1081597X-20110505-01.
; Photorefractive keratectomy (PRK) versus laser assisted in situ keratomileusis (LASIK) for hyperopia correction. Cochrane Database of Systematic Reviews 2009, Issue 2. Art. No.: CD007112. DOI: 10.1002/14651858.CD007112.pub2
; Photorefractive keratectomy (PRK) versus laser-assisted in-situ keratomileusis (LASIK) for myopia. Cochrane Database Syst Rev. 2006 Apr 19(2):CD005135.
A friend of mine has a right eye which goes into a sort of mozaic pattern from time to time. How does this occur and is there anything that can be done about it?TheLordster
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