Refractive surgery is quite successful at treating the basic refractive errors of myopia, hypermetropia and astigmatism. Presbyopia remains somewhat of a challenge. Presbyopia correcting procedures come and go and, in general, have been underwhelming in their efficacy.
You may have even read of some recent advances in presbyopia management in mivision, but it remains to be seen as to whether these procedures will gain widespread popularity.
LASIK is still the most popular option for most refractive patients. It is particularly effective in those with low to moderate degrees of refractive error and normal corneal topography
The most commonly used procedures, which have an excellent track record, are photorefractive keratectomy (PRK), laser assisted in situ keratomileusis (LASIK), phakic intraocular lens implantation and refractive lens exchange (RLE).
Sometimes it is quite easy to decide which of the above methods is most appropriate for the patient seeking to reduce their dependence on glasses or contact lenses. On other occasions, some debate can ensue over which is the best option. Sometimes patient preference dictates the option taken.
Let us examine what most refractive surgeons would do in a number of different case scenarios.
Young low myopes
Let’s say a patient has a low degree of myopia (under -6 diopters) and is aged in his twenties or thirties. That is, he is pre-presbyopic. If his cornea is of sufficient thickness (over 520 micrometres) and has normal corneal topography, LASIK remains the most popular choice of procedure . Though some clinics would favour PRK even in these circumstances, the minimal amount of discomfort and quick visual rehabilitation that is afforded with LASIK makes it a more popular choice amongst refractive surgeons for such patients.
LASIK consists of two stages. The first is the creation of a thin flap in the anterior portion of the cornea. This is now most commonly performed using a femtosecond laser such as the Intralase. Once the flap has been created and flipped aside the excimer laser is used to reshape the underlying corneal stoma. The flap is then returned to its original position and quickly adheres to the reshaped tissue underneath it.
On the other hand, should the same patient have thin corneas but otherwise normal corneal topography, for instance, if his corneal thickness is under 490 micrometers, LASIK would be a less popular choice than surface ablation. More about that in the next case scenario.
Young low myope with mild inferior steepening or thin corneas
Consider the same young low myope that was mentioned above, but with some mild inferior steeping on corneal topography. This would be of concern to me because it may well reflect a predisposition of the cornea to keratoconus. A picture resembling pellucid marginal degeneration would also be worrisome. Others may find consolation in seeing that the cornea is thick centrally and would consider recommending LASIK.
The term forme fruste keratoconus is used to describe inferior steepening on corneal topography with a total absence of any other clinical signs of keratoconus. I would consider surface ablation (and not LASIK) in such cases..
In my own case series of 50 eyes with topographic evidence of forme fruste keratoconus, no cases of keratectasia have developed over the course of five years with surface ablation. I hope they will not appear in the future.
Figure 1. (Click Here) shows a cornea with perfectly normal anterior corneal topography and thickness. No one would doubt that this is a normal cornea and that it would do well with LASIK for low to moderate degrees of myopia.
On the other hand, Figure 2. (Click Here) displays evidence of inferior steepening and many would be concerned that there is an increased risk of keratectasia with LASIK. , Some would even shy away from PRK in such a case.
It has been said that looking at the posterior curvature (‘posterior vault’) is helpful. However, I’m not sure that the Orbscan or even Scheimpflug instruments are good at assessing the posterior cornea. In other words, I don’t think we can rely on them.
In general, corneas under 490 to 500 micrometres are considered to be at increased risk of keratectasia following LASIK and so PRK is the preferred option. There are other risk factors to consider including steepness over 45 diopters, asymmetry of the radial axes in cases of astigmatism, younger age, history of keratoconus in the family and a history of atopy (asthma, hay fever and eczema). Pregnancy has been associated with the onset of keratectasia after LASIK in susceptible corneas.
Keratectasia is quite a serious complication of LASIK (but extremely rare with PRK) and can lead to the need for further surgical intervention. Essentially, an otherwise stable cornea is transformed into a keratoconic one with progressive bulging, irregular astigmatism and disturbance of vision. To improve the quality of vision rigid contact lenses, collagen cross-linking, kera-rings or corneal transplantation may be necessary. As there is less compromise in corneal integrity with PRK and related procedures, a thin cornea, or one displaying inferior steepening on topography, is best served without flap creation.
Surface ablation can be in the form of PRK, LASEK, or epi-LASIK. PRK is the originally described procedure and is performed after manual debridement of the corneal epithelium – in other words, the excimer laser is placed directly onto Bowman’s membrane.
More recently, a transepithelial approach has also been used.
A minor modification of PRK is the use of alcohol to gently loosen the corneal epithelium from underlying corneal tissue but allow the epithelial cells to remain attached to each other, in the form of a sheet, which can be replaced once the excimer laser ablation has been performed. A pretend flap, you could say. This became known as LASEK.
Epi-LASIK is a variation of LASEK where a microkeratome with a plastic separator is used to move the corneal epithelium to one side. In both LASEK and epi-LASIK, the corneal epithelium is then replaced in the form of a pseudo flap mimicking a LASIK flap.
The fact of the matter is that most studies show this carries no advantage over simple PRK , and in my hands, at least, has probably worsened the situation by interfering with the migration of new epithelial cells from the corneal periphery over the ablated surface. I would prefer to remove the epithelium completely to allow fresh cells to move in from the periphery and cover the central defect.
At one stage, surface ablation carried a significant risk of corneal haze. This has been virtually eliminated with the use of intraoperative mitomycin C. Most refractive surgeons performing PRK, certainly for higher degrees of refractive error, would apply mitomycin 0.02 per cent , for anywhere between five seconds to one minute at the time of the procedure. A circular sponge is soaked in a solution of the medication and applied to the cornea right after the laser treatment has taken place. This has done wonders in eliminating the risk of scarring and there have been no cases of corneal thinning with the use of mitomycin as has been reported with its use in pterygium and glaucoma surgery.
One could say that surface ablation has made somewhat of a comeback over the years because of the risk of keratectasia with LASIK, but also as a result of mitomycin C. The non-steroidal anti-inflammatory medications that are used post-operatively have assisted in combating post-operative pain and bandage contact lenses have also helped in this regard.
Some have combated the ‘bladeless’ marketing campaign with IntraLase by claiming that the technique of surface ablation is the equivalent and also bladeless. This is quite cheeky.
Young high myope
Consider a patient in his twenties, thirties or even forties that has a high degree of myopia but an otherwise healthy eye. I think twice about performing LASIK or PRK for very high degrees of myopia. More than -10 diopters of treatment carries a significant risk of reduced contrast sensitivity and unwanted optic phenomena at night when the cornea is ablated. The lower the central thickness of the cornea the more I would consider the alternative of phakic intraocular lens implantation.
Even corneal steepness may lead to the preference for a phakic intraocular lens because the conventional thinking is that a cornea that is any flatter than 34 or 35 diopters (post-operatively) leads to poorer quality of vision than a normally shaped cornea. Put in more technical terms, an oblate cornea is never as good as a prolate one.
Phakic intraocular lenses come in several guises in Australia – the most popular ones being the Alcon AcrySof Cachet, the STAAR ICL and Abbott Verisyse. The Cachet lens is angle supported in the anterior chamber. The ICL sits between the posterior aspect of the iris and the anterior lens capsule, whereas the Verisyse is an iris claw lens that is clipped onto the anterior surface of the iris. All have good track records and the decision as to which one to use is based on surgeon preference.
My argument would be that there is little point in using a phakic intraocular lens if the patient is already presbyopic. There are other contraindications, such as a low endothelial cell count and a shallow anterior chamber. Certainly it would be silly to recommend a phakic intraocular lens if cataract is present and likely to worsen. If emmetropia were to be promised, I would also exercise caution in recommending a phakic intraocular lens to a patient with an abnormal cornea on topography as any fine tuning procedure to be used on the cornea following lens implantation (so called bioptics) will be difficult if the cornea is not suited to laser surgery.
Young low hypermetrope
Now let’s look at a pre-presbyopic patient in his twenties or thirties that has hypermetropia of up to +4 diopters. If his cornea is thick enough and displays no evidence of irregular astigmatism, I would be comfortable recommending LASIK over other procedures to reduce the refractive error.
Hypermetropia and presbyopia usually progress in the late forties and fifties. The alternative of lens based surgery needs to be strongly considered in patients of this age or older. This is also true of higher hypermetropes (over +3 or +4 diopters) in any age group.
Conductive keratoplasty (CK) is one of those procedures that came and went very quickly. Some used it to their benefit for marketing purposes when it first became available in Australia but it did not gain widespread popularity.
Other examples of technology that also made only brief appearances were multifocal corneal ablations and accommodating intraocular lens.
My belief is that IntraCor will not last as a widely acceptable refractive procedure. The fact that so few surgeons are embracing it supports this hypothesis.
Refractive lens exchange (RLE) is essentially a cataract operation in the absence of any visually significant opacity of the lens, otherwise known as a clear lens extraction. Even with refractive lens exchange and implantation of an intraocular lens, there would be several choices.
Monofocal intraocular lenses could be used in a monovision configuration with the dominant eye being set for distance vision and the non-dominant eye for near. The degree of asymmetry that surgeons choose for their patients varies between -0.5 diopters and -2 diopters. Though the adverse effect on distance vision is greater, I prefer closer to -2D as this ensures the patient can read without the need for glasses. The other choice, however, is the implantation of multifocal lenses. Diffractive multifocal lens technology is most popular in Australia and examples of lenses that give excellent near vision (as well as distance) are the Abbott Tecnis multifocal, Alcon ReSTOR and Zeiss Acrilisa.
The halos, glare and suboptimal intermediate vision with these lenses sway some surgeons and patients away from this option in favour of either bilateral, emmetropia or monovision using monofocal intraocular lenses.
Older low myope
One of the most challenging scenarios is the patient who is presbyopic but only has a modest degree of myopia, for instance -2 or -3 diopters. The reason for this is that these patients can see well to read without glasses and aren’t as incapacitated without their glasses or contact lenses when they look into the distance as their higher myopic counterparts.
With a perfectly normal cornea, LASIK, in a monovision configuration, would still be the choice in most patients in their forties and fifties. With thinner corneas, PRK would be used. However, if monovision were not acceptable to the patient, the alternative would be to aim for bilateral emmetropia or use multifocal IOLs. All of these options carry a significant risk of dissatisfaction on the part of the patient.
Older high myope
In the case of presbyopic patients that have a high degree of myopia (over four diopters) I would still consider LASIK a first choice but it would not be unreasonable to look at the alternative of refractive lens exchange. Again there would be the choice of monofocal lens implants, perhaps in a monovision configuration, and multifocal lens implants.
Older high hypermetrope
In patients who are in their presbyopic years but have a high degree of hypermetropia (four diopters or more), I believe refractive lens exchange is the only option that is to be considered.
It must be remembered that presbyopia is probably the most common refractive error presenting to ophthalmologists and refractive surgical centres. It is no wonder that the refractive surgical market is crying out for some help in treating presbyopia. It remains to be seen whether controversial treatments such as femtosecond laser IntrCor , or any number of corneal inlays will reach widespread acceptance.
The difficulties that we need to contend with in treating people with presbyopia include the following:
• Monovision is not acceptable or appropriate in all patients
• In any case, monovision is a compromise
• Pilots and heavy vehicle drivers with monovision would fail their vision tests unless they wear glasses
• With monovision, phorias can decompensate
• Dry eyes are more common in presbyopic patients following laser procedures
• Nuclear sclerosis, though not necessarily visually significant, may be present reducing the longevity of a laser-based procedure
• LASIK and PRK, therefore, are only temporary solutions
Refractive lens exchange is a more permanent solution to laser procedures on the cornea. If multifocal lenses are used there is no need for monovision. Unfortunately, there is still the risk of suboptimal contrast sensitivity with these lenses. Halos and glare may be intolerable and there will be the occasional need for an intraocular lens exchange (with substitution of a monofocal lens). All this represents a major headache for refractive surgeons.
However, the majority of diffractive multifocal lens implants are extremely successful. , , The lenses work on the basis of diffractive rings in the optic such that all parallel rays of light entering the lens are refracted to two distinct focal points – one on the retina, the other into the vitreous cavity. The focal point on the retina serves to focus distant objects on the fovea, the one in the vitreous cavity acts as in the same way a myopic eye does. As the object of regard approaches the eye the ‘near’ focal point approaches the fovea and the object is seen clearly. Essentially these lenses are bifocal and it is not surprising that intermediate vision is less than optimal in eyes implanted with them.
Practically speaking, the only time patients seem to have difficulty is with fonts on computer screens. The optimal near focus is at about 32 to 37 cm from the eyes yet a computer screen is likely to be at 70 or 80 cm from the eyes. If the patient is told before the lens is implanted that they will need to sit closer than usual to a computer screen to see the font clearly, or use a pair of magnifiers to work on computers, there will be no surprises for them. The best refractive error to use these lenses in is presbyopic hypermetropia. I tell my patients these lenses will give them excellent distance vision, excellent near vision but that they may need some help with things in between. If the patient doesn’t accept this limitation perhaps they should be dissuaded from any surgical intervention for their refractive error.
The halos and glare patients experience with multifocal lenses seems to settle over the course of three to six months in the majority. Occasionally, a patient who can notice a reduction in contrast sensitivity will say, ‘doctor, nothing seems to be perfectly clear to me’, which scares me away from implanting multifocals indiscriminately. In such patients, explantation of the multifocal lens and substitution with a monofocal is sometimes necessary.
In summary, PRK, LASIK, phakic intraocular lens implantation and refractive lens exchange remain the most commonly used refractive surgical procedures to cater for the full range of case scenarios in presenting patients.
LASIK is still the most popular option for most refractive patients. It is particularly effective in those with low to moderate degrees of refractive error and normal corneal topography. If the cornea is thin or there is evidence of inferior steepening on corneal topography, then PRK is the preferred option. Even with thick and nicely shaped corneas, surface ablation continues to enjoy widespread popularity.
Phakic intraocular lenses are a better choice for higher degrees of myopia in pre-presbyopic patients. On the other hand, in presbyopic patients, refractive lens exchange needs to be considered. This is an easy option if the error is outside the range treatable with laser. Certainly early cataract or an age over 60 years would make most refractive surgeons lean towards refractive lens exchange.
Multifocal intraocular lenses are an increasingly popular option and available to general cataract surgeons as well as refractive specialists.
Alternatives such as Intacs, corneal inlays, multifocal ablations, IntraCor, scleral implants, and conductive keratoplasty do not have widely accepted roles in refractive surgery at the present time.
Dr. Con Moshegov is the principal surgeon at Perfect Vision Laser Correction and is among Australia’s most highly respected and trusted experts in the field of refractive and cataract surgery. He trained in Sydney and spent two years undergoing an advanced Fellowship in Cornea, External Diseases and Refractive Surgery in the United Kingdom. Dr Moshegov now specialises in refractive and cataract surgery.