Patients who wish to have refractive surgery to correct their presbyopia, and reduce or eliminate the need for spectacles will undergo a multifaceted consultation. It is comprised of a series of diagnostic and data collection procedures as well as some elements of an eye examination and health check and culminating in a detailed discussion about the risks and benefits of surgery.
Pre-operative assessment
As with cataract surgery, there is a holistic approach to pre-operative assessment to ensure that any individual patient needs or concerns prior to undergoing surgery are identified. The different aspects of such an assessment can be seen in Figure 1.
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The details of this assessment and the considerations that need to be made can be found in an earlier series on cataract surgery (Optician, 28.11.14). The assessment will also identify any relative or absolute health contraindications to surgery and these are summarised in Table 1.
In general, patients undergoing refractive lens exchange (RLE) tend to be younger than those with cataract and so usually have fewer co-morbidities and fewer associated risks. However, the expectations of these patients are often more demanding because they are looking to maintain a good level of acuity but be spectacle-free. This means that the surgeon must also carefully evaluate the psychological aspects in each case and discuss the risks with respect to both the surgery and the visual outcome.
Data collection
The diagnostic capability of clinics will vary but the essentials for RLE are biometry and topography. Instruments that can also give detailed information about the posterior surface of the cornea such as the Orbscan or Pentacam are also advantageous because the additional information may help to predict the outcome of surgery more accurately.
Biometry
This diagnostic has become the cornerstone to predictable refractive outcomes with intraocular lens surgery. Decades of cataract surgery experience have allowed complex mathematical formulae to be developed, calculating IOL power to a high degree of accuracy for individual eyes based upon some or all of the following: axial length, anterior chamber depth, corneal curvature and corneal diameter. Patients find it quite comforting to know that unlike spectacles, their lens power is not determined by their subjective (and sometimes inconsistent) refraction. But instead, by applying the principle that if one knows the physical measurements of the eyeball, the power of the lens needed to create a clear image on the retina can be calculated by mathematical means.
Figure 2 shows the first-generation SRK formula which illustrates the relationship of the measured parameters on IOL power. Each IOL type has a set of constant values that are recommended by its manufacturer and applied to the formulae. To improve accuracy of refractive outcomes, the ULIB database allows registered surgeons worldwide to enter their outcomes data. The collective data is then analysed and the constants adjusted. Each surgeon can also factor in their own refractive outcomes and the constant can be ‘optimised’ for individual surgeon and IOL combinations.
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The accuracy of biometry is essential and so the operator must know when the data is reliable and consistent with the other findings for the patient. For example, a difference of more than 0.3mm in axial length would be suspicious unless it was accompanied by a moderate degree of anisometropia. The operator should also be aware of any tear film or corneal warpage issues and take steps to ensure that the readings are not adversely affected. An axial length measurement that is 1mm out could make a difference of between 3 and 7 dioptres to the final refractive outcome, depending upon the length of the eye. The average axial length of a normal eye is about 23.5mm. An eye that is less than 22mm is considered short and greater than 25mm a long eye.
Collective data from the ophthalmology community has also allowed the performance of different formulae at achieving the targeted refraction to be assessed. A surgeon can select which formula to use for various refractive groups, although it is widely acknowledged that accurate biometry measurements will have the greatest impact on outcome rather than formula selection.
Topography
Topography is used to more fully understand the effect of any corneal incisions that are made on the final refractive outcome. In normal phacoemulsification, two incisions are made for entry into the eye and so a small amount of surgeon-induced astigmatism is inevitable. When an astigmatic eye is undergoing surgery, it is useful to know the amount of astigmatism that is due to cornea and/or lens so that modifications to incisions can be made to minimise induced astigmatism postoperatively. For example, the difference between eliminating and doubling a patient’s 1.00DC can be due to simply placing these incisions along either the steepest or flattest meridian.
A topographical map shows the principal meridians of the eye and allows the surgeon to plan the location and the depth of corneal incisions (Figure 3). Surgeons usually incise along the steepest meridian so as to flatten the cornea along that meridian. An incision made at the limbus will induce minimal astigmatism. The further in from the limbus and the longer the incision, the greater this flattening effect.
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Corneal incisions are subject to healing and may not always retain their corneal profile changing effect completely, and so some surgeons prefer to correct astigmatism of more than 1 dioptre with a toric IOL instead, or sometimes a combination of both methods is used. Toric IOLs may seem like the obvious solution and more predictable, but the visual outcome can sometimes be affected by slight IOL rotation inside the capsular bag.
Vision assessment
It is important to understand the visual potential of each eye prior to undertaking any form of refractive surgery and the cause of any reduced vision should be explained. For those patients who have some early cataract, it is important to establish a reliable refractive ocular history. Patients are not always aware of mild amblyopia and so the recorded acuities of previous eye examinations can be useful. Patients with significant amblyopia, or any ocular condition that impacts negatively on the visual acuity or contrast sensitivity, are unlikely to be recommended a multifocal lens implant.
Ocular dominance is assessed, just as with the fitting of multifocal contact lenses, because sometimes the surgeon may opt for some micro-monovision in conjunction with a multifocal IOL to enhance the near vision range. Some IOLs are available in more than one reading addition and so the IOLs can be chosen to suit a range of potential visual tasks or to be more specific to a given range.
Full monovision with monofocal IOLs to provide detailed acuity at near means induced anisometropia of more than 2.50 dioptres if the dominant eye is targeted for emmetropia. Most patients are not able to tolerate this and so may be offered a partial monovision correction (typically around -1.50) with the understanding that reading spectacles may be required for reading small print or in low lighting conditions. As monofocal IOLs do not reduce contrast and are not pupil dependent for performance, they are more suited to those with mild amblyopia.
Clinical examination
A mydriatic is instilled into the patient’s eyes so that the surgeon can conduct a thorough examination of the fundus and the anterior structures of the eye, looking for any pathology which may put the patient at an increased risk during or after intraoculor surgery. Table 2 details the conditions that may potentially contraindicate elective surgery.
There are also some conditions that may not be causing any significant visual loss in a phakic eye with a clear crystalline lens, but when coupled with the loss of contrast through a multifocal IOL, visual performance and patient satisfaction can be hampered. As this surgery is not essential to restore lost vision, but is an elective procedure, additional screening such as OCT, endothelial cell counts and careful assessment of the tear film are useful to help identify those patients that might have lower than expected vision postoperatively. Figure 4 shows an OCT image of an epi-retinal membrane that will very likely degrade the vision through a multifocal. Other examples of potential issues are; maculopathies, macula or corneal dystrophies and ocular surface disease. Where there is significant disruption to the ocular structure or surface, multifocals are likely to be contraindicated. Where the condition can be treated (lid hygiene, tear supplements etc), it should be done prior to surgery.
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Consultation and consent
Patients must weigh up the potential benefits of surgery against the risks and decide whether it is worth it. With any surgery, the benefit must outweigh the perceived risk and this is where a patient’s expectations come in. It is essential that the surgeon understands the visual demands for the patient so that the refractive target can be planned and the IOL type selected carefully. Cataract surgery and refractive lens exchange are essentially the same procedure, so why do patient expectations for the two procedures differ so widely? Figure 5 shows the key differences between them.
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Cataract patients are looking to restore vision that has been lost or compromised and quality of life may have already been affected. RLE patients on the other hand often have an excellent level of best corrected acuity which is adequate for their daily needs but they are spectacle-dependent. It is up to the surgeon to explain the pertinent risks and benefits to individual patients and to establish whether or not the benefits outweigh the risks.
Where a patient needs a significant correction for distance vision, the benefit is more obvious (Figure 6).
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However, where there is low myopia or hypermetropia the level of UCVA after surgery may only be marginally improved and if the patient is emmetropic, the UCVA could be lower postoperatively, which could tip the scales in the opposite direction. Ultimately, any patients wishing to undergo refractive surgery should accept the following:
- An absolute zero prescription is not necessary for good vision;
- Not all patients achieve 6/6 uncorrected vision after treatment;
- The need for spectacle correction is unlikely but cannot be absolutely guaranteed.
The pre-operative consultation usually ends with the surgeon going through the consent process by recapping the treatment expectations, potential risks and alternative treatments available. The patient signs a consent-for-surgery form at a later date when they have had time to reflect on the consultation and after an adequate cooling off period.
Postoperative care
Patients who have undergone RLE are susceptible to the same postoperative complications as those who have undergone cataract surgery and the reader is referred once again to an earlier series on cataract (Optician, 28.11.14). The next section will look at the post-surgery examination of the patient and the issues that potentially crop up in the postoperative period.
Examining the patient
Distance acuity
When measuring acuity, it is important not to jump to the 6/5 line. Not all patients with a good multifocal outcome will see this, especially if they have micro-monovision. Ask the patient what they can see binocularly, after all this is the habitual vision, and patients should be dissuaded from constantly comparing the vision between eyes. If the pre-operative BCVA was previously reduced, then sometimes the patient needs to be reminded of this.
Near acuity
The near vision range can vary greatly from one patient to the next depending upon the IOL type used and the refractive target outcome. Again, it should be measured binocularly at the habitual working distances and then monocularly if the patient has any concerns. For example, a patient who has been implanted with MPlus (effective +3.00 add) in one eye and MPlus Comfort in the other (effective +2.00 add) will have optimal near vision at different distances. The Zeiss AtLisa trifocal aims to give good near vision at 40cm and good intermediate vision at 80cm.
Reduced vision
If a patient is concerned that their vision is not adequate, it is important to establish whether this has been the case since surgery, or whether there was originally a good outcome, but that vision has since deteriorated. The posterior capsule should be carefully examined through a dilated pupil for any signs that might cause visual disturbance. Wrinkles on the posterior capsule surface can sometimes be seen in the early postoperative stage and are due to the capsule being slightly stretched as the IOL is inserted. These wrinkles smooth out spontaneously over a couple of months but can have a significant effect on both the refraction and the visual acuity in the meantime. The lens position should also be checked as capsular changes can sometimes cause the IOL position to decentre. Figure 7 shows anterior capsular phimosis which caused an IOL to decentre.
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Refraction
An autorefractor result may not be the best starting point for a postoperative refraction as these instruments can struggle with the multiple foci of a multifocal IOL implant. If the autorefractor is not aligned correctly when the reading is taken, the results can be particularly inaccurate. Older generation autorefractors tend to over minus, but the cylindrical component tends to be more reliable.
Refractive surprise
Subjective refraction is used to identify any residual spherical or cylindrical component that may detract from the vision. It is always important to push the plus when subjectively refracting the patient to ensure they are not using the intermediate or near portion of the multifocal. The use of refractor heads may also impact on the visual performance of a multifocal IOL as there is less ambient light reaching the eye, thus reducing contrast sensitivity. If the multifocal has any pupil dependent zones the near vision performance may be reduced too. As the refraction may take a few weeks to stabilise, further surgery to correct unwanted refractive error is not considered until at least three months after the primary procedure, and lens capsule problems have either been excluded or treated. An exception to this is if a toric multifocal has been implanted and a significant cylindrical correction is found postoperatively. This may indicate that the toric IOL has rotated after insertion into the capsular bag. If this is indeed the case, then the patient needs to be returned to theatre as soon as possible to have the IOL repositioned.
Unwanted glare or halos
The surgeon discusses the risk of glare with all patients as the multifocal nature of IOLs does cause some light scatter. For most patients, the glare is not a significant nuisance and improves as neuroadaptation continues to occur over a prolonged period of time (six months).
Tear film issues
There are many patients undergoing RLE who have some underlying ocular surface disease that may be related to age, hormonal changes or long-term contact lens wear. A poor tear film can appear to have a disproportionate effect on acuity and/or stability of vision though a multifocal lens and so steps must be introduced where appropriate to treat any meibomian gland dysfunction and address any associated dry eye symptoms.
Further surgery
This is considered where a patient’s visual symptoms persist after a minimum of three months from the time of surgery. If the symptoms are associated with blurred vision due to a refractive surprise then it may be appropriate to carry out further refractive surgery my means of Lasik, Lasek or limbal-relaxing incisions. In some cases an IOL exchange can be considered if there is more than 1.5D of residual hypermetropic error. This latter option can also be considered if the patient is suffering from persistent unwanted glare symptoms associated with the multifocal IOL optics and a monofocal IOL inserted instead. If a further procedure is needed, expectations need to be assessed and risks versus benefits discussed again. The impact of any intended treatment must be checked across all distances. There is no point to making the distance vision better if the near becomes significantly worse and the patient is not prepared to lose the near vision gain.
Patient dissatisfaction
Where a patient remains unhappy despite all possible actions being taken to improve the vision (eg, YAG capsulotomy if indicated), it is sensible to revisit the preoperative discussion and check the surgical plan. The following points should be considered:
- Have the original aims of surgery been met, or is the target moving?
- Is the patient actually struggling with their daily tasks? Look at binocular capability.
- Remind patients if necessary of their pre-op vision by inducing myopic or hypermetropic blur to demonstrate pre-op acuity.
For most patients, the outcomes of refractive surgery are very successful. Good quality pre-operative data, appropriate lens selection, thorough surgery planning and a skilled refractive surgeon will minimise the risk of an unwanted outcome. In addition, management of patient expectations is essential when delivering refractive surgery services and is the key to having happy patients.
Michelle Hanratty is the senior optometrist at Optegra Birmingham and an examiner for the College of Optometrists