204 THE NATIONAL MEDICAL JOURNAL OF INDIA VOL. 27, NO. 4, 2014

Refractive surgery for myopia: Case selection andmanagement options

SRILATHAA GUNASEKARAN, TUSHAR AGARWAL, NAMRATA SHARMA, RADHIKA TANDON

© The National Medical Journal of India 2014

INTRODUCTIONRefraction is the change in direction of light rays when these passfrom one medium to another with a different refractive index.Structures in the anterior segment of the eye, namely cornea andlens, function to focus the incoming rays of light onto the retina.Myopia (near-sightedness) is a refractive state of the eye where ina non-accommodating eye light rays from infinity are focused infront of the retina producing a blurred retinal image. Refractiveerrors account for 70%–90% of visual impairment in Asia,1,2 andof about 20% in the western world (USA, Europe and Australia).3

Spectacles and contact lenses are the principal methods forcorrection of refractive errors. The development of excimer laserin ophthalmology has opened a gamut of safe and effectivetreatment options for surgical correction of refractive errors.Much progress has been made in this field over the past twodecades. This article focuses specifically on the surgical correctionof myopia.

HISTORICAL EVOLUTION OF MYOPIA SURGERYThe first description of refractive surgery dates back to 1885 whencorneal incisions were made for the correction of myopia.4 JoseBarraquer developed the first microkeratome and is consideredthe father of lamellar surgery. The initial techniques ofepikeratophakia and keratophakia, done for aphakia and highmyopia, involved making a lamellar corneal flap and then mouldingthe cap in a cryolathe, according to the degree of refractive error.5

The results of these procedures were unpredictable, and regressionof the refractive correction was common; hence, these procedureswere soon abandoned. Radial keratotomy (RK) involves creationof radial corneal incisions of 90% depth to flatten the centralcornea, thus correcting the myopia.6 In the Prospective Evaluationof Radial Keratotomy (PERK) trial,7 43% of patients showed asignificant hyperopic shift at 10 years. Because of the problems ofunpredictability and lack of stability, the number of these surgeriestoo declined drastically and the procedure is currently obsolete.

With the development of excimer laser systems in the 1990s,surgery for myopia has undergone a transition from incisionalprocedures to ablative procedures. Since then, procedures forlaser vision correction (LVC) have been modified several timesand are now the most commonly performed refractive proceduresfor the correction of myopia.

CLASSIFICATION OF VARIOUS SURGICALPROCEDURESSince cornea and lens are the two main refracting media in the eye,myopia can be corrected by surgery on either of these (Table I).The more commonly performed cornea-based procedures followthe principle of subtraction of tissue by laser ablation. Lens-basedcorrection is reserved for patients in whom the refractive error isvery high or the corneal thickness is inadequate for performing acorneal ablative procedure using laser.

SELECTION OF PATIENTS AND PREOPERATIVEWORK-UPIn view of the elective nature of refractive surgery, the selectionof appropriate surgical candidates is of paramount importance.8

Patient’s historyActive infections or inflammatory conditions of the eye, pasthistory of herpetic keratitis, glaucoma, uveitis, retinal detachment,or ocular trauma necessitating surgical repair are considered ascontraindications for LVC procedures. In patients with history ofdry eyes or allergic conjunctivitis, these coexisting conditionsmust be adequately managed before the procedure. Use of systemicdrugs such as amiodarone and isotretinoin should be enquired

All India Institute of Medical Sciences, Ansari Nagar, New Delhi 110029,India

SRILATHAA GUNASEKARAN, TUSHAR AGARWAL,NAMRATA SHARMA, RADHIKA TANDONDr Rajendra Prasad Centre for Ophthalmic Sciences

Correspondence to RADHIKA TANDON, Room No. 490, Dr RajendraPrasad Centre for Ophthalmic Sciences, All India Institute of MedicalSciences, New Delhi 110029; radhika_tan@yahoo.com

Review Article

TABLE I. Classification of refractive surgeries

Corneal refractive surgeries• Corneal ablation by excimer laser

1. Photo-refractive keratectomy (PRK)2. Laser-assisted subepithelial keratomileusis (LASEK)3. Epithelial laser-assisted stromal in situ keratomileusis (Epi-LASIK)4. Laser-assisted stromal in situ keratomileusis (LASIK)

• Femtosecond lenticule extraction (FLEX) and small incision lenticuleextraction (SMILE)

Lenticular refractive surgeries• Refractive lens exchange• Phakic intraocular lens (IOL)

205GUNASEKARAN et al. : REFRACTIVE SURGERY FOR MYOPIA: CASE SELECTION AND MANAGEMENT OPTIONS

about, since these are known to affect wound healing aftersurgery, as are connective tissue disorders. Patients above the ageof 18 years who have stable refractive error (with variation of upto 0.5D) for at least a year are considered eligible for refractivesurgery. Patients using contact lenses must discontinue their usefor at least a week for soft contact lens and 3 weeks for rigid lenses.

Ocular parameters and examination

The following steps are mandatory before LVC:

• Recording of uncorrected and corrected distance visual acuity,and the power of the spectacles or contact lenses being used.

• Accurate refraction under cycloplegia and precise finalsubjective manifest refraction.

• Measurement of corneal curvature (keratometry) using eithera manual or an automated keratometer; values below 39dioptres (D) or above 47D should arouse suspicion of anabnormal cornea.

• Assessment of corneal topography along with pachymetry andelevation maps to rule out forme fruste keratoconus or otherectatic disorders. Patients with corneal hot spots or irregularastigmatism may need further evaluation before being taken upfor surgery.

• Measurement of corneal thickness (pachymetry) using anultrasonic or slit-scanning technique. If it is below 500 μ,cautious evaluation may be needed to rule out ectatic cornealdisorders.

• Contrast sensitivity and glare acuity.• Mesopic pupil size; since large pupil size has traditionally

been associated with night vision problems though recentstudies have failed to find this association.9,10

• A thorough slit-lamp examination to look for adnexal problems,corneal dystrophy or scars, or lenticular opacities.

• Assessment of ocular surface and tear function using cornealstaining, tear break up time and Schirmer test.

• Screening of retina using indirect ophthalmoscopy to rule outany peripheral treatable lesions as these predispose to retinaldetachment.

• Detailed counselling of the patient, including discussion of thepatient’s expectations and of the anticipated problems.

Special investigation tools

In addition to the above, the following specialized investigationsare an integral part of the preoperative work-up.

• Schiempflug imaging of the cornea (Pentacam): Using the slit-scanning system, anterior and posterior elevation maps, andtopographic and pachymetry maps are prepared. The BelinAmbrosio Display (BAD) map shows posterior elevation datarelative to an enhanced best fit sphere along with cornealthickness spatial profile (CTSP) and percentage thicknessincrease (PTI). This enables early identification of ectasia inpatients undergoing refractive surgery.

• Anterior segment optical coherence tomography (ASOCT): Itallows imaging of the cornea with delineation of the flap andits morphology and thickness.

• Aberrometry: It allows study of optical aberrations of the entireocular system and assessment of the wavefront in the form ofZernike polynomials. Conventional LASIK is known to inducespherical aberration. Hence, newer platforms that providewavefront-optimized (aspheric ablation) and wavefront-guidedablation profiles, which reduce such aberrations after surgery,and hence provide better quality of vision.

TYPES OF KERATO-REFRACTIVE SURGERYSurface ablation proceduresThese include photo-refractive keratectomy (PRK), laser-assistedsubepithelial keratomileusis (LASEK) and epithelial laser-assistedstromal in situ keratomileusis (Epi-LASIK), wherein ablationoccurs just under the epithelium, including the Bowman layer andthe superficial stroma. These procedures essentially lack the risksof flap-related conditions that are associated with LASIK. Hence,these procedures are preferred in persons working in armed forcesand involved in contact sports. Patients with thin cornea andanterior epithelial basement membrane dystrophy are better treatedusing surface ablation. These procedures are approved for thetreatment of mild-to-moderate myopia.

PRK involves removal of the overlying epithelium beforestromal ablation using 15%–20% alcohol or manually using anelectric toothbrush followed by ablation of the stroma usingexcimer laser, followed by application of a bandage contact lens.

LASEK involves removal of the epithelium as a sheet with 20%alcohol only to be placed back at the end of the procedure. Thecorneal epithelial sheet is considered essential in maintainingbalanced epithelial stromal interaction and can produce inflam-matory cytokines and fibroblast transformation when damaged.

Epi-LASIK is based on the same principle as LASEK exceptthat the epithelial sheet is removed using epikeratome—a motorizedmachine similar to that used in LASIK.

Since the epithelial sheet is left behind intact in LASEK andEpi-LASIK, some workers claim that these procedures induceless discomfort than PRK, though this has not been provenconsistently.11 Disadvantages of these surface ablation proceduresover LASIK include the greater postoperative discomfort, longerperiod for visual recovery and corneal haze.

Intrastromal corneal ring segments (ICRS)

They were originally developed to correct low degrees of myopia(–1D to –3D) by an ‘arc-shortening effect’. These rings, made ofpolymethyl methacrylate (PMMA), are available in varyingthicknesses from 250 µ to 450 µ (at 25 µ to 50 µ increments) andare implanted intrastromally at 80% depth in the channels createdmanually or using a femtosecond laser. Presently, the clinical useof ICRS is restricted to the treatment of keratoconus and post-LASIK ectasia.

LASIK

LASIK is the most commonly performed refractive surgery and isthe preferred technique in the low and moderate myopes (<6D)where appropriate preoperative screening has been done. Thesurgery involves creation of a lamellar corneal flap (which includesthe epithelium and superficial stroma) followed by excimer ablationof the stroma using the argon fluoride laser (193 nm). The flap isrepositioned at the end of the surgery allowing it to adhere byfunctioning of the endothelial pump, although the flap no longerprovides any biomechanical strength to the cornea.

Femtosecond LASIK. Femtosecond (FS) has nearly replacedmechanical microkeratomes for flap creation in LASIK. It is anNd-YAG solid-state laser that uses a near-infrared (1053 nm)scanning pulse which is focused to 3 µm spots at a preset depth of1 µm, thus allowing formation of precise lamellar flaps byphotodisruption.12 Each laser pulse creates a tiny 2–3 µm bubbleof carbon dioxide and water vapour. In this procedure, the eye isfixed with a suction ring through which the cornea is applanatedwith a disposable contact lens that couples the eye to the lasersystem during the procedure. The software in the LASER system

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first creates a canal and then the laser pulses are delivered to thestroma in a raster pattern from the hinge across the cornea for theset diameter of the flap (Fig. 1).

Microkeratome versus femtosecond flap. Femtosecond flapsare planar compared to the meniscus-shaped flaps created with amicrokeratome. The flap thickness is consistently within 10 µm ofintended thickness. The flap can be created over a wide range ofthickness profiles compared to only a few modifications that arepossible with the microkeratome. The increased side cut angle infemtosecond flaps act as a barrier to epithelial cells preventingcomplications, such as flap displacement and epithelial ingrowth.In various studies, the final visual outcome with these procedureshave been comparable.

Newer ablation profilesConventional LASIK or standard profile applies a simple sphero-cylindrical correction and is known to induce positive sphericalaberrations dependent on the amount of intended correction.Ablation profiles have been developed to reduce this complicationand provide better postoperative visual outcome. Wavefront-optimized profile designed to reduce or eliminate the inducedspherical aberration of conventional LASIK removes additionaltissue from the periphery to provide smoother transition zones.13

The excimer LASER system can be coupled to patient’s topographymap or wavefront map to generate a complementary ablationprofile; this is referred to as the topoguided LASIK and wavefront-guided LASIK, respectively. Topoguided LASIK is specificallyused in patients with higher amounts of astigmatism or for

enhancement procedures. Wavefront-guided ablation profile aimsto correct all the optical aberrations of the eye and not just thesphero-cylindrical refractive error. The complications of LASIKare listed in Table II.

Refractive lenticule extractionThis is the latest development in the field of keratorefractivesurgery where an intrastromal lenticule corresponding to thedegree of myopia is carved using femtosecond laser and the sameremoved after creation of a flap as was described first (femtosecondlenticule extraction; FLEX) or through a 2 mm side cut incision(small incision lenticule extraction; SmILE).21 It is an allfemtosecond procedure and the excimer platform for the stromalablation is not required. It is a viable option for patients withmyopia –2 to –8D with astigmatism of <1D and is presentlyavailable only in Visumax femtosecond laser platform (Carl ZeissMeditec AG). Since the femtosecond is not coupled with aregistration system, patient fixation throughout the procedure isextremely important. The specific advantages of the SmILEprocedure include probably less dry eye and better biomechanicaleffects as no flap is created. It is considered safer than LASIK/ReLEX for people in the armed forces and those playing contactsports as there is no risk of displacement of the flap.

LENS-BASED REFRACTIVE SURGERIESLens-based refractive surgeries are indicated mainly for patientswith marked myopia who may not be suitable candidates forcorneal ablation procedures.

FIG 1. Intraoperative photographs of a femtosecond LASIK: A. Suction ring applied to the eye tostabilize the globe; B. Femtosecond flap making in progress in the following sequence canal (*)followed by hinge (red arrow) followed by the stromal bed cut (black arrow). The homogeneouswhitish appearance of the cornea underneath the flap is due to formation of gas bubbles as aresult of photodisruption; C. Flap being elevated using Siebels flap elevator; D. Excimerstromal ablation in progress; note the flap and hinge being protected with a flap guard.

207GUNASEKARAN et al. : REFRACTIVE SURGERY FOR MYOPIA: CASE SELECTION AND MANAGEMENT OPTIONS

1. Phakic intraocular lens (IOL) is the most commonly performedsurgery wherein the patient’s crystalline lens is retained andanother IOL corresponding to the amount of refractive error isimplanted in the eye either in the angle, fixed to the iris or inthe sulcus just above the anterior lens capsule. The naturalcrystalline lens is left behind compared to cataract surgerywhere it is extracted and replaced with an IOL.

2. Refractive lens exchange: Clear lens extraction withimplantation of a multifocal IOL/accommodative IOL. It is notdone in pre-presbyopic myopic patients in whomaccommodation is still retained. This procedure has a high riskof retinal detachment (0%–8%) and hence is rarely done.22

The currently available phakic IOL models classified on thebasis of their site of placement are as follows:

Angle-supported phakic IOLs

1. PMMA IOLs: These require a larger incision and may causesurgically induced astigmatism. These models are no longerin use.

2. Foldable angle-supported IOLsa. Acrysof Cachetb. Kelman Duet

Iris fixated anterior chamber IOLs1. PMMA iris claw IOL: Veriseye/Artisan2. Foldable iris claw IOL: Veriflex/Artiflex

Posterior chamber phakic IOLs1. Implantable collamer lens (ICL)2. Phakic refractive lens (PRL)

Of all the designs described, ICL is the most commonly usedbecause of ease of implantation and established safety profile.

ICL descriptionThe ICL (STAAR Surgical Co, Monrovia, California) is bio-compatible, made from a combination of copolymer (pHEMA,

water, benzophenone) and collagen; hence, called Collamer. It isa foldable lens which can be inserted into the eye through a 3.2 mmincision and has a plate haptic design with the haptics beingtucked behind the iris to be placed in the ciliary sulcus (Fig. 2). Itis available in the following range of powers: –18D myopia, +10Dhyperopia and +6D cylinder. The latest design of ICL version V4chas a central hole of 0.3 mm which allows the flow of aqueousfrom the posterior to anterior chamber obviating the need of alaser iridotomy before the surgery.

Patient selection for phakic IOLs

The preoperative work-up is done as described for a cornealrefractive procedure. The specific investigations required are thespecular count of the corneal endothelium as patients with cellcount <2500 cells/mm2 are contraindicated for the procedure. Thepatient’s anterior chamber depth must be adequate (>2.8 mm) andthe white-to-white diameter measured on Orbscan or digitalcaliper should be symmetrical in both eyes.

OUTCOMES OF KERATO-REFRACTIVE SURGERIESThe results of kerato-refractive surgeries are gauged by theirefficacy, stability and safety.

• The efficacy of the procedure is ascertained by the proportionof patients achieving postoperative uncorrected visual acuity(UCVA) of 6/6 or better, and the proportion of eyes withpostoperative manifest refractive spherical equivalent (MRSE)within ±0.5D or ±1.0D of the intended refractive correction.

• Stability is an important parameter to assess as epithelial andstromal tissue remodelling is known to cause regression of therefractive effect. It is measured by determining the meanchange in MRSE over a defined time interval.

• Safety is measured by the percentage of eyes with postoperativeloss of two or more lines of best corrected visual acuity(BCVA) on the Snellen chart and the incidence of surgicalcomplications.

Outcomes of PRK and LASIKEfficacy. Most studies of PRK have reported an efficacy of

70% of patients achieving MRSE within ±1D.23–25 A postoperativeUCVA of 6/12 or better was achieved by more than 80% ofpatients in these studies but outcomes were lower in patients withhigher myopia. A Cochrane review reported no significantdifferences in efficacy outcomes between PRK and LASIK or forlow-to-moderate myopia versus high myopia subgroup.26

Stability. The amount of myopic regression postoperatively ismore frequent and severe with high myopic eyes.24 Post-LASIKMRSE gets stabilized within 3–6 months.27

Safety. In PRK, around 3.2% of eyes lost two or more lines ofBCVA. The mean percentage reported for LASIK is 1.0%–2.3%.28 Post-PRK subepithelial haze peaks at 3–6 monthspostoperatively and clears by 12 months. Comparing LASIKversus PRK with regard to safety, the percentage of eyes losingtwo or more lines of BCVA were significantly lower in LASIKstudies than in PRK studies indicating that LASIK had superiorsafety.23 A Cochrane review found no significant difference insafety between the two techniques.26

Femtosecond LASIK versus microkeratome LASIK

A recent meta-analysis by Zhang et al.29 analysed variousrandomized controlled trials and individual studies30–33 comparingthe femtosecond technology with mechanical microkeratomes.The refractive outcomes were comparable as were the safety

TABLE II. Complications of LASIK14–20

Intraoperative• Flap-related: tear, buttonhole, free cap, eccentric flap• Epithelial defect at flap edge• Suction loss• Decentered ablation• Femtosecond-related: air bubble in anterior chamber, persistent opaque

bubble layer (OBL)

Early postoperative• Dry eyes• Diffuse lamellar keratitits• Pressure-induced stromal keratopathy• Infectious keratitis• Flap striae• Under- or over-correction• Transient light sensitivity syndrome: with femtosecond LASIK

Late postoperative• Dry eyes• Night vision problems: glare, haloes• Epithelial ingrowth• Keratectasia• Regression• Loss of best corrected visual acuity (BSCVA)

LASIK laser-assisted stromal in situ keratomileusis

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profile except that diffuse lamellar keratitis was more likely in thefemtosecond group and the flap buttonhole more in themicrokeratome group. Total higher order aberrations and sphericalaberrations were more common in the microkeratome groupowing to the meniscus-shaped flap resulting in peripheralsteepening and central flattening with the induced sphericalaberration.

Wavefront-guided LASIK versus conventional LASIK

Though the wavefront-guided treatment seemed to offer advantagesover other profiles theoretically, studies34 show that higher-orderaberrations are increased after the procedure; however, less higher-order aberrations occur after the conventional LASIK. In anotherstudy,35 there was no significant difference between wavefront-optimized and wavefront-guided treatment profiles and it wasrecommended that the wavefront-guided profile be reserved forthose with higher-order aberration profiles of >0.3 µ RMS errorpreoperatively.

Low-to-moderate myopia versus high myopiaThe results of trials approved by the US Food and DrugAdministration for low-to-moderate myopia versus high myopiaare given in Table III.36 The efficacy of LASIK in high myopes isevidently less compared with low-to-moderate myopes36 and hencealternative surgical options are usually provided to these patients.

Quality of life after LASIKThe overall patient satisfaction rate after primary LASIK surgeryis reported to be around 95%.37

TABLE III. Outcomes of low-to-moderate myopia versus high myopia36

Parameter Low-to-moderate High myopiamyopia (<6D) (>6D)

MRSE within ±1D 96% 80%MRSE within ±0.5D 81% 61%UCVA >20/40 96% 89%UCVA >20/20 72% 48%

MRSE manifest refractive spherical equivalent UCVA uncorrected visual acuity

OUTCOMES OF LENS-BASED REFRACTIVE PROCEDURESSince ICL implantation is the most common procedure amongphakic IOLs, we have discussed only its safety and efficacy.Numerous studies have proved the efficacy of ICL with the meanpostoperative SE range from 0.02 to –2 and 80%–100% ofpatients within ±1D and 60%–100% patient having UCVA of>0.5%.38 Common complications include anterior subcapsularcataract formation, pupillary block glaucoma and pigmentdispersion. A patent iridotomy or centraflow technology (in ICLV4c) and an adequate postoperative vault (distance between theposterior surface of ICL and anterior lens capsule) of 500–750 µprevents most of these complications. Studies comparing theprevious versions of ICL with V4c show good and comparableoptical quality with safe intraoperative control.39,40

CONCLUSIONThe surgical options for treatment of myopia appear to be safe andeffective subject to the amount of refractive error and cornealthickness, in the absence of any other pre-existing ocular illness.

FIG 2. Steps in the implantation of a collamer lens: A. Loading of the lens in the cartridge; thearrow mark denotes the thin lens and the soft foam tip being used to align it well in the centreof the cartridge; B. Lens being inserted into the eye through a 3.2 mm corneal incision;C. Tucking of the haptics under the iris; D. Postoperative clinical photograph of a patient withICL (V4c) in situ; the central hole in the lens is clearly seen.

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A thorough preoperative screening and counselling regardingpatient expectations can improve outcomes and minimizecomplications.

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