Laser refractive surgery

INTRODUCTION — Laser refractive surgery has long been a popular alternative to traditional spectacles and contact lenses for vision correction. This review will briefly discuss the anatomy and principles behind visual problems and correction and will provide an overview of refractive surgical procedures.

ANATOMY OF THE CORNEA — The transparent cornea is about one-half millimeter thick and has five distinct layers. The epithelium is the most exterior layer providing the smooth refractive surface and barrier against infection. The function of Bowman's membrane, which lies beneath the epithelium and its basement membrane, is unclear. The stroma, made up of intertwining lamellae of collagen fibrils, provides structure and accounts for 90 percent of the corneal thickness. The endothelial basement membrane (Descemet's membrane) and the endothelium form the innermost layers. Endothelial cells, via an active sodium-potassium-ATPase pump, are responsible for the natural corneal dehydration necessary for corneal clarity.

REFRACTIVE ERROR — Refraction is the bending of light rays as they pass from one transparent medium to another medium of a different density. Refraction is measured in diopters (D). The refractive power of a lens is the reciprocal of its focal length in meters. (See "Visual impairment in adults: Refractive disorders and presbyopia".)

In emmetropia (an eye with normal vision), the focusing power of the cornea and lens is perfectly matched to the length of the eye. Parallel light rays from a distant object are brought into focus precisely on the retina and a clear image is perceived (figure 1).

Refractive errors occur when light rays entering the eye do not focus properly on the retina:

●In myopia (nearsightedness), the most common type of refractive error, the cornea is too curved or the lens is too powerful for the length of the globe. Distant objects cannot be seen clearly because light rays are focused in front of the retina (figure 1).

●In hyperopia (farsightedness), the cornea and lens are too weak for the length of the globe. As a result, light rays reach the retina before they are focused to a single point (figure 1). A distant object may be brought into focus by using accommodation, but clear near vision is difficult.

●With an astigmatism, the refractive power of the eye is different in different meridians. Light rays can never be brought to a single point and objects will appear blurry at any distance. Astigmatism may occur with myopia or hyperopia (figure 2).

Presbyopia is a different type of refractive error in which loss of accommodation occurs as the lens hardens with age. This results in an inability to bring a near object into focus on the retina and requires the use of reading glasses, typically in patients in their 40s. Presbyopia is not corrected by laser refractive surgery, and the surgery may in fact hasten the development of presbyopia. (See 'Presbyopia' below.)

CORRECTIVE LENSES — Corrective lenses are the traditional method of treating refractive errors. Myopia is treated with concave lenses with minus or divergent power to focus light rays on the retina. Convex lenses with plus or convergent power help correct vision in the hyperopic eye, and cylindrical lenses are used to neutralize astigmatism.

Spectacle prescriptions are used to correct the sphere and cylinder components of refractive errors. The first part of the prescription is the sphere. A plus number indicates hyperopia and a minus number indicates myopia. The second number of the prescription is the cylinder, otherwise known as the astigmatism, either written in a plus or minus. The third number is the axis of the astigmatism. Spectacle prescriptions are then written in increments of 0.25 D.

As an example, a hyperopic prescription may read +3.00 – 1.50 x 180 and a myopic prescription may read – 4.50 + 2.50 x 090. When treating patients with refractive surgery, the surgeon may also consider the spherical equivalent of a spectacle correction. The spherical equivalent is one-half the cylinder added to the sphere. As an example, the spherical equivalent of +3.00 – 1.50 x 180 = +2.25 by taking one-half of –1.50 which equals –0.75 and adding this number to +3.00.

WAVEFRONT TESTING — In a standard eye examination, the refractive surgeon will test for myopia, hyperopia, and astigmatism. However, patients may have irregular astigmatism defined as higher-order aberrations eg, coma or spherical aberrations. These higher-order optical aberrations can have significant impact on vision. In the past, the ophthalmologist had no way to correct a patient's irregular astigmatism. Spectacles only correct lower-order aberrations such as sphere and cylinder.

Objective techniques exist for a comprehensive measurement of the optics of the eye. The science of wavefront aberrometry is based upon the shape of the wavefront of light reflected from the eye. A beam of light is reflected from the eye and goes through a micro-lens array, producing a spot image array of reflected light. A computer analysis determines the relative displacement of each spot image. The images are then computed to give the local slope and character of the wavefront of light. The analyzed wavefront is then used to derive a correction profile to remove the correct amount of corneal stroma in micron intervals using a guided laser.

The information obtained from wavefront technology enables the refractive surgeon to reduce the natural and surgically induced higher-order aberrations [1,2]. The refractive surgery patient may benefit from the correction of higher-order aberrations by improving best spectacle corrected visual acuity (BSCVA), night vision, contrast sensitivity [3], and reducing glare and halos.

THE EXCIMER LASER — The excimer laser emits an ultraviolet beam that has sufficient energy to break intermolecular bonds within the cornea ("photoablation"). Because there is little or no thermal damage to adjacent tissue, this is often referred to as a "cool laser beam." A computer programmed with the patient's refraction and corneal topography controls the laser beam to precisely remove corneal tissue [4].

The first excimer lasers were approved by the US Food and Drug Administration (FDA) in 1995 and used a broad beam of 4 to 5 mm in width. With improving technology, the width of the laser beam has continued to decrease to less than 100 microns. Some excimer lasers have ablation programs which use both broad beam laser to remove large amounts of corneal stroma and smaller beam laser to remove small amounts of tissue in the same treatment. Each laser pulse removes a discrete volume of corneal stroma. The cumulative ablation is achieved by partial overlap of many laser shots. The greatest corneal stroma removal occurs where laser shot density is the highest. The constantly moving smaller laser beam diameter enables the treatment to avoid revisiting the same site on the cornea for a finite period of time.

In addition, laser eye tracking systems are now available that allow precision corneal ablation during eye movements [5]. The eye can move during laser treatment of the corneal surface, thereby causing a less than desirable ablation. Eye movements such as saccades, high-frequency tremors, and irregular movements can affect the refractive outcome. The speed of the eye tracking systems incorporated in some lasers allows the laser to follow saccadic and involuntary eye movements so that ablation of the cornea can continue while the eye moves, ensuring accurate pulse placement.

In myopia, the laser flattens the central cornea to decrease its focusing power. In hyperopia, the laser indirectly steepens the central cornea by removing tissue from the periphery, thus increasing its focusing power. Astigmatism is treated with an elliptical or cylindrical beam that flattens the steepest corneal meridian.

SURGICAL TECHNIQUES — The majority of refractive surgical procedures can be divided into five broad categories:

●Lamellar procedures: laser-assisted in situ keratomileusis (LASIK), sub-Bowman keratomileusis (SBK)

●Surface ablation procedures: photorefractive keratectomy (PRK), laser epithelial keratomileusis (LASEK), and epithelial laser keratomileusis (epi-LASIK)

●Incisional procedures: radial keratotomy

●Intracorneal procedures: intracorneal ring or corneal inlays

●Intraocular procedures: anterior or posterior chamber intraocular lens.

There do not appear to be significant differences in visual outcomes or quality among most surgical techniques (see 'Outcomes' below). LASIK, PRK, and LASEK are most commonly performed (figure 3) and are discussed in detail below.

Laser-assisted in situ keratomileusis — LASIK, now the most commonly performed refractive surgery, is an effective treatment for low, moderate, and high myopia with and without astigmatism, as well as hyperopia with and without astigmatism [6-12]. LASIK is an outpatient surgery performed with topical anesthesia. A microkeratome, which works like a carpenter's plane, is used to raise a corneal flap about the size of a contact lens. This flap usually averages 160 microns thick and is folded back to expose the underlying stroma [9]. In a refinement of the procedure, a femtosecond laser is used to create the flap without bladed instruments. This method of flap creation is safe and reliable, with more predictable flap dimensions than microkeratome LASIK.

The excimer laser is used to ablate a precise amount of corneal stroma and the flap is irrigated and replaced. The cap is stabilized without sutures by the natural corneal dehydration created by the endothelial pump. Flap stability and adherence to the corneal stroma are checked after surgery, and patients are usually sent home on topical steroid, topical antibiotic, and topical nonsteroidal drops. The patient also is instructed to use an eye shield overnight, with follow-up typically scheduled on postoperative day one and then at one week. The patient is usually seen again at one, three, and six months [13].

LASIK has significant attractions for the patient. It causes little pain, provides quick recovery of vision, and has the potential for treating higher levels of myopia [10]. LASIK has also been found to be safe and effective to treat both eyes on the same day [14]. LASIK produces less stromal haze than PRK and does not require continuous steroid therapy.

Ten-year outcomes for patients with moderate myopia, in a retrospective (nonrandomized) study that compared PRK and LASIK in patients with similar baseline visual acuity, were slightly more favorable for LASIK, with a lower retreatment rate and similar visual acuity [15]. The authors note, however, that there have been subsequent technical improvements that might improve results for surface ablation procedures.

Sub-Bowman keratomileusis — SBK is a modification of LASIK that involves creation of a thin (90 to 100 micron) flap in the corneal stroma, at or just beneath the level of Bowman's membrane. It is proposed as a hybrid between LASIK and surface ablations (PRK, LASEK, or Epi-LASIK) and is made possible by the predictability and accuracy with which the femtosecond laser can create corneal flaps.

The procedure has the advantages in the immediate postoperative period of decreased pain and better vision when compared with surface ablations. Visual acuity is also superior at one month, but by three months and beyond there is no significant difference in visual outcomes between SBK and surface ablations [16-18]. The primary advantage over LASIK is improved biomechanical stability of the cornea, reduced risk of corneal ectasia (iatrogenic keratoconus) [16,19], and perhaps less risk of dry eye.

Femtosecond lenticule extraction and small incision lenticule extraction — Femtosecond lenticule extraction (FLEx) is a procedure first reported in 2008 that can achieve the refractive results solely with a femtosecond laser, without the use of the excimer laser. In FLEx, both the flap (similar to but smaller in diameter to that created in LASIK) and a refractive lenticule are created in a one-step procedure using a femtosecond laser. After removing the lenticule, the flap is repositioned. The procedure was later modified to eliminate the flap and instead remove the lenticule through a small incision. This modification is called small incision lenticule extraction (SMILE). SMILE is performed using the femtosecond laser. The early outcomes of SMILE were first reported in 2011 [20]. SMILE became commercially available in 2012 and the US Food and Drug Administration (FDA) approved its use in the United States in September 2016.

In a review of postoperative outcomes in the initial cohort of SMILE patients, mean spherical equivalent at five years postoperative was -0.375 diopter, remaining close to target refraction (emmetropia); 48.2 percent of eyes were within 0.5 diopter, and 78.6 percent were within 1.0 diopter [21]. Long-term regression after five years was 0.48 diopter compared with six months postoperatively. There was no loss of two or more lines of corrected distance visual acuity over the five-year period. Although dry eye was common in the early healing period following SMILE, no patient needed further treatment after three months or reported significant dry eye at the five-year follow-up. Furthermore, corneal ectasia, cataract formation, or other ocular pathology were not reported. Thus, SMILE appears to be a safe and effective alternative to LASIK and PRK.

Photorefractive keratectomy — PRK effectively treats low to moderate myopia, myopia with astigmatism, and low to moderate hyperopia without astigmatism. PRK is performed as an outpatient procedure using topical anesthetic. First, the corneal epithelium in the ablation zone is removed or pushed to the side to allow a more accurate ablation of the corneal tissue. The laser treatment is then applied to the exposed corneal stroma. Immediately after the laser is completed, the ophthalmologist applies topical antibiotic, topical steroid, and a topical nonsteroidal antiinflammatory drug (NSAID). Then a disposable bandage contact lens is placed over the operated cornea.

In the early postoperative period, patients may have significant tearing, photophobia, blurred vision, and discomfort because of the central corneal abrasion. With the use of the bandage contact lens and topical nonsteroidal antiinflammatory medications, postoperative pain is usually mild to moderate; however, patients occasionally require systemic analgesia for more severe pain.

The contact lens remains in the eye until the epithelial defect is healed, averaging three to four days. Antibiotics are usually continued for two to three days after the defect has healed, and topical corticosteroids may be continued for up to three months postoperatively. Visual acuity improves once the epithelial defect heals, usually within one week postoperatively, and typically fluctuates for several months after the surgery before stabilizing at around three months. Glare, halos, and dry eye symptoms are common in the first month following surgery but usually diminish or disappear entirely by three to six months.

Laser epithelial keratomileusis — In LASEK, preincision of the corneal epithelium is performed using a special microkeratome with a 70 micron depth calibrated blade. The trephine leaves a hinge at the 12 o'clock position, and a few drops of 20 percent alcohol solution are instilled inside the marker and left on the cornea for approximately 30 seconds. The treated area is then washed with water and dried. Next, the excimer laser treats the underlying stroma just like a LASIK treatment [22].

A meta-analysis of 12 randomized trials comparing outcomes between LASEK and PRK in patients with myopia found no difference in visual acuity at any time point after surgery (up to two years), healing time of corneal epithelium, or postoperative pain [23]. However, patients undergoing LASEK had significantly less corneal haze at one and three months after surgery. Another systematic review including 11 randomized trials was unable to reach any conclusions in the difference between LASEK and PRK outcomes for eyes with low to moderate myopia [24]. Because of differences in outcomes measures and follow-up times in the various trials, the review was unable to pool data for meta-analysis.

A 2017 systematic review of four randomized controlled trials concluded that the relative effectiveness of LASEK compared with LASIK in improving refractive error and vision in mildly to moderately myopic patients was uncertain [25].

The benefit of LASEK over LASIK is that the flap is very thin. Having a thin corneal flap avoids the complication of corneal ectasia, which may result when the residual corneal thickness after treatment is less than 250 microns [26]. Without the proper stromal support, the cornea bows anteriorly (ectasia), causing permanent changes in the patient's refraction. Apart from the avoidance of corneal ectasia, LASEK offers other advantages compared with LASIK. LASIK requires the use of more complicated equipment with the risks of higher intraoperative surgical flap complications [27]. The potential risks of infection are also theoretically easier to manage and treat with the relatively superficial flap.

Patients who risk trauma to the cornea postoperatively during recreational sports or professional work also benefit from the LASEK procedure. A dislocated or lost LASIK flap may cause a permanent decrease in vision. By contrast, a LASEK flap complication will only cause a large epithelial defect. LASEK also lowers the risks of diffuse lamellar keratitis because the flap is not located in the stroma.

However, LASEK has some disadvantages compared with LASIK. Due to the thinness of the epithelial flap, it may dislodge postoperatively, causing greater pain. Another disadvantage is the slower optical recovery time compared with LASIK [28]. LASEK also can cause stromal haze in treated high myopic patients similar to PRK.

Epi-LASIK procedure — Epi-LASIK, a hybrid technique, is a newer procedure [29,30]. Rather than using alcohol solution to separate the epithelium from the underlying stroma, Epi-LASIK utilizes an epikeratome, which is a mechanized blunt blade on a handpiece similar to the LASIK microkeratome. The laser ablation is then performed on the surface and the epithelial flap is retained, similar to LASEK.

It has been suggested that this results in less postoperative pain and haze when compared with other surface ablation procedures, including LASEK. An additional benefit of the procedure may be that there is less damage to the underlying cell integrity when compared with alcohol-treated eyes.

Additional techniques

Mitomycin — Mitomycin has become a useful adjunct to a variety of ophthalmic surgical procedures [31,32]. It was first used in glaucoma filtering surgery to reduce the risk of scarring of the filtering bleb, thereby improving the success of the surgical intervention. More recently, interest has turned to its use as a dilute solution in the prevention of haze in post-refractive surgical patients.

The primary use has been in conjunction with PRK, but it has also been expanded to LASEK as well as post-LASIK complications. Postoperative corneal haze is not well understood but is most likely a complex multifactorial wound healing response, where activation of stromal keratocytes and deposition of ground substance and abnormal collagen may result in haze.

Mitomycin modulates wound healing by increasing keratocyte apoptosis and reducing keratocyte reactivation. Although preliminary studies suggest efficacy and safety in dilute applications, the long-term effects associated with mitomycin have not been well defined.

Phakic intraocular lenses — Phakic intraocular lenses are a surgical option for higher degrees of myopia, with or without astigmatism, where the use of excimer laser surgery has been limited by lack of predictability, regression, corneal ectasia, and reduced quality of vision postoperatively [33-35].

In a 2014 systematic review including three randomized trials of patients with moderate to severe myopia (-6 to -20 D), phakic intraocular lenses were associated with fewer adverse effects and similarly effective for visual acuity compared with excimer laser surgical correction [36].

The use of a phakic intraocular lens has been shown to preserve the corneal asphericity, thus eliminating postoperative corneal ectasia. Phakic lenses also provide a better postoperative quality of vision (improved best corrected visual acuity, improved contrast sensitivity, etc) [37].

Phakic intraocular lenses may be classified as either anterior chamber or posterior chamber. The former may be further subdivided into angle- versus iris-supported. The angle-supported anterior chamber phakic intraocular lenses have fallen out of favor because of their higher association of complications such as pupillary ovalization, endothelial cell loss, and decentration. The iris-supported anterior chamber and posterior sulcus-supported intraocular lens are the leading techniques utilized.

A significant downside to posterior chamber phakic intraocular lenses is the risk or pupillary block glaucoma. To mitigate this risk, one or more laser peripheral iridotomies are required preoperatively in each eye before implantation of the lens.

Despite the increase in their popularity, there are still several complications, such as endothelial cell loss, chronic or increased intraocular inflammation, pupillary ovalization, pupillary block glaucoma, cataract formation, intraocular lens dislocation, and retinal detachment, which warrant consideration and may limit their use. Nevertheless, advances in technology (eg, anterior segment imaging technology to properly size the intraocular lens) appear to be decreasing the complication rates significantly. Moreover, newer intraocular lens designs reduce the surgical incision size and reduce risk of damage to intraocular structures.

In March 2022, the FDA approved the EVO Visian ICL and Toric Visian ICL for treatment of myopia and myopia with astigmatism. This modification of the existing Visian ICL has a central port in the optic that allows flow of aqueous through the lens; therefore, the laser peripheral iridotomies are no longer needed. During the FDA clinical trial, no eye experienced pupillary block or required peripheral iridotomy [38,39].

With the technological advances in refractive surgical procedures, we expect to see an increase in the number of phakic intraocular lenses being implanted with excellent reliability, predictability, and safety.

Bioptics — Bioptics is a refractive technique that combines more than one refractive surgical procedure, typically phakic intraocular lens implantation with excimer laser refractive surgery. It has also been used for clear lens extraction followed by excimer laser refractive surgery.

This appears to be an encouraging technique (or combination of techniques) for the management of extremely high levels of ametropia. The results are showing good levels of predictability, reliability, stability, and safety.

PATIENT SELECTION — Not every patient is a candidate for excimer laser treatment. Age, high refractive error, and underlying ocular disease may prevent a patient from obtaining a predictable refractive outcome. A study of laser-assisted in situ keratomileusis (LASIK) surgery for presbyopia compared patients aged 60 to 69 years with patients 40 to 49 years, demonstrating a trend toward higher retreatment rates and more myopia post-procedure in the older age group [40]. However, the procedure was comparably safe for older patients and outcomes at one year were comparable for both age groups.

Proper patient selection is critical for a successful surgery (table 1). The patient's eye must be healthy and the refractive error must be stable over a one-year period of time. The surgeon must select the correct surgical procedure based upon the patient's expectations and ophthalmologic examination with an accurate refraction. After the surgery is scheduled, the surgeon will perform a detailed and accurate surgical plan to include verification on the refraction prior to programming the laser. The surgical plan also includes the detection of complicating factors to maximize the results while minimizing the risks.

Contraindications — There are systemic and ocular contraindications to refractive surgery. Autoimmune, collagen vascular, and immunodeficiency diseases all affect corneal healing. Women who are pregnant or nursing have fluctuating visual acuity due to refractive changes of the eye secondary to corneal hydration. Patients with abnormal wound healing such as keloid or abnormal scar formation may have abnormal corneal healing. Systemic medications such as oral isotretinoin can aggravate dry eye symptoms, while amiodarone can leave transient corneal epithelial deposits.

While labeling from the US Food and Drug Administration (FDA) includes a warning against laser refractive surgery in patients with diabetes mellitus, a literature review has found that LASIK and photorefractive keratectomy (PRK) can be performed without complications in patients with well-controlled diabetes who are without cataract, diabetic retinopathy, or systemic complications of diabetes [41].

Ocular contraindications include severe dry eye from keratoconjunctivitis sicca, exposure keratitis, neurotrophic keratitis, and lid disorders affecting the tear layer. A patient with a history of herpetic keratitis is at risk of virus reactivation and corneal scarring following laser refractive surgery. In addition, any patient with an abnormally shaped cornea such as keratoconus, pellucid marginal degeneration, or keratoglobus may worsen after refractive correction and will not have a predictable refractive outcome.

Patients also may present with anatomic problems that prevent the proper placement of the microkeratome such as deep-set eyes, very narrow palpebral fissures, abnormal lid position, or severe acne rosacea.

PREOPERATIVE EVALUATION — The initial clinical workup includes a complete medical and surgical history. The ophthalmologist reviews the general medical history, particularly looking for a diagnosis of diabetes mellitus, collagen vascular diseases, and immunocompromise of any etiology. The surgeon must also have a detailed ocular history before performing refractive surgery, paying special attention to previous ocular surgery as well as conditions such as glaucoma, strabismus, amblyopia, and dry eye syndrome.

Contact lenses — Prior to refractive surgery, patients need to remove their contact lenses, which can temporarily change the shape of the cornea. Soft contact lenses should be discontinued at least two weeks before the preoperative evaluation and the surgical procedure. Rigid gas permeable (RGP) lenses should be discontinued at least three weeks prior to the preoperative evaluation and the surgical procedure. In addition, the patient must demonstrate a stable keratometry and refraction [42].

Eye examination — The initial patient visit includes a testing of uncorrected visual acuity (UCVA) and a measure of the current spectacle prescription with best spectacle corrected visual acuity (BSCVA). The patient's refractive error must remain stable for one year, defined as no more than a 0.50 D change in either the manifest cylinder or manifest spherical equivalent. The ophthalmologists will review all previous eyeglass prescriptions and prior medical records and measure the current spectacle prescription on the initial preoperative evaluation.

The initial evaluation will include a manifest refraction with BSCVA based on the least minus prescription needed to read the smallest line. The manifest refraction is repeated at least once to confirm the results. The laser treatment is then based on the manifest spherical equivalent and manifest cylinder. The initial visit requires a cycloplegic refraction to paralyze the eye's ability to accommodate. The patient will return on another day for a repeat manifest refraction if there is a greater than –0.50 D difference between manifest and cycloplegic refraction.

●Slit-lamp examination – A slit-lamp examination evaluates the lids for evidence of blepharitis, meibomian gland dysfunction or infection, and the use of cosmetic makeup. The conjunctiva and sclera may reveal injection from chronic irritation or abnormal pooling of the tear film. The cornea may show superficial punctate keratopathy indicative of dry eye syndrome, epithelial irregularities such as epithelial basement membrane dystrophies, scarring from prior trauma, or vascularization into the cornea from previous disease. The cornea may show signs of Fuchs endothelial dystrophy, which can lead to corneal decompensation and poor flap adhesion. The anterior chamber may show signs of inflammation from chronic uveitic processes. The iris may have abnormalities and the lens may have cataractous changes, which will affect the postoperative visual acuity.

●Pupil size – Pupil size evaluation is determined in the initial refractive surgery screening. The pupil is measured with a device called a pupillometer that measures the pupil using infrared light in a dark room. Large pupils are associated with a higher risk of postoperative glare, halos, and night-vision difficulties [43,44].

A typical myopic ablation has an optical zone of 6.0 mm; patients with a pupil size greater than 7.0 mm in dim illumination are at the highest risk for these postoperative complaints. The goal of the ablation is to treat the cornea overlying the pupil in the dark in order to prevent light from entering the eye at the edge of the treated and non-treated cornea [45].

●Dry eye evaluation – The risk of dry eye symptoms increases after laser refractive surgery [46]. Thus, the ophthalmologist must rule out a diagnosis of dry eye before performing surgery. A patient with dry eye syndrome will complain of intermittent blurry vision secondary to poor tear film; sharp, intermittent pain; or foreign body sensation.

The examination for dry eyes includes a basal tear secretion test using Whatman #41 filter paper, 5 mm wide and 35 mm long, placed at the lateral one-third of the lower eyelid with one drop of topical anesthesia. After waiting five minutes, the amount of wetting on the strip is measured; any amount greater than 10 mm is considered normal. The tear breakup time (TBUT) evaluates the oil secretions of the meibomian glands. The tear film must remain intact for greater than 10 seconds. Rose bengal stain is also used to stain conjunctival epithelial cells that are inadequately protected by a mucin tear coating secondary to chronic inflammation or dry eye syndrome.

●Intraocular pressure – The screening examination also includes intraocular pressure (IOP) measurement. An applanation tonometer or pneumotonometer is used to screen patients and document a baseline IOP. Postoperatively, the patient's cornea will be thinner, creating a falsely lower IOP when performing any applanation tonometry. (See 'Glaucoma evaluation and management' below.)

The postoperative photorefractive keratectomy (PRK) patient may develop elevated IOP secondary to topical corticosteroid use. The laser-assisted in situ keratomileusis (LASIK) candidate with high IOP may have undiagnosed glaucoma.

The suction required to hold the cornea during creation of the microkeratome flap during LASIK can elevate the IOP to 65 to 70 mmHg. Thus, any patient with a questionable diagnosis of glaucoma will need a preoperative visual field test.

●Fundoscopic evaluation – A dilated fundus examination may uncover other eye diseases or causes of decreased visual acuity. The lens may have cataract changes, which can prevent optimal results with any laser refractive surgery. Patients with symptomatic cataracts may need a cataract surgery evaluation instead. The patient's vitreous may have opacities such as posterior vitreous detachments that cannot be corrected by refractive surgery. The optic nerve may show signs of atrophy or glaucoma; this will prevent patients from seeing better postoperatively. The refractive surgery candidates may have macular diseases such as macular degeneration or inherited retinal dystrophies. The peripheral retina may have signs of lattice degeneration, retinal tears, retinal holes, or retinal breaks needing further evaluation and treatment prior to undergoing refractive surgery.

●Evaluation of the cornea – The reshaping of the cornea is the fundamental principle of laser refractive surgery. The ophthalmologist must calculate certain corneal measurements. The steepness or flatness of the cornea is defined by keratometry. The shape and contour of the cornea is measured using topography or videokeratography. The keratometry and topography will rule out corneal diseases such as keratoconus or posterior corneal ectasia. The thickness is measured by a pachymeter. Pachymetry is useful for identifying unusually thin corneas and calculating the depth of the ablation.

Glaucoma evaluation and management — Ophthalmologists have become increasingly aware of the role corneal thickness plays in the evaluation and management of glaucoma. Many clinicians managing glaucoma have incorporated corneal pachymetry into their practices. As such, it is important to record corneal thickness before and after refractive surgery, including an estimate or measurement of the amount of tissue ablated during treatment. (See "Open-angle glaucoma: Epidemiology, clinical presentation, and diagnosis", section on 'Pachymetry'.)

It is also important to record intraocular pressure before and after refractive surgery, since corneal thinning will falsely lower the results of applanation tonometry. Nomograms that adjust intraocular pressure for corneal thickness will also aid in this regard.

RISKS — Complications may arise from errors in the planned ablation, intraoperative mechanical factors, postoperative medications, and wound healing (table 2) [47]. The most common subjective complaints in one laser-assisted in situ keratomileusis (LASIK) study were night driving difficulty and glare [48].

Visual acuity — Under- and overcorrections can occur if the excimer laser removes an incorrect amount of corneal stroma. Undercorrections occur more frequently with increasing amounts of myopia. An undercorrection for a myopic patient would leave residual stroma, thereby leaving the patient myopic postoperatively. Enhancements are usually performed within the first year after the original procedure to remove additional corneal stroma and correct for postoperative refractive error [49]. In a prospective study, LASIK retreatment was required in 10.7 percent of eyes [48].

Overcorrections occur when too much corneal stroma is removed and are harder to treat.

Regression — Regression of the postoperative refraction toward the preoperative refractive error can occur over time [50]. Regression may occur due to discontinuation of topical corticosteroids, abnormal wound healing, and pregnancy or other hormonal imbalances.

Visual loss — There is a 0 to 3.6 percent loss of two or more lines of best spectacle corrected visual acuity (BSCVA) reported with laser refractive surgery [48,51-53]. As an example, a patient who sees 20/20 with glasses will have a small chance of not seeing any better than 20/40 with glasses following refractive surgery. There are rare case reports of visual loss and corneal perforations secondary to laser ablation through the corneal stroma or from microkeratome complications [54].

Presbyopia — Patients who are approaching the age at which presbyopia occurs or who are already presbyopic should be educated that correction of a refractive error for clear distance vision may result in an inability to focus and read near objects, which they were able to do before surgery by removing their spectacles [55]. Some patients may be able to avoid this problem by undergoing "monovision" correction, whereby one eye is surgically undercorrected for distance, so that the patient is left with one eye for distance and the other eye for near vision [56]. However, not all patients can tolerate monovision. In one series, 35 percent of patients initially treated with monovision LASIK underwent a subsequent procedure to enhance the other eye [57]. Patients can undergo a trial of monovision with contact lenses prior to refractive surgery.

Dry eyes — Dry eyes are very common following laser refractive surgery [46,58]. During creation of the LASIK flap, the superficial corneal nerves are cut with the microkeratome. The corneal nerves will reinnervate the corneal stroma over a six-month period [46]. The few months post-LASIK may require a patient to use nonpreserved artificial tears. In moderate to severe dry eye, the ophthalmologist may place temporary or permanent punctual plugs into the eyelid punctae to prevent tears from being pumped into the lacrimal system [59]. Patients may experience a foreign body sensation, fluctuating vision, or decreased vision. (See "Dry eye disease".)

Other ocular complications

●Astigmatism – Induced astigmatism occurs with decentered ablations or excessive patient movement during the ablation [60]. Retreatment of postoperative astigmatism can be corrected with glasses, contact lenses, or repeat laser treatments with newer corneal custom ablation profiles [61].

●Irregular astigmatism (central islands) – In some cases, laser treatment or an abnormal healing process may result in astigmatism that is "irregular." This includes "central islands," which are irregular elevated areas in the treated cornea. Such irregularities may result in decreased visual acuity. These "central islands" may not be correctable with laser, and the decrease in visual acuity may not be correctable with glasses or contact lenses [62].

●Glare, halos, diplopia – Glare, halos, and monocular diplopia can occur postoperatively [43,44]; night halos can remain a persistent problem for many years after photorefractive keratectomy (PRK) [63]. The etiology is thought to originate at the interface of the treated and non-treated corneal stroma. Patients with large pupils in dim illumination may develop symptoms as light rays are refracted differently at this junction, thereby causing glare symptoms at night, halos around bright lights, and the perception of double images out of one eye.

The newer-generation excimer lasers have ablation profiles with larger blend zones to minimize the transition of treated and non-treated cornea in an attempt to reduce these symptoms [64].

●Loss of contrast – Some postoperative patients develop a loss of contrast sensitivity following laser refractive surgery [65]. A patient may see Snellen acuity equal to 20/20, but the sharpness and clarity are decreased [66]. Further research to include wavefront testing is ongoing in order to understand the causes of decreased contrast sensitivity.

●Stromal haze – PRK patients also tend to have a higher incidence of stromal haze and scarring, especially when treating myopic patients over –6.0 D [63,67]. The haze peaks between 6 to 12 weeks and then declines.

Epithelial defect — PRK leaves a large epithelial defect postoperatively that causes postoperative pain with delayed corneal epithelial healing. This can cause corneal scarring, corneal melting, and irregular astigmatism.

Since the epithelial barrier to infection is removed, patients also are at higher risk of corneal infections. Infectious keratitis can occur after LASIK and can be vision-threatening. The estimated rate is between 1 in 1000 and 1 in 5000 procedures and can involve bacterial, mycobacterial, or fungal organisms [68].

Flap complications — Creation of the LASIK flap also presents unique risks. During creation of the flap, the microkeratome may create a thin flap, incomplete flap, buttonhole flap, or free flap [27,69,70]. These complications may require the surgeon to stop the procedure and allow the cornea to heal before another attempt at refractive surgery, an average of nine months postoperatively [71]. Postoperatively, the corneal epithelium can grow underneath the flap, requiring another procedure to lift the flap and remove the underlying epithelium [72,73]. The corneal flap may also dislodge spontaneously or with trauma.

Diffuse lamellar keratitis — A sterile inflammatory response without a known etiology called diffuse lamellar keratitis (sands of Sahara syndrome) may occur at the stromal interface beneath the flap. Diffuse lamellar keratitis is treated with topical corticosteroids. Severe cases require the ophthalmologist to lift and irrigate the flap postoperatively [74].

Other keratopathy — Cases have been described of noninflammatory central corneal opacification occurring within two weeks of surgery and regressing 2 to 18 months postprocedure [75]. Lamellar keratitis preceded the corneal opacification in 18 of 19 patients; opacification did not respond to systemic or topical corticosteroids.

OUTCOMES — A 2017 systematic review and meta-analysis of 48 randomized controlled trials concluded that there were no significant differences in visual outcomes or quality among commonly used surgical techniques, which included laser-assisted in situ keratomileusis (LASIK), photorefractive keratectomy (PRK), laser epithelial keratomileusis (LASEK), and epithelial LASIK (Epi-LASIK), among others [76].

Patient satisfaction rates after LASIK are generally high. In a systematic review of 19 articles reporting patient quality of life and satisfaction after LASIK procedure, the overall patient satisfaction with surgical outcome was 95.4 percent [77].

Short-term visual outcomes — Patients can reasonably expect to have a 90 to 99 percent chance of achieving 20/40 or better uncorrected visual acuity (UCVA) [48,78]; 57 to 79 percent achieve 20/20 or better UCVA [6,52,74,79]. The results of every study are based upon each patient's refractive error and the amount of astigmatism. Approximately 85 percent of patients see 20/25 or better UCVA, which enables them to perform the majority of activities without corrective lenses.

Two studies sponsored by the US Food and Drug Administration (FDA) assessed patient-reported outcomes (PRO) after LASIK. PROWL-1 included 262 active-duty Navy personnel (ages 21 to 52 years of age) at a military medical center. PROWL-2 included 312 civilians (ages 21 to 57 years of age) from five United States academic and private practice centers. Although both studies reported very high satisfaction rates (between 96 and 99 percent), a small subset of patients without symptoms prior to LASIK experienced new visual symptoms, such as glare, halos, starbursts, or dry eye symptoms three months after surgery. Overall, visual symptoms and dry eye symptoms after LASIK improved over time, and very few patients reported that their symptoms impacted their daily activities or wellbeing [80].

Outcome in a second eye treated with LASIK may be correlated with the results in the first eye [81]. As an example, one study found a relatively high correlation between the outcomes in the two eyes of patients who had same-session bilateral LASIK and, after controlling for baseline visual acuity, calculated an approximately 20-fold increase in the risk of UCVA of 20/40 or worse in an eye treated with LASIK if such a result occurred in the other eye [82].

The patient factors that affect refractive outcome include low versus high myopia, myopia versus hyperopia, and astigmatism. Patients with low myopia without astigmatism have the best results, whereas high hyperopes with astigmatism have the least predictable results [6-11,83,84].

Additional factors that may affect the surgical outcome include the type of procedure, the surgeon's skill level and experience, and the center's equipment, quality control, and maintenance. The laser characteristics also determine refractive outcomes. The newer-generation small flying spot laser beams (less than 100 microns) with eye tracking systems theoretically offer better predictability than the older-generation wide beam (4 to 5 mm) without eye tracking systems [5]. However, large randomized controlled trials comparing different lasers have not been published.

A systematic review included six randomized trials (417 eyes) comparing outcomes with photorefractive keratectomy (PRK) and LASIK for myopia [85]. Visual recovery was faster with LASIK, but refractive accuracy was similar. At six months, a non-statistically significant greater number of eyes treated with LASIK achieved a visual acuity of 20/20 or better (odds ratio [OR] 1.62, 95% CI 0.75-3.50). In addition to the wide confidence intervals around results, there was significant heterogeneity across studies for some outcomes. No randomized trials were available to compare LASIK with PRK for hyperopia [86]. It remains uncertain whether long-term visual outcomes differ between PRK and LASIK.

Long-term outcomes — Long-term outcome studies for PRK and LASIK are difficult to accomplish because the patient population is largely young, mobile, and generally doing well, resulting in loss to follow-up. Additionally, outcomes related to new technological advances are not reflected in studies of procedures done 10 years earlier.

However, 10-year outcome studies have been published for PRK and LASIK in patients with mild and more severe myopia [87-90]. These studies, involving a combined total of 785 eyes, demonstrate stability for refraction and best spectacle-corrected visual acuity (BSCVA) and overall excellent safety. Myopic regression was more likely to occur in the first two years following surgery, with slower rates thereafter. Retreatment was performed in 20 to 45 percent of patients, commonly beyond two years from the initial treatment, and was well tolerated. In another series of 779 eyes in 402 patients followed for five years following LASIK procedure, the BSCVA remained unchanged, compared with one month after surgery, in 98 percent of patients; 17.5 percent of patients had undergone additional surgery at a mean of 2.5 years after the initial procedure [91]. A case series of 42 eyes followed for 20 years after PRK procedure found there were small (0.5 D) increases in myopic refractive error with no long-term sight-threatening complications [92].

SUMMARY AND RECOMMENDATIONS

●The two most common types of refractive surgical procedures are lamellar procedures: laser-assisted in situ keratomileusis (LASIK) and sub-Bowman keratomileusis (SBK); and surface ablation procedures: photorefractive keratectomy (PRK), laser epithelial keratomileusis (LASEK), and epithelial laser keratomileusis (epi-LASIK). (See 'Surgical techniques' above.)

●Phakic intraocular lenses are a potential option in treating myopia that exceeds the safe limits of excimer laser keratorefractive surgery. (See 'Phakic intraocular lenses' above.)

●Proper patient selection is critical for successful surgery (table 2). The patient's eye must be healthy and the refractive error must be stable over a one-year period of time. Contraindications to refractive surgery include active autoimmune or vascular disease, use of oral isotretinoin or amiodarone, severe dry eyes, keratitis, and other eye conditions. (See 'Patient selection' above.)

●Preoperative evaluation should include slit-lamp examination, computerized corneal topography, fundoscopy, dry eye evaluation, and measurement of intraocular pressure and corneal thickness. (See 'Preoperative evaluation' above.)

●The most common complaints following refractive surgery are dry eyes and difficulty with night driving and glare. Surgical complications include over- or under correction and infectious or inflammatory keratitis. Ten-year outcome studies for PRK and LASIK demonstrate stability of visual correction, following the first two years, and high patient satisfaction. (See 'Risks' above and 'Outcomes' above.)