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Offer realistic expectations to refractive surgery candidates

Take into account refractive error, age and type of surgery and meticulously perform presurgical testing.

by Denise N. Cover-Miller, OD and C. Rob Graef, OD

This course is jointly sponsored by PCON, the State University of New York State College of Optometry and Vindico Medical Education. It is COPE-approved for 2 continuing education credits.

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Laser vision correction (LVC) is one of the most widely performed elective procedures in the United States. More than 14 million Americans have undergone some form of laser vision correction and despite the present economy, approximately 860,000 procedures were expected to be performed in 2009, according to MarketScope. With these numbers, all optometrists will be involved in the care of refractive surgery patients at some point. This article will discuss tests used to screen patients for laser vision correction, follow-up care and postoperative complications, and alternatives to LVC.

Proper candidates for LVC (LASIK or PRK) should be 18 years or older and have stable vision for at least 1 year, healthy corneas, good overall eye health and a treatable refractive error. Patients should be motivated to obtain freedom from their glasses or contact lenses and have realistic expectations.

Tests used to determine candidacy for LVC include a manifest and cycloplegic refraction, eye health assessment, corneal topography/keratometry and pachymetry, scotopic pupil size measurement and aberrometry. Other tests that might be performed include Schirmer’s, vital dye staining and a trial or demonstration of monovision.

Relative contraindications to LVC include, but are not limited to, diabetes, early glaucoma, moderate dry eye, controlled autoimmune disorders and herpes simplex virus keratitis.

Some of the absolute contraindications to elective laser vision surgery include advanced glaucoma, unstable refractive error, visually significant cataract, advanced guttata, unstable autoimmune disorders, uncontrolled diabetes, severe dry eye and pregnant or nursing women.

Tests to determine candidacy: topography

Topography is performed to determine corneal shape, help rule out keratoconus and pellucid marginal degeneration and assess possible contact lens warpage. It is also used to measure corneas that have unusually steep or flat keratometry readings (Ks).

This typical Orbscan quad map printout has an elevation map
This typical Orbscan quad map printout has an elevation map of the anterior corneal surface, an elevation map of the posterior corneal surface, a standard axial topography map and a corneal thickness map.
Images: Cover-Miller D

Flat Ks maybe an issue with treating a high myope. Some surgeons feel a cornea should not be flattened below 34 D to 35 D, although with custom and more prolate ablations this may be more acceptable. It is important to look at steeper Ks when treating hyperopes to avoid steepening the cornea beyond 49 D. According to Wang and colleagues, this can result in reduced optical quality of vision and protracted dry eye symptoms.

Another useful application of topography is to assess possible contact lens corneal warpage. Often this will appear as inferior steepening and may give the appearance of early or subclinical keratoconus. It is important to perform repeat topography until the maps have normalized or stopped changing.

Typical guidelines for discontinuation of contact lens wear prior to refractive surgery are: 1 to 2 weeks for soft daily wear contacts, 2 to 4 weeks for torics, 4 weeks and greater for gas-permeable lenses and up to 6 months or longer for orthokeratology/corneal refractive technology patients.

Many practitioners have topographers in their offices that are based on placido disc technology, while refractive surgery centers typically have an Orbscan (Bausch & Lomb) or Pentacam (Oculus) or both. These instruments provide not only standard topography, but can look at the posterior corneal curvature and pachymetry. Anterior topography alone is not sufficient.

One corneal irregularity is inferior steepening.
One corneal irregularity is inferior steepening.
One corneal irregularity is inferior steepening. An increased I/S ratio is when the inferior (I) corneal curvature at either the 3 mm or 5 mm zone is steeper than the superior (S) value at the same zone along the same meridian.

The Orbscan combines placido disc technology with a scanning slit beam. The scanning slit beam enables measurement of the posterior corneal curvature and corneal thickness. This optically derived corneal thickness measurement may underestimate in thin corneas and overestimate in thick corneas, so often a manual ultrasound measurement is still performed.

When analyzing topographies, we are also looking for slight irregularities that may place the patient at risk for ectasia. One such irregularity would be inferior steepening. An increased I/S ratio is when the inferior (I) corneal curvature at either the 3 mm or 5 mm zone is steeper than the superior (S) value at the same zone along the same meridian.

A value less than 1.5 is considered normal, while a value at or above 1.5 (but below 2.0) can be defined as forme fruste (subclinical) keratoconus (FFKC). An I/S equal to or greater than 2.0 may indicate actual keratoconus.

FFKC is a topographic term used by refractive surgeons when there are no clinical signs of keratoconus such as Vogt’s striae, Fleischer’s ring, apical scarring or advancing irregular astigmatism, but there is an increased I/S ratio on topography. With a topographic finding of FFKC, most refractive surgeons would recommend PRK over LASIK to avoid or greatly reduce the risk of ectasia or additional monitoring in a young patient prior to treatment, according to Schmitt and colleagues and Randelman and colleagues.

Pachymetry more common for screening

The use of ultrasound pachymetry is becoming more common in optometric practices for glaucoma evaluation and refractive surgery screening. Normal central cornea thickness ranges from 500 microns to 600 microns. The common calculation one needs to perform before LASIK is: central cornea thickness minus flap thickness minus tissue ablated equals residual stromal bed thickness.

Visx CustomVue printout quad map.
Visx CustomVue printout quad map.

To calculate the amount of tissue to be ablated, one can use a general rule of 15.5 microns per diopter of treatment. (For hyperopic treatments, it may be closer to 18 microns.) This takes into account larger optical zones, blend zones and custom ablations. Be sure to use the vertex-adjusted spherical equivalent dioptric value for this calculation. For example, -5.00 -1.50 x 180 would be -5.75 D SE vertexed to approximately -5.25 D multiplied by 15.5 microns equals 81 microns to be ablated.

In the past, it was customary to leave a tissue minimum of 250 microns in the stromal bed following LASIK. Now, most practitioners would like to leave as much as 300 microns in the stromal bed for biomechanical strength and for future enhancements. Luckily, with femtosecond laser flap technology we can create very thin, very precise flaps that allow many patients to safely have LASIK and still have room for enhancements.

When PRK is performed on thin or irregular corneas, an acceptable target is to leave 325 microns to 375 microns in the stromal bed below the epithelium.

Performing manifest/cycloplegic refraction

In laser vision correction the treatable refractive error range for myopes includes an Rx of -0.50 D to -12.00 D (assuming adequate pachymetry and normal topographies). Some surgeons do not go above -9.00 D and would switch to a phakic IOL procedure to avoid over thinning or flattening the cornea.

Astigmatism can be treated up to -6.00 D, including mixed astigmatism. Finally, hyperopes can be treated up to +6.00 D; however, many surgeons are reluctant to treat patients above +3.50 D secondary to less predictable results and dryness issues. A younger patient with a flatter cornea and no evidence of dry eye may allow for higher hyperopic treatments.

Both a manifest and cycloplegic refraction should be performed to help guard against over accommodation possibly leading to over corrections. With hyperopes, some degree of latency is acceptable. Larger amounts of latent hyperopia may warrant further testing and repeat refractions. A practitioner may want to conduct a contact lens or glasses trial to try to encourage the patient to accept more plus power before treating. Lastly, comparing the patient’s present refractive error to the year prior is important in checking for stability.

Measure pupil size

It is important to recognize large pupils from a medicolegal standpoint and to counsel patients regarding increased risk of glare and halos, recommend Helgesen and colleagues. Pupil size can be measured with a pupil gauge in a dark room or with a variety of pupil measurement devices. The Colvard pupillometer is an infrared device that can be used in a completely dark room for scotopic pupil size measurement.

Although Schallhorn and colleagues report that pupil size may not be of the level of importance as once thought, it is still necessary to measure and take into account when designing a treatment plan. Custom laser treatments are indicated in patients with large pupils, at or above 7 mm. Custom ablations have demonstrated better night vision and increased contrast sensitivity compared to conventional treatments, according to Hammond and colleagues.

Measuring refraction, aberrations with aberrometry

When a patient is seen at a refractive surgery center, an aberrometer is used to assess refractive error and higher-order aberrations (HOAs), both type and magnitude. The device used by Visx is a Hartmann-Shack style aberrometer that directs infrared light into the eye and then measures how the light reflects off the retina and exits the pupil. It is then refracted by a series of lenslets, and this image is captured by a camera. Next, a series of calculations tabulates an RMS value (root mean square) or total quantity of HOAs.

A normal range for HOAs RMS for all pupil sizes is 0.10 microns to 0.40 microns. Above 0.40 microns, the patient may have a corneal topographic irregularity or cataracts or may have undergone previous refractive surgery.

With elevated HOAs, a wavefront-guided treatment would be recommended to address aberrations that may contribute to night glare. A few exceptions, according to Kuroda and Koh, would be unreliable HOAs secondary to cataracts, dry eye or anterior basement membrane disease changes, pupils too small for reliable or usable custom calculations and when the manifest refraction does not equal the aberrometry refraction due to a patient over accommodating.

Testing for dry eye

Pre-existing dry eye needs to be recognized and treated before refractive surgery. Postoperative dry eye is common and can slow healing, interfere with vision and make the patient uncomfortable. Discontinuation of contact lenses, use of artificial tears and a course of Restasis (cyclosporine ophthalmic emulsion 0.05%, Allergan) leading up to and after laser vision correction can properly prep a patient for laser surgery.

Remember to educate these patients on improved hydration and a diet high in essential fatty acids and omega-3s. Blepharitis is particularly common and should be addressed with lid hygiene and in some cases may warrant Azasite (azithromycin ophthalmic solution 1%, Inspire Pharmaceuticals) therapy or oral doxycycline.

Diffuse lamellar keratitis
It is important to recognize and treat visually significant striae early
Diffuse lamellar keratitis is seen as a diffuse layer of fine white grainy cells that have the appearance of waves or sand. It is important to recognize and treat visually significant striae early to avoid permanent irregularities within the flap that may lead to decreased best-corrected visual acuity..

Image: Harris A
Behind the slit lamp, ingrowth appears as whitish bubbles or swirls
If haze is seen behind the slit lamp, confirm that the patient is taking 1,000 mg of vitamin C per day
Behind the slit lamp, ingrowth appears as whitish bubbles or swirls under the flap at the interface. If haze is seen behind the slit lamp, confirm that the patient is taking 1,000 mg of vitamin C per day and protecting themselves from ultraviolet light exposure with sunglasses.

Schirmer’s or phenol red thread testing can be done on patients where low tear volume is suspected and it is necessary to rule out the severe dry eye patient or the patient who would benefit from pre-op Restasis and punctal plugs. A careful slit lamp exam checking for decreased tear break-up time, obvious superficial punctate keratitis, conjunctival chalasis, elevated pinguecula or pterygia, or lagophthalmos and other anatomical risk factors for dryness should be performed.

Vital dye staining including fluorescein, lissamine green or rose bengal can be used to look for more subtle dry eye findings such as conjunctival staining where extra measures and counseling about prolonged dry eye symptoms are needed.

Monovision discussion/trial

It is necessary to discuss presbyopia and the need for reading glasses with patients older than 40. A demonstration or trial of monovision with those interested or appropriate for monovision is warranted. This can be done with a trial frame or more accurately with soft contact lenses.

In comanagement settings, the comanaging doctor becomes the key person in discussing, trialing and finalizing the monovision decision.

Perform postoperative counseling preoperatively

Postsurgical dryness, temporary night glare, fluctuation of vision, compliance with eye drops and follow-up care and avoidance of swimming, eye rubbing and eye makeup are all important things to counsel patients about before their procedure. Let patients know it may take 3 months for vision to stabilize with most myopes and possibly 4 to 6 months in higher myopes, hyperopes and large astigmats. The possibility of reading glasses, night driving glasses and enhancements should also be covered.

With PRK patients it is helpful to discuss the longer recover period. This may include 3 to 5 days of burning, stinging and light sensitivity, 1 to 3 weeks of moderately blurred vision and up to 6 months for complete vision stabilization.

LVC technique: PRK

PRK has become more popular over the last 5 years. About 15% of procedures are now surface-based. Reasons for PRK’s growing popularity include the accepted safety and efficacy of mitomycin C (MMC) to reduce the chance of corneal haze, report Lee and colleague. There are better fitting and oxygen transmitting bandage soft contact lenses, better pain management with topical nonsteroidal anti-inflammatory drugs and corneal cooling techniques, and smoother laser ablations allowing quicker re-epithelialization.

With these updated techniques combining to make PRK a better, safer, faster healing procedure, the term advanced surface ablation is now used as an alternate or updated term for PRK.

Other reasons to consider PRK include occupation (certain units within the military allow only PRK), hobbies (boxing and competitive martial arts carry a higher risk of eye trauma) and patient preference (some do not want a flap).

LVC with LASIK

LASIK preserves the epithelium and allows for quicker vision recovery and less discomfort than PRK. The microkeratome works well, but disadvantages include flaps with variable thicknesses, a higher chance of an abrasion and higher chance of partial flaps or buttonholes within the flap. Microkeratome flaps have a meniscus shape, meaning thinner in the center and thicker in the periphery, which may lead to a less ideal optical outcome.

All-laser LASIK

Over the last several years, the use of a femtosecond laser to create the flap has become more common. Also called all-laser-LASIK, this technique uses a high-frequency, short pulsed laser. The laser is set to a precise depth within the anterior stroma to determine the flap thickness. The laser energy creates a series of small cavitation bubbles that, when lined up, create the incision.

Intralase is the most widely used system and has been on the market the longest. There are newer femtosecond units from Ziemer and Zeiss. With a technique termed sub-Bowman’s keratomileusis, a thin (90 microns to 110 microns) smaller-diameter flap is created with a femtosecond laser just below Bowman’s membrane. Some surgeons feel this thinner, smaller diameter flap is less likely to cause biomechanical weakening of the cornea and can help reduce the chance of ectasia and dryness.

Infiltrates will appear as a concentration of small whitish fluffy round subepithelial cells
Intacs intrastromal corneal ring segments
Infiltrates will appear as a concentration of small whitish fluffy round subepithelial cells. Intacs intrastromal corneal ring segments are small implantable/removable PMMA segments originally designed for nearsightedness that are now being used for keratoconus or post-laser ectasia patients.

These Orbscan maps show keratoconus
These Orbscan maps show keratoconus.

The femtosecond flap is also planar (uniform thickness across), so it is less likely to induce optical aberrations. Also, the side-cut, or outer border of the flap, can be set at 90 degrees or as an inverse-bevel, creating a more stable fit with less likelihood of slippage.

With femtosecond, losing suction during the procedure is no longer a reason to abort and perform PRK a few months later. With femtosecond we have the ability to re-apply suction with the same patient interface cone and continue to create the flap, reports Binder.

Wavefront-guided, wavefront-optimized ablations

Custom wavefront laser ablations use wavefront principles to design and deliver the laser ablation to the cornea. With a wavefront-guided treatment, an aberrometer measures an eye’s aberrations or optical distortions, calculates a treatment plan, then transmits that data to the laser “guiding” the ablation. This treatment takes into account both a patient’s lower-order aberrations (such as myopia, astigmatism, hyperopia) and higher-order aberrations (such as spherical aberration, coma, trefoil).

Wavefront-guided treatments rely on a minimum pupil size (usually 5.0 mm to 5.5 mm) to obtain a reliable capture for calculation. Lenticular changes may cause erroneous wavefront measurements and are subject to change as the lens changes. It is important when performing such treatments to ensure the wavefront reading closely matches the manifest refraction to avoid overcorrections and achieve a more predictable outcome.

In a wavefront-optimized treatment, the ablation takes into account an individual’s corneal curvature and prescription. By delivering extra pulses into the corneal periphery during treatment, a wavefront-optimized treatment minimizes the induction of spherical aberration, which has been implicated as the main culprit in causing halos at night following conventional (non-custom) treatments. A wavefront-optimized treatment attempts to preserve a more natural, prolate corneal shape, resulting in better night vision.

The advantages of custom wavefront laser treatments (either wavefront-guided or -optimized) include higher rates of achieving 20/20, better contrast sensitivity and fewer complaints of halos and glare. These treatments are particularly useful for those with large prescriptions, large pupils and night-time critical occupations such as firefighters, police, military, truck drivers and pilots.

Postoperative care

LVC patients should expect to have several follow-up visits. Typically the patient is seen at 1 day, 1 week, 1 month and 3 months postoperatively. A 6-month and a 1-year postop visit can also be performed.

At each visit it is important to check the unaided vision and perform a careful slit lamp exam. At the 3-month visit a careful refraction is helpful and can be used to update an individual laser’s nomogram. At each of these follow-up visits it is important to check for complications.

Dryness: most common side effect

The most common side effect of laser vision correction is dryness. Often the treatment is limited to nonpreserved artificial tears and gels several times a day to possibly every hour to improve patient comfort. However, some patients require Restasis and punctal plugs to reduce the signs and symptoms of dryness. Adjunct therapy of lid hygiene, omega-3s and increased hydration are always beneficial.

Remember, it is imperative to treat any dryness or blepharitis prior to treatment to ensure the patient has the best results with a comfortable, uneventful recovery.

Diffuse lamellar keratitis

Diffuse lamellar keratitis (DLK) is a complication seen after LASIK or LASIK enhancement that varies in degrees of severity. DLK is seen as a diffuse layer of fine white grainy cells that have the appearance of waves or sand. It is caused by the body’s reaction to something in the interface below the flap or an epithelial defect overlying the flap. Femtosecond laser energy, interface debris, bacterial endotoxins or contaminants from a sterilizer have all been implicated as possible causes of DLK.

If left untreated, DLK can cause decreased vision, light sensitivity and foreign body sensation. Treatment is typically an increase in topical steroid drops with close monitoring. If resolution is not seen in a few days, oral steroids or flap elevation and irrigation may be necessary to avoid a stromal melt, reported Gil-Cazorla and colleagues.

Displaced, wrinkled flaps

Another postoperative concern is a patient dislodging a flap or causing the flap to become wrinkled. Flap striae can typically be repaired by elevating and repositioning the flap. Sometimes bandage contact lenses are placed to help further stabilize it.

Flap striae are seen less often now with the use of Intralase and the steeper side cut of the laser than with flaps created with a microkeratome. It is important to recognize and treat visually significant striae early to avoid permanent irregularities within the flap that may lead to decreased best-corrected visual acuity.

Epithelial ingrowth

Epithelial ingrowth is a rare complication of LASIK and LASIK enhancements. Ingrowth is seen when epithelial cells get trapped beneath the flap and continue to proliferate. These cells can cause irregular topography, decrease in vision and mild foreign body sensation for the patient.

Behind the slit lamp, ingrowth appears as whitish bubbles or swirls under the flap at the interface. It typically begins at the flap edge and can migrate centrally if not removed. It is important to elevate the flap and remove epithelial ingrowth before it encroaches on the visual axis.

This patient must be monitored carefully for recurrence or possible flap melt. PRK would be indicated should there ever be any need for enhancement.

Ingrowth is more common with enhancements when lifting older flaps. Eyes with flaps older than 3 years may be best enhanced with PRK to avoid ingrowth, reports Wang.

PRK haze

Surface ablation treatment carries an increased risk of postoperative haze. Even with advanced techniques and the use of MMC intraoperatively, patients can develop haze post laser. MMC is a potent anti-tumor antibiotic that acts as a DNA alkylating agent, cross-linking DNA, thus inhibiting DNA synthesis. The topical application of MMC reduces the risk of corneal haze post laser.

If haze is seen behind the slit lamp, confirm that the patient is taking 1,000 mg of vitamin C per day and protecting themselves from ultraviolet light exposure with sunglasses.

Treatment is typically an extended course of steroid drops several times per day. This patient will require close monitoring of haze resolution and IOP measurements. If the steroid drops are ineffective, rarely these patients need to undergo a haze removal procedure with the surgeon, according to Roh and colleagues and Verweij and Pinedo.

Slow healing epithelial defect

By day 4 or 5 postoperatively, the epithelium should be healed or closed. If there is still an open defect beyond day 5, one may want to exchange the bandage contact lens if coated with debris.

Make certain the patient is using plenty of preservative-free artificial tears and consider tapering the topical steroids. Also, placing temporary punctal plugs in the lower and/or upper lids may promote faster healing. Careful monitoring until the epithelium is intact is important.

Biggest concern: infection

The biggest postoperative concern with laser vision correction is infection. Patients generally take an antibiotic four times a day for a week to help reduce the risk of infection. At each postop visit, the cornea should be carefully checked for any sign of infiltrate or epithelial defect. Infiltrates will appear as a concentration of small whitish fluffy round subepithelial cells. If there is an overlying epithelial defect with fluorescein staining or pooling, the infection is considered ulcerative.

Treatment is typically an increase in antibiotic coverage every 1 to 2 hours and possibly adding a fortified antibiotic. This requires close observation and several follow-up visits until reepithelialization occurs and the infiltrate resolves.

For health care workers, it is recommended to use a later-generation fluoroquinolone, possibly in combination with Polytrim (polymyxin B-trimethoprim, Allergan) or something similar, to include broad-spectrum coverage against methicillin-resistant Staphylococcus aureus/methicillin-resistant Staphylococcus epidermidis. It is important to involve the surgeon for suspect infections, because he or she may want to perform cultures and sensitivity analysis.

Under/overcorrections

The majority of patients heal as we expect and meet their targeted visual outcome. Depending on the surgeon and the facility, the average retreatment rate is 5% to 10%. Rarely, patients will need to wear temporary prescription glasses or contact lenses until an enhancement is recommended. It is important to explain to the patients that there is fluctuation during the healing process and that it can take up to 6 months to reach their final visual acuity.

Enhancements are indicated when the benefits outweigh the risks and when there is enough residual stromal bed thickness to undergo another treatment. It will be important to reassess ocular health as well as patient lifestyle and functional vision before any enhancement is performed.

Alternatives to LVC

As mentioned earlier, not all patients are excellent candidates for LVC. At times, it will be necessary to educate patients on risks, benefits and alternative means of vision correction. For the patients who fall outside the parameters of safe LVC, we need to be able to guide them into making an educated decision about what therapy or surgery may be right for them.

Intacs intrastromal corneal ring segments (Addition Technology) are small implantable/removable PMMA ring segments originally designed for nearsightedness that are now being used for keratoconus or post-laser ectasia patients. These rings work by restoring the cornea to a more natural shape with hopes to achieve more functional vision and possibly delay the need for a corneal transplant.

With the use of a femtosecond laser and its ability to create a tunnel within the cornea, implanting Intacs is now a minimally invasive procedure, report Siganos and colleagues.

For highly nearsighted patients and patients with thin corneas, phakic IOLs are now being successfully utilized. P-IOLs are implants that sit in front of the crystalline lens, either in front of or behind the iris, depending on the manufacturer.

According to Vilasec and colleagues, patients need to have adequate anterior chamber depth and a normal endothelial cell count for safe lens implantation. P-IOLs are currently approved by the U.S. Food and Drug Administration for up to -20 D, with astigmatic lenses close to approval.

Cataract surgery/refractive lens exchange

Patients with visually significant cataracts are not candidates for LVC and should instead consider cataract surgery. Older patients with refractive errors that are high and difficult to treat with LVC, but without visually significant cataracts, may opt for a refractive lens exchange (RLE). This is essentially the same surgery as cataract extraction, but for elective, refractive purposes and is not covered by health insurance.

Multifocal and pseudo-accommodating lenses are becoming more popular for patients who want freedom from glasses. For example, a +5.00 D 55-year-old female with early lens changes and dry eye and a desire to be free from glasses is a poor candidate for LVC, but may be a candidate for a multifocal IOL. It is important to educate these patients on the risks and costs of available lens procedures.

The primary care optometrist will be the one to conduct much of the candidacy screening for LVC. He or she will be working closely with a surgeon or refractive surgery referral center, so it is important to be familiar with that center’s lasers, protocols, surgeon preferences and selection criteria. It is equally important to know what to look for postoperatively when comanaging these patients. The optometrist should know how to treat any complications and when to involve the surgeon.

Remember, refractive surgery is for motivated patients with realistic expectations who are prepared to invest both the time and money into an elective procedure. It is important to educate these patients on what to expect based on their refractive error, age and type of surgery. If surgery is not in the patient’s best interest, then glasses, contact lenses or perhaps corneal refractive therapy would be more suitable choices.

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