LASIK Complications: Etiology, Management, and Prevention

Monday, October 20, 2014

LASIK Complications: Etiology, Management, and Prevention Samir A. Melki, MD, PhD, and Dimitri T. Azar, MD

Cornea and Refractive Surgery Service, Massachusetts Eye & Ear Infirmary, and Schepens Eye Research Institute, Harvard Medical School, and Boston Cornea Center, Boston, Massachusetts, USA

Abstract.

Laser in situ keratomileusis (LASIK) is a rapidly evolving ophthalmic surgical procedure. Several anatomic and refractive complications have been identified. Anatomic complications include corneal flap abnormalities, epithelial ingrowth, and corneal ectasia. Refractive complications include unexpected refractive outcomes, irregular astigmatism, decentration, visual aberrations, and loss of vision. Infectious keratitis, dry eyes, and diffuse lamellar keratitis may also occur following LASIK. By examining the etiology, management, and prevention of these complications, the refractive surgeon may be able to improve visual outcomes and prevent vision-threatening problems. Reporting outcomes and mishaps of LASIK surgery will help refine our approach to the management of emerging complications.

Laser in situ keratomileusis (LASIK) is a relatively new ophthalmic procedure that represents a combination of previously used techniques in refractive surgery. It involves the use of a microkeratome to create a thin corneal flap followed by excimer laser ablation of the corneal stroma and repositioning of the flap.

Corneal lamellar dissection was first described in 1949 by Jose I. Barraquer as part of keratomileusis surgery, and it was later modified to become an integral component of automated lamellar keratoplasty.

Corneal excimer laser ablation has been used in photorefractive keratectomy (PRK) since 1983. Further advances in excimer laser technology and the development of safer microkeratomes have allowed lamellar refractive surgery to expand from being a procedure that is performed by few experts to one that the general ophthalmic surgeon is now undertaking.

LASIK is now a widely performed refractive procedure with an estimated 1.5 million annual procedures performed worldwide.

New complications are bound to occur when a surgical procedure is performed with increasing frequency. Unforeseen events might be encountered as patients with yet unidentified contraindications undergo the surgery.

Effective means have emerged to manage several LASIK complications, whereas others are still subject to investigation. A thorough understanding of the potential complications of LASIK and the various strategies to manage them is essential for surgeons performing the procedure. This helps in improving surgical outcomes and in offering patients comprehensive informed consent. In this article, we review the currently known complications of LASIK as well as various strategies to prevent and manage them.

I. Anatomic Complications
A. THIN/IRREGULAR/BUTTONHOLED FLAP

The incidence of thin flaps after LASIK has been reported to vary between 0.3% and 0.75% in the three major studies.

Lamellar flap creation in LASIK should be deeper than Bowman’s layer, sparing it from laser ablation. A flap is considered thin when the keratome cuts within or above the 12m-thick Bowman’s layer. This is recognized by a shiny reflex on the stromal surface. The use of pachymetry before and after lifting the flap may be helpful in recognizing this occurrence. A measurement below 60m is suspicious, as the thickness of the corneal epithelium is approximately 50m.

The definition of an irregular flap varies according to the study. It can refer to a bi-leveled flap, a bisected flap, or a flap with a notch.

Its incidence is lower than that of thin flaps (Table 1). This can lead to scarring and irregular astigmatism.

A buttonholed flap occurs when the microkeratome blade travels more superficially than intended and enters the epithelium/Bowman’s complex.

This can provide a channel for epithelial cells to infiltrate the flap–stroma interface. Buttonholes may be partial thickness if they transect Bowman’s layer or full thickness if they exit through the epithelium. The incidence of buttonholes ranges between 0.2% and 0.56% (Table 1). This was the most common complication resulting in loss of best corrected visual acuity (BCVA) among 1062 eyes studied by Stulting et al.

A flap buttonhole is more likely to occur when LASIK is performed on eyes with previous incisional keratotomy. Thompson described a flap hydrodissection technique to minimize incision separation in this situation.

His technique involves placing a 27-gauge cannula under the flap, near the hinge to hydrodissect the flap. Once the flap floats on a bed of balanced salt solution, it can be lifted with minimal stress to the radial keratotomy (RK) incisions.

1. Etiology

Steep corneas have been compared to tennis balls that would buckle centrally upon applanating pressure. This results in a central dimple missed by the blade, leading to a buttonhole. Another theory is that higher keratometric values offer increased resistance to cutting when applanated, leading to upward movement of the blade. Leung et al reported six cases of buttonholed flaps with a mean keratometric value of 44.20 D.

Instead of high keratometric values, they believe that a lack of synchronization between translational flat keratome movement and oscillatory blade movement results in forward displacement of corneal tissue and stepped, thinned, or buttonholed flaps. Flat corneas may result in a thin and/or small flap, as they could be below the adequate cutting plane in certain locations locations (Table 2).

Irregular flaps (abnormal shape/diameter) or buttonholes with or without abnormal thickness may result from damaged microkeratome blades, irregular oscillation speed, or poor suction. The latter is more likely to occur in the setting of deep set eyes or small diameter corneas with inadequate suction ring placement or conjunctival incarceration in the suction port.

LASIK COMPLICATIONS

Blade positioning in the microkeratome and the preset space for the blade in the microkeratome may affect flap thickness in the absence of irregular flap shape. Blade damage may happen either during manufacturing or at the time of usage.

Another possible risk factor for flap buttonhole occurrence may be previous ocular surgery, as suggested by Stulting et al. However, this did not reach statistical significance (P 0.09).

2. Management

The safest way to proceed when a thin, irregular, or buttonholed flap is encountered is to reposition the flap and abort the procedure. A deeper flap may be recut (20–60 _ m deeper) approximately 10–12 weeks later with a different microkeratome or a larger diameter flap size. While some advocate proceeding with scraping the epithelium and performing a PRK laser ablation, this approach may not be feasible in higher myopes due to the appearance of subepithelial haze.

Kapadia and Wilson advocate using a no-touch transepithelial PRK within 2 weeks of the initial irregular cut to prevent irregular astigmatism formation from the uneven ablation profile resulting from any late scar formation.

The recommendation to perform PRK over a LASIK buttonholed flap to avoid scar formation is contradictory to the common belief that haze occurs following PRK treatment on top of a LASIK flap. A transepithelial PRK may prevent the development of a buttonhole-related scar. This is different from PRK after a normal LASIK flap because the scarring in the latter situation would not occur in the absence of PRK.

3. Prevention

The incidence of thin, irregular, and perforated flaps may be reduced if the surgeon ensures adequate suction, inspects the blades, adjusts the plate thickness according to corneal curvature, and pays attention to the following guidelines:
1. Avoid cutting the flap if the intraocular pressure (IOP) is low due to low suction. A pressure above 80 mm Hg may be essential for safest flap creation. Measurement is probably most valuable with a pneumotonometer, as other means may provide imprecise readings at times.

Care should be taken to avoid pseudosuction, often caused by conjunctival clogging of the suction port, which could lead to discrepancy between the intraocular pressure and the suction pressure recorded on the microkeratome vacuum console.

2. Set the microkeratome to a deeper cutting depth if keratometry readings show evidence of a steep cornea, assuming that the amount of intended myopic correction to be treated allows such modification. Most refractive surgeons follow such an approach, setting the cut-off point at 46–48 D, although no definitive supportive study exists in the literature.

3. Use larger suction rings in flat corneas to prevent small flaps.

4. Inspect the microkeratome blade under the operating microscope before engaging it in the suction ring in order to rule out manufacturing or other preoperative damage to the blade. Keep the microkeratome away from hard surfaces after assembly to avoid subsequent blade damage.

5. Inspect previous keratotomy incisions to ensure adequate healing and lack of epithelial plugs prior to proceeding with LASIK. This can prevent intraoperative separation of incisions.

B. INCOMPLETE FLAP

Incomplete
flaps are created when the microkeratome blade comes to a halt prior to reaching the intended location of the hinge. Visual aberrations are more likely to occur when the created hinge results in scarring in proximity to the visual axis (Fig. 2). The incidence of this complication reported in large series ranges between 0.3% and 1.2% (Table 1).

1. Etiology

Microkeratome jamming due to either electrical failure or mechanical obstacles may be the most common cause of incomplete flaps. Lashes, drape, loose epithelium, and precipitated salt from the irrigating solution have been recognized as possible impediments to smooth keratome head progression. Incomplete flaps also occur when the gear advancement mechanism jams or is inadequate.

Loss of adequate suction in some microkeratomes may lead to automatic abortion of the dissection or force the surgeon to premature arrest of the microkeratome head.

2. Management

Unless enough space exists for ablation, incomplete flaps are best managed by immediate repositioning and postponing the procedure. Resuming forward cutting after a stop may result in an irregular stromal bed and irregular astigmatism. It is advisable to achieve a deeper and more peripheral cut during the retreatment. If the created hinge is beyond the visual axis, some surgeons may instinctively consider manually extending the dissection with a blade. Caution is advised when attempting such a maneuver due to the risks of uneven bed creation and flap buttonhole formation. When the laser ablation is performed, the flap should be protected from laser exposure. This may be more important in hyperopic treatments, given the larger diameter ablations.

3. Prevention

Microkeratome jamming can be minimized by meticulous cleaning of its components and by inspection of its electrical connections. The manufacturer’s recommendations for cleaning procedures and solutions may differ over time as more is learned about a particular machine. A clear cutting path for the microkeratome can be achieved through adequate draping (to prevent lashes from getting into the cutting field), adjustable eye speculae (to provide as wide of an interpalpebral opening as tolerated by the patient) and gentle lifting of the globe after vacuum activation (to provide better exposure and unhindered gear progression). In addition, variable tilting of the suction ring and eye can be utilized to obtain a clear cutting path. This should be followed by the IOP measurement step to rule out inadvertent loss of suction pressure when lifting the globe.

Deep orbits may represent a challenge, as some keratome heads may not fit and may be stopped by the eyelid speculum. We believe that the risks and discomfort associated with more invasive techniques, such as retrobulbar saline injections and lateral canthotomies, may not be justified, given the elective nature of the procedure. Poor or loss of suction can be prevented through measures discussed in section IA.

C. DISLODGED FLAP

A dislodged flap is an emergency. It should be repositioned as soon as possible to prevent infection, fixed folds, and epithelial ingrowth. This displacement can occur as late as many months after the procedure. The incidence of perioperative flap dislocation has been reported to vary between 1.1% and 2.0% in large series.

The relative high rate of dislodged flaps after LASIK in earlier publications has prompted many investigators to refine their techniques of flap repositioning with resultant positive impact on lowering the incidence of this complication.

1. Etiology

Mechanical displacement by lid action is the main factor in the early period, especially if the ocular surface is dry. This may follow eyelid rubbing or squeezing. Larger diameter and thinner flaps are more prone to be displaced, especially if the hinge is small. The flap remains vulnerable to traumatic displacement several months after surgery.

Two reports described dislocation of a LASIK flap during vitrectomy surgery. LASIK corneal flaps can be lifted for retreatment even later than 12 months after the primary procedure. This is in agreement with histological studies showing minimal healing at the stromal interface after LASIK.

2. Management

The flap should first be reflected and the interface (stromal bed and stromal aspect of the flap) carefully examined for epithelial cells or other debris. It should be aggressively scraped prior to repositioning the flap. A contact lens can be applied to provide added protection from further displacement and to protect the flap from eyelid movement. Stromal scarring in an incomplete flap after an aborted LASIK.

LASIK COMPLICATIONS

Tecniques described below to flatten any associated folds should be used. This is important to prevent epithelial cell migration from the healing periphery toward the flap interface under the tented folds.

3. Prevention

Prevention of dislodged flaps rests mainly on the use of protective measures such as the superiorly hinged flaps, which were designed to circumvent upper flap edge displacement through blinking.

It is not clear yet whether they have achieved their intended purpose. Other preventive steps include applying a contact lens after the procedure, lid taping, and encouraging eyelid closure in the first few hours following surgery. We advise our patients not to apply eyedrops soon after surgery to avoid any early mechanical disturbances.

For 1–3 weeks after the procedure, a protective shield can be worn when sleeping to minimize traumatic displacement through unintentional rubbing or mechanical pressure on the eyelids. Patients involved in contact sports and similar activities should be thoroughly counseled about the added risk of late flap displacement with LASIK. PRK might be a better alternative, if judged feasible, in these situations.

D. FREE CAP

If the cap cannot be retrieved, attempts at fashioning a lamellar flap from a donor cap should not be attempted as a primary procedure. The corneal epithelium is allowed to heal as in PRK with possibly a more profound central applanation effect. The excimer laser treatment should be aborted and retreatment deferred until refractive stability is achieved.

3. Prevention

The same measures described to prevent thin and small flaps also will help avoid a free cap. We believe that presurgical fiducial marking may facilitate proper flap orientation during cap repositioning and avoid induced irregular astigmatism.

E. FLAP FOLDS

Flap folds can induce irregular astigmatism with optical aberrations and loss of BCVA, especially if they involve the visual axis.

Macrofolds are easily seen by slit-lamp examination and represent fullthickness flap tenting in a linear fashion. On the other hand, microfolds within the flap itself may represent wrinkles in Bowman’s layer or in the epithelial basement membrane. They are easily visualized as negative staining lines with sodium fluores- Fig. 3. Dislocation of a corneal flap 3 weeks post-LASIK secondary to trauma by finger to the eye. (Courtesy of Nada S. Jabbur, MD).

Carpel et al described 5 types of folds and striae in LASIK flaps. While confocal microscopy reveals microfolds at the Bowman’s layer in 97% of cases, the incidence of folds requiring intervention ranges between 0.2% and 1.5%.

It is not clear why some folds may adversely affect vision while others with similar appearance may be asymptomatic.

1. Etiology

Flap folds result from uneven alignment of the flap edge and the peripheral epithelial ring. Thinner and larger flaps tend to shift more readily with resultant surface wrinkling. Uneven sponge smoothing can result in radial (with centrifugal movement) or circumferential folds (with centripetal movement). A higher incidence of flap folds is usually found in higher myopes and hyperopes and is sometimes unavoidable. This is due to the altered central convexity and stromal support resulting in flap redundancy that may be quite difficult to flatten. The latter is referred to as the tenting effect.

2. Management

The management of flap folds ranges from stroking the flap with a moist microsponge at the slitlamp to simple lifting and refloating of the flap and to placement of sutures to stretch a recalcitrant flap into position.

It is likely that the earlier a flap is attended to, the higher the chances of quick resolution.

Fixed folds probably occur when epithelial hyperplasia has time to form in the crevices formed by the folds. Flattening should aim toward an even distribution of forces applied to the flap.

This can be performed with methylcellulose sponges or their equivalent. Instruments such as the Pineda LASIK Flap Iron (Asico, Westmont, IL) can also be used to flatten isolated flaps at the slit-lamp or under the operating microscope by gently pressing on them. Recalcitrant folds may respond well to placement of running antitorque sutures at the flap edge.

However, this may result in significant astigmatism. Another strategy is to make superficial epithelial incisions, phototherapeutic keratectomy (PTK), or frank epithelial debridement over the wrinkled area. This may relieve contractures that occur secondary to epithelial hyperplasia in longer standing folds. Probst et al described a technique using the red reflex as a way to better detect mild irregularities.

Other reported strategies include hydrating the flap with hypotonic saline (60–80%) or deionized water,153,188 which may facilitate flattening. In extreme cases, removal of the corneal cap has been reported to be successful.138 Suturing of the flap may be the procedure of choice for recalcitrant cases that do not respond to the measures listed here. The sutures, however, should be removed in the first few days after surgery, especially if suture-induced folds are evident postoperatively. We have noted few instances where mere flap ironing will result in refractive error shift of more than one diopter.

3. Prevention

Preplaced surgical landmarks straddling the flap edge permit accurate repositioning of the flap in the immediate operative and postoperative period. Examination at the slit-lamp 20 min after the procedure is useful to ensure adequate flap positioning. Care should be taken to ensure even distribution of the gap “gutter” between the flap edge and the peripheral epithelial ring. This is noted after the procedure and usually disappears by the first postoperative day. This gap is probably due to biomechanical retraction of the collagen lamellae or to flap dehydration and subsequent retraction. The dehydration effect alone may not explain the gutter formation as the gutter can be seen immediately after the cut where dehydration may not have yet occurred. Contraction of intercellular adhesion complexes secondary to mechanical trauma might also contribute to the retraction of the flap. There has been no histologic confirmation of these theories.

Placing a wet microsponge on the stromal aspect of the flap during long ablations might minimize the dehydration effect. However, this may introduce fibrils and debris in the interface. We currently favor spreading 1–2 drops of fluid on the stromal aspect of the reflected flap after lifting. Other surgeons prefer folding the flap while performing the laser ablation.

F. EPITHELIAL IMPLANTATION AND INGROWTH

Implantation of epithelial cells in the interface occurs either due to seeding during surgery or migration under the flap.88 Epithelial cells can be seen as tongues or pearls (Fig. 4). Connection to the outside epithelium might be conspicuous or undetectable at the slit-lamp. Most isolated nests of cells will disappear without consequences. More concerning is epithelial ingrowth that is contiguous with the flap edge. This can progress to involve the visual axis with irregular astigmatism and possible overlying flap melting.40 The epithelial cells at the interface may block aqueous diffusion, which may compromise the nutrition of the flap and result in corneal melting. The migrating epithelial cells may also produce proteolytic enzymes that may further contribute to stromal melting of the flap. Epithelial growth at the interface may be more common after enhancement procedures, as the lifting of the flap can induce adjacent epithelial abrasions with increased cell proliferation.

LASIK COMPLICATIONS

1. Etiology

Helena88 described four mechanisms by which epithelial cells could reach the lamellar interface. These include mechanical dragging by the keratome blade during keratectomy, backflow during irrigation carrying floating epithelial cells, outgrowth from epithelial plugs in eyes with previous incisional keratotomy, and ingrowth at the junction of the epithelium and keratotomy. The latter is believed to be the most frequent cause of epithelial ingrowth. Other mechanisms include:

1. Implantation of epithelial cells when the interface is manipulated with instruments that touch the surrounding epithelium.
2. Cell migration under a fold extending to the flap edge. This is more likely to occur if an epithelial defect is induced at the edge of the flap during the procedure, leading to greater mitotic activity.
3. Buttonholed flap epithelial infiltration at the edges of the perforation.278 Wang et al reported an incidence of 0.92% in a cohort of 3786 eyes that underwent primary LASIK.279 The incidence after enhancement was 1.7% (480 eyes). The authors hypothesize that epithelial ingrowth is secondary to postoperative invasion under the flap by surface epithelial cells rather than intraoperative implantation of epithelial cells.

2. Management

Epithelial cells under the LASIK flap should be managed aggressively if they progress toward the visual axis or induce stromal melting.150 The flap is lifted, the stromal bed and the flap undersurface are thoroughly irrigated and scraped, and the flap is repositioned. 278 Epithelial cell debridement can be achieved mechanically with a #15 Bard-Parker blade or with dedicated instruments such as the Yaghouti LASIK Polisher (Asico, Westmont, IL), or by using excimer laser bursts in phototherapeutic keratectomy (PTK) mode.16,88,260 Haw et al successfully treated aggressive epithelial ingrowth in four eyes with 50% ethanol.85 Nonprogressive isolated epithelial cells should be monitored. Hyperopic shift is an early indicator of possible underlying stromal melt. This may result in irregular astigmatism and loss of BCVA.

3. Prevention

Close inspection for epithelial ingrowth is essential for patients with surgically induced epithelial defects, especially when the defects are adjacent to the flap edge. Considering PRK rather than LASIK in patients with a history of poorly adherent epithelium (e.g., history of recurrent erosions) may help reduce epithelial defect formation.49 Other preventive measures include dedicating instruments for interface manipulation that do not come in contact with the surrounding epithelium. Similarly, meticulous attention should be paid to avoid flap folds, especially those extending toward the periphery, providing a conduit for cell infiltration. It is reasonable to speculate that the introduction of epithelial cells in the interface during LASIK enhancement may be significantly minimized by lifting the flap edge with a dedicated forceps rather than extensive edge dissection around the flap circumference. Alternatively, the gutter can be dissected with a Sinskey hook prior to separating the stromal lamellae. This may prevent large epithelial tears. Walker et al advocate the use of an aspirating lid speculum and a bandage contact lens for the first day after surgery during LASIK and LASIK enhancement.

G. INTERFACE DEBRIS

Interface debris should be distinguished from inflammatory or infectious reactions. This distinction may be difficult at times. An in vivo confocal microscopy study revealed corneal flap interface debris in 100% of 62 eyes that had undergone myopic LASIK. As a rule, debris is usually inert with no progression or deleterious effects on vision unless present in large quantities. Nevertheless, it must be kept in mind that some patients might be more susceptible than others and may present with an inflammatory response to a variety of debris.

1. Etiology

Several possible sources of debris have been identified at the LASIK flap interface.270 These include metallic fragments from blade shattering during the dissection,57 oil material from the microkeratome, meibomian gland secretions, powder from gloves,252 air bubbles, central interface opacification of unknown etiology,68 and lint fibers. Lint fibers settle on the stromal bed prior to flap repositioning. They can be released from clothes, eye patches used to cover the unoperated eye, and gauze close to the operative field. Hirst93 reported brown interface deposits from dry methylcellulose sponges used to protect the hinge during laser ablation.

2. Management

If an inflammatory reaction is suspected secondary to interface debris, the flap should be lifted and copious irrigation applied. We examine our patients 20 min after the procedure and proceed to early flap lifting and irrigation if debris or fibers are noted.

3. Prevention

Lint fibers may be minimized by the use of scrub suits by the surgical team and by having the patient wear a scrub-like cover over their clothes to minimize floating fibers in the atmosphere. Moistening any gauze material in the surgical field will achieve a similar result. Other measures include using powder- free gloves, draping the lashes, and applying a fibrocellulose ring (LASIK Eye Drain [Chayet], Visitec, FL) around the limbus to provide a barrier from surrounding ocular secretions. Oblique illumination under the operating microscope can reveal very small fibrils and other debris in the interface after flap repositioning, which can be removed with generous interface irrigation.

H. EPITHELIAL DEFECT

On postoperative day 1, dilute sodium fluorescein can aid in detecting epithelial defects that might have occurred during or after the procedure. Many patients will demonstrate mild staining at the edge of the flap. Larger defects are more worrisome, especially those with a connection to the flap edge. The incidence of epithelial defects with LASIK was reported to be around 5%.51 As mentioned above, the proliferating epithelial cells might migrate under the flap edge. Associated inflammation can also lead to melting of the surrounding flap tissue. We and others have observed an increased risk of diffuse lamellar keratitis in patients with epithelial defects.

1. Etiology

Patients with a history of recurrent erosions or anterior basement membrane dystrophy (ABMD) are at higher risk of developing epithelial abrasions, especially with LASIK, and would probably be better PRK candidates. In fact, surface excimer laser ablation is used to treat patients with recurrent erosion syndrome.

2. Management

If an epithelial defect is noted intraoperatively, a higher index of suspicion for epithelial ingrowth should be maintained. An attempt at repositioning the loose epithelium should be performed. Alternatively, the epithelium can be gently debrided and a contact lens applied. These measures help in pain control as well as improving flap adherence and preventing epithelial cell ingrowth. Topical nonsteroidal anti-inflammatory drugs (NSAIDs) may also be useful to ease the associated discomfort, but they may be associated with the induction of sterile infiltrates.

3. Prevention

Candidates for LASIK surgery should be questioned for prior history or symptoms of recurrent erosion syndrome. Slit-lamp examination should include careful inspection of the epithelial surface for signs of ABMD. Even when the corneal surface appears clear, negative or asymmetric fluorescein staining should alert the observer to an abnormality in corneal surface integrity. Some investigators recommend touching the corneal epithelium at the slitlamp with a microsponge applicator in patients with suspected loose epithelium. If movable epithelium is noted, PRK may be a more appropriate procedure.

I. CORNEAL PERFORATION

This devastating complication occurs mainly as a result of faulty microkeratome assembly. A handful of studies in the literature report cases of anterior chamber penetration during lamellar dissection10,11,73 or through laser ablation.100,111 The original ACS microkeratome (Bausch & Lomb Surgical, Rochester, NY) keratome models require the placement of a thickness foot-plate to determine the depth of dissection. If it is left out, a full-thickness corneal incision with serious damage to anterior segment structures could ensue. This should be managed as any traumatic ruptured globe situation. Meticulous adherence to the instructions of keratome assembly to prevent this catastrophic event cannot be emphasized enough. In one published case, a thin preexisting keratoconic cornea was suspected as the etiology for the full-thickness laser ablation.

J. CORNEAL ECTASIA

Corneal ectasia is a devastating complication as it induces a keratoconus-like condition with all its attendant complications. Only a relatively small number of cases have been reported in the literature to date.Initially, patients may be managed with hard contact lens wear but many progress to require penetrating keratoplasty. The true incidence of this iatrogenic complication might not emerge until longer term follow-up studies are conducted. The safe limit of residual stromal bed thickness after refractive surgery remains subject to speculation. The current consensus is a minimum of 250m, while Barraquer has recommended a minimal thickness of 300m of stress-bearing corneal stroma.

1. Etiology

Although a thin stromal bed is suspected to be the culprit in all reported cases of corneal ectasia after LASIK, none of the cases had a reliable measurement of residual bed thickness. Other factors, such as late stromal melting, cannot be ruled out at this time. One report showed no underlying inflammation in an excised ectatic corneal button after LASIK, suggesting biomechanical corneal weakening as the cause of the ectasia. Patients who develop postoperative ectasia can be detected by conventional videokeratography or through slit-lamp findings. Orbscan technology (Bausch and Lomb, Rochester, NY) can image posterior corneal curvature and total corneal pachymetry, which may be useful in following patients after LASIK.

2. Prevention

Safeguarding 250m of stromal bed is the current standard of care to prevent this complication. The surgeon should, however, keep a high index of suspicion as this value could be proven inadequate when longer follow-up on better documented cases become available. Preoperatively, keratoconus suspects, may be detected by characteristic patterns on videokeratography such as inferior corneal steepening, and should be approached cautiously. Amoils et al recommend a preoperative corneal thickness of 500m, and Seiler believes that it is safer to use a percentage of the corneal thickness as a minimal residual stromal thickness rather than an absolute number. Orbscan topography can provide information on posterior corneal curvature before and after refractive surgery, as well as detailed pachymetry that can help detect forme fruste keratoconus. A combination of clinical and topographic criteria is currently used at the Massachusetts Eye and Ear Infirmary, which serves as the basis for diagnosing keratoconus and keratoconus suspects . High-frequency ultrasound corneal analysis may be able to resolve flap from residual stromal bed with a 2-micron precision. Similarly, optical coherence tomography may have some promise in providing detailed analysis of flap and stromal bed thickness postoperatively.These modalities could turn out to be useful tools in the analysis of cases with poor postoperative optical quality or unexpected ectasia.

Calculations should be made prior to surgery to determine if a safe corneal bed thickness can be achieved. It must be stressed that there can be considerable variation in the actual flap thickness, compared to the expected one (based on microkeratome manufacturer’s specifications).In high refractive error cases, pachymetry should be performed on the stromal bed after the flap is lifted. By factoring the amount of tissue removed by the laser, a more accurate assessment of residual bed thickness can be made. This can guide the surgeon later if enhancement is necessary. Establishing a registry of corneal ectasia cases could help collect much needed information on this concerning aspect of LASIK and prevent a late discovery of a potentially disastrous longterm outcome of the procedure. A safe approach would involve Orbscan videokeratography prior to enhancement ablations to differentiate between early ectasia versus regression due to stromal or epithelial remodeling.

II. Refractive Complications
A. CENTRAL ISLANDS

Patients with topographic central islands often report visual fluctuations, ghost images, and monocular diplopia. Both uncorrected and best-corrected vision may be affected. Wilson suspected that the incidence of central islands after LASIK is higher than that after PRK, but no data have been reported. Laser manufacturers have modified their software to allow additional central pulses, which appears to have minimized the incidence of this complication. Other factors, such as pattern of laser ablation and surgical technique, may also influence the development of central islands.

1. Etiology

No consensus exists about the true etiology of central islands after PRK or LASIK. They are thought to result from shielding of the central stroma by pulverized tissue plume or central collection of fluid as the ablation is performed. Degradation of the laser optics has also been implicated in causing central islands.

2. Management

Central islands in LASIK have less tendency to spontaneously resolve than in PRK. This is probably due to minimal epithelial remodeling after LASIK. Central island treatment is usually based on the last topography performed, as described by Rachid et al. Manche et al1 also described a technique to treat central islands after refractive surgery but cautioned against associated hyperopic shifts. Successful treatment protocols after PRK may not apply to LASIK patients.

Custom ablations solely based on videokeratography data might erroneously treat areas of overlying epithelial hyperplasia over areas of stromal depression, not achieving the intended stromal smoothness. In recalcitrant cases, hard contact lenses may be the only currently available means of regaining lost visual acuity and relieving visual aberrations.

3. Prevention

Both the Summit company (APEX, Waltham, MA) and the VISX Laser company (VISX, Sunnyvale, CA) have developed anti-island software for additional central ablation. There is growing evidence of minimal occurrence of this complication with scanningslit beam and flying spot lasers. Some surgeons believe that corneal fluid collects on the surface of the central stroma and suggest wiping the bed surface with a sponge or spatula every 40 to 50 laser bursts. Others only dry the surface when moisture is visible. To date, there are no studies to evaluate the benefit of these techniques.

B. DECENTRATION

Corneal surgical procedures should always be centered over the pupil. Astigmatism due to decentration of the ablation is probably the most difficult problem to correct. Although patient fixation might be more difficult under a dissected flap, Pallikaris et al reported similar centration between their LASIK and PRK groups.206 Decentration results in an uneven ablation area with the flatter treatment zone shifted peripherally, leaving the central area of the ablation zone with a higher corneal surface power difference. Consequently, the area of greatest flattening of tangential curvature is shifted away from the center of the ablation zone, resulting in uneven and undercorrected corneal surface over the pupillary axis. Decentration that involves laser application to the stromal side of the flap results in significant asymmetric flattening. This is translated into irregular astigmatism, causing glare, monocular diplopia, and halos. Decentration is best measured with tangential topography. Decentration not exceeding 0.3 mm is rarely visually significant.

1. Etiology

Decentration of excimer laser ablation may occur secondary to either treatment displacement (shift) or intraoperative drift. Shift refers to a decentered treatment throughout the ablation. This can occur due to poor fixation or surgeon’s error. Drift occurs when the eye moves involuntarily during treatment or when the surgeon attempts to correct apparent decentration during treatment. Azar and Yeh have shown that the visual outcomes of patients with treatment displacement were better than those with intraoperative drift.

2. Management

Theoretically, the flap can be lifted, and the patient retreated with decentration of the treatment in the opposite direction to the previous ablation, using a wide optical zone. This is more easily performed with decentered PRK after transepithelial PTK with the epithelium being used as a masking agent over the already ablated area. Such a technique yields quite satisfactory results. It is not clear whether other masking agents at the LASIK interface would be as effective. Currently developed modalities, such as wavefront or topography-guided ablations, may yield more accurate results. Miotics can be tried to constrict the pupillary axis to the central smooth ablation and minimize optical aberrations. This might be more useful if both the decentration and the pupil shift with pharmacological miosis are superonasal. A hard contact lens can alleviate the symptoms by neutralizing optical aberrations resulting from irregular astigmatism.

3. Prevention

The risk of decentration can be minimized by performing the ablation under the lowest illumination possible to improve the patient fixation. Meticulous attention should be directed toward adequate centration from the onset of ablation. Recentration should be avoided when possible, in view of the drift effect on visual outcome. When recentration is attempted early in the procedure, the drift effect might not be as significant as that in the late stages of the ablation. Continuous verbal encouragement can help patients maintain fixation, especially during deep ablations.

Lasers with a pupillary-tracking ability are designed to prevent decentered ablations (drift).17,264 The effects of the microsaccades of the eye may be abolished, in principle, with these lasers. Miotics or high illumination are preferably avoided. They can shift the pupil superonasally, resulting in decentered ablations that are apparent only after surgery.

C. OVER- AND UNDER-CORRECTION

Variations in corneal healing, atmospheric pressure, humidity, and ambient temperature are among the many factors that contribute to the relative unpredictability of refractive surgical procedures. Some patients develop unintentional refractive overor under-corrections following LASIK, often affecting uncorrected visual acuity (UCVA). Myopic patients with a hyperopic result can suffer from quite unsatisfactory UCVA both at near and distance, especially if they belong to the presbyopic age group.

1. Etiology

Surgical procedures based on inaccurate refractions could result in significant residual or induced postoperative refractive errors. These include erroneous refraction, relying on non-cycloplegic refraction in an accommodating patient, and wrong information input into the laser secondary to human error. Failing to reexamine a contact lens wearer until a stable and reproducible refraction is obtained may result in unexpected refractive outcomes. In the case of rigid or gas permeable contact lenses, it can take 5 weeks to achieve preoperative refractive stability. Planocylindrical corrections have been associated with a higher incidence of over-correction. This is probably due to the fact that flattening of the intended steep meridian is accompanied by the unintentional flattening of the flat meridian (but to a lesser degree).

2. Management

Over- or under-correction can be corrected by lifting the flap (even months after the surgery) and applying additional laser ablation. Multiple procedures have been developed to correct hyperopia, whether induced or native. The FDA-approved treatments in the USA at the time of this publication are hyperopic PRK and laser thermo-keratoplasty (LTK). The latter is approved only for the temporary reduction of low levels of hyperopia. In a study of 13 overcorrected eyes after LASIK, Ismail reported a mean increase of 4.1 D in central keratometric power with use of the noncontact holmuim:YAG (Ho:YAG) LTK (18-month follow-up). Goggin et al treated 11 eyes with induced hyperopia after myopic PRK and reported satisfactory results as late as 1 year after surgery. As mentioned above, a high level of suspicion for keractesia disguised as myopic regression is vital, especially in patients with higher levels of initial corneal ablation.

3. Prevention

Accurate preoperative manifest and cycloplegic refractions are essential for reliable assessment of the patient’s refractive error. Reexamination of patients with fluctuating or unstable refractions prior to performing primary or secondary treatments may help avoid unexpected outcomes. We advocate a conservative approach in treating plano-cylindrical errors, erring on the side of under-correction.

D. RESIDUAL/INDUCED ASTIGMATISM

Correction of preoperative astigmatism can result in incomplete resolution, worsening and/or shift in axis. Spherical corrections can also lead to postoperative cylindrical refractive errors.

1. Etiology

Inaccurate refraction and contact lens-induced corneal warpage can easily lead to unexpected cylindrical residual or induced errors. Similar errors may occur following rotational ocular shift or drift during laser ablation. A axis error results in loss of 50% of the cylindrical correction, while a axis error results in no change of the magnitude of the cylinder (but with a rotation in the axis of astigmatism). Other factors, such as cyclotorsion between the sitting and supine position and the lamellar cut, may contribute to astigmatism alteration.

2. Management

Depending on the amount of residual refractive error, an enhancement procedure can be contemplated. Some currently available software programs do not allow treatment of plano-cylindrical errors. Maneuvers to bypass the built-in software could lead to unexpected over-correction.

3. Prevention

As noted above, meticulous refractions and ensuring refractive stability improve the predictability of refractive outcomes. Marking of the cardinal meridians at the slit-lamp prior to the procedure and constant monitoring during the ablation to ensure proper globe orientation may help reduce the effect of ocular rotation.

E. REGRESSION

Regression seems to be reported with higher frequency after high-myopia correction and after hyperopic LASIK. Often the underlying stromal bed is too thin to permit additional laser ablation. Regression may be differentiated from natural progression of refractive error by analyzing difference maps by corneal topography. Hyperopic treatment has been plagued by regression due to peripheral epithelial hyperplasia counteracting the laser-induced corneal steepness. It must be noted that, in principle, an over-corrected myopic treatment may require less peripheral and hyperopic ablation for a specific desired long-term outcome than primary hyperopic treatments. This is because of the associated amelioration of the steep zone at the junction of the treated and untreated areas.

1. Etiology

Postoperative epithelial or subepithelial and stromal hyperplasia leading to postoperative corneal steepening have been implicated in the etiology of postoperative refractive regression. It is not clear yet which layer plays a more prominent role in this postoperative complication. Occasionally, patients whose refractive error is erroneously believed to have reached the plateau stage prior to the surgery will exhibit progressive refractive change even months after the procedure. This is more likely to occur in younger patients.

2. Management

It is not clear if regression after LASIK is as amenable to pharmacological manipulation as in PRK. There are no data to confirm the benefits of topical steroids for corneal steepening secondary to the healing response. Additional laser ablation must be guided by careful and conservative calculations of residual stromal bed thickness. Guell suggested treating regression after LASIK with intraepithelial PRK. The temptation of not disappointing a demanding patient should be tempered by the increased risk of excessive corneal thinning (see section IIJ). Sophisticated instruments, such as the very high-frequency ultrasound, might be able to accurately measure residual stromal thickness and better guide the surgeon in deciding whether or not to perform additional surgery.

3. Prevention

Development of surgeon-specific nomograms might allow better tailoring of ablations for higher correction. The variety of factors that influence the outcome (age, corneal hydration, ambient temperature, etc.) makes this task relatively imprecise.

F. HALOS AND GLARE

Visual aberrations have plagued most refractive procedures, sometimes permanently affecting the quality of vision. It is not clear to what extent pupil size plays a role in the pathogenesis of glare and halos. Generally, these symptoms abate over time. It is not clear if this is due to resolution of an underlying anatomic irregularity or to patient’s adaptation. A small subset of patients report no significant improvement and can be substantially incapacitated under various lighting situations, such as night driving, despite good uncorrected visual acuity at high contrast levels. This may be due to loss of contrast sensitivity and may pose a hazard for night driving.

1. Etiology

There is growing evidence that main reasons for higher order aberrations resulting in glare and halos are subclinical decentration (less than 1.0 mm) and/or wide-area laser ablation profiles solely based on Munnerlyn’s recommendation. Similarly, when pupils dilate to a diameter larger than the optical treatment zone, rays of light refracted by the untreated peripheral cornea are not focused at the same position as the central rays and result in blur circles (negative clearance phenomenon). These symptoms are more pronounced after treatment of cylindrical errors due to the oval shape of laser treatment with inherently smaller optical zone in the steep meridian. In addition, correction of higher refractive errors is associated with increased aberrations due to the larger refractive differential between the ablated and the intact cornea. Irregular astigmatism due to flap folds, topographic abnormalities, or simply residual myopia can also result in these symptoms. Other contributing factors include dry eyes and irregular epithelial surface.

2. Management

Optical aberrations after refractive surgery may be significantly reduced through enlargement of the ablation zone by means of the currently developed wavefront- or topography-guided lasers. Currently, conservative management, such as mild miotics, is prescribed, which can help for night activities, especially driving. Leaving the car dome’s light on when driving at night has also been reported to improve symptoms through pupillary constriction. Anecdotal evidence suggests a reduction in mesopic pupillary dilation with topical brimonidine (Alphagan; Allergan, Irvine, CA). Tinted contact lenses with artificial pupils and yellow-tinted eyeglasses might occasionally provide significant relief. Ocular surface lubrication with artificial tears or punctal plugs can occasionally result in dramatic improvement of symptoms related to ocular surface dryness. Other strategies include prescribing correcting spectacles, which can sometimes be the easiest solution to relieve poor night vision due to residual myopia. Similarly, a well-centered hard contact lens will enlarge the optical zone and could be helpful in select situations.

3. Prevention

Pupil size can be gauged with use of a Rosenbaum near card scale or an infrared Colvard infrared pupillometer. Room lights should be dimmed in both situations to replicate mesopic conditions en108 Surv Ophthalmol countered by the patient at night. Patients with pupil diameter of more than 6.0 mm should be informed of the significant risk of night vision disturbances after LASIK. Larger ablation zone diameters have been associated with decreased incidence of night glare. The development of software allowing effective larger ablation diameter with adequate preservation of stromal tissue could help lower the incidence of this problem. Measures discussed above to prevent decentration and central islands may also help diminish the incidence of these symptoms. Several investigators encourage their patients to look for symptoms of glare, halos, and starburst effects at night, prior to surgery. This may help reduce the possibility of attributing preexisting visual aberrations to the surgical procedure.

G. LOSS OF CONTRAST SENSITIVITY

Holladay et al recently showed worsening in functional vision as the target contrast diminishes and the pupil size increases. They concluded that the oblate shape of the cornea following LASIK is the predominant factor in the functional vision decrease. On the other hand, Perez-Santonja reported improvement in contrast sensitivity at certain frequencies 6 months after LASIK in eyes with moderate to high myopia. It is difficult to compare studies measuring contrast sensitivity due to the different methods used. It is unclear to what extent loss of contrast sensitivity overlaps with patients’ complaints of other optical disturbances, such as glare and halos.

III. Loss of Best Spectacle-Corrected
Visual Acuity (BSCVA)

The incidence of loss of 2 or more lines of best spectacle-corrected visual acuity (BSCVA) after LASIK is reported to be about 4.8%. It is more frequent with correction of larger refractive errors256 and with correction of compound astigmatism compared to spherical corrections. Comparing various studies could prove difficult, as some report BCVA while others only measure spectacle-corrected visual acuity. Lin et al reported less than two lines of loss of BSCVA, which were secondary to flap complications in lamellar surgery but did not mention other types of complications. Davidorf reported higher loss of BSCVA with hyperopic treatment.51 Most complications described in this report could potentially affect BSCVA either temporarily or permanently. Early recognition and heightened levels of suspicion are valuable for the prevention of unnecessary loss of vision in an otherwise healthy eye.

IV. Dry Eyes

A majority of patients complain of dry eye symptoms after LASIK. It is not known whether PRK patients report these symptoms as frequently. Many present with superficial punctate keratopathy.51 A recent study reported that 35 (42.2%) of 83 eyes displayed a distinctive brown-colored corneal iron line of variable density in a ring or patch configuration near the margin of the ablated zone in the overlying corneal flap epithelium after LASIK. The appearance of this iron line correlated positively with time after surgery (3 months) and preoperative spherical equivalent (4.5 D). This probably reflects an alteration in the surface tear dynamics due to the central corneal flattening. It can also be associated with central island formation.135 Similarly, an iron ring is usually noted after hyperopic excimer ablation.

1. Etiology

The dry eye condition after LASIK may be due to decreased corneal sensation, resulting from severing of corneal nerves, with subsequent decreased blinking rate. Another theory implicates suction ring damage to the keratolimbal area with subsequent damage to goblet cells.

2. Management

Most patients will notice an improvement in their symptoms a few weeks after the procedure. Meanwhile, surface lubrication will alleviate the sensation of ocular irritation. A recent report showed better results with carmellose-based artificial tears than with balanced salt solution. Temporary collagen plugs or longer lasting silicone lacrimal punctal plugs also provide symptomatic relief in the postoperative period after LASIK.

3. Prevention

Prophylactic placement of temporary lacrimal punctal plugs at the end of the procedure has been used by some surgeons to enhance the tear lake in the early postoperative period. A slow taper of the postoperative topical steroids may also provide relief from dry eye symptoms in the early postoperative period.

V. Infectious Keratitis and Sterile Infiltrates

Lin et al reported the incidence of bacterial keratitis after LASIK to be 0.1%. The low rate of infection may have encouraged some surgeons to abandon sterile techniques while performing the procedure. It is not known whether this has resulted in a higher rate of infection. Nevertheless, this approach is inherently risky from the medical and legal standpoints. LASIK should be approached in a manner similar to other surgical procedures, given that bilateral69,102,282 corneal infections and endophthalmitis have been reported.186 Similarly, cases of fungal47,251 and micobacterial keratitis have been repor ted. Karp et al reported two cases of delayed infectious keratitis (1 month and 3 months) after LASIK.115 Occasionally, sterile infiltrates may be seen at the edge of the flap. Blepharitis with or without rosacea, dry eyes, the use of topical NSAIDs, and undiagnosed connective tissue diseases may predispose to the formation of such infiltrates. These cannot be reliably differentiated solely by their appearance from infectious infiltrates; a high level of suspicion should be maintained.

1. Etiology

Sources of contamination include the ocular flora, any instruments or sponges used to manipulate the eye, the surgeon’s hands, or airborne contaminants. Several organisms have been implicated in post-LASIK infectious keratitis. A known history of herpes simplex virus (HSV) is a contraindication for LASIK50 especially in the presence of corneal findings because of the risk of virus reactivation.

2. Management

Treatment should be initiated promptly after corneal cultures are obtained whenever possible. Unique features to post-LASIK infectious keratitis include the possibility of lifting the flap and culturing/ irrigating the interface if deemed useful. In addition, the flap can be disinserted and excised in extreme cases of nonresponsive infections leading to severe flap melting.

3. Prevention

The following measures may help reduce the incidence of post-operative infectious keratitis:
1. Strict aseptic technique.
2. Using one microkeratome blade per eye in cases of simultaneous bilateral LASIK.
3. Warning patients about the possibility of recurrence if a questionable history of herpes infection is elicited and if signs of prior HSV keratitis are not seen. Anesthesiometry prior to surgery could help in assessing the risk of persistent epithelial defects. Systemic antiviral prophylaxis could also be considered to minimize the risk of surgically triggered reactivation.

VI. Diffuse Lamellar Keratitis

Diffuse lamellar keratitis (DLK) is a recently described syndrome117,152,249 characterized by proliferation of presumably inflammatory cells at the LASIK interface . It occurs in approximately 0.2– 3.2% of cases . It can lead to stromal corneal melting with induced hyperopia or hyperopic astigmatism. Additional symptoms include loss of BCVA with optical aberrations secondary to irregular astigmatism. Lyle et al reported a case associated with interface fluid accumulation and epithelial ingrowth. Other reported associations include corneal epithelial defects, micropannus hemorrhage, and concomitant contact dermatitis of the eyelids. In their initial report about DLK, Smith and Maloney noted several characteristics defining the infiltrate associated with this new syndrome. Since then, we and others have noted major differences as compared to the criteria described by Smith et al (in italics hereafter). Our patients include cases where the patients presented with DLK as early as the first postoperative day (day 2), asymptomatic (pain/photophobia), extending beyond the interface (confined to the interface), more prevalent with epithelial defects (no overlying epithelial defect), and associated with an inflamed conjunctiva (no ciliary flush). It has been anecdotally reported to happen sporadically or in clusters with primary procedures or in cases of LASIK enhancement. No single agent has been demonstrated to be responsible for this relatively rare but potentially serious complication. A relationship to endotoxins released from sterilizer reservoir biofilms has been described. Left: Diffuse lamellar keratitis, 2 days post-LASIK. Right: Diffuse lamellar keratitis, central coalescence with scarring and stromal melt, 5 days post-LASIK. The natural history of DLK appears to involve central coalescence of the inflammatory cells if not resolved by the 5th postoperative day (personal observations). This may lead to central stromal melting and scarring (Fig. 8, right). Our current protocol involves flap lift, scraping and irrigation by the 4th postoperative day at the latest if the inflammation is judged to progress despite hourly topical prednisolone acetate 1% with broad-spectrum topical antibiotic coverage. Any signs of stromal melting should prompt earlier surgical intervention. We have used intensive perioperative topical steroids when retreating at least 5 patients after an episode of DLK with no recurrence (unpublished observations). Peters et al propose the use of topical intrastromal steroid during LASIK to reduce the incidence and severity of DLK.

VII. Other Complications

Other complications include reports of increased risk of cataract formation, effect on endothelial cell count, unilateral or bilateral macular hemorrhage, difficulty in contact lens fitting, and difficulties in IOL power calculations in patients undergoing cataract extraction. A large study of eyes who have undergone LASIK showed a low incidence of vitreoretinal pathology (0.06%), confirming earlier reports of infrequent serious retinal complications.

VIII. Conclusions

LASIK refractive surgery is a relatively new technique with very high success rate. The demand for high standards of safety for this surgery is mandated by the fact that relatively healthy eyes are placed at risk every time the procedure is performed. These risks can be minimized by learning from our mistakes, analyzing outcomes, and prodding new territory thoughtfully and ethically. Investigators have helped advance our knowledge of unexpected results and prompted indepth procedural reviews of this relatively recent surgical procedure by reporting their complications and sharing their experience with the rest of the refractive community. This has allowed for continuous refinements in LASIK surgical technique and provided the basis for new and improved future vision correction strategies.

Method of Literature Search

In this review, we identified pertinent articles on LASIK published in the peer-reviewed journals through a multistaged, systematic approach. In the first stage, a computerized search of the PUBMED database (National Library of Medicine) was performed to identify all articles about LASIK published up to February 2000. The term laser in situ keratomileusis and the text word keratomileusis were used for a broad and sensitive search. In the second stage, all abstracts were carefully scanned to identify articles, written in English, that described either the complications of LASIK or the results of a clinical series. Non-English articles were included when deemed necessary. Copies of the entire articles were obtained. Bibliographies of the retrieved articles were manually searched with use of the same search guidelines. In the third stage, articles were reviewed and complications of LASIK were compiled and incorporated into the manuscript. We have derived the incidence of various LASIK complications primarily from three large studies (1000 eyes). Due to the rapid evolving nature of the subject, we included some information gathered from anecdotal reports, selected presentations at scientific meetings, and from our personal experience. Additionally, stages 1–3 were repeated upon final revision of the manuscript to include important articles published in the February 2000 to February 2001 interval.



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