Preoperative Evaluation and Diagnostic Approach

The first step in the evaluation should be to determine the goals of the patient in seeking refractive surgery and assess whether the patient has realistic expectations. Patients should understand the risks, benefits, and alternatives to the LASIK procedure. A stable refraction is important. Most surgeons now limit the upper range of correction to treatment of 8–10 D of myopia even though the lasers are capable of treating higher corrections.

Also important is a review of ocular and systemic conditions. Visual acuity is measured using manifest and cycloplegic refraction. The refraction is compared with prior spectacle corrections to assess the stability of refraction for the given eye. In addition, the wavefront refraction can be used as the baseline to obtain the wavefront-adjusted manifest refraction (WAMR). Pupil size, ocular dominance testing, and distance and near vision with and without correction should also be documented. Anterior and posterior segment examinations are performed to rule out other conditions that may adversely affect the surgical result.

Measurement of the central corneal thickness is an important element of the preoperative evaluation. WFG and optimized procedures usually remove more stromal tissue compared with conventional treatments. An estimated residual stromal bed is needed for surgical planning, because it is one factor that may predict postoperative ectasia. It is computed by subtracting the anticipated flap thickness and maximum ablation depth from the central corneal thickness measurement. The minimal residual bed for LASIK remains controversial, although 250–300 µm is generally recognized as a minimum.

Ectasia can occur with even thick residual stromal beds in eyes with keratoconus features. To account for variations in actual flap thickness, the stromal bed can be measured after the flap has been retracted to allow for a more precise determination of the residual bed. A percentage of anterior tissue depth altered (PTA) ≥40 at the time of LASIK was significantly associated with the development of ectasia in a study of eyes with normal preoperative topography.

Computer-assisted videokeratoscopy, corneal tomography, and Scheimpflug imaging are now available and used routinely in the preoperative and postoperative assessments of refractive surgery patients. These tools can help to screen for subclinical keratoconus or other corneal diseases.

Rigid contact lens wearers should remove their lenses for 3–4 weeks before examination, and soft contact lens wearers should have 2 weeks without lenses.

The possibility of monovision should be discussed with patients near or of presbyopic age; the discussion should include glare and halos, the possibility of under- and overcorrection, and any special considerations. Appropriate reading materials are helpful for patient education.

Informed consent should include a discussion of the most frequent side effects and potential risks involved with the surgery.

Alternative refractive treatments such as PRK should be discussed and may be preferred to LASIK in patients with anterior basement membrane dystrophy (ABMD), thin corneas, small and deep-set orbits, anterior scleral buckles, glaucoma after trabeculectomy, optic nerve disease, a risky occupation or activity, and corneal ectasia.

The option of phakic IOL implantation can be offered to patients with high myopic corrections, very thin corneas, or abnormal corneal topography and/or tomography. Natural lens replacement is an option for patients nearing cataract age or requiring higher hyperopic corrections.

Limitations and Contraindications

Laser vision correction may have a higher risk in patients with collagen vascular disease, patients with autoimmune or immunodeficiency diseases, women who are pregnant or nursing, patients with signs of keratoconus, and those taking isotretinoin or amiodarone. Other conditions potentially associated with more adverse outcomes include Fuchs’ corneal dystrophy, strabismus, ophthalmic herpes simplex or herpes zoster, and other systemic diseases likely to affect healing, such as diabetes and atopic disease. Caution should be exercised for patients with abnormal corneal topographies or with ocular abnormalities as well as systemic conditions that are likely to affect wound healing.

Microkeratome Surgical Technique

Topical anesthesia is applied to the eye. The eyelids are prepared with dilute povidone–iodine solution. A lid speculum is inserted to open the eyelids. Eyelashes should be kept away from the surgical field by the use of adhesive drapes or a closed bladed lid speculum. The contralateral eye is taped shut to prevent cross-fixation and drying. The microkeratome should be inspected for any defects in the blade or function of the moving parts. It is vital to confirm that the excimer laser will be able to deliver treatment after the corneal flap is reflected.

The cornea can be marked with ink before creating the corneal flap with the microkeratome to more easily realign the corneal flap in the event that a free flap is created.

The suction ring is placed using a bimanual technique in which the shaft of the suction ring is held in the fingers of one hand and a finger from the other hand provides additional support on the ring itself. Once adequate placement has been achieved, the suction is engaged by foot control (usually done by the technician). Adequate intraocular pressure (above 65 mm Hg) is then verified, with one useful method being an applanation lens (Barraquer tonometer). When adequate suction is achieved, the patient will confirm the temporary loss of visualization of the fixation light.

Before the pass of the microkeratome, several drops of artificial tears are placed on the cornea. This lubrication reduces the likelihood of a corneal epithelial defect occurring during the microkeratome pass. If using a two-piece microkeratome, the head is slid onto the post of the suction ring and advanced until the gear on the microkeratome head engages the track. It is important to again verify that the suction ring is still firmly attached to the globe at this point by gently lifting the suction ring upward, making sure that the suction is not lost. The surgeon then activates the microkeratome using forward and reverse foot control, the suction is turned off after the microkeratome pass, and then the suction ring can be carefully removed. Prompt attention at this point is extremely important in the case that a free cap or buttonhole has been created. In cases in which the stromal bed is too small or irregular for a good result, the laser ablation should not be performed, and the flap should be placed carefully back into position.

Before the flap is lifted, a wet cellulose sponge is used to prevent any cells, debris, or excess fluid from getting onto the stromal bed. With use of a flat cannula, iris sweep, or smooth forceps, the flap can be lifted and directed toward the hinge. Assessment of the thickness of the residual corneal bed may be performed by using ultrasound pachymetry. Microkeratome flap thickness may vary, with differences in thickness occurring even with the same microkeratome, making this measurement more important in eyes that may require a deeper ablation with the excimer laser, thus leaving less tissue in the residual corneal bed.

Femtosecond Laser Flap Creation

The femtosecond laser is a highly precise, computer-guided laser that enables surgeons to program flap parameters, such as diameter, depth, hinge location, and edges, based on patients’ anatomical characteristics or surgeons’ preferences (Video 3.4.1 ). Flap parameters such as pattern, hinge position, flap depth, flap diameter, bed energy and spot separation, side cut angle and energy, hinge angles, and pocket parameters need to be carefully programmed or reviewed by the surgeon before the surgical procedure is initiated.

A suction ring is applied to the eye with slight downward pressure, and suction is then enabled to affix the ring firmly to the eye. Proper centration is extremely important and is a critical step that ensures greater accuracy of all subsequent steps of flap creation ( Fig. 3.4.4 ). With the eye fixated, the cornea is then fully applanated. Centration and flap size should be confirmed on the monitor before the laser is initialized for treatment. It is critical to instruct the patient to abstain from eyelid squeezing during this step to prevent suction loss ( Fig. 3.4.5A ).

Suction Ring Centration Is a Critical Step That Ensures Accuracy of All Subsequent Steps of Femtosecond Laser Flap Creation.

Raster pattern flap creation with Intralase femtosecond laser (A). Femtosecond flap dissection with blunt spatula. (B).

Once the procedure is complete, suction can be released and the same technique can be repeated on the contralateral eye.

Lifting femtosecond flaps can be quite different from lifting a microkeratome flap and is rather a blunt dissection, best accomplished with the use of a spatula ( Fig. 3.4.5B ). The spatula should be entered near the hinge and gently elevated in one to four swipes. The flap is then reflected, and the surgeon can proceed with excimer laser treatment.

Excimer Laser Ablation

The patient should be positioned under the microscope with the head carefully aligned to make sure that the iris is perpendicular to the laser beam. Careful centration with the eye aligned in the x, y, and z planes is crucial. Alignment is even more critical with wavefront-guided laser treatments because most HOA are not radially symmetrical. Torsional misalignment—either cyclotorsion or head tilt—during surgery can result in undercorrection of the aberration or even in the induction of additional aberrations. The most basic technique to ensure alignment is to mark the limbus, typically at the 3 and 9 o’clock positions, immediately before surgery while the patient is seated. These marks are then used to align the head when the patient is lying under the laser. Most modern laser systems are now equipped with either pupil tracking or iris registration.

The surgeon should always verify the entered computer data before starting the ablation. The microscope should be adequately focused on the corneal surface. A dry cellulose sponge is then used to carefully remove any excess fluid from the stromal surface. Hydration of the stromal bed needs to be adjusted evenly and consistently in all cases. It is important at this point to minimize the procedure time in order to prevent stromal dehydration and subsequent overcorrection. Uneven hydration can lead to central islands and/or irregular astigmatism. Excess pooling of fluid often can be found near the hinge after folding back the flap and should be wicked away. The patient should be instructed to fixate on the target light. Adequate centration over the pupil should be reassessed ( Fig. 3.4.6 ).

Patient Fixation on Target Light Under Laser and Tracker Fixation on Pupil Width Should Be Maintained at All Times During Excimer Treatment.

The laser eye tracker and iris registration are activated, and the laser ablation initiated. If fluid starts accumulating over the stromal surface, the laser ablation can be halted, and the fluid should be removed with a cellulose sponge. The hinge should be protected if it is within the ablation zone.

After the ablation, the flap is then repositioned onto the bed using an irrigation cannula or an iris sweep. Saline solution is used to remove debris from the interface ( Fig. 3.4.7 ). A wet cellulose sponge is then used to realign the flap. Sweeping movements should be performed from the hinge toward the periphery of the flap.

Irrigation Under the Flap Can Remove Debris From the Interface.

Care must be taken not to overirrigate, as this can increase the risk of flap striae from overhydration.

Good adhesion of the flap is verified by stretching the flap toward the gutter. If good adhesion is present, there is minimal space in the gutter and no movement of the flap occurs when stroking the flap with a dry sponge. When the flap is felt to be securely in position, a drop of an antibiotic, a corticosteroid, and a lubricating agent may be applied to the cornea before removal of the speculum. If bilateral LASIK will be performed, the operated eye is covered and the procedure repeated in the contralateral eye. Both eyes are then protected with transparent plastic shields until the following day.

Intraoperative Complications

An incomplete flap may result from the premature termination of the microkeratome advancement or ineffective passes or suction loss during a femtosecond flap creation. Reasons for a microkeratome incomplete pass include inadequate globe exposure due to interference of eyelid, lashes, speculum, and/or drape as well as loss of suction during the pass. If there is not enough room beneath the flap to perform the ablation, then the surgeon should reposition the flap and abort the surgery. Incomplete flaps also can result when the femtosecond laser cannot photodisrupt the corneal stroma in portions of the intended interface or if there is resistance within the interface from scar tissue. Typically, surface laser treatment with mitomycin-C can be used later to complete the refractive correction.

When suction loss occurs during a femtosecond LASIK flap creation, resulting in an incomplete flap, a second femtosecond pass can create an intact flap. Tomita and colleagues reported successful immediate lamellar recut in their series of eyes with suction loss. Some controversy surrounds the rationale for immediate recut, as other authors have demonstrated the potential for creating new cleavage planes if immediate recut is undertaken. When suction loss occurs during the side cut or just before starting the side cut, a repeat side cut can be performed with a smaller diameter.

A flap buttonhole is one of the most serious flap complications, and the excimer laser ablation should be aborted. The flap should be repositioned and smoothed into place. Treatment of the second eye is not advisable at the same setting, as the same complication is likely to happen in the presence of a steep cornea or poor suction. However, these almost always occur in the second eye with a thinner flap in mechanical microkeratomes. Femtosecond laser flaps also are prone to similar complications, albeit usually from a different mechanism: vertical gas breakthrough. Epithelial ingrowth or haze may occur in the area of the buttonhole and may require further intervention. Typically, retreatment can be performed immediately or later with phototherapeutic keratectomy (PTK)/PRK and mitomycin-C.

A free cap can occasionally occur when using a microkeratome to create a flap, and the surgeon should be prepared to deal with this problem. If the cap is small and/or decentered, it should be replaced without ablation and the procedure aborted. If it is well centered and of adequate size, the cap is typically placed on the conjunctiva with the epithelial side down during the photoablation. Care must be taken to reposition the cap into the same orientation after the ablation. Adequate drying time should be allowed for the cap to adhere without sutures. The most frequent cause of a free cap is a flat or small cornea in which there is less tissue to be brought forward into the microkeratome. Poor suction also can cause small free flaps. Marking of the cornea before flap creation can facilitate alignment in the event of a free cap.

Epithelial defects can be prevented with adequate lubrication of the cornea before the microkeratome pass. Also, toxic anesthetics should be kept to a minimum before the procedure. If a large epithelial defect occurs during the microkeratome pass, the ablation should be aborted in the affected eye. It is not recommended to proceed with flap creation in the contralateral eye, as the same complication is likely to occur. On the other hand, if the epithelial defect is small, a contact lens can be placed over the cornea to decrease significant discomfort to the patient. An epithelial defect may lead to greater flap edema with poorer adherence in the area of the defect, increasing the risk of epithelial ingrowth and diffuse lamellar keratitis.

Epithelial breakthrough is a rare complication during femtosecond laser corneal flap creation and is generally observed in patients with prior corneal incisions in the ablation zone. In these situations, increasing femtosecond energy or microkeratome flap creation should be considered. In patients with severe corneal scars that obscure visualization of anterior segment structures, femtosecond flap creation should be avoided. It is important to be attentive for this complication intraoperatively and to abstain from lifting the flap to prevent more serious flap problems such as buttonholes or flap tears.

An opaque bubble layer (OBL) and anterior chamber gas bubbles can present during corneal flap creation with a femtosecond laser. An OBL forms when gas bubbles created by laser photocavitation are trapped inside the stoma. Gas also can appear in the anterior chamber and persist up to several minutes or even hours. The OBL can make lifting of the flap difficult, and care must be taken to perform a gentle blunt dissection to prevent a flap tear in the area of the OBL. If the OBL or anterior chamber bubbles are over the pupillary area, they may interfere with an eye-tracking mechanism during excimer laser ablation. In these situations, it is usually recommended to wait for bubbles to dissipate before proceeding with laser treatment. Other factors associated with increased frequency of OBL include steeper, thicker corneas and a hard docking technique. Although the OBL makes the LASIK procedure more difficult, it does not appear to affect the postoperative optical quality and visual outcome ( Fig. 3.4.8 ).

Opaque Bubble Layer (OBL).

Note gas bubbles trapped inside the stoma. The OBL can make lifting of the flap difficult and, if over the pupillary area, may interfere with an eye-tracking mechanism during excimer laser ablation.

Ablation Complications

Central islands are small central elevations in the corneal topography that may occur for a variety of reasons. Beam profile abnormalities, increased hydration of the central corneal stroma, or particulate material falling onto the cornea may block subsequent laser pulses. This was more common with broad-beamed lasers. Typically these central islands resolve with time as epithelial remodeling fills in the surrounding area.

Decentration ( Fig. 3.4.9 ) can result from poor fixation and alignment, eye movement during the laser procedure, significant pupil shift with light, or asymmetrical hydration of the cornea. The higher the myopic correction, the greater the risk of a decentered ablation, which can result in glare, irregular astigmatism, and a decrease in BCVA. Low-contrast visual acuity is a more sensitive measurement of visual function than high-contrast Snellen acuity and can be used to assess these patients more accurately. Decentration may be decreased with the use of current lasers with incorporated eye-tracking systems and iris registration, yet careful attention must still be paid to patient fixation. Typically, if the ablation is more than 1 mm decentered, the irregular astigmatism that occurs is symptomatic. Management of decentration by treatment based on wavefront or topographical information may decrease symptoms in patients with an unsatisfactory outcome with the first procedure.

Superior Decentration on Scheimpflug Imaging Following Myopic Excimer Ablation.

Under- and overcorrection may result from errors of refraction, improper surgical ablation, malfunctioning of the excimer laser, abnormal corneal hydration status, or an excessive or inadequate wound-healing response. It is crucial to maintain consistent hydration of the cornea, because excessive fluid on the cornea results in an undercorrection. If desiccation of the corneal stroma is present, then overcorrection and haze may occur. The higher the refractive error, the greater the chance of regression. Many surgeons find that adjusting the amount of treatment using a nomogram based on their actual surgical results improves their refractive outcomes.

Postoperative Complications

Interface debris is common even with aggressive interface irrigation. The most frequent source of debris is meibomian gland material from the lids that is trapped in the interface. Careful draping of lashes and cleaning of the flap interface with balanced salt solution before and after the flap is floated into position can help to reduce the incidence of this problem.

Flap displacement usually occurs in the first 24 hours postoperatively. When a flap displacement occurs, it should be lifted and repositioned as soon as possible. The epithelium at the flap edge grows remarkably fast to cover the stromal bed. Care must be taken to clean the bed and back of the flap of debris and epithelial cells. Stroking the cap with a cellulose sponge can minimize persistent folds in the flap and properly line up the cap with the bed.

Punctate epithelial keratopathy (dry eye) is the most frequent complication of LASIK. Several possible mechanisms contributing to LASIK-induced dry eye have been proposed. The mechanisms include injury to the afferent sensory nerve fibers, a reduction in neurotrophic influences on epithelial cells, a decreased blinking rate, decreased tear production, altered tear-film stability and distribution, increased tear evaporation, and injury to limbal goblet cells. The preponderance of data supports the hypothesis that the most important factor in the pathophysiology of LASIK-induced dry eye is the transection of afferent sensory nerves in the anterior third of the stroma during the lamellar cut. The disorder tends to be more common and more severe in the context of underlying chronic dry eye. It has become evident, however, that the disorder is multifactorial. Studies in patients in whom the flap was created with a microkeratome showed significantly more signs and symptoms of LASIK-induced dry eye than those in which a femtosecond laser was used for flap creation. Treatment involves frequent lubrication of the ocular surface with artificial tears, topical anti-inflammatory therapy, and punctual plugs; management of any eyelid disorder also may be of benefit.

Diffuse lamellar keratitis (DLK), also known as Sands of Sahara syndrome, is an interface inflammatory process that occurs in the early postoperative period after LASIK ( Fig. 3.4.10 ). Patients are initially asymptomatic and often have no visual impairment. A fine granular-appearing infiltrate that looks like dust or sand typically presents initially in the interface periphery. If left untreated, the inflammation can progressively worsen and may lead to corneal scarring with resultant irregular astigmatism. The cause of DLK is likely multifactorial. Bacterial toxins or antigens, debris on the instruments, eyelid secretions, or other factors may play a role. Femtosecond LASIK flap creation has been associated with a higher risk for DLK than microkeratome flap creation. Studies suggest that newer generation femtosecond lasers with higher frequency are associated with decreased DLK rates because lower energy settings are used in creating LASIK flaps.

Diffuse Lamellar Keratitis (DLK).

The image shows stage II DLK, and identification of this should be followed by increased topical corticosteroid administration and close follow-up.

Treatment involves frequent topical corticosteroids, and if severe enough, interface irrigation. However, some authors have reported excellent results in the treatment of severe DLK with high-dose topical and oral corticosteroids without flap lifting and interface irrigation.

Flap striae and microstriae are common complications after LASIK. Most striae are asymptomatic and can be visualized if the flap is carefully examined with retroillumination. When microstriae occur over the pupil or when macrostriae exist, irregular astigmatism with visual aberrations and monocular diplopia may result. In such cases, the flap should be lifted again, hydrated, and stretched back into position.

Epithelial ingrowth into the interface between the flap and the stromal bed occurs in up to 3% of patients following myopic LASIK surgery ( Fig. 3.4.11 ). Known risk factors for this complication include epithelial defects at the time of surgery, history of recurrent corneal erosions, corneal basement membrane epithelial dystrophy, history of ingrowth in the other eye, hyperopic LASIK correction, flap striae or folds (as when the flap dislocates after surgery because of trauma or poor adhesion), flap instability, type 1 diabetes, and repeated LASIK surgeries. Rarely, the epithelial ingrowth progresses into the central visual axis, causing irregular astigmatism and loss of BCVA. In some cases, the epithelial cells will block nutritional support for the overlying stroma and lead to flap melt. If this is the case, the flap should be lifted, and careful scraping of the epithelium should be performed at the stromal bed as well as under the flap. In recurrent cases, suturing or flap gluing may help to reduce the incidence of recurrent epithelial ingrowth. Nd:YAG laser application has been described as a technique for the treatment of epithelial ingrowth after LASIK surgery.

(A) Central epithelial ingrowth under LASIK flap following traumatic flap displacement. (B) Peripheral epithelial ingrowth with associated flap melting.

Corneal haze can occur following LASIK. With fears of corneal ectasia following LASIK, surgeons continue to aim for thinner flaps. Although thin-flap LASIK has been reported to be successful, there have been reports of interface haze formation with thin-flap femtosecond LASIK (<90 µm), especially in young patients and could be related to focal disruptions of the Bowman’s layer and injury to the epithelial basement membrane.

Infectious keratitis after LASIK is a devastating, vision-threatening complication. Fortunately, the estimated incidence is low and reported to be between 1 in 1000 and 1 in 5000 procedures. Reported organisms include Mycobacterium spp., fungi, Nocardia spp., Staphylococcus aureus, Streptococcus viridans, coagulase-negative Staphylococcus spp., and Streptococcus pneumoniae. The most common organism cultured in a worldwide survey was methicillin-resistant Staphylococcus aureus (MRSA). Symptoms may include pain, photophobia, watering, decreased visual acuity, ghost images, and halos. Slit-lamp examination may reveal ciliary injection, epithelial defect, anterior chamber reaction, and hypopyon. In the case of mycobacteria and fungi, presentation is usually delayed several weeks after the LASIK procedure and then has a smoldering course. Clinically, the mycobacteria and fungi usually are seen in the interface, often with a feathery or indistinct margin. Gram-positive infections are usually seen shortly after the procedure, often at the flap margin, and usually have distinct, sharp margins.

It is important to maintain a high suspicion for atypical organisms. The flap should be lifted and cultures should be inoculated on blood, chocolate, Sabouraud and Lowenstein–Jensen agar, and blood–heart infusion. Smears should also be taken for Gram, Giemsa, and Calcofluor white stains as well as Ziehl–Neelsen for acid-fast bacteria.

The mycobacteria species associated with keratitis following LASIK belong to the nontuberculous mycobacteria group. These species are resistant to chemical disinfectants such as chlorine, which is probably why such infections may occur after surgical procedures. Clarithromycin and amikacin are the antibiotics of choice, but poor penetration of topical medications leads to persistent infection. Early flap lifting and soaking of the flap and bed with amikacin 0.08% and/or clarithromycin 1% followed by aggressive topical therapy leads to the best results.

Perhaps the most important factor within our control is prevention. Meibomian gland disease should be treated before LASIK. Instruments must be properly sterilized, and intraoperative sterile techniques should be employed, including the use of sterile gloves and drapes and disinfection of the skin and eyelids with povidone–iodine. During the procedure, instruments should be sterile and sterile plastic bags used for the nonsterile portions of the laser. Efforts should be made to avoid irrigating meibomian secretions into the interface. Suction lid specula may be helpful in removing excessive fluids and debris. Postoperatively, the patient should be instructed to wear shields and not to rub the eye. Prophylactic antibiotics should be used for 1 week postoperatively.

Keratectasia

Retrospective analysis of patients with ectasia suggests the following risk factors for progression of ectasia after laser vision correction: (1) abnormal preoperative topography, (2) low residual bed thickness, (3) young age, (4) low preoperative corneal thickness, and (5) high myopia. General agreement exists on leaving 250–300 µm of untouched posterior cornea stroma. Ablation below that limit may cause biomechanical weakening, causing the cornea to bulge forward. Another important cause of iatrogenic keratectasia is LASIK performed on unrecognized keratoconus suspects. Videokeratographic clues for a keratoconus suspect may include steep keratometry, inferior steepening of the cornea, asymmetry of the corneal curvature, or nonorthogonal astigmatism. Iatrogenic keratectasia has been reported as early as 1 week and as late as 2 years postoperatively.

Although ectasia is rare after laser vision correction, many management options are available. Most are similar to those available for naturally occurring keratoconus. Most patients can still function well with nonsurgical options, such as contact lenses. Intracorneal ring segments may be used to manage patients with ectasia after refractive surgery. Corneal collagen crosslinking (CXL) with riboflavin and ultraviolet-A (UVA) radiation is now being used to treat keratoconus and post-LASIK ectasia; the principal goal of the technique is to stabilize the progression of these corneal diseases. In previous studies of CXL outcomes, patients had improvement in corrected distance visual acuity (CDVA), uncorrected distance visual acuity (UDVA), maximum and average keratometry (K) values, several corneal topography indices, and corneal and optical HOA.

Although corneal transplantation (penetrating or lamellar) is not often required, transplantation does have a high rate of success in eyes with keratoconus and would likely yield similarly good results in eyes with ectasia after refractive surgery.