eMedicine Specialties > Ophthalmology > Refractive Disorders

Myopia, LASIK: Treatment

Author: Michael Taravella, MD, Director of Cornea and Refractive Surgery, Rocky Mountain Lions Eye Institute; Professor, Department of Ophthalmology, University of Colorado School of Medicine
Coauthor(s): Timothy A Perozek, MD, Consulting Ophthalmologist, Private Practice, Perozek Professional Corporation and Westfield Eye Center; Scott A Thomas MD, Staff Physician, Department of Ophthalmology, University of Colorado, Rocky Mountains Lions Eye Institute
Contributor Information and Disclosures

Updated: May 11, 2009

Treatment

Preoperative Details

To a large extent, patient selection for LASIK often determines the overall success of the procedure; therefore, it is crucial that a thorough preoperative examination be performed, accompanied by appropriate counseling. Contact lens wear should be discontinued prior to the examination; ideally 2 weeks for soft contact lens wear and 3 weeks for rigid gas permeable lenses.

A complete eye examination, including manifest and cycloplegic refraction, slit lamp examination, dilated fundus examination, and corneal topography, is recommended. Wavefront measurements can also be taken as part of the initial screening examination and are helpful in determining if the patient is a candidate for custom treatment and as a comparison to the current glasses prescription and refraction. In addition, an estimate of scotopic pupil size is helpful in screening candidates who may be at risk for postoperative glare.

Poor surgical candidates include patients with a refraction out of the recommended correction range, patients with active inflammation of the external eye or iritis, and patients with cataracts or retinal holes or tears. Although LASIK surgery has only rarely been associated with vitreoretinal pathology, retinal detachments following surgery have been reported.⁹ Therefore, screening with indirect ophthalmoscopy is advisable. Dry eye is a relative contraindication as well, and every effort should be made to improve the health of the ocular surface prior to performing any refractive procedure.¹⁰ Patients with chronic punctate keratitis, meibomitis, and blepharitis are generally poor candidates unless these conditions can be resolved prior to surgery. A short trial of Restasis, artificial tears, and even tetracycline (for meibomitis) often results in a significant improvement of the ocular surface.

The refraction should be stable prior to performing surgery. Stability can be assessed by serial refractions and an evaluation of medical records and old glasses. Any change greater than 0.50 D compared to the above is suspect and suggests that the current refraction is not stable.

Corneal topography is essential to rule out keratoconus and irregular astigmatism. These problems tend to make the surgical outcome unpredictable. In particular, keratoconus patients may be more prone to the development of ectasia or thinning following LASIK; refractive surgery on this group of patients is considered investigational. Several topography units come with built-in screening programs based on criteria developed by Rabinowitz and Klyce to aid in the detection of keratoconus.⁸ Corneal topography also is helpful in evaluating contact lens-induced corneal warping. Patients with irregular corneas and a history of contact lens wear should be observed with serial refractions and topography until both stabilize.

Finally, ultrasonic pachymetry is necessary to determine if enough corneal thickness is present to create a flap, ablate the cornea, and still leave enough tissue behind to prevent structural weakening and ectasia. Current guidelines recommend leaving at least 250 µm of cornea untouched.

Intraoperative Details

The procedure usually is performed under topical anesthesia, but it can be supplemented by intravenous or oral conscious sedation.

A sterile drape and lid speculum is placed carefully to maximize exposure and to isolate the lashes. The patient is positioned underneath the microscope of the laser so that the flap can be cut under direct visualization.

The cornea is marked. A radial keratotomy marker and optic zone marker (placed eccentrically) dipped in methylene blue or gentian violet can be used. The marks allow replacement and alignment of the flap in the event that a nonhinged free flap is cut by the microkeratome.

Balanced salt solution (BSS) is used to rinse the ocular surface and to moisten the conjunctiva. Excess solution can be removed from the conjunctival fornices with Weck-cel sponges or a suction speculum. This rinsing removes mucus and debris from the ocular surface decreasing the chance that this material will find its way under the flap at the end of the procedure.

The following technique applies to the use of the Amadeus microkeratome. However, the principles are similar no matter which microkeratome is used. The combined suction ring and microkeratome is placed on the eye and centered over the limbus with slight nasal displacement. Unlike the older style of microkeratomes, the Amadeus microkeratome does not require an on-eye assembly; this is particularly advantageous for novice surgeons. Nasal displacement ensures that the hinge of the flap will be clear of the path of the excimer laser ablation, but it increases the risk of a free flap. (This technique does not apply to the Hansatome microkeratome because the hinge is located superiorly with this device.) Suction is turned on by the surgeon or assistant.

The pressure in the eye is checked with a tonometer confirming that the intraocular pressure is at least 60 mm Hg. The pupil often can be seen to dilate, and the patient's vision will black out momentarily. Note that pupil dilation cannot be used as a sign of good suction for eyes treated with the Alcon LADARVision system, as dilation is performed prior to treatment. Intraocular pressure with the suction ring applied is between 60-90 mm Hg. High pressure is necessary to hold the suction ring firmly in place and to properly expose the cornea to the cutting mechanism of the microkeratome.

A depth plate in the microkeratome determines the planned thickness for the flap resection (140 µm and 160 µm depth plates are the most common for the Amadeus microkeratome). Factors that affect the selection of the depth plate include degree of myopia to be corrected and preoperative corneal thickness as measured by ultrasound pachymetry. A residual (untouched) corneal thickness of about 250 µm is desirable to prevent possible structural weakening of the cornea and progressive ectasia. Therefore, a patient with high myopia and thinner corneas would require a thinner depth plate.

Once good suction is confirmed, a foot pedal is used to simultaneously switch on the motorized vibrating blade that cuts the corneal flap and the mechanism that advances the microkeratome. The microkeratome should not be manipulated during the flap cutting phase, and it is important to remind the patient not to move or attempt to squeeze the eye shut during the cut. The hinge width can be programmed on the microkeratome computerized interface and is set based on the corneal curvature and the inner diameter of the chosen ring size. The microkeratome automatically reverses, suction is turned off, and the microkeratome assembly is removed.

Inspect the flap prior to lifting. In general, thin or irregular flaps are left in place with minimal manipulation. A spatula is placed between the flap and the stromal bed, and the flap is reflected nasally.

The laser is focused and centered, and the planned refractive ablation takes place. Most lasers have a tracking mechanism that tracks eye movements and locks onto the pupil.¹¹ The tracker is engaged prior to performing the planned laser ablation. Upon completion of the ablation, the flap is swept back into position with a spatula and then floated into position with irrigation under the flap. This irrigation also helps keep the interface between the flap and the corneal bed free of debris. A moistened Weck-cel sponge is lightly stroked once or twice over the corneal flap to squeeze out excess moisture in the bed, being careful not to apply so much pressure as to induce wrinkles in the flap.

The previously placed radial keratotomy marks and the eccentric zone mark are used to ensure that the flap is aligned properly. Once the surgeon feels alignment is satisfactory, the flap is allowed to remain undisturbed for several minutes. Endothelial pump pressure is the initial force that holds the flap in place.

The lid speculum and draping is removed carefully from the eye. The patient is allowed to blink to ensure that the flap remains in place. Immediate slit lamp examination is useful in detecting misplacement or wrinkles in the flap. The flap can be refloated and repositioned, if necessary.

Postoperative medications (eg, topical antibiotic drops, topical steroid drops) are administered. Then, a see-through bubble shield is placed over the eye to prevent inadvertent rubbing of the eye.

Enhancements

One of the great advantages of LASIK over other refractive procedures is the ease and safety of performing enhancement surgery.¹² Enhancements should be postponed until the refractive error is stable, usually about 3 months postoperatively. It is common to wait longer, up to 6 months, for patients who experience an overcorrection because this will often regress.¹³ The corneal flap can usually be lifted easily within the first several years after surgery; beyond this time period, consideration should be given to cutting a new flap with the microkeratome.

Enhancement surgery is performed by first positioning the patient at the slit lamp. A special shaped spatula is used to gently lift the edge of the flap and to find the corneal plane of the original cut. Then, the patient is positioned under the laser. The cornea is marked as usual. There is no risk of a free flap, but these marks help in realigning the flap. A blunt spatula is passed under the flap and swept gently back and forth, almost to the edge of the flap but avoiding breaking through the edge to the surface. The flap is grasped firmly with non-tooth forceps and peeled back, creating a clean epithelial edge. The laser treatment proceeds as usual, replacing the flap after the procedure is complete. Some practitioners prefer to use a contact bandage lens to protect the flap and for patient comfort after surgery since the epithelial edge tends to be more irregular than if the flap were cut with the microkeratome.

Noting the original depth plate and the hinge location is helpful when primary LASIK is performed; this helps prevent tearing the hinge accidentally. Tearing can occur if the surgeon anticipates a nasal hinge, but a superior hinge was used previously.

Occasionally, a patient will not have enough residual corneal stroma in the flap to allow for an enhancement. In these patients, the correction can be applied to the surface on top of the flap by performing PRK. In general, this requires the use of mitomycin C to prevent corneal scarring and haze after treatment.

In patients whose records may not be available, the use of a device, such as the Visante OCT, may aid in determining the thickness of the original flap.

Postoperative Details

The patient usually is seen within the first 24 hours following surgery to check visual acuity, to inspect flap position, and to ensure that no signs of infection or inflammation in the cornea are present.

A regimen of postoperative antibiotics, given 4 times a day for 1 week, is recommended. Fourth-generation fluoroquinolones are a good choice because of excellent corneal penetration and broad-spectrum coverage. Currently, there are 2 commercially available preparations: moxifloxacin 0.5% and gatifloxacin 0.3%. There is much debate in the ophthalmic community as to which of these topical antibiotics is best for this setting. Similarly, consensus on the use of topical steroids does not exist. However, most surgeons prescribe their use for the first week after surgery, discontinuing or tapering rapidly thereafter. A potent and penetrating steroid, such as prednisolone acetate 1%, commonly is used. This helps prevent inflammation under the flap. The role of topical steroids in influencing postoperative healing and regression has not been determined.

Patients who are undercorrected or who appear to be regressing rapidly (increasing myopia), as determined by serial refractions, may benefit from more prolonged treatment with topical steroids and a slower tapering off of these drops. Overcorrected patients may benefit from discontinuing steroids early in the postoperative period and by the use of topical nonsteroidal drops. These pharmaceutical maneuvers have not been studied in any controlled or randomized fashion.

Follow-up

Follow-up examinations are performed on day 1, week 1-2, 3 months, 6 months, and 1 year after surgery. The examination should include uncorrected and best-corrected visual acuity, slit lamp examination, and tonometry (this examination is crucial if the patient is still on topical steroid drops). The corneal thinning associated with LASIK surgery can result in falsely low tonometry readings. It is important to also note that this is not the same as congenitally thin corneas, and nomogram adjustments for corneal thickness versus pressure have not been worked out for postrefractive patients. The Tono-Pen may be preferred over applanation for pressure measurements, since it seems to be less sensitive to corneal thickness variations.¹⁴

Corneal topography is a useful adjunct in assessing postoperative results and planning enhancements and should be performed between week 1 and month 6. Centration and ablation pattern can be assessed best with topography; it is especially useful in patients who have an unexplained decrease in best-corrected visual acuity.

Repeat wavefront analysis prior to performing an enhancement is helpful to confirm the refraction, especially astigmatism and cylinder axis.

For excellent patient education resources, visit eMedicine's Eye and Vision Center. Also, see eMedicine's patient education article Vision Correction Surgery.

Complications

Complications can be divided into intraoperative (usually microkeratome related) and those that occur postoperatively.^15,16,17 The following list outlines the more common complications, the time period in which they are likely to be seen (ie, immediate, early postoperative, late postoperative), and an approximate incidence of occurrence. Each complication will be discussed in more detail in the following section.

Intraoperative microkeratome-related complications include the following:

• Entry into eye (intraoperative; rare)¹⁸
• Thin, irregular, or perforated flap (intraoperative; <0.2%)¹⁹
• Free flap (intraoperative; rare; 0.2%)

Laser-related complications include the following:

• Decentration (<1%)
• Irregular astigmatism (<1%)

Other postoperative complications include the following:

• Visually significant wrinkles or striae in the flap (1%)
• Dislocated flap (early postoperative period)
• Infection (early postoperative period; very rare; <0.02%)^20,21,22
• Diffuse intralamellar keratitis (<0.1%)^23,24
• Epithelial ingrowth (early to late postoperative; 1-2%)²⁵
• Under/overcorrection (see results)
• Ectasia (incidence unknown; <0.01%)

• Perforation and entry into the eye: Probably the most dreaded complication related to use of the ACS is perforation and entry into the eye. Because the eye is pressurized to about 60 mm Hg, entering the eye at this pressure is particularly hazardous. Case reports of iris and lens injury occurring at the time of entry are well documented. The cause is improper assembly of the ACS unit; specifically, leaving the depth plate out. True incidence of this rare complication is unknown. Newer microkeratomes from most manufacturers (eg, Hansatome unit from Bausch & Lomb Surgical, Amadeus unit from AMO) have a built in depth plate to prevent this assembly error.
• Thin or perforated (poor) flap
  • Another feared complication is a thin or perforated flap.
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    Thin, perforated flap.

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    Thin, perforated flap.

  • It usually occurs with loss of suction or poor suction when the suction ring is applied. Steep corneas with average K readings of 46.00 D or greater are also at higher risk for perforated buttonhole flaps. When suction is turned on, the suction ring presses down around the limbus, causing distortion and an abrupt increase in pressure inside the eye. Characteristically, the pressure will rise to more than 60 mm Hg. A handheld tonometer is used to check the level of pressure in the eye. The pupil dilates, and the patient's vision blacks out as a result of temporary ischemia. Lifting the ring will correspondingly lift the eye.
  • All of the above are signs of good suction. If the surgeon detects poor suction, the procedure should be aborted and performed another day. In general, no attempt should be made to immediately replace the suction ring since the initial placement often causes slight conjunctival chemosis, precluding the possibility of obtaining good suction. Experienced surgeons may feel comfortable reapplying suction and attempting to cut the flap; however, the risk for a thin or poor flap is probably higher when repositioning the ring is attempted.
  • Another reason for poor flaps is patient eyelid squeezing during the microkeratome pass. This action pushes the microkeratome suction ring up and results in a thin flap or buttonhole. Squeezing can be prevented by adequately preparing the patient for the sound of the microkeratome and asking the patient to be especially careful about movement and eyelid closure at that moment. In general, lid blocks are not necessary.
  • The second eye in a bilateral case often has a slightly thinner flap as measured by subtraction pachymetry. Using one blade for both eyes in a given patient is common practice. The blade may dull slightly on the first eye; therefore, in patients with very steep corneas (46-47 D), using a new blade for the second eye may be helpful. This has not been shown to decrease the incidence of a thin or irregular flap.
  • Treatment of a perforated or buttonhole flap consists of immediately replacing the flap with as little manipulation as possible. If a buttonhole flap is recognized prior to lifting it, it should be smoothed back into position with Weck-cel sponges. Subsequent treatment depends on how the flap heals. If the flap adheres smoothly and heals without complications and if there is a good return of BCVA, then no immediate intervention is necessary. After 6 months, a flap can be recut. An alternative would be to consider subsequent surgery with the femtosecond laser cutting a flap well beneath the plane of the original cut. If the flap appears to have significant central irregularity, transepithelial PRK with adjunctive mitomycin C can be performed relatively early in the postoperative period.^26,27 The author has used this technique on occasion with good results.
• Thin or irregular flap: Other microkeratome-related problems that can result in a thin or irregular flap include binding, jamming, or a jerky pass of the microkeratome over the corneal surface. Such problems often are caused by poor maintenance and inspection of the microkeratome by the surgeon or technician. It is the surgeon's responsibility to confirm a smooth pass of the microkeratome while it is engaged in the suction ring prior to making a corneal flap. The blade should be inspected carefully under the surgical microscope by the surgeon or the technician to confirm its proper insertion into the microkeratome and to ensure that edge abnormalities are not present. For instance, a notch in the blade has been shown to cause a divided flap, according to Robert Maloney, MD, in a presentation at the University of Colorado in 1997.
• Free or partial flap: Free flaps occur for various reasons. Flat corneas with average K readings of 41.00 D and below are at risk for this complication. Excessive decentration of the suction ring on the cornea also can result in a free or partial flap. The key to managing this complication is composure and planning. First, good precut marks (usually made with a radial keratotomy marker and optical zone marker) on the cornea are essential for helping realign a free flap. Next, an assessment of the quality of the underlying stromal bed needs to be performed. If the bed is of good quality and appropriate size and position, the ablation is performed as usual.
• Handling a free flap
  • The recommended method of handling the flap is that, in general, less manipulation of the flap is required if no attempt is made to remove it from the microkeratome and place it in a desiccation chamber. Instead, the flap is left in the microkeratome. After the ablation is performed, the unit is repositioned in the same orientation as the cut was made. The flap is removed gently from the microkeratome by grasping it with toothless forceps and sliding it onto the stromal bed (premoistened). If performed properly, minimal rotation of the flap is required to align it with the marks made on the corneal surface at the beginning of the procedure. The flap is allowed to settle onto the corneal surface for a few seconds; then, it is smoothed gently into position with very light strokes of a moistened Weck-cel sponge.
  • Suturing the flap usually is not necessary, although a suture can be placed in the flap following replacement and drying to create a pseudohinge. A bandage contact lens is not necessary.

Laser-related complications

• [#target1]Decentration
  • Experienced laser surgeons recommend centering the laser ablation pattern over the pupil. All lasers currently approved for use in the United States are able to track the center of the pupil. A brief discussion of tracking technology appears in Future and Controversies. Despite tracking, however, decentration of the ablation can still be a significant problem with all excimer laser systems.²⁸
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    Decentered flap and ablation.

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    Decentered flap and ablation.

  • Factors that probably contribute to decentration include the following: (1) surgeon experience, (2) degree of myopia to be corrected, and (3) location of the visual axis line of sight versus the center of the pupil. The more myopic a patient is, the greater difficulty the patient may have in seeing fixation lights. Turning down external light sources (eg, oblique and ring lights on the VISX laser) aids patient fixation. Some controversy remains over whether it is better to center laser ablations over the pupil or the patient's line of sight. Normally, little clinical difference exists between the two methods; however, occasionally, patients have a large positive or negative angle kappa, and the decision on where to focus the laser becomes problematic. At present, no clinical studies that compare the two methods of laser alignment exist.
  • Compounding the problem is the way in which decentration is measured clinically. In general, topography maps of the cornea are used to assess alignment. However, topography centers around the line of sight and is based on patient fixation. This alignment is often slightly different than the corneal apex (highest point on the cornea) and the center of the pupil. If the difference between the line of sight and the center of the pupil is relatively large, the ablation pattern will appear decentered on the topographic map.
  • Actual decentration is characterized clinically by poor uncorrected and best-corrected vision, complaints of glare, "ghosting" around images and haloes, and refractive astigmatism (usually plus cylinder) in the axis of decentration. Light scatter occurring at the edge of the ablation zone causes the above symptoms. Normally, the pupil would mask light scatter; however, if the edge of the ablation pattern is near the center of the pupil, it becomes readily evident to the patient. Wavefront analysis may be helpful in establishing the diagnosis of decentered laser ablation since higher order aberrations, such as coma, may be more prevalent. Customized corneal ablations or topographic linked systems offer the best hope for correcting this problem and have been shown clinically to improve uncorrected and best-corrected vision while decreasing symptoms of glare and haloes associated with decentration.
• Irregular astigmatism
  • Irregular astigmatism can be caused by various intraoperative and postoperative complications. The most common complications include the following: (1) decentration of the ablation pattern, (2) problems with beam homogeneity, (3) irregular healing, and (4) scar formation from flap complications. The symptoms are similar to decentration: poor vision and optical aberrations (eg, glare, haloes).
  • Beam homogeneity can be assessed best by ablating thin films and looking for hot or cold spots. Subtraction topography (preoperative minus postoperative ablation) also can be useful in assessing this problem. A smooth ablation pattern should be evident after subtraction is performed since it will "subtract" preexisting topographic abnormalities from the postoperative topography. Meticulous laser maintenance with careful attention to the optical system is necessary to prevent this problem.
  • Central islands are a special case of irregular astigmatism and represent areas of unablated tissue in the central cornea. This problem has largely disappeared with the introduction of newer technology and software on all laser platforms. Patients may complain of poor vision, and undercorrection may be evident on refraction. Topography typically reveals a central area of elevation. Many central islands simply resolve over time and require no treatment. Again, a customized treatment approach or topographic linked lasers may offer the best hope of treating this condition.
  • Use of a rigid gas permeable contact lens should optically correct irregular astigmatism and can be used as a short-term solution (as well as a diagnostic aid for irregular astigmatism). Corneal transplantation offers good results and can be used, if necessary, but it should be considered a last resort for those patients who are contact lens intolerant, who are significantly visually impaired, and who cannot wait for future technological fixes.

Other postoperative complications

• Dislocated flaps
  • Dislocated flaps usually occur in the early postoperative period (first 48 hours) and can result in poor vision, pain, and permanent striae, if not treated aggressively and appropriately.²⁹ Prevention is paramount and is accomplished by meticulous alignment of the flap at the time of surgery and checking the flap again at the slit lamp prior to allowing the patient to leave the laser center, usually within 20 minutes. If flap dislocation is noted, the flap can be refloated and repositioned easily before the patient leaves.
  • Patients leave the eye center with plastic bubble eye shields and are instructed not to remove them for the first day and evening after surgery, except to instill drops. They also are instructed not to touch or rub the eye.
  • The flap is inspected again at the slit lamp within 24 hours and any misalignment, significant striae or folds, or dislocation is treated immediately by refloating the flap.
  • Late dislocation is uncommon and usually involves significant eye trauma. Striae in the flap can occur despite the most careful alignment of the flap and vigilant postoperative care. Thicker flaps (180-200 µm) may be less prone to this problem. If outside the visual axis and center of the pupil, they can be ignored. However, if they appear to be central and are associated with a loss of best-corrected visual acuity, they should be treated. Note that no topographic abnormalities may be present despite the slit lamp appearance.
  • Various methods have been described to remove visually significant striae from the flap. These methods include simply lifting and smoothing the flap with multiple strokes of a spatula over the surface, suturing the flap, and thermal ironing of the flap.^30,29 Unfortunately, no consensus currently exists on the treatment of striae. Generally, a stepwise approach is used, and suturing or thermal ironing procedures are reserved for long-standing striae or those that do not resolve after a simple lift and smooth technique is tried. No attempt is made to mark the flap since alignment marks made prior to performing this stretching maneuver will not correspond to the actual flap alignment noted after stretching is complete.
  • Striae may still be present immediately after flap stretching, but they usually will be improved or resolved within 24 hours. Creating an epithelial defect directly over the striae may be helpful in recalcitrant cases. A bandage lens in this situation also may be helpful because it will likely induce flap edema and further stretch the cornea.
• Epithelial ingrowth
  • Epithelial ingrowth under the LASIK flap has been reported to occur in 1-2% of patients. Fortunately, significant epithelial ingrowth requiring treatment is rare.
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    Epithelial ingrowth.

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    Epithelial ingrowth.

  • Poor technique and adhesion of the flap can be associated with this complication. Grasping the flap with forceps or pinching the flap also may allow an avenue for ingrowth to occur. Mild, stable, ingrowth at the edge of the flap extending no more than 1 mm from the edge does not require treatment. However, sheets of epithelium growing in from the edge or epithelial "nests" involving the central visual axis or inducing topographic abnormalities and irregular astigmatism should be treated as soon as possible. Untreated sheets of epithelium with poor adherence of the flap edge can lead to corneal flap melting and permanent damage to the flap.
  • Usually, epithelium can be seen easily on slit lamp examination and in retroillumination. Fluorescein staining of the cornea can reveal communication of a pocket or sheet of epithelium with the flap edge.
  • Treatment involves lifting the flap and mechanically scraping both the stromal surface and the back of the flap. (A Paton spatula or 69 Beaver blade can be used.) Alcohol or cocaine on the stromal surface or the flap usually is not necessary and is not advised due to potential toxicity to the cornea and the endothelium. Sealing the edge of the flap with fibrin glue can be a useful adjunct in recalcitrant cases with multiple recurrences.³¹
• Diffuse lamellar keratitis
  • Diffuse lamellar keratitis, also known as Sands of the Sahara syndrome, represents a sterile inflammation occurring in the flap interface.³² Maddox first described this complication, and Hatsis subsequently classified it into 4 grades based on severity.³³ The etiology is unknown, but it is believed to be due to the introduction of toxins under the flap at the time of surgery. Gram-negative endotoxin from dead bacteria and hydrocarbon contamination from the microkeratome motor or head lead the possible suspects. Milder cases have been associated with epithelial defects that sometimes occur on the surface of the flap following surgery.
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    Diffuse intralamellar keratitis (day 5).

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    Diffuse intralamellar keratitis (day 5).

  • Grades I and II are characterized by asymptomatic patients with normal vision on the first postoperative day. Slit lamp examination may reveal a fine, diffuse, powdery infiltrate (sandlike in color and appearance) confined to the interface. Grade I partially covers the interface, while grade II covers the entire interface and is associated with a denser infiltrate. Sometimes, wavy, fine lines with intervening clear areas can be seen. If untreated, the infiltrate typically worsens during the first postoperative week.
  • Grade III is associated with worsening vision and focal plaquelike infiltrates against a background of diffuse infiltration.
  • The most aggressive stage, grade IV, can be accompanied by significant visual loss and inflammatory signs (eg, lid edema, profound photophobia, perilimbal injection, flare and cell) in the anterior chamber. Typically, the infiltrate is dense and associated with large focal clumps of cells. Corneal topographic changes reflect the severity of the inflammation and become more marked as enzymatic digestion of the flap and the stromal bed progress. This process can result in permanent corneal changes.
  • The key to treatment is early recognition and intervention. Topical therapy consists of prednisolone acetate 1% or a steroid drop of equal potency given hourly. Topography and visual acuity are helpful in assessing progression. The trend in treatment has been toward early intervention before progression to grade III or IV. This consists of lifting the flap and thoroughly irrigating the interface. Cultures can be performed to rule out infectious keratitis if suspected.
  • The usual outcome is gradual resolution and return of best-corrected visual acuity, even in more severe cases. Complete resolution may take weeks to months. As noted, permanent corneal topographic changes due to melting of the cap and the stromal bed are possible and can result in corneal scarring and irregular astigmatism.
  • Efforts to reduce the incidence of diffuse lamellar keratitis focus on prevention, and they are updated on a continual basis. Particular attention has been given to the cleaning of instruments, especially the microkeratome, and sterilization technique. The author's center uses sterile distilled water in the steam autoclave, which may help prevent the buildup of gram-negative endotoxin. The author's center also uses a filter capable of removing bacteria from any solution used to irrigate under a flap. Balanced salt solution is used for irrigation under the flap and is placed on a syringe, with the filter interposed between the syringe and the irrigation cannula. Disposable cannulas only are used for irrigation, since endotoxin and biofilm can build up on the inside of reusable cannulas.
• Infectious keratitis
  • Infectious keratitis after LASIK is exceedingly rare. This finding is despite the fact that LASIK usually is performed in outpatient centers not subject to the rigid sterility protocols in force for the operating room. Surgeons often do not wear gloves during the procedure. The low infection rate may be due in part to the fact that epithelial integrity is relatively well maintained (compared to PRK).
  • Other factors that may contribute to the low incidence of infection are the limited use of topical steroids (usually 1-2 wk) and the routine use of potent topical antibiotics (eg, fluoroquinolones) during the perioperative period. However, infections have been reported and tend to be serious. This finding is partly due to the fact that when infection does occur, the invading organism has already gained access to the deep corneal stroma.
  • Organisms that have been reported to cause infectious keratitis following LASIK include Streptococcus pneumoniae, Staphylococcus aureus, Mycobacterium chelonae, and Nocardia asteroides. Atypical mycobacterial infections represent about one half of all reported cases. Mycobacterial infections may be more frequent when cold or chemical sterilization techniques are used for the microkeratome. The actual source of mycobacteria is often contaminated tap water or ice. These organisms seem to have a predilection for the relatively anoxic environment that exists in the flap interface.
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    Bacterial keratitis following LASIK.

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    Bacterial keratitis following LASIK.

  • Symptoms of infection include poor vision, pain, and redness. Signs include infiltration under the flap with possible anterior chamber reaction. Patients with diffuse intralamellar keratitis can present with similar findings, but the eye tends to be quiet, eliciting minimal pain and redness. Mycobacterial infections often present 2-4 weeks following surgery and are characterized by multiple discrete interface infiltrates with indistinct and feathery edges.
  • The principles of diagnosis and treatment remain the same as with any bacterial or fungal corneal infection; identify the organism and treat aggressively with appropriate broad-spectrum antibiotic drops based on Gram stain and culture results. However, management of the flap can be problematic. In general, cultures should be obtained from under the flap. Sometimes, cultures can be performed atraumatically by gently lifting an edge of the flap and inoculating a calginate swab soaked in culture media broth (eg, BHI, thioglycolate). If an infection appears to be progressing despite aggressive antibiotic treatment, the flap should be lifted, cultures should be repeated, and antibiotics should be irrigated in the flap interface. Initial therapy could consist of a fluoroquinolone combined with a fortified cephalosporin drop. This treatment provides adequate coverage for most bacteria. Infections that do not respond may benefit from therapeutic penetrating keratoplasty.
  • Mycobacterial infections may require prolonged antibiotic treatment over a course of weeks to months. The current antibiotics of choice are fortified amikacin or clarithromycin. They penetrate the flap poorly. Fourth-generation fluoroquinolones have significant activity against mycobacteria and much better penetration; however, there are no reported cases to date treated successfully with these antibiotics as a single agent. Cases of confirmed mycobacterial infection that do not respond to antibiotic treatment may require very aggressive treatment with amputation of the LASIK flap and the addition of systemic antibiotics.³⁴
  • LASIK has not been reported to cause damage to the corneal endothelium, and, in fact, several studies have shown no decrease in average endothelial cell density following LASIK.
• Ectasia
  • Ectasia refers to the apparent postoperative biomechanical weakening of the cornea following LASIK or, more rarely, PRK. Ectasia is characterized by poor vision and topographic findings resembling that of keratoconus.
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    Postoperative ectasia: Orbscan. Note the elevation on anterior and posterior floats and the thinning of the central cornea on the pachymetry map.

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    Postoperative ectasia: Orbscan. Note the elevation on anterior and posterior floats and the thinning of the central cornea on the pachymetry map.

  • Although the etiology remains unknown, several important risk factors for the development of ectasia have been outlined by Randleman and colleagues.^35,36,37,38 These risk factors include abnormal topographic findings suggestive of forme fruste keratoconus, a residual corneal stromal bed thickness of less than 250 µm, high myopia, a thin cornea (<500 µm) preoperatively, and an age of 21 years at the time of surgery. Of these, abnormal topographic findings appear to be the most important, and, in fact, there are many examples of patients with thin corneas who have undergone uncomplicated LASIK procedures.
  • Treatment modalities range from the use of contact lenses to corneal transplantation. Intacs have been used with some success to mechanically bolster the cornea. More recently, cornea collagen cross-linking has been tried. In this technique, the cornea epithelium is removed, and the stroma is exposed first to riboflavin and then to ultraviolet light. The concentration of riboflavin and the time of exposure to ultraviolet light determine the extent of cross-linking. This treatment results in stiffening and variable flattening of the cornea. Its use in the treatment of post-LASIK ectasia at present remains investigational.

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