Ординатура / Офтальмология / Английские материалы / LASIK and Beyond LASIK Wavefront Analysis and Customized Ablation_Boyd_2001

stroma; it is diffuse and scattered through a large area; there are multiple faint foci; there is little or no anterior chamber reaction, no overlying epithelial defect, and little or no ciliary flush10. DLK can be distinguished from infectious infiltrates by clinical presentation and close follow-up. Patients with this syndrome may show spontaneous slow and progressive resolution over the next two to three weeks.

Treatment

After the clinical and microbiologic work-up has been completed, a therapeutic plan is developed to most effectively manage the infected cornea. The goals of therapy include the eradication of viable bacteria and rapid suppression of the inflammatory response elicited by the microorganisms. These goals are best achieved by prompt initiation of specific antimicrobial therapy29.

In cases of corneal infections, not only after LASIK, it is advocated to initiate broad spectrum antibiotics, with modifications of this therapy based on the clinical impression and culture results. Broadspectrum antibiotics should be used until organisms are identified on culture.

Based on data that fortified antibiotic eyedrops are as effective as subconjunctival injections, the standard route of administration of antibiotics for the treatment of corneal infections is topical30. Subconjunctival administration of the drug is appropriate when compliance is in question, there is a delay or inability to administer topical medications, or in very severe cases of infectious keratitis29. Numerous reports have shown that systemic administration of antibiotics is ineffective in treating bacterial keratitis31,32. This route of administration is reserved for perforated corneal ulcers.

Double therapy with fortified cefazolin (50 mg/ml) plus fortified tobramycin (15 mg/ml) or ciprofloxacin (3 mg/ml) plus fortified cefazolin (50 mg/ml), are first choice regimens to which the most common bacteria responsible for bacterial keratitis are sensitive30. An alternative therapy in patients allergic to cephalosporins is vancomycin (25-50 mg/ml) 30. Initial therapy regimens should be

modified after isolation of the organism and if there is evidence of deterioration. Change in therapy should also be made if the corneal infection continues to worsen and the laboratory reports a pathogen resistant to the initial combination of antibiotics.

One large multi-center study33 concluded that fluoroquinolone monotherapy (ciprofloxacin) was equivalent to fortified cefazolin-tobramycin in treating bacterial keratitis. However, this monotherapy regimen is not recommended in view of the rapid emergence of resistant strains to the fluoroquinolones34.

Cefazolin, a first generation cephalosporin, has an excellent coverage for streptococcus and peni- cillinase-producing organisms. Cefazolin is usually active against gram-positive cocci, including betalactamase producing Staphylococcus aureus and S. Epidermidis, group A beta-hemolytic streptococci

(S. Pyogenes), group B streptococci, and Streptococcus pneumoniae. It has limited activity against gram-negative bacteria, and is not effective for the treatment of methicillin-resistant staphylococci. Tobramycin, an aminoglycoside antibiotic, is usually effective against most gram-negative organisms (e.g., Pseudomonas, Proteus, Klebsiella, E.

Coli, H. Influenzae, Serratia) and some gram-posi- tive organisms (S. Aureus and S. Epidermidis, including beta-lactamase-producing strains). Tobramycin is ineffective against Streptococcus. Ciprofloxacin, the most active fluoroquinolone, is highly effective against a broad spectrum of gram-positive (Staphylococcus and methicillin-resistant Staphylococcus), gram-negative (Haemophylus, Pseudomonas, Proteus, E. Coli, Serratia, Klebsiella, Moraxella), mycobacteria (including Mycobacterium chelonei), and chlamydial organisms29,30. The recommended therapy for the initial treatment of bacterial keratitis after LASIK includes irrigation of the interface with two fortified antibiotics after lifting the flap and obtaining specimens for microbiologic work-up. Topical fortified antibiotic should be given every hour alternating with another topical fortified antibiotic every hour (on the half hour) for 48 hours. If clinical improvement occurs after 48 hours, the antibiotic therapy can be reduced to two topical fortified anti-

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Chapter 26

biotics every 2 hours, 5 minutes apart. After 72 hours topical fortified antibiotics frequency may be reduced to every 3-4 hours, and one fortified antibiotic can be discontinued. After 5 days, regular strength antibiotic drops may be used (if available), and this medication is slowly tapered off based on clinical improvement. This suggested schedule is similar to that recommended for regular bacterial keratitis29. (Table 5).

Table 5

Suggested Schedule for the Initial Treatment of Bacterial Keratitis after LASIK

In conclusion, infectious keratitis is a serious complication after LASIK. However, if treated promptly and appropriately, it is possible to achieve a good visual outcome.

Prevention of Infectious Keratitis

Following LASIK

Laser in-situ keratomileusis is an invasive surgical procedure which creates a flap in the cornea. Several factors may contribute to infectious keratitis following LASIK, and certain preoperative, intraoperative and postoperative measures can be taken for prevention (Table 6).

Table 6

Preoperative, Intraoperative and Postoperative Measures for Prevention of Infection following LASIK

Preoperative

1.Treatment of eyelid, conjunctival or adnexal infection prior to LASIK

2.Proper sterilization of the instruments

3.Proper intraoperative sterile techniques

4.Wearing of powderless sterile gloves

5.Disinfection of the skin of the eyelids and lid margin with Betadine

6.Applying a sterile drape over the lid margin

Intraoperative

1.Irrigation with BSS without recycling of the fluid from the ocular surface to under the flap

2.Use of a fresh, sterile bottle of BSS

3.Use of a sterile cannula

4.Avoiding touching the eyelashes

Postoperative

1.Proper instruction to patient not to rub the eye

2.Use of postoperative prophylactic antibiotics for 3 to 5 days

In the preoperative care, patient should be examined properly to rule out a nasolacrimal duct obstruction, a sub-clinical dacryocystitis or chronic conjunctivitis35. Patients with lid margin disease such as Staphylococcal blepharitis should be treated properly prior to LASIK. Culture from the lids is obtained and appropriate topical antibiotics are used to eradicate the infection from the lid margin. Patients with chronic meibomianitis may have to be given systemic antibiotics before LASIK. Ocular and periocular infections are considered contraindications to LASIK. In these cases, appropriate antibiotic therapy should be given prior to any surgical intervention. During the preoperative stage of LASIK, all instruments should be properly sterilized. In the preoperative preparation, the surgeon should be dressed with an operating room attire and sterile coat and gloves should be worn. The preoperative preparations should include scrubbing of the eyelids and lid margins with Betadine.

The lid margins and the eyelashes should be covered with a sterile drape. A lid speculum can be placed in such a way so that the drape can be turned over the lid margin covering the lid margin. The blade should be used once only. Irrigation of the flap should be brief without allowing pooling of the Balanced Salt Solution (BSS) over the eye and back under the flap. Irrigation should be one way from the cannula to under the flap and to the outside without allowing the recycling of the BSS under the flap. BSS should not be stored, and a fresh bottle of BSS (BSS 15 ml, Alcon) should be used for each case. The corneal flap should be well approximated at the end of the procedure and allowed to dry in place. A broad-spec- trum antibiotic may be instilled at the end of the procedure, such as ofloxacin 0.3% eyedrops or lomefloxacin 0.3% eyedrops.

Following surgery, the patient may be placed on topical broad-spectrum antibiotics every 2 hours for the first 12 hours and then four times daily for a period of 3 to 5 days. Steroids are not indicated in the early postoperative period and may be started 12 to 24 hours after the procedure. Furthermore, bilateral simultaneous LASIK should be avoided in patients who are HIV positive or who are immunologically compromised27,28.

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Chapter 26

LASIK is an elective procedure and should be performed under optimal conditions. Any patient with evidence of ocular surface or adnexal infection should be cancelled and rescheduled after the infection has been eradicated.

Conclusion

LASIK is an elective procedure and should not be performed unless optimal ocular surface conditions are present. Ocular surface or adnexal infections are considered to be contraindications to LASIK. Prevention of infection is much better than managing the cure of infection that may follow the surgical procedure. Early recognition and prompt therapy of infections following LASIK may lead to full visual recovery and rehabilitation. Although laboratory testing has been the basis for identifying many types of infections, the clinician is often the first to recognize a very early infection following LASIK.

REFERENCES

1.- Pallikaris IG, Siganos DS. Excimer laser in situ keratomileusis and photorefractive keratectomy for correction of high myopia. J Refract Corneal Surg. 1994; 10:498-510.

2.- Knorz MC, Liermann A, Seiberth V, Steiner H, Wiesinger B. Laser in situ keratomileusis to correct myopia of –6.00 to –29.00 diopters. J Refract Surg. 1996; 12:575-584.

3.- Fiander DC, Tayfour F. Excimer laser in situ keratomileusis in 124 myopic eyes. J Refract Surg. 1995; 11(suppl):S234-S238.

4.- Kremer FB, Dufek M. Excimer laser in situ keratomileusis. J Refract Surg. 1995; 11(suppl):S244S247.

5.- Güell JL, Muller A. Laser in situ keratomileusis (LASIK) for myopia from -7 to -18 diopters. J Refract Surg. 1996; 12:222-228.

6.- Pérez-Santonja JJ, Linna TU, Tervo KM, Sakla HF, Alió JL, Tervo TM. Corneal wound healing after laser in situ keratomileusis in rabbits. J Refract Surg. 1998; 14:602-609.

7.- Pérez-Santonja JJ, Bellot J, Claramonte P, Ismail MM, Alió JL. Laser in situ keratomileusis to correct high myopia. J Cataract Refract Surg. 1997; 23:372-385.

8.- Gimbel HV, Anderson-Penno EE. Early postoperative complications: 24 to 48 hours. In: Gimbel HV, AndersonPenno EE, eds. LASIK complications, prevention and management. Thorofare, NJ: Slack Inc.; 1999; 81-91.

9.- Pérez-Santonja JJ, Sakla HF, Abad JL, Zorraquino A, Esteban J, Alió JL. Nocardial keratitis after laser in situ keratomileusis. J Refract Surg. 1997; 13:314-317.

10.- Smith RJ, Maloney RK. Diffuse lamellar keratitis. A new syndrome in lamellar refractive surgery. Ophthalmology. 1998; 105:1721-1726.

11.- Nascimento EG, Carvalho MJ, de Freitas D. Campos M. Nocardial keratitis following myopic keratomileusis. J Refract Surg. 1995; 11: 210-211.

12.- Watanabe H, Sato S, Maeda N, Inoue Y, Shimomura Y, Tano Y. Bilateral corneal infection as a complication of laser in situ keratomileusis. Arch Ophthalmol. 1997; 115: 1593-1594.

13.- Lin RT, Maloney RK. Flap complications associated with lamellar refractive surgery. Am J Ophthalmol. 1999; 127:129-136.

14.- Kaufman SC, Maitchouk DY, Chiou AGY, Beuerman RW. Interface inflammation after laser in situ keratomileusis. Sands of the Sahara syndrome. J Cataract Refract Surg. 1998; 24:1589-1593.

15.- Aras C, Ozdamar A, Bahçecioglu H, Sener B. Corneal interface abscess after excimer laser in situ keratomileusis. J Refract Surg. 1998; 14:156-157.

16.- Reviglio V, Rodriguez ML, Picotti GS, Paradello M, Luna JD, Juárez CP. Mycobacterium chelonae keratitis following laser in situ keratomileusis. J Refract Surg. 1998; 14:357-360.

17.- Kim HM, Song JS, Han HS, Jung HR. Streptococcal keratitis after myopic laser in situ keratomileusis. Korean J Ophthalmol. 1998; 12:108-111.

18.- Al-Reefy M. Bacterial keratitis following laser in situ keratomileusis for hyperopia. J Refract Surg. 1999; 15(suppl):S216-S217.

19.- Webber SK, Lawless MA, Sutton GL, Rogers CM. Staphylococcal infection under a LASIK flap. Cornea. 1999; 18:361-365.

20.- Condon PI, Mulhern M, Fulcher T, Foley-Nolan A, O’Keefe M. Laser intrastromal keratomileusis for high myopia and myopic astigmatism. Br J Ophthalmol. 1997; 81:199-206.

21.- Gussler JR, Miller D, Jaffe M, Alfonso EC. Infection after radial keratotomy. Am J Ophthalmol. 1995; 119:798799.

22.- Grimmett MR, Holland EJ, Krachmer JH. Therapeutic keratoplasty after radial keratotomy. Am J Ophthalmol. 1994; 118:108-109.

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23.- Hoffer KJ, Darin JJ, Pettit TH, Hofbauer JD, Elander R, Levenson JE. Three year experience with radial keratotomy: the UCLA study. Ophthalmology. 1983; 90:627636.

24.- Amayem A, Tawfik Ali A, Waring III GO, Ibrahim O. Bacterial keratitits after photorefractive keratectomy. J Refract Surg. 1996; 12:642-644.

25.- Sampath R, Ridgway AEA, Leatherbarrow B. Bacterial keratitis following excimer laser photorefractive keratectomy: a case report. Eye. 1994; 8:481-482.

26.- Mulhern MG, Condon PI, O’Keefe M. Endophthalmitis after astigmatic myopic laser in situ keratomileusis. J Cataract Refract Surg. 1997; 23:948-950.

27.- Hovanesian JA, Faktorovich EG, Hoffbauer JD, Shah SS, Maloney RK. Bilateral bacterial keratitis after Laser in-situ keratomileusis in a patient with human immunodeficiency virus infection. Arch Ophthalmol. 1999;117:968-970.

28.- Quiros PA, Chuck RS, Smith RE, Irvine JA, McDonnell PJ, Chao LC, McDonell PJ. Infectious Ulcerative keratitis after laser in situ keratomileusis. Arch Ophthalmol. 1999; 117:1423-1427.

29.- Abbott RL, Zegans M, Kremer PA. Bacterial corneal ulcers. In: Tasman W, Jaeger EA, eds. Duane’s Clinical Ophthalmology. 1998 edition. Philadelphia, Pennsylvania: Lippincott Williams and Wilkins; 1998; Vol 4: chap 18.

30.- Baum JL. Antibiotic use in ophthalmology. In: Tasman W, Jaeger EA, eds. Duane’s Clinical Ophthalmology. 1998 edition. Philadelphia, Pennsylvania: Lippincott Williams and Wilkins; 1998; Vol 4: chap 26.

31.- Woo FL, Johnson AP, Insler MS, George WJ, LaCorte WS. Gentamicin, tobramycin, amikacin and netilmicin levels in tears following intravenous administration. Arch Ophthalmol. 1985; 103: 216-218.

32.- Davis SD, Sarff LD, Hyndiuk RA. Comparison of therapeutic routes in experimental pseudomonas keratitis. Am J Ophthalmol. 1979; 87:710-716.

33.- Hyndiuk RA, Eiferman RA, Caldwell DR, Rosenwasser GO, Santos CI, Katz HR, Badrinath SS, Reddy MK, Adenis JP, Klauss V. Comparison of ciprofloxacin ophthalmic solution 0.3% to fortified to- bramycin-cefazolin in treating bacterial corneal ulcers. Ophthalmology. 1996; 103:1854-1862.

LASIK surgery is a highly successful surgery. However, like all surgical procedures, complications can occur. Knowledge of their causes and careful attention to details can prevent the vast majority of complications. Furthermore when complications do occur, if appropriate management is employed, good outcomes are still achievable.

INTRAOPERATIVE

COMPLICATIONS

Flap Complications

Thin Flap and Buttonholes

Thin flaps and buttonholes may result from one of several causes. These include inadequate suction, a damaged blade, or an excessively steep cornea. Intraocular pressure must exceed 65 mm Hg for an optimal cut to be made by the microkeratome. Confirmation of adequate pressure can be confirmed by intraoperative Barraquer tonometry. Other signs supportive of adequate suction include ability to elevate the globe with the suction ring, pupil dilation, and the patient’s report that his/her vision has dimmed or blacked out. The keratome blade should be carefully inspected under the microscope for nicks or poor finishes. Steep corneas (curvature > 45 or 46 D) may buckle during the microkeratome pass, resulting in a buttonhole or thin flap. In these cases, a smaller ring size should be chosen.

Thin flaps can also occur in cases of steep corneas. A thin flap is more difficult to reposition and is more likely to wrinkle. (1) If the flap is com-

plete enough to allow repositioning, then the case can proceed. When possible, a deeper of depth plate of 180 mm rather than 160 mm should be chosen for patients with keratometry readings > 46 D. If buttonholing or a break in Bowman’s layer occurs within the optical zone, then the flap must be repositioned as well as possible and the procedure postponed for 3 to 6 months.

Free Cap

Free caps can occur in cases of equipment malfunction or excessively flat corneas. When using the Chiron Automated Corneal Shaper, improper setting of the stopper can lead to this complication. The more frequent cause of this complication, however, is a very flat cornea. With corneas flatter than 41 D, less tissue protrudes through the suction ring, thereby presenting a smaller cap diameter to the microkeratome blade. Hence, a free cap may result.

It is still possible to complete a successful LASIK procedure in certain cases of free caps. As long as the stromal bed beneath the cap is centered and of adequate size for the ablation, then surgery can proceed. During the ablation the cap can be stored in an antidesiccation chamber or on the conjunctiva with the epithelial side down. After the completion of the laser treatment, the bed is irrigated with sterile balanced salt solution. The cap is then repositioned into the same orientation as before the surgery. The use of orientation marks can be helpful in realigning the cap. Five to 10 minutes of drying time allows the cap to adhere without sutures. Some surgeons tape the lid shut overnight.

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If the microkeratome prematurely stops in its pass across the cornea, then an incomplete flap is created. Advancement of the microkeratome may be halted by its obstruction with various objects such as the lids, speculum, or drape. Debris or crystals from incompletely removed balanced salt solution can block the proper advancement of the microkeratome gears.

Good exposure is crucial to LASIK surgery. In cases of blepharospasm, deep-set orbits, or small lids, one should consider a lid block, retrobulbar block (to proptose the globe), or a lateral canthotomy. It is necessary to keep the microkeratome clean and check that it runs without resistance in both the forward and reverse pass.

With an incomplete pass occurs, the surgeon must decide whether the ablation can be completed. If the hinge is near the edge of the planned ablation, then laser treatment can proceed while the flap is protected with a moist Merocel sponge. If the hinge is close to or crosses the pupil, the flap should be repositioned and allowed to heal for 3-6 months before creating a new flap.

Figure 27-2. Epithelial defect near flap hinge.

As with any corneal abrasion, an epithelial defect can heal with excess anterior basement membrane material and irregular astigmatism. Treatment depends on the size and location of the defect. For small defects, the loose epithelium may be simply removed or repositioned and copious lubrication administered postoperatively. For larger defects, placement of a bandage contact lens should also be done. In some cases, phototherapeutic keratectomy might be considered to remove excess anterior basement membrane material and allow good adhesion of the

Figure 27-3. Corneal topography of central island showing small area of elevation.

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occur for a variety of reasons.(4, 5) Beam profile abnormalities, increased hydration of the central corneal stroma, or particulate material falling onto the cornea may block subsequent laser pulses. A flat ablation beam may direct stromal fluid into the central area of ablation, and the hydrated tissue is ablated at a slower rate. This is more common with broad-beam lasers and rare with scanning-beam lasers. (6) The presence of more hydrated central stromal tissue can result in less effective ablation in the central 1 to 3 mm of the cornea. Laser software can add extra pulses in the central cornea to compensate for this, yet careful monitoring of the hydration of the central cornea is important. If excessive hydration is noted, the procedure should be paused and excess fluid removed from the center of the cornea.

Typically these islands resolve with time as epithelial remodeling fills in the surrounding area. These resolve more slowly after LASIK than after PRK. If resolution has not occurred by 3 months, then the flap can be lifted and the island retreated to reduce irregular astigmatism. Elevation topography should be used to measure the diameter and height of the island. Typically these islands are 2 to 3 mm in diameter and 7 to 15 mm in height.

Figure 27-4. Corneal topography of treatment decentered nasally in the left eye.

Decentration

Decentration of the refractive excimer laser ablation (Figure 27-4) can result in irregular astigmatism causing glare, halos, diplopia, and a decrease in best corrected visual acuity (7). Controversy exists as to whether refractive procedures should be centered on the pupil or the corneal light reflex (8). The patient should be continuously coached throughout the ablation to look at the fixation target. Minimizing sedation will also help the patient to remain alert and cooperative. If the patient is unable to maintain fixation despite these measures, then the suction ring or a Fine-Thornton fixation ring may be used to stabilize the globe. Some lasers now have pupil trackers available to adjust for small eye movements.

Typically if the excimer ablation is decentered by more than 1 mm, adverse visual consequences can result. Greater impairment occurs with smaller total ablation areas than larger ones. The management of this complication requires a retreatment with ablation applied asymmetrically to the previously undertreated area, to enlarge and smooth the optical zone (9). This can be accomplished by masking the previously treated area or by purposely decentering the excimer beam. Acceptable result have been reported after PRK but are certainly quite challenging (10).

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POSTOPERATIVE COMPLICATIONS

Interface Debris

Metal filings, sponge fibers, or meibomian secretions can become trapped in the interface of the flap and stromal bed (Figure 27-5). This may occur even with appropriate irrigation, use of a wick, or aspiration. In some instances, overly aggressive irrigation may carry debris from the conjunctival fornices onto the stromal bed. These particles rarely incite inflammation or affect vision. However, in cases of large amounts of debris, the flap can be relifted and the debris irrigated. Operating in a lintfree environment, the use of nonfragmenting sponges, draping of the lashes, and careful irrigation can limit this complication.

Figure 27-5. Debris in the flap interface.

Flap Displacement

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Figure 27-6. Displacement of a nasally hinged flap.