Ординатура / Офтальмология / Английские материалы / Jaypee Gold Standard mini Atlas Series Lasik_Aragawal, Jacob_2009
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surgical day. They found that if they inoculated a clean sterile sterilizer with the bacteria, biofilm would redevelop and cultures would again become positive within 6-11 days. Their findings give strong evidence that epidemics of DLK can be caused by bacterial endotoxin released from gram-negative bacterial biofilms in sterilizer reservoirs which survive short cycle steam sterilization, and the toxin incites a polymorphonuclear (PMN) inflammatory reaction in susceptible individuals resulting in DLK.
A more simple change in protocol was effective in controlling an outbreak reported by Yuhan in which the only changes in protocol were to switch the distilled water in plastic surgical bowls after each patient instead of at the end of each day, secondly to replace the plastic surgical bowls at the end of each day instead of at the end of each week and thirdly to eliminate the use of ultrasonic cleaning of the keratome head. There was no change in maintenance of the autoclave reservoir, and the autoclave was refilled with distilled water when the water level was low or empty as during the outbreak.
More recently Villarrubia and coworkers controlled an outbreak at their center by instituting the following changes in their protocol. Sterile water instead of distilled water was used to clean instruments prior to placing them in the
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sterilizer. Sterile water instead of distilled water was used in the sterilizer reservoir, and the instruments were steam sterilized between each patient instead of daily. Chemical sterilization of the instruments in peracetic acid bath between patients was diskontinued. At the end of each surgical day, the reservoir was drained, and any residual fluid was removed by aspiration or by using a wet-dry vacuum. Also at the end of the surgical day, the instruments were cleaned with sterile water and air dried using compressed air and left in a dry autoclave cassette until the following surgical day. At the beginning of the next surgical day, the reservoir was filled with sterile water and the instruments were sterilized. Their findings highlight the importance of air drying the microkeratome head and surgical instruments, draining the reservoir of the steam sterilizer, and aspirating the sterile water at the end of each surgical session.
While the femtosecond LASIK flap eliminates the need for sterilizing the microkeratome, a sterile flap lifting instrument like the Seibel IntraLASIK Flap lifter and a sterile irrigating cannula are used for each case, and thus attention to cleaning protocols remains important for centers with femtosecond lasers as well. The femtosecond laser, for example, the IntraLase (IntraLase Corp. Irvine,
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CA) can itself cause DLK Javaloy and coworkers in a prospective masked study of 200 eyes of 100 consecutive patients comparing Moria microkeratome LASIK to IntraLase femtosecond LASIK found DLK in 16% of eyes in the IntraLase laser group and none in the microkeratome group. When they subsequently reduced the side cut energy from 1.6 to 1.2 mJ, their incidence of DLK decreased to 3.5%. A 15 KHz femtosecond was used in the study, instead of 60 KHz which is now available. Optimizing the energy parameters for the femtosecond laser can help prevent DLK.
Intraoperative epithelial defects are a major cause of spontaneous cases of DLK. The incidence of DLK in patients with epithelial defects may exceed 50%. Preventing epithelial defects would be the ideal solution, but that may not be possible. Preoperative screening for epithelial basement membrane dystrophy (with maps, dots or fingerprints) can help identify those at risk for an epithelial defect. Patients with epithelial basement membrane dystrophy or recurrent corneal erosions are not good candidates for LASIK but may be considered for surface laser treatment (PRK). For patients with minimal or equivocal findings of epithelial basement membrane dystrophy, the provocative test described by Anthony
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Aldave can help make the diagnosis. Topical anesthetic is instilled. The test involves gently pressing a Weck cell sponge on the suspicious part of the corneal surface. If an abrasion or a slough is created the patient is at risk for an intraoperative epithelial defect and is not a good candidate for LASIK. (The patient also needs to be treated for the abrasion created during the test.) If an intraoperative epithelial defect does occur during the LASIK procedure, then prophylactic steroids should be instituted at a more frequent dosing regimen, for example, prednisolone 1% every 1-2 hours while awake. Other preventative measures include covering Meibomian gland orifices, avoiding getting povidone iodine under the flap, and to irrigating the interface after laser treatment.
Treatment of Diffuse Lamellar Keratitis
A short course of strong topical corticosteroids like prednisolone acetate 1% every 1-2 hours while awake with close observation is indicated. Usually topical corticosteroids are sufficient. Adding oral corticosteroids in severe cases has also been effective. The disease should start to improve within a day or two. When the density of inflammatory infiltrates has significantly decreased, then taper off the corticosteroids. If the infiltrates are worsening
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despite treatment, then reconsider the possibility of an infectious etiology and lift the flap, culture for bacteria, mycobacteria and fungus, irrigate the interface and increase the frequency of topical antibiotics. While the cultures are pending, the patient can usually be continued on the short course of corticosteroid. If fungal or mycobacterial keratitis is high on the differential, however, then corticosteroid should be stopped. It is worth noting that most of the patients in the initial report on DLK had resolution of DLK even without use of corticosteroid. DLK is not a chronic condition. Corticosteroid treatment should last no more than a couple of weeks.
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Figure 3.11: Central toxic keratopathy (Courtesy: Ronald J Smith)
Central toxic keratopathy is a rare complication of LASIK presenting as a triad of central stromal haze, deep stromal folds and flattening with hyperopia (Figure 3.11). The largest and most detailed clinical description of the condition has been published by Sonmez and Maloney. The condition presents 3-9 days after LASIK and is usually preceded by DLK, but persists for months after the infiltrates are gone. The central haze and folds extend posteriorly into the stromal bed. The pathology is not
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confined to the interface. The haze and scarring are most prominent in the area of greatest laser treatment which led them to postulate that the etiology is related to the laser-corneal interaction. After several months to a year or more, the haze clears and the refraction stabilizes— sometimes achieving the patient’s refractive goal after a long period of hyperopia. In others, the refraction is stable and amenable to further laser treatment. If flap folds persist and are causing irregular astigmatism along with negative fluorescein staining, the flap can be lifted and smoothed after the haze has subsided. Lindstrom found that the patients who had the best outcomes were those who refused early surgical intervention. He had three patients whose haze cleared, and hyperopia and irregular astigmatism resolved over the course of several years in a manner similar to patients with PRK haze. Importantly, the condition does not respond to corticosteroid treatment, and a pitfall of misdiagnosis is long-term corticosteroids causing steroid induced glaucoma.
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Figure 3.12A
Figure 3.12B
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Figure 3.12C
Figures 3.12A to C: Femtosecond laser complications (Courtesy: William Culbertson). (A) Gas bubbles in the anterior chamber obscuring the patient’s view of the laser fixation light;
(B) Gas bubbles deep to the interface in the anterior stromal bed (“deep OBL”); (C) Flap torn during attempt to forcefully dissect flap with spatula.
The near infrared femtosecond laser is a unique instrument which can produce incisions and lamellar interface planes in the cornea by the process of photodisruption. Contiguous plasma gas bubbles are created in the cornea which expand and cause micro-delamination of the corneal collagen. At the same time complications and nightmares can occur.
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Intraoperative Complications
Suction Loss
During the creation of the flap the Intralase suction ring may lose vacuum and the applanation plate may become separated from the cornea. If this occurs during the propagation of the lamellar interface there is no serious consequence to the flap except that the interface is incomplete. In this case the suction ring is reapplied, the interface cut is performed again and the side cut is made at the end. If suction is lost during the side cut then the diameter of the side cut is decreased by 1.0 mm, the suction ring is reapplied and the side cut is performed just inside the outside diameter of the lamellar cut.
Interference by Gas Bubbles
Gas Bubbles in the Anterior Chamber
Occasionally the gas bubbles generated from the intrastromal photodisruption can dissect from the interface through the peripheral cornea and into the anterior chamber via the trabecular meshwork. With the patient supine and the anteroposterior axis of the eye oriented vertically in preparation for flap lifting and excimer laser treatment, the bubbles collect and coalesce in the apex of
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