Ординатура / Офтальмология / Английские материалы / Jaypee Gold Standard mini Atlas Series Lasik_Aragawal, Jacob_2009

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most. Haze can be removed by a second laser ablation in a therapeutic fashion, but this approach also is often ineffective because laser ablation generated the haze in the first place and may induce haze recurrence. A very effective option to manage intraoperative flap complications involves surface ablation with adjunctive mitomycin C (Figure 3.9).

MMC is an antibiotic/chemotherapeutic agent with alkylating properties, which enables it to inhibit DNA synthesis. It is commonly used topically after glaucoma surgery, pterygium excision, in the treatment of conjunctival and corneal intraepithelial neoplasia, and in the treatment of ocular pemphigoid. Rationale to its use relies on its long-term, possibly permanent, cytostatic effect on tissue. More specifically, its use after surface ablation is intended to inhibit subepithelial fibrosis as the result of an abnormal activation or proliferation of stromal keratocytes following laser ablation. This use was originally proposed by Talamo and associates on an experimental model. Haze reduction following mitomycin C administration was also documented by Xu and associates in rabbit eyes. More recently, Majmudar and coworkers reported a successful series of eyes treated using a 0.02% (0.2 mg/ml) mitomycin C solution, to remove haze after PRK and radial keratotomy.

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Even when used prophylactically, results of surface ablation MMC-assisted are astonishing. MMC does not interfere with re-epithelialization or early wound healing period. Haze rates are extremely low, whenever present, also for high corrections and ablation depths. The accuracy of the procedure is reported as much higher than for surface ablation without the use of MMC, with lower standard deviations. All the published series report a marked trend to overcorrection (in the range 10-15%, according to the laser used and individual nomograms), thus suggesting a programmed undercorrection when using MMC.

Given these very positive results, the association of surface ablation with MMC for the treatment of intraoperative flap complications and performing the excimer laser correction seems very viable. The technique to be adopted has not been widely accepted, given also the wide range of potential complications and their different outcomes.Basically, the two major issues are the time when to perform the treatment, and the modality for removing the epithelium. In the event a button-hole or an incomplete flap occurs, the shift to surface ablation may be done immediately, or after a certain healing period. For both options there are advantages and disadvantages.

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surface is smooth, the flap is adherent (such as after 2-3 weeks after the attempted surgery), and the flap has not been intersected (like in a button-hole or a partially damaged flap), all epithelial removal techniques may be safely used. Mechanical scrap should be carefully performed, paying attention to scrap in the direction where the hinge was intended, without applying rotational forces. A diluted 20% alcohol solution applied for 20-25 seconds may be very beneficial in such cases to reduce the adherence between the epithelium and Bowman’s membrane, to make the scraping process much more gentle and less aggressive. Brushes look less indicated due to the lower control they have on epithelium removal, and to the torsional movement they apply onto the flap which may determine some shift and/or flap displacement. Laser epithelium removal in a therapeutic fashion seems the technique with less chances of displacing the attempted flap, though a precise way to remove the whole epithelium in a homogeneous way over the entire ablation surface is still difficult to perform, due to the different thickness the epithelium has. The observation of fluorescence during the ablation helps the surgeon in evaluating this process.

For all flap complications where the flap has been intersected, it seems more reasonable to perform

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irregularity in a localized fashion. The following refractive ablation to correct pre-existing error has to be adjusted according to the amount of phototherapeutic ablation performed, in order to avoid overcorrection. Again, it is very difficult to give generic advices, being the refractive effect of phototherapeutic keratectomy very technique, ablation diameter, and laser-dependent.

Once the ablation has been performed, MMC has to be applied on the stromal surface to avoid haze formation. There are different techniques to apply MMC. Some surgeons use marking trephine to be filled with MMC solution, most of them use circular sponges soaked with MMC solution. The concentration of MMC in its dilution, and the application time are also controversial. The original studies performed by Majmudar and colleagues suggested a 0.02% (0.2 mg/ml) concentration for a two-minute application time. Most of the published literature assesses this concentration and application time to be effective in avoiding haze formation, and no side effects or complications related to the use of MMC have been reported over a follow-up period longer than 10 years. Shorter application times and lower dosages have been investigated on laboratory animals and on patients for MMC when applied prophylactically to avoid haze