Henry Journal of Ophthalmology & Optometry

PhotoRefractive Keratectomy (PRK) procedure is commonly used to correct myopia and astigmatism. PRK involves the use of an excimer laser to reshape the anterior corneal surface [1]. The excimer laser alters the refractive state of the eye by removing tissue from the anterior cornea through a process known as photoablative decomposition.

This process uses ultraviolet energy from the excimer laser to disrupt chemical bonds in the cornea without causing any thermal damage to surrounding tissue [2]. Laser Epithelial Keratomileusis (LASEK) is a modified PRK technique introduced by Camellin in 1999 [3] based on the detachment of an epithelial flap after the application of an alcohol solution, and then the repositioning of this flap following laser application.

The popularity of LASEK has been gaining momentum among refractive ophthalmologists after a few clinical series showed that LASEK might have significant clinical advantages over PRK [4]. Autrata et al. compared the clinical results (efficacy, safety, stability, and postoperative pain or discomfort) of LASEK and conventional PRK for the correction of low to moderate myopia and they showed that there were no statistically significant differences in the safety and efficacy indices at 2 years [5]. Lee et al. learned that LASEK-treated eyes had less significant postoperative pain and corneal haze than PRK-treated eyes in the early postoperative period [6].

LASEK consists on the detachment of an epithelial flap after the application of an alcohol solution, and then the repositioning of this flap following laser application. From the viewpoint of the decreased corneal haze after LASEK, although the details of underlying cellular events remain unclear, we speculate that if an epithelial flap is made, it becomes loose and lengthens enough to cover the cut epithelial border. It seals up the bare stroma. That prevents the release of cytokines and growth factors from the stroma and damaged epithelium, which decreases the initial inflammatory damage to the stroma. This may reduce the apoptosis of anterior stromal keratocytes and subsequent replenishment with activated keratocytes, later decreasing the synthesis of collagens [7].

Ethanol was initially used in refractive surgery to assist in the removal of the epithelium before PRK [8,9] and has been shown to enhance corneal flap lifting without significant loss of flap viability. Alcohol acts on the corneal epithelium-basement membrane complex by splitting the epithelial basement membrane without affecting the anchoring of the basement membrane to the underlying Bowman’s layer [10].

However, alcohol is known to be cytotoxic; among a multiplicity of toxic effect mechanisms, the predominant mode of action appears to derive from protein coagulation/denaturation [11], which takes place at the cell membrane and among the various plasma proteins. Coagulation of enzymatic proteins leads to the loss of cell functions [12].

Hence, caution is required when applying alcohol to the cornea. Removal of the corneal epithelium before PRK with 18% or 20% ethanol left for 20 to 40 seconds is safe to the underlying corneal stroma and is an effective alternative to scraping (ie, mechanical debridement) [13,14]. Even an exposure time of 3 minutes with 25% alcohol appears safe, effective, and predictable without stromal dehydration or toxic effects and is not associated with significant loss of CDVA after PRK [15]. However, higher alcohol concentrations such as 50% [16] and 100% [17] ethanol can lead to substantial damage to the underlying stroma.

Soma et al. evaluated the effect of mechanical epithelial separation with an epikeratome on the histologic ultrastructure of epithelial flaps and stromal beds from human corneas. He showed that on scanning electron microscopy, the cleavage planes of epithelial flaps and stromal beds were relatively smooth; on transmission electron microscopy, epithelial flaps were separated partially within the lamina fibroreticularis and partially within the lamina lucida; immunofluorescence showed positive staining for type VII collagen and discontinuous staining for type IV collagen in stromal beds. Discontinuous linear staining for types IV and VII collagens was observed in epithelial flaps. Staining for integrins alpha 6 and beta 4 was positive in some regions and discontinuous in other regions of epithelial flaps. In stromal beds, integrins alpha 6 and beta 4 had a patchy expression pattern. Staining for laminin 5 was intermittently positive along the basal side of epithelial flaps and stromal beds [18].

In 1995 we examined four human corneas that had undergone PRK and subsequent penetrating keratoplasty by means of light and electron microscopy in an attempt to detect possible causes for complications after PRK and despite recovery of a continuous epithelial layer as early as 3 days after PRK, abnormalities of both epithelium and superficial stroma could be detected in all specimens, including the one obtained 13 months after the refractive procedure were observed [19]. Cui et al., using immunohistochemical staining and Western Blot analysis, observed and compared the accurate dynamic changes of type I, III, V, VI collagen in the wound healing processes of the rabbit cornea which underwent LASEK or PRK to investigate the possible mechanism of corneal haze and myopic regression.

They showed that after LASEK, the corneal wound healing with type I and III collagen were much faster than PRK, and the wound response was also much weaker. The value of these two types of collagen after PRK were higher than LASEK. Moreover there were significant differences between LASEK and PRK on type V and VI collagens in the time of reacting, reaching an apex and returning to normal. LASEK had slighter intensity of reaction and there was an excessive aggregation of collagens after PRK that it may be the histological foundation of obvious haze and myopic regression in PRK [20].

Zhou et al evaluated short-term corneal endothelial changes after LASEK and they documented that acute endothelial changes occur on specular microscopic examination after LASEK. When taken as a whole, LASEK-treated eyes had a significant increase in postoperative Coefficient of Variation (CV) of cell size and a significant decrease in Endothelial Cell Density (ECD) and cell hexagonality at 15 min post-operatively. These findings indicate that, immediately after LASEK, the number of cells/mm² decreases and the endothelial cells became much more swollen compared with their preoperative size. But these changes were transient; ECD and variations in cell area returned to near baseline (pre-operative) levels by 1 day postoperatively. An increased CV of cell size would be expected if there was a decrease in the percentage of hexagonal cells, as observed by Zhou.

The percentage of hexagonal cells also returned to near the baseline (pre-operative) level by 1 week post-operatively, suggesting that endothelial cell function recovered 1 week after LASEK [21]. Studies that have evaluated the endothelium after PRK have reported little or no endothelial change, with no clinically significant decrease in central ECD [22,23].

The vitality of the epithelial flap is probably a crucial factor in the dampened wound response in LASEK versus PRK. Gabler et al [24]. Investigated the vitality of the corneal epithelium after exposure to 20% ethanol during LASEK and they demonstrated that after 15 and 30 seconds of exposure to 20% ethanol, the epithelium is intact and most corneal epithelial cells are alive. They also recommend 20to 30-second exposure to 20% alcohol (ethanol) for LASEK. At 30 seconds, they found predominantly vital epithelial cells, whereas after 45 seconds, the fraction of dead cells increased substantially to about 50%. Predominantly dead epithelial cells are seen after 60 and 120 seconds of exposure.

There might have been an overestimation of the fraction of dead cells because of the time between the donor’s death and the beginning of the study. However, their experiments confirmed that after exposure of the cornea to 20% ethanol for up to 30 seconds, the epithelial flap contained predominantly vital cells, which is probably one of the crucial factors in the dampened flap contained predominantly vital cells, which is probably one of the crucial factors in the dampened wound response in LASEK compared to that in PRK. The exposure of the human cornea to ethanol reduces the number of vital epithelial cells rapidly [25] and increases cell death in a doseand timedependent manner. Chen et al. studied the effect of dilute alcohol on human corneal epithelial cell morphology and viability with electron microscopy and they showed that the conventional concentrations and duration of alcohol treatment (20%, 25 seconds) resulted in varying morphological changes in the basement membrane zone by electron microscopy and varying viability in standard tissue culture conditions.

Their electron microscopic findings showed morphological differences in the plane of cleavage among several patients, in whom the same technique was used for creating the epithelial flap. This may be due to variability between individuals in relation to the adhesion of the epithelium to the basal membrane or to the variability of the effect of alcohol on adhesion of epithelial cells. Electron microscopy showed varying degrees of basement membrane alterations after alcohol application, including disruptions, discontinuities, irregularities and duplication.

Cellular destruction and vacuolization of basal epithelial cells associated with absent basement membrane were also observed. Their studies in vitro suggest a doseand time-dependent effect of alcohol on epithelial cells. The 25% concentration of ethanol was the inflection point of epithelial survival. Significant increase in cellular death occurred after 35 seconds of ethanol exposure. Forty seconds of exposure further increased apoptosis after 8 hours of incubation. These findings are consistent with the clinical observations of varied epithelial attachment to the stromal bed after LASEK surgery.

Then they demonstrated that alcohol diluted in Keratinocyte Serum-Free Medium (KSFM) had no effect on cellular survival and apoptosis. At this time, it is not clear whether modification of the preparation of dilute alcohol, used during LASEK and PRK, would allow for better cell survival and adhesion in vivo [26]. The dilution of alcohol in Balanced Salt Solution (BSS), physiologic solution, or sterile water, thus obtaining different osmolarities, is an area of active debate but none of the LASEK studies has shown a definite advantage of a specific formulation.

Camellin strongly points out the importance of a hypotonic solution obtained by diluting alcohol in distilled water for facilitating epithelial detachment [4].

Yuksel et al. evaluated clinical and confocal results of alcoholassisted LASEK for correction of myopia in twenty-two eyes with a mean follow-up duration of 45 months and they showed that LASEK offered safe and effective correction of myopia in the long term [27].

In a retrospective study the stability of visual acuity and refraction, the predictability, corneal keratometry, safety, efficacy, and postoperative complications after 10 years after excimer laser surface ablation performed on thin corneas were evaluated. It was demonstrated that surface ablation seems to be safe and effective to correct myopia in corneas thinner than mm500, with stable visual and refractive outcomes [28].

In 2016 Li SM et al. performed a Cochrane study to compare LASEK versus PRK for correction of myopia by evaluating their efficacy and safety in terms of postoperative uncorrected visual acuity, residual refractive error, and associated complications. They concluded that uncertainty surrounds differences in efficacy, accuracy, safety, and adverse effects between LASEK and PRK for eyes with low to moderate myopia. Future trials comparing LASEK versus PRK should follow reporting standards and follow correct analysis. Trial investigators should expand enrollment criteria to include participants with high myopia and should evaluate visual acuity, refraction, epithelial healing time, pain scores, and adverse events [29].

In 2015 we performed a study evaluating the effectiveness and safety of no-alcohol LASEK after long follow-up of 60 months and comparing the obtained results with no-alcohol standard PRK [30]. Twenty-five eyes were treated with LASEK and twenty-five eyes with standard PRK. Twenty-one eyes and 22 eyes completed follow-up of 60 months in LASEK and PRK group respectively. Manifest refraction at 60 months follow-up was -0.01 and 0.26 in LASEK and PRK group respectively. In the LASEK group mean Uncorrected Distance Visual Acuity (UDVA) and mean Corrected Distance Visual Acuity (CDVA) were 20/22 and 20/20 respectively (p>0.01). In the PRK group mean UDVA and mean CDVA at 60 months follow-up were 20/20 and 20/20 after 60 months (p>0.01). The efficacy indexes were 0.87 and 0.95, and the safety indexes were 1.25 and 1.4 respectively for LASEK group and PRK group. Therefore both standard PRK and no-alcohol LASEK offered safe and effective results in the long term period without any statistically significant difference between the two groups.

In conclusion, this long term study, LASEK with mechanical deepithelialization without the use of alcohol solution demonstrated to be a safe and effective technique to correct low to medium myopia.