LASIK induced changes in the GCD and N/C ratio of epithelial cells. In this study, the results showed the decrease in GCD to be related to suction time and the SE before surgery. We observed in myopic eyes that the decrease in GCD with both flap creation techniques for the LASIK procedures was related to the degree of preoperative myopia and the depth of the laser treatment. This effect has been observed in other studies
20 37 and suggests that the destruction of the corneal tissue due to LASIK directly or indirectly affects the population of conjunctival GCs. Conjunctival GCD and N/C ratio of epithelial cells were significantly reduced after procedures with both the M2 microkeratome and the IL. These findings confirm previous reports on other microkeratomes and refractive surgery procedures.
11 19 20 21 The IL produced a greater decrease in GCD than did the M2 manual microkeratome; however, the N/C ratio decreased with both techniques but without significant differences between them.
The reduction in GCD after IL was probably due to several factors such as toxicity of topical medication, damage of the corneal and conjunctival nerves, inflammation, or mechanical trauma produced by the suction ring. To find differences between the two groups, we compared both techniques, procedures, management of patients, and time of suction. Both groups were homogeneous regarding age, type of ablation, CL users, and preoperative SE. The only difference was gender-related; in the IL group there were more women, although before surgery the GCD in both groups were very similar (
P = 0.82), and thus this finding seems to be irrelevant. All procedures in each group were performed by the same experienced LASIK surgeon, to avoid possible differences in the procedure for making the flap. Topical medication was very similar in both groups. Iodine solution was used only on the periocular skin, never on the conjunctiva, and the management of the ocular surface after LASIK was very similar. All patients used nonpreserved artificial tears after surgery and received the same treatment after surgery with tobramycin 0.3%-dexamethasone 0.1% for 1 week—the only difference being that the application after M2 surgery was four times daily and after IL surgery was six times daily. There was a more frequent application of anti-inflammatory medication in the IL group because in the first patients, in whom we had used similar doses, we had observed a postoperative inflammation that was more severe with the IL than with the M2 (Alió JL, unpublished data, 2005). Because inflammation on the ocular surface can result in decreased conjunctival GCD, the greater decrease in the IL group could be related to this effect, in that a longer suction time may also affect the ocular surface and induce inflammation changes. Such changes are frequently found in the form of traumatic conjunctivitis during the early follow-up of these patients. Chronic inflammatory infiltrates of lymphocytes were observed on the conjunctiva of both groups of patients (scores >3) before surgery, possibly because most of the patients were long-term CL users and had chronic inflammation. There were no significant differences between both groups after surgery. Regarding the trauma caused by the suction ring on the conjunctiva, we observed that the time required to create the flap by the IL was much longer than was required with the M2, and we found a high correlation between this time and the decrease in GCs after surgery. This damage may be related to the more fragile nature of the GC, which, being a secretory cell, has a less stable cell membrane that degranulates with vacuum pressure. In contrast, corneal nerve density decreased after LASIK, whether microkeratomes or bladeless flap creation techniques are used (Erie JC et al.
IOVS 2006;47:E-Abstract 516). However, in this study, there were no differences between treatments, which supports the idea that the corneal nerves are damaged by the LASIK effect, regardless of the technique used to create the flap. The decrease in the GCD may have occurred due to the effect of the suction ring on the conjunctival nerves that regulate GC secretion.
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The results of this study are based on the use of a femtosecond power of 15 KHz, and at this speed, the mean suction time was 108 seconds. Today, the same technology (IntraLase Corp.) uses an updated technology with a power of 60 KHz, which decreases the photograph disruption time, with a consequent decrease in the suction time, and also decreases slightly the amount of energy needed to create the photograph disruption plane inside the cornea. These occurrences should have an effect on the amount of GCs affected by femtosecond LASIK and deserve further investigation.
In conclusion, analyses of the ocular surface by impression cytology before and after LASIK showed a correlation between the degree of correction performed with LASIK and decreased GCD. These changes in GCD may contribute to the development of the ocular surface syndrome that occurs after LASIK. The greater reduction in the GC population with IL than with the M2 microkeratome, however, is more likely to be related to the suction time than to the technology.