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DISCUSSION
LASIK has become the most popular surgery procedure for the correction of myopia, hyperopia, and astigmatism. Complications after LASIK have become evident [111]. One of the most devastating complications after LASIK is corneal ectasia which was first reported in 1998 by Seiler[3,4] and Speicher[22]. Signs of keratectasia typically appear from 1 week to 4 years after LASIK, and keratectasia appears bilaterally in about two thirds of patients [23,24]. Our patients time to onset of keratectasia was 29 months postoperatively, which was within the reported period.
The etiology of iatrogenic keratectasia after LASIK is not known, other than to state that intraocular pressure causes forward bowing and thinning of a structurally compromised cornea[25,26]. In order to prevent this complication, most doctors suggest that the residual stroma should not be less than 250μm or should be at least half the original thickness, which is considered as the relatively safe thickness[2730] , but in recent years, many reports revealed that the “safe thickness” can not prevent patients from developing keratectasia after refractive surgery in some cases, especially in LASIK patients[2,7,8,10,23]. In our patient, preoperative corneal thickness was 526μm in the right eye and 541μm in the left eye, the theoretical flap thickness was 160μm in both eyes, and the ablation was 102μm and 86μm, resulting in a residual posterior corneal thickness of 264μm and 295μm, which is thicker than the literature advises. But as we know, inaccurate microkeratome cuts that result in thicker flap and deeper laser ablation than prediction are also risk factor for keratectasia after LASIK[3135], which will lead to a discrepancy between the calculated and real residual bed thickness. In our patient, the residual corneal thickness of 264μm OD and 295μm OS is the calculated value, the true thickness was not known, as we did not do the intraoperative measurement, which might be one defect of our operation. Except for the thin residual thickness, forme fruste keratoconus is also a risk factor for keratectasia[3638], but in our patient, the preoperative topography showed no signs of forme fruste keratoconus, the central keratometry was 44.30×87°/42.63×176° and 44.12×93°/42.19×7° . In our patient, there was a flap displacement caused by an unexpected force in the right eye on the operative night. We consider this could not be the reason that lead to keratectasia in the right eye, as keratectasia also occurred in the left eye; the more pronounced condition in the right eye may due to the thinner residual stroma.
In order to prevent postoperative ectasia, preoperative screening to exclude patients who are at high risk of experiencing this condition is very important. In 2006, Tabbara and Kotb AA[38]established a grading system and assessed the cumulative risk score. In this system, corneal keratometry, oblique cylinder, pachymetry, posterior surface elevation, difference between the inferior and superior corneal dioptric power, and posterior best sphere fit (BSF) over anterior BSF were given a grade of 1 to 3 each. The results showed that patients who had a grade of 7 or less showed no evidence of corneal ectasia, whereas 59% (16 of 27) patients who had a grade of 8 to 12 had corneal ectasia; all(21 of 21) patients with a grade of more than 12 had corneal ectasia after LASIK. In our patient, as there were no OrbscanⅡsystem results, we can not use this grading system to evaluate the cumulative risk score.
Options to rehabilitate poor visual acuity in keratectatic patients include contact lens fitting, intracorneal rings, keratoplasty and collagen crosslinking; glasses are of little use in most cases. Among the options, most doctors prefer contact lens because they are easily available and the method is not invasive[1221]. In keratoconus eyes, most apexes are located in the inferotemporal quadrant [1215], while in some iatrogenic keratectasia patients, the apex and corneal thinning are in the center which make it difficult for a contact lens to fit for this kinds of cornea, because the central apex was so steep and small that it would trap airbubble formation below the apex to disturb the visual acuity. In our patients, the apexes were in the inferior halfpart, which were like that in ordinary keratoconus eye. We tried the contact lens designed for keratoconus patients which exhibited an apical clearance fluorescein pattern; and the patient could tolerate allday wearing. The BCVA was 20/25 OD and 20/20 OS, which was the same as preoperative BCVA.
In the first two followup years, the topographical map of this patient showed that the keratectasia maintained relatively stable; in the third followup year, the topographical map showed that the steepening area in both eyes were larger than before, while the maximal keratometry had no much changes, it were 62.04D OD and 54.25D OS respectively. In the three years followup time, the thinnest pachymetry was stable, which ranged from 342μm to 363μm in the right eye and from 375μm to 385μm in the left eye. The little difference might due to the measurement error and/or some extension of corneal epithelium edema.
As the contact lens can still enhance the BCVA to 20/20, we think that there is no need to do any surgical procedure, such as intracorneal ring implantation or keratoplasty, except close monitoring of the patient during the following years to avoid other serious complications. In the more follow up time, if cone of the keratoconus would not permit to fit RGP, other options such as penetrating keratoplasty or intracorneal ring segments should be considered. Talking about monitoring, our patients followup was not very well. The patient left our department on the third postoperative day and did not come to check up until 29 months later; after the first pair of contact lens was given, she did not go to any eye department for about two years until she felt decreased visual acuity in the right eye in March 2006.
The main complications of contact lens wearing include epithelial damage, apex scarring, corneoscleral junction pannus, intolerances of allday wearing and contact lens displacement [13, 39, 40]. In our patient, the cornea apex of the right eye had some edema and opacity in March 2006 and this led to the decreased UCVA and BCVA, while in the last examination time, the edema had disappeared and the opacity had faded in the right eye, the BCVA reached to 20/20 again, and there were no problems in the patients right eye. In the left eye, there were some epithelial defects in the centre when the patient was examined in April 2007; they healed with sodium hyaluronate eye drops. Except for the epithelial changes, this patient had no other complications during the three years followup time.
Based on this case, we think that recommendations can be given to a surgeon as following: a) normal topography has some limitations in decision making for refractive surgery, other examination such as OrbscanⅡsystem should be included; b) intraoperative pachymetry in patients that undergo LASIK surgery with borderline pachymetry is necessary; c) early recognition and proper management are essential to prevent progression and to promote visual rehabilitation for patient with keratectasia. d) suitable selection of lens design and modification of parameters can help to obtain comfortable visual rehabilitation in eyes with keratectasia after LASIK. Intracorneal ring segments implantation and keratoplasty can probably be avoided by careful monitoring of the contact lens fit in this patient.
【参考文献】
1 Sugar A, Rapuano CJ, Culbertson WW, Huang D, Varley GA, Agapitos PJ, de Luise VP, Koch DD. Laser in situ keratomileusis for myopia and astigmatism: safety and efficacy: a report by the American Academy of Ophthalmology. Ophthalmology2002;109(1):175187
2 Ou RJ, Shaw EL, Glasgow BJ. Keratectasia after laser in situ keratomileusis (LASIK): evaluation of the calculated residual stromal bed thickness. Am J Ophthalmol2002;134(5):771773
3 Seiler T, Koufala K, Richter G. Iatrogenic keratectasia after laser in situ keratomileusis. J Refract Surg1998;14(3):312317
4 Seiler T, Quurke AW. Iatrogenic keratectasia after LASIK in a case of forme fruste keratoconus. J Cataract Refract Surg1998;24(7):10071009
5 Geggel HS, Talley AR. Delayed onset keratectasia following laser in situ keratomileusis. J Cataract Refract Surg1999;25(4):582586
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