Ординатура / Офтальмология / Английские материалы / Hyperopia and Presbyopia_Tsubota, Boxer Wachler, Azar_2003

Figure 11 Spherical aberration after myopic treatment showing increased positive asphericity, as represented by a sombrero hat.

REFERENCES

1.Applegate RA, Thibos LN, Hilmantel G. Optics of aberroscopy and super vision. J Cataract Refract Surg 2001; 27:1093–1107.

2.Maeda N. Wavefront technology in ophthalmology. Curr Opin Ophthalmol 2001; 12:294–299.

3.Huang D. Physics of customized corneal ablation. In: MacRae SM, Krueger RR, Applegate RA, eds. Customized Corneal Ablation: The Quest for Supervision. Thorofare NJ: Slack, 2001: 51–62.

4.Mrochen M, Kaemmerer M, Seiler T. Wavefront-guided laser in situ keratomileusis: early results in three eyes. J Refract Surg 2000; 16:116–121.

5.Kaemmerer M, Mrochen M, Mierdel P, Krinke HE, Seiler T. Clinical experience with the Tscherning aberrometer. J Refract Surg 2000; 16:S584–S587.

6.Krueger RR, Mrochen M, Kaemmerer M, Seiler T. Understanding refraction and accommodation through “retinal imaging” aberrometry. Ophthalmology 2001; 108:674–678.

7.Artal P. Understanding aberrations by using double-pass techniques. J Refract Surg 2000; 16: S560–S562.

8.Schwiegerling J. Theoretical limits to visual performance. Surv Ophthalmol 2000; 45(2): 139–146.

9.Applegate RA. Limits to vision: Can we do better than nature? J Refract Surg 2000; 16: S547–S551.

10.Williams D, Yoon GY, Porter J, Guirao A, Hofer H, Cox I. Visual benefit of correcting higher order aberrations of the eye. J Refract Surg 2000; 16:S554–S559.

11.Thibos LN. The prospects for perfect vision. J Refract Surg 2000; 16:S540–S546.

12.Thibos L. Wavefront data reporting and terminology. J Refract Surg 2001; 17:S578–S583.

13.Oshika T, Klyce SD, Applegate RA, Howland HC, Danasoury MAE. Comparison of corneal wavefront aberrations after photorefractive keratectomy and laser in situ keratomileusis. Am J Ophthalmol 1999; 127:1–7.

14.Mrochen M, Kaemmerer M, Mierdel P, Seiler T. Increased higher-order optical aberrations after laser refractive surgery. A problem of subclinical decentration. J Cataract Refract Surg 2001; 27:362–369.

15.Mrochen M, Kaemmerer M, Seiler T. Clinical results of wavefront-guided laser in situ keratomileusis 3 months after surgery. J Cataract Refract Surg 2001; 27:201–207.

16.Howland HC. The history and methods of ophthalmic wavefront sensing. J Refract Surg 2000; 16:S552–S553.

17.Mrochen M, Kaemmerer M, Mierdel P, Krinke HE, Seiler T. Principles of Tscherning aberrometry. J Refract Surg 2000; 16:S570–S571.

18.Platt BC, Shack R. History and principles of Shack-Hartmann wavefront sensing. J Refract Surg 2001; 17:S573–S577.

19.Krueger RR. Technology requirements for Summit-Autonomus CustomCornea. J Refract Surg 2000; 16:S592–S601.

20.Thibos L. Principles of Shack-Hartmann aberrometry. J Refract Surg 2000; 16:S563–S565.

21.Roberts C, Dupps Jr WJ. Corneal biomechanics and their role in corneal ablative procedures. In: MacRae SM, Krueger RR, Applegate RA eds. Customized Corneal Ablation: The Quest for Supervision. Thorofare, NJ: Slack, 2001:109–131.

22.Argento CJ, Consentino MJ. Laser in situ keratomileusis for hyperopia. J Cataract Refract Surg 1998; 24:1050–1058.

23.McDonald M. Summit—Autonomus CustomCornea Laser in situ keratomileusis outcomes. J J Refract Surg 2000; 16:S617–S618.

24.Pettit GH, Campin J, Liedel K, Housand B. Clinical experience with the CustomCornea measurement device. J Refract Surg 2000; 16:S581–S583.

Correction of hyperopia has been a constant concern of ophthalmologists over the past decades. Some of the surgical procedures that have been developed—hexagonal keratotomy (4,5), keratophakia, keratomileusis, and epikeratophakia (6–9)—have been abandoned because of limited applicability or side effects. Among current corrective procedures undoubtedly laser-assisted in situ Keratomileusis (LASIK) and Ho:YAG laser thermal keratoplasty (LTK) are the most widespread. Recently published clinical results emphasize the fact that LASIK is a procedure with good predictability, stability, efficacy, and safety for the correction of low to moderate spherical hyperopia (10). Long-term predictability with occurrence of undercorrection is influenced by the preoperative keratometric values and ablation zone diameter (11). Other studies point out the importance of corneal thickness and width of the flap for LASIK feasibility (12). The effectiveness of LASIK for severe hyperopia and hyperopic astigmatism is reduced (13,14). For treatments over 5.00 D, the incidence of loss of best-corrected visual acuity was increased. Current nomograms require the cut of a larger flap in order to enlarge the ablation zone and to decrease the risk of halos, glare, and night vision difficulties for patients with high hyperopia and astigmatism (15). A lower predictability for astigmatic corrections was also reported after LASIK for myopia (16) in spite of in situ axis alignment (17,18). Encouraging results have been reported with respect to the safety, predictability, and stability of LASIK correction, for small degrees of hyperopia that were secondary to previous radial keratotomy (RK), and for automated lamellar keratoplasty (ALK) (19). The degree of regression after H-LASIK was reported to be higher relative to myopic corrections but lower, even in high hyperopia, than with the PRK procedure (20). Flap irregularities, epithelium, infection, or nonspecific inflammation at the flap interface have been reported complications of the LASIK procedure (21). Loss of vision can occur in cases of buttonholes, free cap, or amputation of the flap (22).

Correction of hyperopia and astigmatism by thermal keratoplasty was reported more than 100 years ago (23–25). The actual mechanism by which this procedure alters the anterior corneal curvature has been clarified with the discovery of shrinkage temperature of corneal collagen by Stringer and Parr (26). In 1970s and 1980s, keratoconus was the focus of theromokeratoplasty technology. A number of clinical studies done have evaluated thermal keratoplasty potential to replace penetrating keratoplasty in keratoconus treatment (27–30). In spite of the fact that initial flattening of the cone followed the procedure, regression occurred within a few weeks postoperatively. It was not uncommon for these keratoconus treatments to be accompanied by complications such as corneal scarring, vascularization, and bullous keratopathy. Additionally, poor predictability and stability contributed to the withdrawing of the procedure from clinical use for keratoconus.

A more recent approach to thermal keratoplasty is credited to Fyodorov, who developed a technique, using controlled thermal burns of corneal stroma with a retractable probe tip heated to 600 C and applied in a radial pattern. The procedure was eventually abandoned because of the high incidence of postoperative regression (31). In spite of repeated challenges to achieve predictable and stable refractive outcomes, researchers did not give up on probe technology but took another avenue, which was the use of lasers to deliver controllable amounts of energy to the stroma.

Lasers such as continuous CO2 and cobalt magnesium fluoride have been used in experimental studies on rabbit corneas, with transient results (32,33). Reports of clinical studies that used the erbium:glass laser (34) have shown good results for hyperopia higher than 3.00 D. Over the past decade, ophthalmologists in the United States have directed their work at evaluating two Ho:YAG laser systems: the noncontact system (Sunrise Tech-

nologies, Fremont, CA) and the contact system (Summit Technologies, Waltham, MA). The Sunrise Ho:YAG is a pulsed laser that emits laser light at a wavelength of 2.13 m. Other technical characteristics include pulse repetition frequency of 5 Hz and pulse energy in the range of 226 to 258 in correlation with the amount of refractive correction required. The energy is applied to the cornea in a noncontact mode through a fiberoptic slit-lamp system; the treatment pattern is represented by rings of spots concentric to the pupil (35). Sand, who was granted a patent for performing infrared LTK, was an important contributor to the development of this technology. Initial in vitro investigations have been made on swine and human cadaver eyes (36,37) in an attempt to establish a treatment protocol. Further studies done on human poorly sighted eyes showed a mean change in corneal curvature of 1.10 D followed by some amounts of regression (38). Results of clinical trials done outside the United States, which used the eight-spot treatment pattern applied at different diameters (6, 7, or 8 mm), had shown that the procedure works best up to 3.00 D. They also proposed a treatment algorithm adjusted to variables such as age and central corneal thickness (39). Other studies have demonstrated that the amount of refractive change is increased when a two-ring treatment is applied at the 6- and 7-mm center line in a radial instead of a staggered pattern (40,41). The U.S. phase III study protocol has defined the efficacy criteria for the LTK procedure as improvement in distance UCVA and reduction in hyperopia manifest refraction spherical equvalent (MRSE) 0.5 D. Evaluation at 2 years showed that 69.4% of patients had more than two lines of improvement in distance uncontrolled visual acuity (UCVA) and no eyes had lost more than two lines of best spectacle corrected visual acuity (BSCDVA) (35).

B. CONTRAST SENSITIVITY IN LASIK AND LTK

In understanding the outcomes of contrast sensitivity, we conducted a study to evaluate the quality of vision through its changes in LASIK and noncontact Ho:YAG LTK for the correction of low to moderate spherical hyperopia. We analyzed the results of two groups of patients who had LASIK and LTK, respectively, as primary procedures. There was no history of ocular diseases or surgery. We compared best-corrected contrast sensitivity values preoperatively and at 3 months postoperatively. Contrast sensitivity was measured with the self-calibrated, internally luminated CSV-1000E Vector Vision (Dayton, OH) at 12 cycles per degree (cpd) spatial frequency. The patient was instructed to identify whether the bars were in the top circle, bottom circle, or neither. The last correct identification has been taken as the contrast sensitivity. On the contrast sensitivity chart the numbers represent normalized ratios where values greater than 1.0 correspond to percent contrasts sensitivity above the population average and values below 1.0 represent percent of the average contrast sensitivity below the population average (42). Visual acuity was measured with the Vector Vision acuity chart using a scoring method of the U.S. Food and Drug Administration for refractive surgery clinical trials (43). All visual function tests were done with best spectacle-corrected visual acuity.

Data were analyzed with the StatView (SAS Institute Inc., Cary, NC) statistical package. Visual acuity data were analyzed in logMAR values. Normalized contrast sensitivity values were converted to log values and used for statistical analysis.

The LASIK study group comprised 94 eyes of 49 patients, 21 men and 28 women. Mean patient age was 59.67 years 7.95 SD, range 44 to 78 years. Preoperatively, mean deviation from target manifest refraction was 2.4 D 1.2 D, SD, (range 0.37 to5.60 D). LASIK procedures were performed by the same surgeon (B.B.W.) using the

Moria LSK (Doylestown, PA) microkeratome and the Summit Apex Plus Laser (Summit Technology Inc., Waltham, MA); the treatment zone was centered on the pupil. Results have shown a mean postoperative deviation from target manifest refraction of 0.09 D0.88 D, SD, (range 2.25 to 2.00 D) at 3 months. Table 1 shows the mean preoperative and postoperative log contrast sensitivity values, standard deviations, and maximum and minimum values. At 3 months postoperatively the mean log contrast sensitivity value was not statistically significantly different compared to preoperative levels (p 0.18). The mean best spectacle-corrected logMAR visual acuity value at 3 months was statistically significantly worse relative to preoperative value (p 0.008). However, the change was not clinically significant, as the logMAR conversion was a loss of 1.5 letters on the acuity chart. There was a statistically significant correlation between achieved refraction and changes in log contrast sensitivity values (p 0.006) (Fig. 1) (r 0.29, p 0.006). This indicated that higher amounts of hyperopic correction were associated with greater loss of best-corrected contrast sensitivity. No statistically significant correlation was ob-

Figure 1 Correlation between changes in log contrast sensitivity values and achieved refraction in the H-LASIK group.

Figure 2 Correlation between changes in best spectacle-corrected logMAR visual acuity values and achieved refraction in the H-LASIK group.

served between achieved refraction and changes in best spectacle-corrected logMAR visual acuity (r 0.05, p 0.58)(Fig. 2).

The LTK study group comprised 55 eyes of 35 patients, 16 males and 19 females. Mean patient age was 57.61 years 7.35, SD, with a range of 39 to 71 years; mean deviation from target manifest refraction of treated eyes was 1.5 D 0.59 D, SD, range 0 to 3.00 D. Noncontact Ho:YAG LTK treatments were performed by the same surgeon (B.B.W.) using the Sunrise Hyperion Holmium Laser Corneal Shaping System (Sunrise Technologies Inc., Fremont, CA). The treatment was centered on the corneal purkinje image of the patient fixation light. The light reflex closely approximates the visual axis. Therefore, in cases of positive angle kappa, the treatment was not centered on the pupil. Laser parameters included wavelength, 2.13 m; pulse duration, 250 s; pulse repetition frequency, 5 Hz; pulse energy, adjustable from 226 to 258 mJ/pulse. In the current study we used a two concentric radial 8-spot ring treatment pattern centered around the fixation light reflex on the cornea. Postoperatively, results showed a mean deviation from target manifest refraction of 0.36 D 0.84 D, SD, range 3.50 to 1.25 D. Mean log contrast sensitivity value was not statistically significantly decreased (p 0.07) (Table 2) and mean best spectacle-corrected logMAR visual acuity value was statistically signifi-

Table 2 LTK Group—Preoperative and Postoperative Log Contrast Sensitivity Values and Best Spectacle-Corrected LogMAR Visual Acuity Values

Figure 3 Correlation between changes in log contrast sensitivity values and achieved refraction in the LTK group.

cantly worse (p 0.0067) relative to preoperative values. The change in acuity was not clinically significant as the change represented approximately four letters on the acuity chart. No statistically significant correlation (R 0.16, p 0.25) was found between achieved refraction and changes in log contrast sensitivity values (Fig. 3). Figure 4 shows the lack of correlation between achieved refraction and best-spectacle corrected logMAR visual acuity values (r 0.15, p 0.26).

Figure 4 Correlation between changes in best spectacle-corrected logMAR visual acuity values and achieved refraction in the LTK group.

C. DISCUSSION

As new surgical procedures are added to the refractive surgery armamentarium, assessing visual outcome becomes more difficult. Information regarding postoperative visual acuity and refractive changes is no longer satisfactory to evaluate the quality of the image projected on the retina (44). Contrast sensitivity, as a functional method, has been shown to be directly affected by the distorted image following excimer laser surgery (45). Using digitized retroillumination, Vinciguerra has shown that corneal distortion arising from prominent flap striae may be overlooked by the customary slit-lamp examination (46). Our results have shown a slight decrease in contrast sensitivity at 12 cpd spatial frequency postoperatively after LASIK procedure. However the difference was not statistically significantly different (p 0.18). Previous literature data that have demonstrated that spatial frequency of 12 cpd is mostly affected by degradation in optics, such as aberration or blur (47). Other studies reported a loss of contrast sensitivity at 12 months after LASIK of up to one line for low hyperopia and of more than two lines for high hyperopia with no statistical significance (13). An interesting finding in the LASIK group was the significant correlation between achieved refraction and change in contrast sensitivity, demonstrating that larger amounts of correction are accompanied by larger loss of contrast sensitivity. This indicates that with the Summit Apex Plus laser used for LASIK and centered on the pupil, higher degrees of hyperopic treatment as associated with a higher risk of loss of best-corrected contrast sensitivity.

Contrast sensitivity showed little change after the LTK procedure. The minimal decrease observed was not statistically significant (p 0.07). Furthermore, contrast sensitivity changes showed no correlation with the amount of spherical correction attempted. Clinical trials at 1 and 2 years after LTK reported that mean contrast sensitivity increased at all follow-up visits for the two-ring treatment group at Regan charts (40,48). Postoperatively visual acuity did not vary significantly (p 0.0067) and was not influenced by the amount of correction, although the amount of hyperopia corrected in the LTK group was less than that corrected in the LASIK group.

We conclude that measuring contrast sensitivity after refractive surgical procedures should be encouraged and further developed in order to assess the limits of safety for given procedures and devices used for such procedures. Studies should be directed at identifying laser characteristics and treatment patterns that are able to optimize the optical system of the eye, thus increasing safety.

REFERENCES

1.Palmer SE. Vision Science—Photons to Phenomenology. Cambridge, MA: MIT Press, 1999: 146–198.

2.Blakemore C, Campbell FW. On the existence of neurons in the human visual system selectively responsive to the orientation and size of retinal images. J Physiol 1969; 203:237–260.

3.Shapley R, Man-Kit Lam D. Contrast Sensitivity: Proceedings of the Retina Research Foundation Symposia. Vol. 5. Cambridge, MA: MIT Press, 1993: 253–266.

4.Werblin TP. Hexagonal keratotomy. Should we still be trying? J Refract Surg 1996; 12: 613–620.

5.Grandon SC, Sanders DR, Anello RD, Jacobs D, Biscaro M. Clinical evaluation of hexagonal keratotomy for the treatment of primary hyperopia. J Cataract Refract Surg 1995; 21:140–149.

6.Swinger CA, Barraquer JI. Keratophakia and keratomileusis—clinical results. Ophthalmology 1981; 88:709–715.