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

ture and smaller amounts of topographical regression were noted when a two-ring laser treatment pattern was applied. When the topography was analyzed, several forms of induced astigmatism were observed: bowtie (both symmetrical and asymmetrical), irregularly irregular, and semicircular patterns. Only one eye in the entire study group was observed to have a homogeneous pattern. At present, noncontact LTK appears to be most promising for low hyperopia up to approximately 2 D. Regression of the effect appears to limit the procedure’s usefulness for refractive errors higher than 2 D. Furthermore, factors such as younger age (less than age 30) and increased preoperative corneal thickness may also contribute to faster rates of regression (5).

Early hyperopic photorefractive keratoplasty (H-PRK) ablations consisted of small optical zones (approximately 4.0 mm) with small transition zones, creating an overall treatment zone diameter of 7 to 8 mm. Small optical zones increase the patient’s sensitivity to small decentrations. Likewise, small transition zones produce abrupt topographical and refractive changes between treated and untreated tissue. This “lack of smoothness” promotes more aggressive stromal and epithelial regeneration and thus refractive regression

(6). It should also be noted that in myopic PRK, significant decrements in the character and magnitude of corneal optical aberrations have been found with larger optical and transition zones. Larger optical and transition zones result in a more natural physiological pattern of measured aberrations in myopic PRK (6), and a similar result would be expected in approaches to correct hyperopia. These considerations have led to larger optical zones of 6.0 mm, with overall hyperopic ablations now reaching 9.0 mm.

With these considerations, induced aberrations after H-PRK have been carefully evaluated (7). Corneal topography after H-PRK showed a change from positive to negative spherical aberration on the order of 3 D. It is known that the positive spherical aberration of the cornea and the spherical aberration of the crystalline lens act in concert to decrease the overall aberrations of the eye. However, if hyperopic procedures over correct for corneal spherical aberration, a negative impact on visual function is expected. This effect can be seen in Figure 1.

Even with larger ablation sizes, difficulties remain. By the nature of the procedure, the functional optical zone becomes smaller as the attempted correction increases in size. This is undoubtedly one of the most significant factors contributing to the poor success rate of both H-PRK and hyperopic laser assisted in situ keratomileusis (H-LASIK) for the correction of 5.00 D or greater. Moreover, Choi et al. (8a) report an increased risk of irregular astigmatism based on topographic analyses when corrections above this level are attempted. The comfort level in this respect seems to be surgeon-related; therefore some surgeons limit attempted corrections to 4.0 D or less.

In reference to H-LASIK, a 9.0-mm ablation size requires the creation of a 9.5-mm flap. Although modern microkeratomes may provide for this flap size, some patients with small eyes or thin corneas are unsuitable candidates for this treatment. Larger flap diameters and larger amounts of correction increase the chances of striae formation, which can translate to irregular astigmatism on corneal topography.

H-LASIK is gaining widespread use as a procedure to correct primary hyperopia as well as to modify consecutive hyperopia after overcorrection from LASIK for myopia; it is said to be safe and effective (8). Two typical case reports are given below to illustrate the topography obtained. Each patient underwent hyperopic LASIK with the VISX, Inc., Star Excimer Laser System. The diameter of the optical zone was 5.00 mm, with a total treatment zone of 9.00 mm OU.

Figure 1 H-LASIK effect on corneal topography and total eye aberration measured with NIDEK OPD-Scan. (1) (top left panel) standard corneal topography showing off-center treatment; (2) skiascopic (pointwise refraction) map: in the postoperative period, corneal aberrations for this eye account for the bulk of the total ocular aberrations; (3) placido image; (4) wavefront map showing induction of excess negative spherical aberration and coma.

Case 1. A 66-year-old woman with no prior history of ocular surgery underwent H-LASIK for a refraction of 0.75 1.00 170 OD and 0.25 1.25 180 OS. Her best spectaclecorrected visual acvity (BSCVA) was 20/20 ( 2) OU. The patient requested refractive surgery for monovision. Her preoperative K-readings were 44.3/44.5 at 118 OD and 44.4/44.8 at 163 OS. The laser was programmed to correct OD for 1.00 1.25 170 and OS for 2.501.50 180. The total ablation depth was 20 m OD and 38 m OS. Optical zone diameter was 5.00 mm. Her visual acuity without correction on postoperative day 1 was 20/200 OD and 20/80 OS. Two weeks postoperatively, her visual acuity without correction was 20/70 ( 1) OD and 20/200 OS and her BSCVA was 20/25 OD and 20/40 OS. The manifest refraction was 0.75 1.00 050 OD and 2.50 0.75 055 OS. At 4 weeks postoperatively, her visual acuity without correction was 20/30 ( 2) OD and 20/25 ( 2) OS. Her refraction at this time was 0.75 0.75 055 OD and 1.50 0.75 165 OS, with BSCVA being 20/25 OD and 20/25 ( 2) OS. Postoperative corneal topography

A

B

Figure 2 H-LASIK 1-month postoperative topography for 66-year-old requesting monovision.

(A) OD; attempted correction: 1.00 1.25 170. (B) OS; attempted correction: 2.50 1.50180.

showed the extent of induced cylinder, revealed a steepening of the central 5 mm of the cornea, and produced simulated keratometry readings (SimKs) of 46.13/44.17 at 96 with a potential visual acuity (PVA) of 20/25 to 20/30 OD and 47.41/46.68 at 94 with a PVA of 20/20 to 20/30 OS (Figure 2).

Case 2. A 26-year-old woman presented for refractive surgery evaluation. She had a refractive error of 4.75 0.50 083 OD and 5.25 0.75 095 OS. Her BSCVA was 20/25 OU. Her keratometry readings were 44.1/45.6 at 091 OD and 44.1/46.0 at 099 OS. The desired correction for the right eye was 6.00 0.50 083 and for the left eye was 6.00 0.50 105. Total ablation depth was 65 m OU. Optical zone diameter was 5.00 mm. On postoperative day 1 her uncorrected visual acuity was 20/30 OD and 20/40 OS. Six months postoperatively, BSCVA was 20/25 ( 1) OD with no improvement with manifest refraction. BSCVA OS was 20/25 ( 1) with a manifest refraction of 1.00 0.75 165. There was some evidence of regression OS. Postoperative keratometry readings were 48.70/49.18 at 058 OD and 47.24/48.07 at 051 OS (Figure 3).

Hence, H-LASIK seems a good choice of procedures at least for the temporary correction of hyperopia. Long-term stability will need to be demonstrated for this approach, as for others discussed in this chapter.

Conductive keratoplasty (CK) is being developed as an alternative procedure for treating hyperopia. It is argued that if the collagen is heated to a carefully controlled critical temperature, the shrinkage and changes in corneal shape might be more permanent.

Figure 3 Six-month postoperative corneal topography of H-LASIK patient showing good centration OU. (Central green irregularities OS are temporary, from tear film breakup.)

Figure 4 Preoperative and postoperative topographies after CK. Note large uniform area of increased power.

Conductive keratoplasty uses radiofrequency energy to generate heat in the corneal periphery. As with LTK, the shrinkage of the collagen occurs from the production of a ring pattern of treatment spots around the corneal periphery. This shrinkage creates a pursestring effect to steepen the central cornea. One of the immediately appreciated benefits of CK over H-LASIK is the larger functional optical zone (Figure 4).

C. MULTIFOCAL EFFECTS

As the number of patients undergoing refractive surgery expands, the curious phenomenon of presbyopic patients presenting with functional near and far vision after refractive surgery is being more frequently reported for both myopic and hyperopic corrections. Described as a “multifocal” effect, this side effect of the surgery deserves scrutiny.

It was Benjamin Franklin who conceived the first bifocal spectacle in 1874, initiating what is perceived to be a sequence of developments (Figure 5). Deliberate multifocality was introduced to the contact lens field prior to 1967 (9) and to the intraocular lens (IOL) arena before 1987 (10). While early models of IOLs and contact lenses exhibited pronounced aberrations that reduced contrast sensitivity, current renditions have enjoyed a measure of patient acceptance, at least with contact lenses. Unintentional iatrogenic multifocality was first identified with corneal topography in 11 eyes after radial kerato-

Figure 5 Historical use of multifocality in vision correction: Ben Franklin’s bifocal spectacles, bifocal contact lenses, bifocal IOLs, multifocality in radial keratotomy (11) and in photorefractive keratectomy for myopia (13).

tomy, and although the possibility of complications from degradation of contrast sensitivity as well as monocular diplopia was anticipated, no patient complaints of this type were in fact reported (11). However, shortly after this report, additional analysis showed that certain patients with the multifocal effect after radial keratotomy could experience visually debilitating irregular astigmatism. This should be considered a complication of surgery (12). Multifocal effects have also been found following PRK (13) for myopia and contribute to a form of artificial accommodation in pseudophakic eyes (14).

It is well known that patients with an extreme amount of irregular corneal astigmatism often refract over a large range of powers. This is the basis for the so-called multifocal effect; in spectacles, distinctly separate areas of the lens are prepared with different specific powers, whereas the power distributions of the multifocal cornea are more continuously graded and are analogous to gradations of refractive powers of the Varilux contact lens system. It might therefore be more accurate to describe the multifocal property as one of varifocality.

A topographical multifocal effect can be assessed by direct examination of the distribution of corneal powers over the entrance pupil. The standard statistical metric for measuring the width of such distributions is the coefficient of variation; hence, an appropriate topographic definition of corneal multifocality is the coefficient of variation of corneal power (CVP) (15). The increase in the range or width of the distribution of central corneal

Figure 8 After correction for distance vision with conductive keratoplasty for hyperopia, uncorrected near vision (UNVA N) also improves (p 0.001). (Data courtesy of Refractec, Inc., lrvine, CA).

in contrast sensitivity and symptomatic vision. The effect of varifocality in corneal surgery can be evaluated mathematically by fitting the surface with Zernike polynomials and calculating from this the modulation transfer function. This will give the global optical characteristics of the corneal surface and allow the simulation of multifocal effects on vision, as shown in Figure 7.

With hyperopic keratorefractive surgery, there is another effect that comes into play under the guise of multifocality. In one cohort of patients undergoing the conductive keratoplasty procedure for the correction of low hyperopia, postoperative near vision either remained constant or was enhanced at 1 month for every eye in the study (Figure 8). The average improvement was statistically significant (p 0.001). This is a striking effect that generally contrasts with myopic keratorefractive surgeries, where functional near vision typically worsens in the presbyopic patient population (16). This effect can be explained. Presbyopes who are mildly myopic often have excellent near vision without correction. With keratorefractive surgery, near vision is sacrificed for improved distance vision. On the other hand, presbyopic hyperopes have very poor uncorrected near vision, and when keratorefractive surgery is used to correct their distance vision, this brings their near focal point closer to the eye and improves vision at the near reading distances.

Multifocality and better than expected near vision after keratorefractive surgery for the correction of hyperopia are due to a combination of factors. Focus over a range of distances is made possible by the varifocal nature of some postoperative corneal topographies. Residual accommodation in younger patients can enable uncorrected near vision. Use of the pinhole effect and bright illumination make a contribution as well. Finally, improvements in uncorrected near vision can be expected after hyperopic corrections because the near focal plane is brought closer to the eye, whereas with myopic corrections, it is moved further away.

D. DIAGNOSTIC IMPLICATIONS

Several approaches have been developed to provide for the automatic interpretation of corneal topography (17). Among these, neural networks appear to have the greatest poten-

E. SUMMARY

Corneal topographic analysis is helpful in elucidating the strengths and weaknesses of refractive surgical procedures, and surgery for hyperopia is no exception. Centration is critical, and a large treatment zone size is technically difficult to achieve. A hyperopic procedure’s stability can be objectively and precisely measured with corneal topography. However, the results of stability measurements may be confounded by the fact that people in this age group (50–65 years) are undergoing progressive hyperopia naturally; this must be taken into account. Several factors, including varifocality of corneal topography, contribute to better than expected near visual function after the surgery. With advancing age, qualities of the tear film diminish, and this leads to fine surface irregularities, while the induction of coma results from global asymmetrical changes in shape.

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