Proper evaluation of accommodating IOLs

LETTERS

‘‘a steepening of anterior central lens curvature’’ should not have been included in our introduction. As suggested by Galand, a forward shift of the IOL optic is the appropriate statement regarding this topic.dLuis G. Vargas, MD

Long-term results of implantation of phakic posterior chamber IOLs In their article about the long-term results of phakic posterior chamber intraocular lens (IOL) implantation, Lackner et al.1 present the complication rates of cataracts in patients with a phakic posterior chamber IOL with 3 years of follow-up. It is noted that in 76 eyes of patients with a mean age of 48.3 years, there were 11 cases (14.5%) of cataract, of which 5 (45.5%) were progressive and 3 of the 5 eventually required cataract extractions. The authors conclude that the age of the patient is an important risk factor for the development of lens opacification. They also conclude that if one were to follow the suggested criteria in which surgery is performed only in those younger than 45 years of age, the opacification rate would decrease to 5.0%. Whereas 6 cases were presented with early opacification implying intraoperative trauma, 5 cases had late presentation with no apparent predisposing factor. This represented a 6% rate of late cataract complication in which age appears to be the only factor. Considering the number of phakic IOLs implanted each year, it is likely that cataracts can present at a much earlier age in this group of patients as they get older. We think that in contrast to the authors’ conclusion, excluding patients over a certain age may only reduce the incidence of cataracts in the medium term. Long-term results must be validated. ARTHUR C.K. CHENG, MRCSED KENNETH S.C. YUEN, MRCSED DENNIS S.C. LAM, FRCS, FCROPHTH Hong Kong SAR, People’s Republic of China REFERENCE 1. Lackner B, Pieh S, Schmidinger G, et al. Long-term results of implantation of phakic posterior chamber intraocular lenses. J Cataract Refract Surg 2004; 30:2269–2276

Reply: The observation of Cheng et al. is not as far from our conclusions as they may think. We agree that the incidence of late opacifications is associated with a more vulnerable lens in advanced age. However, with the case numbers in this study, it would be inappropriate to apply inferential statistics to this association. At present, the assumption that the lens is constantly traumatized and therefore opacifies more readily after intraocular contact lens (ICL) implantation irrespective of patient age at the time of implantation is as valid as the assumption that it is patient age at the time of surgery that is the risk factor and the effect of surgery decreases with time. In light of recent publications that report significantly lower midterm rates of opacification in patients younger than 45 years (at the time of implantation), the second option may still prove to be more valid. We are as convinced as Cheng et al. that only long-term investigations can tell us more about the reaction of the crystalline lens to ICL implantation, preferably from the centers that have reported

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a higher incidence of opacification in short-term results.dBirgit Lackner, MD

Choroidal neovascularization after LASIK in a patient with low myopia Saeed et al.1 report a single case of choroidal neovascularization (CNV) 3 months after laser in situ keratomileusis (LASIK) for low myopia. After a discussion featuring indefinite terms such as could, probably, and possibility, they conclude: ‘‘.the potential risks of LASIK outweigh its benefits.’’1 I believe there are 2 errors in the author’s analysis. First, they group the affected eye, which had
2.75 diopters (D) of myopia, with eyes that had more than
13.00 D of myopia. Second, they define a risk for all eyes with low myopia having LASIK on the basis of a single case. Eyes with greater than
6.0 D of myopia in which the basic abnormality is excessive axial elongation are known to be at increased risk for CNV.2,3 The authors admit an incidence of CNV in high myopia of 5% to 10% but do not present evidence for a higher incidence in highly myopic eyes that have had LASIK. Also, CNV is common in some general populations with an incidence of between 0.4% and 1.1% in patients older than 43 years4 and up to 3.5% in the elderly.5 Without proper assessment of the incidence of CNV after LASIK for low myopia and comparison with a matched control group, this report remains an isolated novelty. Therefore, I suggest that the authors’ conclusions are currently unfounded and unjustified and should be moderated. C. STEVEN BAILEY, FRCS, FRCOPHTH London, United Kingdom REFERENCES 1. Saeed M, Poon W, Goyal S, et al. Choroidal neovascularization after laser in situ keratomileusis in a patient with low myopia. J Cataract Refract Surg 2004; 30:2632–2635 2. Avila MP, Weiter JJ, Jalkh AE, et al. Natural history of choroidal neovascularization in degenerative myopia. Ophthalmology 1984; 91:1573– 1580. discussion by BKJCurtin, 1580–1581 3. Hotchkiss ML, Fine SL. Pathologic myopia and choroidal neovascularization. Am J Ophthalmol 1981; 91:177–183 4. Tomany SC, Wang JJ, van Leeuwen R, et al. Risk factors for incident agerelated macular degeneration; pooled findings from 3 continents. Ophthalmology 2004; 111:1280–1287 5. Gregor Z, Joffe L. Senile macular changes in the black African. Br J Ophthalmol 1978; 62:547–550

Proper evaluation of accommodating IOLs In their article on the changes in accommodation postimplantation of the Crystalens AT-45 intraocular lens (IOL), Marchini et al.1 show paired photographs from the unaccommodated and accommodated states of the same eye. The authors show (Figure 1) that both the corneal surface and the anterior surface of the

J CATARACT REFRACT SURG - VOL 32, JANUARY 2006

LETTERS

Unaccommodated

Accommodated Anterior and Posterior Corneal Surfaces are Steeper

Anterior Surface of the IOL is Steeper

Anterior and Posterior Corneal Surfaces are Flatter

Anterior Surface of the IOL is Flatter

Figure 1. The anterior and posterior corneal surfaces and the anterior surface of the Crystalens AT-45-IOL are steeper in the UBM image of the unaccommodated eye. Images are from Figure 1 of Marchini et al.1

Crystalens AT-45 IOL are flatter in the accommodated condition than in the unaccommodated condition. Comparison with Figure 2 shows a difference in corneal thickness between the 2 images. This requires some discussion. Corneal curvature and thickness changes are not associated with the accommodative process.2–4 Moreover, the direction of change in curvature in the corneal and lenticular surfaces is opposite that expected to increase refractive power. Because the eye converges and excyclotorts during accommodation,4,5 the position of the eye relative to the ultrasound biomicroscopy (UBM) may change during accommodation. It is likely that this extraocular movement of the eye relative to the UBM imaging device is responsible for the observed curvature and thickness changes. This movement is not random and cannot, therefore, be simply

Unaccommodated

removed by averaging. It is a systematic error, introducing confounding bias. Another example, highlighting the problem of extraocular movement, is associated with the change in the scleral–ciliary process angle during accommodation (Figure 2). Comparison of the unaccommodated and accommodated images shows a difference in corneal thickness. Because corneal thickness does not change during accommodation,2–4 this difference in corneal thickness reflects eye rotation between UBM images. The 3-pixel difference in corneal thickness noted between these images represents an eye rotation of 25.84 degrees, cos
1 27/30. Eye rotation will significantly affect the measurement of the scleral–ciliary process angle. This can be demonstrated readily using the geometric relationship below.

Accommodated Surface is not horizontal 27pixels 30pixels

Figure 2. The thickness of the corneal images of the unaccommodated and accommodated eyes appears different. The surface of the examined region of the accommodated eye was not perpendicular to the probe. Images are from Figure 2 of Marchini et al.1

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LETTERS

The projected angle of an angle that has been rotated around 1 of its sides is equal to

Projected angle Z tan
1 ½ tanðoriginal angleÞ  cosðangle of rotationÞ

ð1Þ

Substituting the calculated eye rotation of 25.84 degrees for the angle of rotation and the authors’ measurement of the unaccommodated scleral–ciliary process angle of 39.95 degrees for the original angle in equation 1, the projected scleral–ciliary process angle is 37.01 degrees as a result of this example of eye rotation. Therefore, eye rotation can account for all the measured change in scleral–ciliary process angle associated with accommodation. The incorporation of positional controls in the UBM images are required to differentiate between movement artifact and a physiological process.3 Without the inclusion of nonchanging references in the UBM images, the etiology of the authors’ observations cannot be determined. RONALD A. SCHACHAR, MD, PHD Dallas, Texas, USA FARHAD KAMANGAR, PHD Arlington, Texas, USA

the model.2–5 These advantages have led to the widespread use of Rasch analysis in ophthalmology.2–5 Indeed, in a recent review of the psychometric properties of existing vision-related qualityof-life questionnaires, the use of Rasch analysis was 1 of the criteria proposed to identify questionnaire quality.5 Velozo et al.2 found that there were not enough ‘‘difficult’’ items in the VF-14, and there were gaps in the scale, which suggested additional items were required. They added 10 questions and included 2 in a final VF-10. Mallinson et al.3 used Rasch analysis to determine whether shortening the VF-14 resulted in a loss of measurement precision. They found that items could be removed without losing precision as long as the ‘‘easier’’ tasks were removed. If the ‘‘harder’’ tasks were removed, an unacceptable loss of measurement precision occurred. They also found redundancy within the VF-14; for example ‘‘reading small print’’ or ‘‘reading the newspaper’’ was predictable based on the response to the other question so it was beneficial to remove 1 of these items. Notably, both remained in Pager’s VF-7. Pager justified the 7-item scale by its correlation with the 14-item scale. However, a high correlation does not imply interchangeability; a Bland-Altman limits of agreement analysis would be required to demonstrate interchangeability.6 In addition, few of the psychometric properties suggested by de Boer et al.5 to evaluate vision-related questionnaires were included in Pager’s report. Clinicians and scientists looking to use a shortened version of the VF-14 would be well advised to consider the work of Velozo et al. and Mallinson et al. and use a Rasch analyzed shortened version of the VF-14.

REFERENCES 1. Marchini G, Pedrotti E, Sartori P, Tosi R. Ultrasound biomicroscopic changes during accommodation in eyes with accommodating intraocular lenses; pilot study and hypothesis for the mechanism of accommodation. J Cataract Refract Surg 2004; 30:2476–2482 2. Schachar RA. Effect of accommodation on the cornea [letter]. J Cataract Refract Surg 2004; 30:531–533 3. Schachar RA, Kamangar F. Computer image analysis of ultrasound biomicroscopy of primate accommodation. Eye, (in press) 4. Buehren T, Collins MJ, Loughridge J, et al. Corneal topography and accommodation. Cornea 2003; 22:311–316 5. Steffen H, Walker MF, Zee DS. Rotation of Listing’s plane with convergence: independence from eye position. Invest Ophthalmol Vis Sci 2000; 41:715–721

Shortening the VF-14 visual disability questionnaire Pager 1 should be commended for his article on assessing visual satisfaction and function after cataract surgery because it tackles some important and difficult issues. However, the work on shortening the VF-14 deserves further comment. Pager claims ‘‘only 1 proposal to shorten the VF-14 has been advocated,’’ but 3 other reports introducing shortened versions of the VF-14 have been published including a 10-item version by Velozo et al.2 and five 7-item versions by Mallinson et al.3 that were evaluated with Rasch analysis. Rasch analysis examines the pattern of questionnaire responses using an iterative probabilistic model to determine the calibration of person ability and question (and response scale) difficulty along the same linear scale. This provides truly linear measurement and a powerful insight into the questionnaire’s internal consistency by reporting questions fit to

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KONRAD PESUDOVS, PHD Bedford Park, South Australia, Australia DAVID B. ELLIOTT, PHD West Yorkshire, United Kingdom REFERENCES 1. Pager CK. Assessment of visual satisfaction and function after cataract surgery. J Cataract Refract Surg 2004; 30:2510–2516 2. Velozo CA, Lai JS, Mallinson T, Hauselman E. Maintaining instrument quality while reducing items: application of Rasch analysis to a selfreport of visual function. J Outcome Meas 2000–2001; 4:667–680 3. Mallinson T, Stelmack J, Velozo C. A comparison of the separation ratio and coefficient a in the creation of minimum item sets. Med Care 2004; 42 (suppl 1):I17–I24 4. Pesudovs K, Garamendi E, Elliott DB. The Quality of Life Impact of Refractive Correction (QIRC) questionnaire: development and validation. Optom Vis Sci 2004; 81:769–777 5. de Boer MR, Moll AC, de Vet HCW, et al. Psychometric properties of vision-related quality of life questionnaires: a systematic review. Ophthalmic Physiol Opt 2004; 24:257–273 6. Bland JM, Altman DG. Measuring agreement in method comparison studies. Stat Methods Med Res 1999; 8:135–160

Amaurosis and anesthesia technique We read with interest the discussion by Greenbaum and Yoshimoto 1 on amaurosis after various forms of local ophthalmic anesthesia. Both authors discussed the effect of different types of local anesthesia on patients’ visual acuity. However, because the stated aim of the mode of anesthesia was to avoid perception of surgical instruments during cataract surgery, we feel that it is equally

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