185LETTERS

in a system using a geometrical progression for visualacuity chart letter sizes. Because of this geometricalprogression, mathematical manipulations such as cal-culating visual acuity means and SD values must usea geometrical expression of individual visual acuityvalues to arrive at a meaningful result.3 Using the dec-imal notation for visual acuity (eg, 20/40 0.5 and20/200 0.1) to calculate a mean visual acuity valuewill lead to an overestimation of the mean, and addi-tional manipulations such as the difference between2 means can compound such errors in the estimates.

Recognizing the kind of calculation errors that canresult from using decimal notation alone, this journalhas published details on the use of the logMAR systemand other geometrical methods to properly calculatemean visual acuity values.4 Since January 2000, thelogMAR equivalent values for visual acuity havebeen listed in each issue, in part to encourage readersand authors to use these values in calculating aggre-gate visual acuity scores.5

The utility of using the logMAR system to accountfor the geometrical progression of visual acuity chartlines should not be overlooked. In the Llovet et al.article, it is unclear whether the proper geometricalcalculation method was used to arrive at mean visualacuity values and the key derived outcome measures.It remains perfectly acceptable to collect and reportvisual acuity values using common systems such asdecimal notation or Snellen fractions that are easilyinterpreted by readers. However, during the analyti-cal process itself, any manipulations of visual acuitydata must account for the geometrical scale andthese techniques should be described explicitly in themethods.

Jan S. Peterson, MS, CCRA, RACPotomac, Maryland, USA

REFERENCES1. Llovet F, Galal A, Benitez-del-Castillo J-M, Ortega J, Martin C,

Baviera J. One-year results of excimer laser in situ keratomileusis

for hyperopia. J Cataract Refract Surg 2009; 35:1156 1165

2. Koch DD, Kohnen T, Obstbaum SA, Rosen ES. Format for report-

ing refractive surgical data [editorial]. J Cataract Refract Surg

1998; 24:285 287

3. Holladay JT, Prager TC. Mean visual acuity [letter]. Am J Oph-

thalmol 1991; 111:372 374

4. Holladay JT. Visual acuity measurements [guest editorial]. J Cat-

aract Refract Surg 2004; 30:287 290

5. Peterson JS. Visual acuity equivalents [letter]. J Cataract Refract

Surg 2000; 26:11

REPLY: In our article, the visual acuity data werecollected in logMAR charts and the means ob-tained by the statistics were converted into decimalvalues. In the article, when we referred to Snellen

J CATARACT REFRACT SURG

values, we meant that the data were reported indecimal values; indeed, we should have describedthe conversion process that we used. We agreewith Peterson about the importance of collectingthe data using logMAR charts for both far andnear and think all articles published in the journalshould follow this recommendation. Ahmed Galal,MD, PhD

Additional complications of corneal crosslinkingWe were very interested in the study of the com-

plication rate after corneal collagen crosslinking(CXL) by Koller et al.1 It is very important to warnthe scientific community about the possible compli-cations and failure rate of CXL since it is a relativelynew technique.

We think it is necessary to point out 2 complicationsthat have been reported in the literature but were notincluded in the article. The first is a case of herpetic ker-atitis with iritis after CXL,2 which led us to believe thatCXL can induce herpetic keratitis with iritis even inpatients with no history of herpetic disease. The sec-ond is the induction of diffuse lamellar keratitis afterCXL in a patient with post-laser in situ keratomileusis(LASIK) ectasia,3 which was successfully treated withintensive topical corticosteroids. Early diagnosis andproper treatment of both complications (such as anti-viral treatment in patients with a history of herpeticdisease or intensive topical corticosteroids in patientswith post-LASIK corneal ectasia) are essential fora favorable outcome.

Corneal CXL probably represents the future treat-ment of corneal ectatic disorders as it may minimizethe percentage of patients who need penetrating kera-toplasty. We strongly encourage surgeons to report allpossible complications of this new technique since ap-propriate patient selection and technique performancecan improve success rates.

George D. Kymionis, MD, PhDDimitra M. Portaliou, MD

Ioannis G. Pallikaris, MD, PhDHeraklion, Crete, Greece

REFERENCES1. Koller T, Mrochen M, Seiler T. Complication and failure rates after

corneal crosslinking. J Cataract Refract Surg 2009; 35:1358 1362

2. Kymionis GD, Portaliou DM, Bouzoukis DI, Suh LH, Pallikaris AI,

Markomanolakis M, Yoo SH. Herpetic keratitis with iritis after cor-

neal crosslinking with riboflavin and ultraviolet A for keratoconus.

J Cataract Refract Surg 2007; 33:1982 1984

3. Kymionis GD, Bouzoukis DI, Diakonis VF, Portaliou DM,

Pallikaris AI, Yoo SH. Diffuse lamellar keratitis after corneal

crosslinking in a patient with post-laser in situ keratomileusis cor-

neal ectasia. J Cataract Refract Surg 2007; 33:2135 2137

VOL 36, JANUARY 2009