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Total, Corneal, and Internal Ocular Optical Aberrations in Patients with Keratoconus

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Total, Corneal, and Internal Ocular Optical Aberrations in Patients with Keratoconus Total, Corneal, and Internal Ocular Optical Aberrations in Patients With Keratoconus Zuzana Schlegel, MD; Yara Lteif, MD; Harkaran S. Bains; Damien Gatinel, MD, PhD he front corneal surface is a major refracting com- ABSTRACT ponent of the eye and is considerably distorted in 1 PURPOSE: To measure and compare total, corneal, and T patients with keratoconus. Asymmetric corneal internal ocular aberrations using combined wavefront protrusion is the primary cause of irregular astigmatism in analysis and corneal topography in eyes with keratoco- keratoconus.2 This deformity affects both the anterior and nus and eyes with normal corneas. posterior corneal surfaces.3-5 Recent studies have investigated the contribution of the posterior surface to the overall corneal METHODS: This prospective study comprised eyes of optical performance by analyzing data obtained with slit- patients with keratoconus and myopic patients seeking 6 7 refractive surgery. Patients diagnosed with keratoconus scanning or Scheimpfl ug topography. Signifi cantly larger and with a classifi cation of “normal” or “keratoconus” amounts of posterior corneal aberrations and higher compen- on the NIDEK Corneal Navigator corneal disease screen- sation effects were observed in keratoconic eyes compared ing software were selected for inclusion in this study. to normal eyes.6,7 Both the posterior corneal aberrations and The normal group comprised eyes with a “normal” clas- crystalline lens aberrations contribute to the internal aberra- sifi cation with 99% similarity. In the normal group, only one eye per patient was randomly selected based on tion component. a randomization schedule. Corneal, internal, and total Recent studies have found that in pre-presbyopic patients, wavefront measurements were provided by the NIDEK the magnitude of higher order aberrations for the cornea or OPD-Scan II. the lens individually are larger than for the entire eye.8-12 Despite variation in size and shape, the average magnitude RESULTS: One hundred eyes with keratoconus and of aberrations in emmetropic eyes is similar to that found in 155 normal eyes were enrolled in the study. Statisti- 12 cally signifi cant higher corneal and internal higher order eyes with mild to moderate myopia and hyperopia. Artal et 13 aberrations were observed in the eyes with keratoconus al propose a passive, simple geometric model for the ocular (PϽ.05). However, an increase in ocular higher order compensation of aberrations. They suggest that the compo- aberrations proportional to corneal higher order aberra- nents of the eye are similar to an autocompensating design tions was not observed in the keratoconus group. producing similar overall average optical quality for different refractive errors, despite large structural variations. Keratoco- CONCLUSIONS: A compensatory effect of increased anterior corneal aberrations by internal aberrations in nus represents an acquired structural change, and currently, keratoconic eyes was present for some aberrations. The no study has investigated the ocular, anterior corneal, and origin of this compensation and the optical mechanism internal higher order wavefront aberrations in a large number behind it requires further study. [J Refract Surg. 2009;25: of eyes with keratoconus using a combined anterior corneal S951-S957.] doi:10.3928/1081597X-20090915-10 topographer and aberrometer (OPD-Scan II; NIDEK Co Ltd, Gamagori, Japan). From the Department of Ophthalmology, Rothschild Foundation; AP-HP Bichat-Claude Bernard Hospital; and Center for Expertise and Research in Optics for Clinicians, Paris, France (Schlegel, Lteif, Gatinel). Mr Bains is a consultant to NIDEK Co Ltd. The remaining authors have no proprietary interest in the materials presented herein. Presented at the 2009 French Society Meeting; May 7-11, 2009; Paris, France. Correspondence: Damien Gatinel, MD, PhD, Rothschild Foundation, 25 rue Manin, 75019 Paris, France. Tel: 33 1 48 03 64 82; Fax: 33 1 48 03 64 87; E-mail: [email protected] Journal of Refractive Surgery Volume 25 October (Suppl) 2009 Commercially Sponsored Section S951 Ocular Aberration Measurement Using the OPD-Scan II/Schlegel et al fractive surgery,” “hyperopic refractive surgery,” and “un- classifi ed variation” (Fig 1). These diagnostic results are es- timated based on the relation- ship between many corneal indices and cases. For each diagnostic condition, the per- centage of similarity is indi- cated, and the value can vary from 0 to 99%. The result for each topography classifi ca- tion is independent of other categories. Only eyes classifi ed as “normal” or “keratoconus” by the Corneal Navigator were selected for inclusion in this study (see Fig 1). INCLUSION AND EXCLUSION CRITERIA FOR EYES WITH KERATOCONUS For patients with keratoco- nus, bilateral data were used when available; however, in some cases, keratoconus was more advanced in one eye compared to the fellow eye. In- clusion criteria were a positive Figure 1. Screen image of the Corneal Navigator (NIDEK Co Ltd, Gamagori, Japan) software of an eye (Ͼ80%) score for manifest ker- with keratoconus. KC = keratoconus, KSI = keratoconus severity index atoconus similarity using the Corneal Navigator with no pre- PATIENTS AND METHODS vious eye disease, injury, contact lens wear, or surgery. This prospective study included eyes of patients Other inclusion criteria were one or more of the follow- with keratoconus and eyes of myopic patients seeking ing obtained with OPD-Scan II or Orbscan IIz (Bausch refractive surgery at the outpatient clinic of the Depart- & Lomb, Rochester, NY) corneal topography: an area of ment of Ophthalmology at the Rothschild Foundation, central, inferior, or superior marked steepening; topo- Paris, France, between October 2007 and October 2008. graphic asymmetry; oblique cylinder Ͼ1.50 diopters This research adhered to the tenets of the Declaration (D); steep keratometric curvature Ͼ47.00 D; or mini- of Helsinki. Verbal informed consent was obtained mal central corneal thickness Ͻ500 µm. All eyes with from all patients after an explanation of the risks and a positive keratoconus classifi cation with the Corneal benefi ts of the study. Navigator were examined by an ophthalmologist for The Corneal Navigator (NIDEK Co Ltd) software was central thinning of the stroma, with a Fleischer’s ring, used to analyze the data obtained via the OPD-Scan II Vogt’s striae, or both observed on slit-lamp examina- anterior Placido-based topographer. The Corneal Navi- tion, which were considered confi rmatory criteria, not gator uses artifi cial intelligence to train a computer required criteria, for inclusion in this study. Patients neural network to recognize specifi c classifi cations of with corneal scarring, cataract, or other ocular diseases corneal topography. The Corneal Navigator fi rst calcu- and eyes with advanced keratoconus from which reli- lates various indices representing the characteristics of able topography or wavefront measurements could not corneal shape and then classifi es the cornea into one be obtained were excluded from the study. Patients clas- of the following nine types: “normal,” “astigmatism,” sifi ed as “keratoconus suspect” by the Corneal Naviga- “keratoconus suspect,” “keratoconus,” “pellucid mar- tor and those with a forme fruste keratoconus pattern on ginal degeneration,” “post-keratoplasty,” “myopic re- Orbscan IIz topography were excluded. A forme fruste S952 Commercially Sponsored Section journalofrefractivesurgery.com Ocular Aberration Measurement Using the OPD-Scan II/Schlegel et al pattern on the Orbscan IIz included focal or inferior cor- based aberrometry as opposed to position-based aber- neal steepening and/or central keratometry Ͼ47.00 D on rometry. This allows the instrument to have a broader the keratometric map. Orbscan IIz was used as a sec- dynamic range, higher resolution, increased accu- ondary confi rmation of the topography patterns associ- racy, and is particularly adapted to the measurement ated with suspicious or forme fruste topographies. We of highly aberrated eyes compared to position-based routinely use corneal topography measurements from aberrometers such as Hartmann-Shack aberrometers.14-17 two different topographers (OPD-Scan II and Orbscan The corneal topography is measured using Placido- IIz) to confi rm topographic patterns. If both topogra- disk technology and the ocular wavefront is measured phy measurements gave a similar pattern indicative of using the principle of skiascopic phase difference. The forme fruste, the patient was excluded from the study. retina is scanned with an infrared light slit beam and Additionally, the Orbscan IIz provides elevation to- the refl ected light is captured by an array of rotating pography of the posterior corneal surface, which could photodetectors over a 360° area. Measurements are indicate early changes associated with keratoconus. taken within 0.5 seconds for 1440 pupil positions. It Such patients were also excluded. Using this method, is particularly adapted to the measurement of highly we ensured that the corneas were considered normal aberrated eyes partly due to the semi-sequential acqui- based on currently available technology. sition (meridian by meridian), which reduces the risk To be included in the study, measurements be- of error in wavefront reconstruction. The OPD-Scan II fore any surgery using the OPD-Scan and Orbscan IIz has similarities with Placido-disk type keratographers (Bausch & Lomb) had to be acquired successfully. The in that measurement at any pupil position is in a radial
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