ROTATING SCHEIMPFLUG TOPOGRAPHIC PARAMETERS IMPORTANT IN DISTINGUISHING NORMAL FROM KERATOCONIC CORNEAL MORPHOLOGICAL FEATURES

Clayton Falknor, MD, Orkun Muftuoglu, MD, Steven Verity, MD, James P. McCulley, MD

Some of the authors have received consultant reimbursement from Alcon Labs, Inc.

None of the authors have financial interest in the subject matter of this poster.

Keratoconus

Essential to identify keratoconus prior to keratorefractive surgery

Keratoconus is characterized by: Non-inflammatory, progressive corneal disease Stromal thinning and anterior bulging of cornea Irregular astigmatism and myopia Potentially severe corneal scarring

Keratoconus is identified by: Fleisher ring (corneal iron line at base of cone) Vogt striae (stromal stress lines within cone) Scissoring of retinoscopic reflex Apical scarring and/or subepithelial fibrosis Central or paracentral steepening on topography

Subclinical (forme fruste) keratoconus is difficult to identify

Identification of keratoconus Traditional method to identify subclinical

keratoconus is corneal topography Placido disk-based Measures slopes of anterior corneal surface only Axial curvature method subject to misalignment of

corneal apex and corneal sighting point May lead to misleading maps, eg normal eyes may show

asymmetric bow-tie or inferior steepening Contribution of posterior corneal surface important

Orbscan (Bausch and Lomb, Salt Lake City, Utah, USA) Slit-scanning beam combined with Placido ring technology Posterior surface recreated with triangulation algorithms Not all images include central cornea

Pentacam (Oculus Optikgeraete GmbH, Germany)

Pentacam Comprehensive Eye Scanner Rotating Scheimpflug camera Monochromatic slit light source rotates with

camera 25-50 slit images per acquisition Eye movement monitoring by 2nd camera

Less than 0.6mm decentration Rotates 180º in 2 seconds All images include central cornea Corneal elevation data independent of visual axis

and corneal apexhttp://www.oculus.de/chi/downloads/dyn/sonstige/sonstige/pentacam_aao_2006.pdf

Purpose

Evaluate parameters obtained by Pentacam important in distinguishing keratoconus from normal

Pentacam parameters to detect keratoconus

Patients

Keratoconus (Diagnosed clinically with topography support) 108 eyes of 54 patients (34 men, 20 women) Inclusion: distorted keratometry mires, abnormal retinoscopic

reflex, Vogt’s striae, Fleischer’s ring, corneal scarring, available topography maps

Exclusion: prior corneal surgery, extensive corneal scarring Controls (normals presenting for keratorefractive surgery)

72 eyes of 36 patients (16 men, 20 women) Inclusion: underwent pre-operative screening for

keratorefractive Sx, normal corneal exam, available topography maps

Exclusion: prior ocular surgery or trauma, suspicion for keratoconus or pellucid marginal degeneration by topography

Age-matched Keratoconus group 36.4 ± 11 Control group 43 ± 14

Posterior corneal elevation

Mean posterior elevation Keratoconus 98.7 ± 46.3

µm Control 11.8 ± 6.12 µm Difference significant

(p<0.001)

With Orbscan IIz, posterior elevation optimal cutoff

point to discriminate keratoconus and

keratoconus suspect versus normal corneas was 40 µm

(Rao et al & Fam et al)

Pentacam keratoconus parameters

Corneal variance indices

Summary of results

Mean posterior corneal elevation significantly higher in keratoconus compared to controls For cut-off of 35 µm, sensitivity 93% & specificity 95%,

comparable to Orbscan Progression index minimum, average, and maximum

all significantly different in keratoconus vs. controls Other significant parameters:

All corneal variance parameters (all based on anterior surface) Most sensitive: ISV, Abr, IVA, KI, IHD Most specific: CKI, KI, IVA

Pachymetry at pupil center and thinnest, flat and steep keratometry, AC depth, corneal volume of central 7mm diameter

Not significantly different: Keratometry axis, AC volume, AC angle

Zernike analysis Both anterior and posterior elevation data decomposed into Zernike higher-order aberration

polynomials Real differences between keratoconus and controls within the third through sixth orders

Trefoil, coma, fourth-order astigmatism, spherical aberration all differ both anteriorly and posteriorly

For both anterior and posterior surfaces, vertical coma most important HOA

Conclusions

Pentacam is useful for identifying keratoconus

Both anterior and posterior corneal surface parameters are important

Advantages over other instruments Measures central corneal zone Elevation data independent of reference

axis Zernike analysis of anterior and posterior

corneal HOA