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 2 nd 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 apex http://www.oculus.de/chi/downloads/dyn/sonstige/sonstige/pent acam_aao_2006.pdf

Purpose



Evaluate parameters obtained by Pentacam important in distinguishing keratoconus from normal



Pentacam parameters to detect keratoconus

       Pachymetry Progression index of corneal thinning Corneal volume within fixed diameter Simulated keratometry AC volume, depth and angle Posterior elevation over best-fit sphere (option of toric ellipsoid) Zernike HOA of anterior and posterior surfaces  Corneal variance indices        ISV (index of surface variance) IVA (index of vertical asymmetry) IHA (index of height asymmetry) IHD (index of height decentration) Rmin (radius minimum) KI (keratoconus index) CKI (center keratoconus index)

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) Table 1. The sensitivity and specificity of different posterior corneal elevation levels to distinguish between keratoconic eyes and controls. Keratoconus Controls Cut-off point (µm) Sensitivity Specificity 10 15 100 100 0 12.3

20 25 30 35 40 45 100 100 96.4

93.2

89.0

86.2

53.4

66.2

84.5

94.9

98.9.

100

Pentacam keratoconus parameters

Table 2. Comparison of Pentacam parameters between Keratoconic eyes and normal controls Keratoconus Controls P* Mean ± SD Mean ± SD

PROGMIN PROGAVG PROGMAX CVolume7mm (mm 3 ) K1 K2 Kmean Kaxis PACHYPupil (µm) PACHYThinnest (µm) ACVol (mm 3 ) ACDepth (mm) ACAngle (deg) 1.79 ± 2.50

2.75 ± 2.71

3.85 ± 3.62

22.92 ± 1.85

46.68 ± 6.03

50.81 ± 6.45

50.2 ± 6.1

86.93 ± 55.56

481.00 ± 60.64

450.72 ± 70.67

193.8 ± 35.8

3.4 ± 0.4

39.8 ± 6.9

0.71 ± 0.41

0.97 ± 0.48

1.26 ± 0.62

24.23 ± 1.65

42.46 ± 2.36

43.49 ± 2.25

43.5 ± 2.3

79.51 ± 64.13

533.19 ± 44.19

530.18 ± 43.17

183.2 ± 56.1

3.1 ± 0.6

39.8 ± 8.8

<0.001

<0.001

<0.001

<0.001

<0.001

<0.001

<0.001

0.422

<0.001

<0.001

0.081

<0.001

0.886

Corneal variance indices

Table 3. Comparison of corneal variance indices between keratoconic eyes and normal controls Keratoconus Mean ± SD Controls Mean ± SD P*

ISV IVA KI CKI IHA IHD RMIN ABR 94.34 ± 51.90

0.96 ± 0.57

1.23 ± 0.18

1.03 ± 0.11

22.11 ± 19.72

0.09 ± 0.07

6.14 ± 0.85

2.51 ± 1.10

26.41 ± 29.46

0.24 ± 0.39

1.01 ± 0.07

1.00 ± 0.02

5.39 ± 3.90

0.01 ± 0.02

7.46 ± 0.39

1.46 ± 0.57

<0.01

<0.01

<0.01

<0.01

<0.01

<0.01

<0.01

<0.01

Sensitivity

98 95 92 66 46 92 86 100

Specificity

65 86 88 89 78 63 46 0

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