Mathematical Description of the Ocular Response Analyzer Applanation Signal in Post-LASIK

Ectasia

Mujtaba A. Qazi, M.D.1,2 David A. Luce, Ph.D.3 Jay S. Pepose, M.D., Ph.D.1,2

1. Washington University School of Medicine 2. Pepose Vision Institute, St. Louis, Missouri 3. Reichert Inc.

Dr. Pepose has received research support Dr. Pepose has received research support from Reichert.from Reichert.

Dr. Luce is an employee with Reichert.Dr. Luce is an employee with Reichert.

Financial DisclosureFinancial Disclosure

Ocular Response AnalyzerOcular Response Analyzer

• Uses air pulse to produce 2 (inward and outward) applanation events:

AIR PULSEAIR PULSE & INFRAREDINFRARED Signals

• IOP-g (Goldmann equivalent)IOP-g (Goldmann equivalent)

• IOP-cc (Cornea Compensation)IOP-cc (Cornea Compensation):: IOP calculated as a function of CH IOP calculated as a function of CH

• Corneal Hysteresis (CH)Corneal Hysteresis (CH):: ability of cornea to dampen or absorb applied force = ability of cornea to dampen or absorb applied force = P1-P2 (mean value in normal population P1-P2 (mean value in normal population ~ 10)~ 10)

• Corneal Resistance Factor (CRF)Corneal Resistance Factor (CRF):: ability of cornea to resist or rebound from ability of cornea to resist or rebound from applied force = P1-0.43xP2 applied force = P1-0.43xP2 (mean value in normal population ~10)(mean value in normal population ~10)

Study DesignStudy Design ORA waveforms captured in series of eyes with clinical ORA waveforms captured in series of eyes with clinical

diagnosis of Post-LASIK Ectasia (n=14)diagnosis of Post-LASIK Ectasia (n=14)

Compared to pachymetry-matched:Compared to pachymetry-matched: Pre-LASIK, Control eyes from pool of preoperative keratorefractive Pre-LASIK, Control eyes from pool of preoperative keratorefractive

surgery patients (n=7)surgery patients (n=7) Post-LASIK, Control eyes (n=17)Post-LASIK, Control eyes (n=17) Keratoconus eyes (n=22) Keratoconus eyes (n=22)

Along with IOP & Biomechanical metrics, Along with IOP & Biomechanical metrics, 38 novel ORA signal parameters evaluated38 novel ORA signal parameters evaluated

ORA signals then entered into 11 x 36 Matrix ORA signals then entered into 11 x 36 Matrix Neural Net Neural Net

ORA Measurements Pachymetry-ORA Measurements Pachymetry-matched KCN, pre- and post-LASIK matched KCN, pre- and post-LASIK Controls, and post-LASIK EctasiaControls, and post-LASIK Ectasia

Pre-LASIKPre-LASIK Post-LASIKPost-LASIK KCNKCN LASIK LASIK EctasiaEctasia

N 7 17 22 14

CCT µm 458.8 ± 15.4 420.8 ± 30.6 434.3 ± 37.4 409.0 ± 58.9

IOP-g mmHg 12.0 ± 3.4 10.0 ± 2.4 9.6 ± 2.7 8.2 ± 1.3

IOP-cc mmHg

14.5 ± 2.7 13.7 ± 2.2 13.8 ± 2.3 13.2 ± 2.2

CH mmHg 8.0 ± 1.1 7.3 ± 0.9 7.6 ± 1.2 7.1 ± 1.5

CRF mmHg 7.0 ± 1.6 5.9 ± 1.0 5.8 ± 1.5 5.4 ± 1.3

CRF:CRF: Pre-LASIK to LASIK Ectasia approaches statistical significance (p=0.06, ANOVA with Bonferroni correction)

Applanation Waveform analyzed using Applanation Waveform analyzed using 38 mathematical metrics, such as:38 mathematical metrics, such as:

Height of Peak 1 & 2Height of Peak 1 & 2

Width of Peak 1 & 2Width of Peak 1 & 2

Ratio of Height : Width of Peak 1 & 2Ratio of Height : Width of Peak 1 & 2

Slope of the rising and descending portions of Peak 1 & 2Slope of the rising and descending portions of Peak 1 & 2

Acceleration of the slopes of Peak 1 & 2Acceleration of the slopes of Peak 1 & 2

Slope of upper half of Peak 1 & 2Slope of upper half of Peak 1 & 2

Ascending segments minus descending segments of rising portion of Ascending segments minus descending segments of rising portion of Peak1 & 2 (“chatter”)Peak1 & 2 (“chatter”)

Ratio of width of upper half : width of lower half of Peak 1 & 2 Ratio of width of upper half : width of lower half of Peak 1 & 2

Pre-LASIK Pre-LASIK ControlControl

Post-Post-LASIK LASIK ControlControl

Post-Post-LASIK LASIK EctasiaEctasia

Neural Net AnalysisNeural Net Analysis

There was a statistically significant difference in the mean Neural Net Scores of these 4 populations (p<0.01)

Neural Net Scores

Unoperated NormalPost-LASIK Normal KCNPost-LASIK Ectasia

Summary:Summary: There is a trend for lower CRF (and CH) values in the Post-

LASIK Ectasia group compared to pachymetry-matched control and Keratoconic eyes.

There is no difference in mean IOPcc for all groups evaluated.

The applanation signals are altered by corneal pathology or surgery. This may be due to a smaller area of applanation or a dynamic misalignment due to non-uniform deformation of the asymmetric corneal surface with air-pulse application.

Novel ORA signal metrics, applied through a Neural Net, may be used to assist in the diagnosis of corneal ectasia.

A larger sample size is needed to fully evaluate the utility of this technique for differentiating between early or forme fruste ectasia and normal eyes and to validate the Neural Net.