Retinal Nerve Fiber Layer Measurements by Scanning Laser Polarimetry With Enhanced Corneal Compensation in Healthy Subjects Harsha L. Rao, MD, DNB,*w Chirravuri R. Venkatesh, B Opt,* Kelli Vidyasagar, B Opt,* Ravi K. Yadav, B Opt,* Uday K. Addepalli, B Opt,* Aarthi Jude, B Opt,* Sirisha Senthil, MS, FRCS,* and Chandra S. Garudadri, MD, FRCS* Purpose: To evaluate the (i) effects of biological (age and axial length) and instrument-related [typical scan score (TSS) and cor- neal birefringence] parameters on the retinal nerve fiber layer (RNFL) measurements and (ii) repeatability of RNFL measure- ments with the enhanced corneal compensation (ECC) protocol of scanning laser polarimetry (SLP) in healthy subjects. Methods: In a cross-sectional study, 140 eyes of 73 healthy subjects underwent RNFL imaging with the ECC protocol of SLP. Linear mixed modeling methods were used to evaluate the effects of age, axial length, TSS, and corneal birefringence on RNFL measure- ments. One randomly selected eye of 48 subjects from the cohort underwent 3 serial scans during the same session to determine the repeatability. Results: Age significantly influenced all RNFL measurements. RNFL measurements decreased by 1 mm for every decade increase in age. TSS affected the overall average RNFL measurement (b =  0.62, P = 0.003), whereas residual anterior segment retard- ance affected the superior quadrant measurement (b = 1.14, P = 0.01). Axial length and corneal birefringence measurements did not influence RNFL measurements. Repeatability, as assessed by the coefficient of variation, ranged between 1.7% for the overall average RNFL measurement and 11.4% for th nerve fiber indicator. Conclusions: Age significantly affected all RNFL measurements with the ECC protocol of SLP, whereas TSS and residual anterior segment retardance affected the overall average and the superior average RNFL measurements, respectively. Axial length and cor- neal birefringence measurements did not influence any RNFL measurements. RNFL measurements had good intrasession repeatability. These results are important while evaluating the change in structural measurements over time in glaucoma patients. Key Words: scanning laser polarimetry, enhanced corneal com- pensation, predictors, repeatability (J Glaucoma 2014;23:589–593) S canning laser polarimetry (SLP) is a noncontact imaging technology used in glaucoma practice to measure the retinal nerve fiber layer (RNFL) birefringence in vivo. It is based on the principle that polarized light passing through the birefringent RNFL undergoes a measurable phase shift, known as retardation, which is linearly related to the RNFL tissue thickness. 1 The cornea also exhibits birefrin- gent properties, which are significantly neutralized with the recent versions of SLP. 2 A more recent confounder detected in the RNFL birefringence measurements with SLP was the presence of atypical birefringence pattern (ABP) images. 3 ABP is characterized by irregular patches of high retarda- tion that do not match the expected retardation based on the RNFL anatomy. Although the exact cause of ABP is not known, it is thought to be predominantly due to a low signal-to-noise ratio. 3 The recently introduced SLP proto- col to overcome the problem of ABP images, called the enhanced corneal compensation (ECC) protocol, optimizes imaging by improving the signal-to-noise ratio. The ECC protocol introduces a predetermined birefringence bias to shift the measurement of the total retardation to a higher value region to remove noise and reduce atypical patterns. The amount of birefringence bias is then removed mathe- matically point by point from the total birefringence pat- tern of the image, to improve the signal and obtain a retardation pattern of the RNFL with the least noise. 4–6 Although there are studies reporting better performance of the ECC protocol compared with the earlier protocol for glaucoma detection [variable corneal compensation (VCC)], 7,8 there are no comprehensive studies on the biological and the instrument-related characteristics that affect the measurement of RNFL birefringence by the ECC protocol of SLP. The purpose of this study was to evaluate the (i) effects of bio- logical (age and axial length) and instrument-related parame- ters [typical scan score (TSS) and corneal birefringence meas- urements] on the RNFL birefringence measurements and (ii) repeatability of RNFL measurements with the ECC protocol of SLP in healthy subjects. METHODS This was an observational, cross-sectional study of 140 eyes of 73 healthy subjects conducted at a tertiary eye care facility in South India between November 2010 and June 2011. Informed consent was obtained from all participants and the Ethics Committee of L V Prasad Eye Institute approved all methodologies. All methods adhered to the tenets of the Declaration of Helsinki for research involving human subjects. Inclusion criteria were age above 18 years, a best corrected visual acuity of 20/30 or better, spherical refrac- tion within ± 5.0 D, and cylinder correction within ± 2.0 D. Eyes with media opacities precluding clinical examination or SLP imaging were excluded. Subjects were Received for publication August 3, 2012; accepted November 19, 2012. From the *VST Glaucoma Center; and wCenter for Clinical Epidemiology and Biostatistics, L V Prasad Eye Institute, Banjara Hills, Hyderabad, Andhra Pradesh, India. Disclosure: The authors declare no conflict of interest. Reprints: Harsha L. Rao, MD, DNB, L V Prasad Eye Institute, Ban- jara Hills, Hyderabad 500034, Andhra Pradesh, India (e-mail: harsha@lvpei.org; harshalaxmanarao@gmail.com). Copyright r 2013 by Lippincott Williams & Wilkins DOI: 10.1097/IJG.0b013e318286ffa5 ORIGINAL STUDY J Glaucoma  Volume 23, Number 9, December 2014 www.glaucomajournal.com | 589