Preperimetric Glaucoma Assessment With Scanning Laser Polarimetry (GDx VCC) Analysis of Retinal Nerve Fiber Layer by Sectors Begon ˜a Baraibar, BSc, Ana Sa ´nchez-Cano, BSc, Luis E. Pablo, MD, and Francisco M. Honrubia, MD Purpose: To evaluate the capability of the GDx VCC nerve fiber analyzer to detect preperimetric glaucoma across 12 retinal nerve fiber layer (RNFL) peripapillary sectors. Methods: Data were obtained in a cross-sectional, hospital clinic- based study; 699 eyes from 699 glaucoma suspects were enrolled in this protocol. All subjects underwent ophthalmologic examina- tion, static automated perimetry [Humphrey 24-2 Swedish interactive threshold algorithm (SITA) Standard], optic nerve stereoscopic photographs, red-free digital RNFL photographs and GDx VCC examination. Group S included 283 normal eyes and 39 preperimetric glaucoma eyes with RNFL superior or diffuse defects in the fiber layer photographs. Group I included 324 normal subjects and 24 with preperimetric glaucoma eyes with RNFL inferior or diffuse defects in fiber layer photographs. Results: Mean values of the area under the curve (AUC) for receiver operating characteristic analysis for inferior average (Inf Avg), temporal-superior-nasal-inferior temporal average (TSNIT Avg), superior average (Sup Avg), and the nerve fiber indicator were significantly less in the eyes with RNFL defects than the control group compared with the AUC for thickness at hour 12 and at hour 6 calculated from the RNFL sector density. The AUC for receiver operating characteristic analysis of the new parameters improved by 12% with respect to the best GDx VCC standard values. Conclusions: Our results confirm that the 12 sector divisions of the GDx VCC have better diagnostic reliability in preperimetric glaucoma, and are able to improve the discrimination capability between normal and early damaged RNFLs. Key Words: retinal nerve fiber layer, GDx VCC, scanning laser polarimetry, preperimetric glaucoma (J Glaucoma 2007;16:659–664) G laucoma is an optical neuropathy resulting in the loss of ganglion cells and their axons in the retina. 1,2 The functional defect is usually manifested by the loss of the visual field (VF) owing to diffuse or wedge defects of the ganglion cells and retinal nerve fiber layer (RNFL). These defects may be evident at least 6 years before the field alterations. 3–9 Some authors have stated that up to 50% of the nerve fibers can be lost before any functional failure is detected. 5 However, it is not yet proven that structural changes always precede functional damage. This impression could come from the fact that structural assessment techniques are at the moment more accurate that the functional ones. The stage before functional loss detection is known as preperimetric glaucoma. Diverse techniques have been used to distinguish normal eyes from the glaucomatous eyes in preperimetric stages. 4,5 RNFL photography is widely used as a qualitative detection method owing to difficulties in measuring the thickness of the RNFL. 6–9 Both analog and digital photographs are usually done in black and white using a funduscopic camera and a red-free filter. This method has been validated and is widely used for detecting early changes in the RNFL loss. 9 The use of a laser polarimeter with corneal compensation GDx VCC (Laser Diagnostic Technologies Inc, San Diego, CA) is reliable for the measurement of the RNFL peripapillary thickness in daily practice. 10–13 The GDx VCC is a scanning confocal laser that uses polarized light to measure the RNFL thickness. 10,14–17 Owing to the birefringent nature of the ocular structures, there is a variation in the propagation velocity of 1 of the 2 components of light which consequently results in a signal delay that is measured by a detector. This delay is proportional to the RNFL thickness after compensating for the delay produced by the anterior segment of the eye (mainly cornea and lens). 11,12 The GDx VCC has a diode laser with a wavelength of 780 nm, a power of 40 mW (classified as class I 18 ), and a scanning time of 0.8 seconds. The signal delay, as measured by the detector, is proportional to the RNFL thickness using a conversion factor of 0.67 nm/mm. Each image is formed by 256 (horizontal)  128 (vertical) pixels, generating a total of 32,768 pixels. For an emmetropic eye, one pixel is 0.0465mm with a scanning total field dimension of 11.9 mm (horizontal)  5.9 mm (vertical). Copyright r 2007 by Lippincott Williams & Wilkins Received for publication October 9, 2006; accepted April 23, 2007. From the Department of Ophthalmology, Miguel Servet University Hospital, Instituto Aragone´s de Ciencias de la Salud, Zaragoza, Spain. This paper has been financed in part by a RTIC-C (Red tema´tica de Investigacio´n Cooperativa, Instituto de Salud Carlos III) grant (number C03/13 (2003-2006). Reprints: Begon˜a Baraibar, BSc, Department of Ophthalmology, Miguel Servet University Hospital, 50.009 Zaragoza, Spain (e-mail: iodb325@io.cfmac.csic.es). ORIGINAL STUDY J Glaucoma  Volume 16, Number 8, December 2007 659