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R. P. Tornow, R. Laemmer, F. K. Horn, F. E. Kruse, C. Y. Mardin; Imaging and Quantifying Retinal Nerve Fiber Loss in Glaucoma Patients Using Routine High Resolution Spectral Domain Optical Coherence Tomography. Invest. Ophthalmol. Vis. Sci. 2008;49(13):1862.
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To evaluate the use of routine spectral domain optical coherence tomography (SOCT) to quantify retinal nerve fiber loss and to compare the results with scanning laser polarimetry (SLP).
40 normal subjects and 20 glaucoma patients with perimetric defects were scanned using a commercially available SOCT (Spectralis HRA+OCT, Heidelberg Engineering). Infrared images and OCT scans (40,000 A-Scans/sec) of the dual laser scanning systems are acquired simultaneously. 20 consecutive circular B-scans (3.4 mm diameter, 768 A-scans) around the optic disc were automatically averaged to reduce speckle noise. An online tracking system compensates for eye movements. To assess even very thin parts of the nerve fiber layer (NFL) the images were analysed manually. In addition, all subjects were examined with SLP (GDxVCC, Carl Zeiss Meditec).
The NFL can be distinguished from the neighbouring layers due to high contast (0.3) in most B-scans. However, some areas have low contrast (< 0.08). The position of the retinal vessels in relation to the NFL (above, within, below) can be clearly determined. This is important for the development of a powerful automatic segmentation algorithm. Compared to SLP, SOCT shows higher NFL thickness. Mean thickness over a complete 360° ring in normals: 87 µm (OCT), 58 µm (GDx). Patients: 61 µm (OCT), 44 µm (GDx). Mean thickness over temporal quadrant (90° segment): normals 72 µm (OCT), 34 µm (GDx). Patients: 45 µm (OCT), 33 µm (GDx). The difference concerning NFL thickness in normals is possibly caused by different calibration of the instruments. Despite different calibration, the ratio of the GDx measure (retardance of NFL) and the OCT measure (thickness of NFL) along the circular scan is a measure of relative retardance of the NFL per unit of thickness. In Glaucoma patients, the retardance per thickness is increased in the temporal quadrant by a factor of 1.5 compared to normal subjects but not in the other quadrants. In the temporal quadrant, the NFL thickness decreases while the retardance remains constant or even increases.
Spectral domain optical coherence tomography is a powerful tool to image and quantify the NFL in normals and glaucoma patients. Due to the high resolution and the large number of A-scans even areas with very thin NFL can be analyzed. The increased relative retardance in Glaucoma patients in the temporal region might be the reason why at this location SLP measurements do not reveal NFL loss. In contrast, SOCT clearly shows NFL thinning corresponding with histological data.
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