Retinal Nerve Fiber Layer Assessment

Abstract

Retinal nerve fiber layer is the innermost layer of the retina, it is formed of the axons of ganglion cells. Attrition of nerve fibers in the anterior visual pathways is reflected directly in the nerve fiber layer of the retina. These retrograde degenerative changes are subtle but they can be seen with an ophthalmoscope and can be photographed with a fundus camera. In routine clinical practice identifying pathologic alterations in the RNFL ophthalmoscopically necessitates optimal viewing conditions, familiarity with the appearance of the normal RNFL, and a high index of suspicion. So, the importance of the new objective quantitative technologies for RNFL assessment which are the optical coherence tomography, the confocal scanning laser ophthalmoscopy, the scanning laser polarimetry and the retinal thickness analyzer had been proved. Retinal nerve fiber layer (RNFL) can be evaluated both structurally and functionally. This is helpful for early diagnosis and following up of the optic nerve diseases . For structural evaluation, RNFL can be evaluated by clinical examination of the optic disc, stereoscopic optic nerve photography, red-free monochromatic RNFL photograph as well as computer-based devices such as: optical coherence tomography, Confocal scanning laser ophthalmoscopy, and scanning laser polarimetry, that provide quantitative assessments of structural damage. For functional evaluation, RNFL can be evaluated by perimetry as well as electrophysiology; there are now many different methods for performing visual field testing including standard automated perimetry (SAP) as well as selective techniques including short wavelength automated perimetry (SWAP), frequency doubling technology (FDT) perimetry, high pass resolution perimetry (HRP) and motion perimetry. Glaucoma is a multi-factorial optic neuropathy characterized by Irreversible progressive loss of the retinal ganglion cells (RGCs) and thinning of the retinal nerve fiber layer (RNFL), leading to visual field loss and eventually, total loss of vision. 40% to 50% of the RNFL could be lost before visual field defects are detected by conventional perimetry. Thus, RNFL assessment had emerged as an important parameter for pre-perimetric diagnosis of glaucoma.