The thinning of the
retinal nerve fiber layer (NFL) is one of the earliest alterations in glaucoma,
and is a basical diagnostic sign, specially as long as visual field defects are
not yet present. A qualitative analysis can be performed at the slit lamp using
red-free light, but this can acceptably assess only the presence of possible
focal defects. As an alternative, scanning laser ophthalmoscope (SLO) provides
high definition images with better visualization of both diffuse and focal
defects. Quantitative measurements of the peripapillary NFL thickness are
obtained by using more specific instruments. GDx and OCT are the more used in
the clinical practice. GDx measures the phase shift between polarized light
beams reflected from the NFL, while OCT provides a very high resolution
tomography of the retinal layers by means of low coherence interferometry. Both
techniques can detect and quantify diffuse or focal NFL thickness defects,
allowing to monitor morphological changes over time.