Overview of the GDx Nerve Fibre Analyser

Melinda Whitton, Gwen Stead, Anna Sclavos, Margaret Doyle, Julia Kelly

Glaucoma is a multifactorial optic neuropathy in which there is a characteristic acquired loss of the optic nerve fibres; this loss of fibres typically eventuates in visual field loss. However, glaucoma is now known to ‘cause the loss of a substantial number of optic nerve fibres and ganglion cells without detectable anomaly in standard Perimetry. Several clinical trials have demonstrated that thinning of the retinal nerve fibre layer (RNFL) precedes visual field defect.
The Nerve Fibre Analyser instrument referred to in this paper is the updated ‘GDx Glaucoma Scanning System.’ The GDx is a scanning laser Polari meter. It is a diagnostic instrument which provides sensitive testing for the early detection of glaucoma. Around the region of the optic nerve head, the RNFL is known to be the most vulnerable to damage in the glaucoma process. Therefore, by analysis of the thickness of the RNFL around this region, and by comparing the thickness with an age/race matched normative data base, glaucomatous damage can be identified.
The GDx gives precise, reproducible, quantitative, objective measurements of the RNFL. It is important to understand that the GDx measures the thickness of the RNFL rather than the surface topography of the retina. The birefringent nerve fibres cause a low power infra-red polarised light directed towards the eye to split into two parallel rays that travel at different velocities. The light undergoes a wavelength shift proportional to the thickness of the RNFL. The retardation between the rays emerging from the RNFL directly correlates to the nerve fibre layer thickness. Since the cornea also has birefringent properties, the GDx incorporates a “corneal compensator” to correct for the effect this may have on the retardation measurements.