SLP

SLP (commercially-available as the GDx range) estimates the thickness of the peripapillary RNFL, based on the retardation of polarised light passed through the RNFL. SLP also provides other quantitative information including modulation measurements, ratio measurements and a value giving an indication of the likelihood of glaucoma.

The RNFL, like the cornea and the Henle fiber layer in the macula, is birefringent. This means that a light beam polarized in one direction travels more slowly through it than a beam polarized perpendicular to it. The result is that one beam is retarded relative to the other. The GDx measures this retardation. The amount of retardation is directly proportional to the thickness of the RNFL.

This technology has undergone a number of changes since the first instruments were introduced. The first commercial instrument, manufactured by Laser Diagnostic Technologies (now Carl Zeiss Meditec, Inc.) was the Nerve Fiber Analyzer, Mark I. This had a manually adjustable compensator designed to overcome the effects of corneal birefringence. This was subsequently replaced by a fixed compensator in the Mark II instrument (GDx), which compensated population average corneal birefringence. In 2003, an automated Variable Corneal Compensator instrument (GDx VCC), able to accurately compensate corneal birefringence in all eyes, replaced the fixed compensator instrument.

In 2007, a further software upgrade (Enhanced Corneal Compensation) was introduced. This largely overcomes artifact in some images (atypical scans) in regions of low retardation signal.

Figure: GDx VCC (Carl Zeiss Meditec, Inc.) View large