RNFL measurements using laser polarimetry altered by refractive overcorrection

Retinal nerve fiber layer measurements made using scanning laser polarimetry can be significantly altered by excessive refractive correction, according to a study by researchers in Sweden.

"A perpendicular incident laser beam in the RNFL is a prerequisite of a correct measurement of the thickness of this layer," the authors said. "An error in refractive correction could cause an obliqueness of the laser beam and consequently a deviation of the RNFL thickness reading."

Enping Chen, MD, PhD, of St. Erik's Eye Hospital, Stockholm, and colleagues used the GDx (Carl Zeiss Meditec) scanning laser polarimeter with variable corneal compensation (VCC) to measure RNFL thickness in 15 glaucoma patients and 17 healthy controls. Measurements were first made with appropriate correction of patients' refractive errors. Investigators then made four additional measurements with refractions overcorrected by ±1 D and ±2 D, according to the study.

Researchers found that the excessive refractive corrections caused significant deviations in RNFL thickness measurements and nerve fiber index readings in both groups, the study authors said.

"A careful correction of refractive error is important for glaucoma diagnosis and follow-up with GDx VCC," they said.

The study is published in the March issue of the Journal of Glaucoma.