Clinical Study
Eye (2007) 21, 333–340. doi:10.1038/sj.eye.6702187; published online 9 December 2005
Accuracy of GDx variable corneal compensation polarization measurements in normal human eyes: effect of accommodation, cycloplegia, focus, pupil size, and eye selection on reproducibility
The authors have no proprietary interest in any aspect of the study. This work was presented, in part, as a poster at the American Academy of Ophthalmology annual meeting in New Orleans, LA, 2004
1Florida Ophthalmic Institute, Gainesville, FL, USA
Correspondence: NS Levy, The Florida Ophthalmic Institute, 7106 N. W. 11th Place, Gainesville, FL 32605, USA. Tel: +1 352 331 2020; Fax: +1 352 331 2019; E-mail: afn22025@afn.org
Received 20 May 2005; Revised 29 September 2005; Accepted 8 October 2005; Published online 9 December 2005.
Abstract
Purpose
To evaluate the reproducibility of variable corneal compensation (VCC) and the effect of accommodation, cycloplegia, eye selection, focus, and pupil size on this measurement of polarization.
Methods
Using a GDx scanning laser polarimeter, multiple measurements of the VCC were obtained from each eye of 33 healthy, young adults under differing conditions. Pupil size and refraction were independently measured with a pupillometer and an autorefractometer. The effects of eye, instrument focus, pupil diameter, cycloplegia, and accommodation were statistically assessed.
Results
The reproducibility of a single retardation measurement and its axis, as determined by the standard deviations (SD) of repeated measurements, is 
1.58 nm and 2.10°, respectively. There is a difference in retardation between right and left eyes, of 5.269 nm, P=0.002. Increasing pupil size increases retardation. Cycloplegia or defocusing decreases retardation, and pharmacologically induced accommodation has no effect on retardation.
Conclusions
The retardation and its axis are highly reproducible measurements when the pupil is of physiologic size and the GDx is properly focused. There is a consistent difference in VCC retardation between the paired right and left eyes. This difference may reflect equipment-induced measurement artefact and/or an anatomic asymmetry between the paired eyes of the subjects studied. Clinicians should be cautious when comparing interocular VCC measurements between paired right and left eyes and using data pooled from both eyes for age-adjusted, normalized standards.
Keywords:
scanning laser polarimetry, pupil size, focus, accommodation, cycloplegia, GDx VCC
