Zeiss 3000 - Page 107

Stratus OCT User Manual PN 2660021134133 A
Analysis Protocols
6-27
regression analysis and reports whether or not any observed RNFL loss over the included
visits is statistically significant with 95% or higher confidence. The analysis can be applied
to one or both eyes and can include up to 8 exams (from 8 visits) for each eye—up to 16
total exams per analysis. If you select two scans taken on the same day, Stratus
automatically selects the best scan from that day (in terms of scan quality factors).
Notes on Using GPA in Clinical Practice
Stratus OCT GPA Advanced Serial Analysis does not evaluate “progression of
glaucoma,” which can only be assessed through evaluating changes in several clinical
factors including optic nerve head appearance and visual fields. GPA only refers to
change in the nerve fiber layer thickness as assessed by statistical analysis. Such change
or progression of RNFL thickness may or may not be clinically relevant.
GPA is not meant to diagnose. Diagnosis is the responsibility of the practitioner, who
should base diagnosis upon many parameters, many or most of which are not assessed
by Stratus.
GPA Advanced Serial Analysis provides a statistical analysis (linear regression) of RNFL
thickness average versus age. Linear regression fits test data to a linear model,
assuming that the measurements are independent, normally distributed, and that
variability does not depend on the size of the measurement. Rates of change estimated
by linear progression may provide useful information, even in cases where change has
not been completely linear. Projections based upon linear regression analysis should be
used to alert the practitioner to what
might happen in the future assuming linearity, but
not to predict future change. In any case, linear regression is a statistical analysis, and
cannot replace clinical evaluation of the patient’s status and progress.
RNFL thickness is expected to decrease slowly as a function of normal aging. Literature
1
shows an age-related loss as low as 0 μm per year to as high as 0.31 μm per year. All of
these results are based on cross-sectional data. An individual patient’s normal aging
rate may vary.
Because the exact rate of change for any individual is unknown, Stratus reports
statistical significance if the 95% confidence limits on the slope exclude zero, rather
than determining if they exclude normal age-related loss. If Stratus reports that a rate of
change is statistically significant, but the 95% confidence limits include a rate
consistent with normal aging, the observed change may be due to normal aging.
1. Ramakrishnan R, Mittal S, Sonal A, et al. Retinal nerve fibre layer thickness measurements in normal Indian
population by optical coherence tomography. Indian J Ophthalmol. 2006;54:11–15.
Sony P, Sihota R, Tewari Hem K, et al. Quantification of the retinal nerve fibre layer thickness in normal Indian
Eyes with optical coherence tomography. Indian J Ophthalmol. 2004;52:303–309.
Hougaard JL, Ostenfeld C, Heijl A, et al. Modeling the normal retinal nerve fiber layer thickness as measured
by Stratus optical coherence tomography. Graefes Arch Clin Exp Ophthalmol. 2006.
Budenz DL, Anderson DR, Varma R, et al. Determinants of normal retinal nerve fiber layer thickness by Stratus
OCT. Ophthalmology. 2007;114:1046–1052.
Parikh RS, Parikh SR, Sekhar GC, et al. Normal age-related decay of retinal nerve fiber layer thickness. Ophthal-
mology. 2007;114:921–926.
Ronald S. Harwerth Age-Related Losses of Retinal Ganglion Cells and Axons. Investigative Ophthalmology &
Visual Science, October 2008, Vol. 49, No. 10.