Does reduction in retinal nerve fiber layer (RNFL) birefringence precede change in RNFL thickness in glaucoma?

Abstract

 

Purpose

 

Previous experimental studies suggest that loss of RNFL birefringence measured with scanning laser polarimetry (SLP) is evident before reduction in RNFL thickness measured with optical coherence tomography (OCT) after optic nerve injury. However, clinical data corroborating this observation is still lacking. In this prospective study, we compared the performance of SLP and OCT RNFL map analyses in detecting RNFL progression in glaucoma patients.

 

Methods

 

248 eyes of 151 glaucoma patients were followed at 4-month intervals for RNFL imaging with SLP (GDx ECC) and OCT (Cirrus HD-OCT) for ≥36 months. Only images meeting the quality criteria (signal strength ≥8 for OCT; quality score ≥8 for SLP) and collected during the same visits were included for progression analysis. RNFL progression was evaluated by the Guided Progression Analysis (GPA) using serial RNFL birefringence (SLP) and RNFL thickness maps (OCT). Progression was defined when there were more than 5 pixels encoded in red in the RNFL change map (i.e. repeatable reduction in RNFL birefringence / thickness greater than the test-retest variability). The RNFL change maps in the latest follow-up were exported to a computer for measurement of area of change.

 

Results

 

The mean follow-up time was 62.7 months (range: 36.1- 82.8 months). 29 eyes (11.7%) of 27 patients (17.9%) had RNFL thickness progression detected by OCT alone whereas only 8 eyes (3.2%) of 8 patients (5.2%) had RNFL birefringence progression detected by SLP (p<0.001, Fisher exact test). 7eyes of 7patients had RNFL progression detected by both instruments. Notably, all eyes had RNFL thickness progression prior to RNFL birefringence progression. The area of RNFL thickness and RNFL birefringence progression ranged between 0.34 and 3.12 mm², (mean, 1.20±0.67 mm²), and between 1.12 and 4.23 mm², (mean, 2.41±1.32 mm²), respectively. The inferotemporal sector was the most frequent location where progression was detected by both SLP and OCT.

 

Conclusions

 

Both SLP and OCT can detect RNFL progression. In glaucoma, reduction in RNFL thickness detected by OCT occurs before reduction in RNFL birefringence detected by SLP.

 

 

Keywords: 550 imaging/image analysis: clinical