RNFL thickness maps help track location, patterns of glaucoma progression
Analysis of retinal nerve fiber layer thickness maps enabled detection and long-term surveillance of the morphology of retinal nerve fiber layer defects associated with glaucoma progression, a study found.
Retinal nerve fiber layer (RNFL) thickness maps generated with spectral-domain optical coherence tomography (SD-OCT) provide a unique opportunity to track glaucoma progression, Christopher Kai-Shun Leung, MD, the corresponding study author, said.
“Longitudinal analysis of the RNFL thickness maps provides an important tool to visualize not only the location and distribution of RNFL defects, but also the patterns of RNFL progression,” Leung said in an email interview. “Most commercially available SD-OCT instruments only provide average and sectoral RNFL thicknesses derived from a circle scan, which is inadequate to detect RNFL defects or progression outside the circle measurement.”
Patients and methods
The prospective, longitudinal study, published in Ophthalmology, included 186 eyes of 103 glaucoma patients.
The Cirrus HD-OCT platform (Carl Zeiss Meditec) was used to image both eyes of each patient. Guided Progression Analysis (Zeiss) was used to analyze RNFL morphology and track RNFL progression based on consecutive thickness maps.
The Humphrey Field Analyzer II-i (Zeiss) was used to perform standard automated white-on-white perimetry. OCT imaging and visual field assessment were performed every 4 months for at least 36 months. Imaging and visual field testing were performed separately and masked from other clinical data.
A total of 2,135 OCT images were taken. The mean interval between OCT examinations was 4.2 months; an average of 11.5 images were analyzed for each eye.
RNFL maps generated with SD-OCT provide RNFL thickness measurements in an area of 6 × 6 mm2 , or 200 × 200 pixels around the optic nerve head, Leung said.
“By aligning serial RNFL thickness maps, it is possible to track pixel-to-pixel changes of the RNFL thickness, thereby increasing the sensitivity to detect RNFL change,” he said.
Results and conclusions
RNFL thickness mapping detected RNFL progression in 28 eyes. Concomitant visual field progression was identified in 13 eyes; eight of these eyes (61.5%) had RNFL progression that occurred before or concurrently with visual field progression.
Forty-two eyes had visual field progression without evidence of progression on RNFL thickness maps.
“RNFL progression was most frequently detected in the inferotemporal sector, at a distance approximately 2 mm from the optic disc center,” Leung said. “Progression may be missed if the RNFL is only examined with a circle scan of a default radius of 1.73 mm.”
The mean interval between the first baseline RNFL thickness deviation map and the first RNFL thickness change map that showed evidence of progression was 27 months.
“The rate of RNFL progression varies with the level of intraocular pressure and the stage of disease,” Leung said. “It is possible to identify RNFL progression within 6 months.”
Further study is needed to determine whether RNFL mapping would improve the agreement between RNFL progression and visual field loss, he said. – by Matt Hasson