Estimating OCT Structural Measurement Floors to Detect Progression in Advanced Glaucoma
American Journal of Ophthalmology, March 2017
In retinal imaging, the “floor effect” is defined as the point at which no further structural damage can be measured. Bowd et al. sought to identify an anatomic structure that did not thin to its measurement floor in patients with glaucoma. Using spectral-domain optical coherence tomography (SD-OCT), they estimated the floor for several retinal structures and found that ganglion cell–inner plexiform layer thickness (GC-IPLT) remained above the measurement floor after other retinal tissues had thinned to the extent of reaching their floor. They suggested that this makes the GC-IPLT a better candidate than other retinal structures for detecting progression in advanced glaucoma.
The researchers evaluated 86 patients with glaucoma (128 eyes) as part of the prospective longitudinal UC San Diego Diagnostic Innovations in Glaucoma Study. Of these, 41 eyes (27 patients) with moderate or advanced glaucoma underwent SD-OCT repeatedly for 5 weeks to estimate measurement floors and variability of the measurements. The other 87 eyes (59 patients), which had advanced or severe glaucoma, were imaged twice—at baseline and 2 years later—to calculate rate of change in tissue thickness and percentage of the image area that remained above the measurement floor at baseline.
The minimum rim width (MRW), GC-IPLT, and circumpapillary retinal nerve fiber layer thickness (cpRNFLT) were determined. For each of these structures, the authors plotted the distribution of variability of the linear regression slopes obtained from various locations on SD-OCT scans and used the data to define the measurement floor.
Average measurement floors were 105 μm for MRW, 38 μm for GC-IPLT, and 38 μm for cpRNFLT. Mean rates of change were −1.51 μm/year for MRW, −0.21 μm/year for GC-IPLT, and −0.36 μm/year for cpRNFLT (all p < .05). The mean percentage of the image area remaining above the floor at baseline was 19% for MRW, 36% for GC-IPLT, and 14% for cpRNFLT.
The authors concluded that for patients with advanced glaucoma, GC-IPLT may be the most suitable metric for determining structural disease progression on SD-OCT.
The original article can be found here.