Abstract
Purpose: :
Preliminary evidence exists that following experimental optic nerve injury, retinal nerve fiber layer (RNFL) birefringence declines prior to RNFL atrophy. The purpose of this study was to examine the hypothesis that RNFL birefringence improves following surgical reduction of IOP.
Methods: :
Glaucoma patients requiring trabeculectomy or drainage implant meeting eligibility criteria were prospectively enrolled. All patients underwent complete examination including intraocular pressure (IOP) and arterial blood pressure (BP) measurements, standard automated perimetry (SAP), optical coherence tomography (Stratus OCT) and scanning laser polarimetry (GDxECC) prior to surgery and at three months post-operatively. Sixty-four corresponding RNFL segments were extracted from the GDxECC and OCT measurement circles and RNFL birefringence values were calculated for each segment (GDxECC retardance divided by OCT-derived RNFL thickness). Values corresponding to 120° superior and inferior quadrants were calculated. Variability estimates were generated for each segment using 4 pre-operative birefringence measurements. A significant birefringence change was defined as 1.96 x the weighed test-retest SD in 4 contiguous segments with change occurring in the same direction.
Results: :
Twenty-nine patients (age 70.7±12.0) were enrolled. Pre-operative IOP (19.0±6.0mmHg) was significantly (p<0.001) reduced compared with post-operative IOP (10.3±3.6mmHg). The average (62.6±15.6) and inferior (62.3±19.2) postoperative OCT-derived RNFL thickness significantly (p=0.003, p= 0.04) increased compared to preoperation (59.3±15.6, 59.2±19.4 respectively), and were associated with an increase in mean ocular perfusion pressure (r=-0.58, p=0.001; r=-0.41, p=0.03 respectively). A significant focal increase in retardance, RNFL thickness and birefringence occurred in 8/29 (28%), 18/29 (62%), and 8/29 (28%) patients, respectively. Focal decrease in retardance, RNFL thickness and birefringence occurred in 3/29 (10%), 3/29 (10%) and 5/29 (17%) patients, respectively.
Conclusions: :
Focal improvement in peripapillary RNFL retardance, thickness and birefringence may occur in some patients following surgical IOP reduction. These data suggest that reversal of axonal microtuble dysfunction may be quantified using this methodology.
Keywords: nerve fiber layer • imaging/image analysis: clinical • imaging methods (CT, FA, ICG, MRI, OCT, RTA, SLO, ultrasound)