Purpose:
Blood vessels (BVs) contribute to the shape and thickness of the retinal nerve fiber layer (RNFL) profile measured with time domain optical coherence tomography (tdOCT). [1,2] The purpose is to obtain a better estimate of this contribution using frequency domain optical coherence tomography (fdOCT) in patients with severe visual field losses.
Methods:
27 eyes of 24 patients (3 ION and 21 glaucoma) and 27 eyes of 15 controls were tested with fdOCT (Disc 3D-Scan, 3D-OCT1000, Topcon). All patients had advanced field losses in one (19 eyes) or both (8 eyes) hemifields; the average total deviation of these hemifields was -22.3 ± 5.2dB as measured by 24-2 Humphrey visual fields. BVs were clearly visible on the fdOCT scan of patients with severe losses. (Figure) RNFL thickness was measured at points where the superior temporal (ST) and inferior temporal (IT) veins (V) and arteries (A) crossed a circle, 3.4mm in diameter, centered on the optic disc. This is the same diameter as the tdOCT analysis circle. 15 patient eyes and 14 control eyes also had tdOCT scans (Fast RNFL Scan, OCT3, Zeiss Meditech).
Results:
For controls, the average fdOCT RNFL thickness at the location of the BVs was 144.7 (STA), 131.2 (STV), 153.9 (ITA), and 131.1µm (ITV). For the severely affected hemifields, the average fdOCT RNFL thickness (essentially BV thickness) was 110.6 (STA), 103.9 (STV), 106.4 (ITA), and 115.3µm (ITV); an overall average of 109.6µm or 78% of control values. At the BV locations, for controls, the overall average RNFL thickness of tdOCT and fdOCT profiles was similar [151.7 (tdOCT) vs 143.0µm (fdOCT)]. However, for the severely affected hemifields, the overall average was significantly smaller for tdOCT [61.2 (tdOCT) vs 107.7µm (fdOCT)].
Conclusions:
BVs make a substantial contribution to the shape and thickness of the OCT RNFL profile. [1,2] This contribution will vary depending upon the algorithm used to identify the RNFL.1. Hood & Kardon, PRER (2007); 2. Hood et al, J. Glau. In press.
Keywords: nerve fiber layer • neuro-ophthalmology: optic nerve • imaging methods (CT, FA, ICG, MRI, OCT, RTA, SLO, ultrasound)