Letter to the Editor
Chan and Miller (1) recently reported that in eyes with nonglaucomatous optic neuropathies and no light perception, the “peripapillary retinal nerve fiber layer thickness (PRNFLT)” seemed not to diminish below approximately 45 microns in thickness on averaged optical coherence tomography (OCT) measurement.
The authors offered three explanations for this phenomenon. Although the first two explanations (remnant or reactive glial tissue and remnant of non-functioning axons) are certainly possible, it is incorrect to attribute the measurement to an artifact generated by the built-in software of the OCT. This software measures the width of a signal peak above a specific or calculated threshold intensity, often represented by white or red on the pseudo-colored images. This band is generally assumed to be generated by the retinal nerve fiber layer and has a thickness that can be measured by the built-in software as well as third-party imaging software.
In a study addressing the interpretation of retinal OCT images (2), we ablated the surface of cadaveric retinas with an excimer laser, progressively removing retinal layers from the inner retinal surface towards the retinal pigment epithelium. A high-intensity inner band of signal at the surface of the retinal specimen consistently measuring up to 36 microns was observed throughout. This is similar to the PRNFLT measurements reported by Chan and Miller (1) as well as by Sihota et al (3). OCT is an optical imaging system in which signal is generated by back-scattering of the incident light (4). The high-intensity signal we observed from the innermost aspect of the retina was generated by the interface between the immersion fluid (corresponding to vitreous in vivo) and the surface of the retina, independent of the anatomical layer exposed by the excimer laser. This measured up to 36 microns because that was the thickness of the signal peak at the threshold set for “RNFL” measurement in our study (2). Signal from the layers just beneath the surface is likely to be swamped by this peak.
Chan and Miller (1) also found it difficult to explain the difference between PRNFLT measurements of the two eyes in their Case 3. Once again, although the explanations offered are reasonable, it is worth noting that the in vivo axial resolution of the Stratus OCT is, at best, on the order of 15 microns, and that the effects of polarization on the images from each eye are not known (2). Thus, a difference of less than 14 microns between eyes is outside the resolution limits of the instrument. A similar argument applies to the study of Fisher (5) showing a correlation between the reduction of 1 line of low contrast letter acuity per 4 microns of lost PRNFLT, to which the Chan and Miller (1) referred.
These issues of tissue specificity of signal origin and axial resolution apply to all time OCT and spectral domain OCT instruments currently available commercially and will only be addressed by empirical correlations of histology with images using ultra-high resolution OCT machines currently being investigated (6). Until this has been achieved, quantitative assessment of PRNFLT in optic neuropathy is fundamentally flawed.
Devinder S. Chauhan, MBBS MD FRCOphth FRANZCO
Victoria, Australia email@example.com
1. Chan CKM, Miller NR. Peripapillary nerve fibre layer thickness measured with optical coherence tomography in patients with no light perception from long-standing nonglaucomatous optic neuropathies. J Neouro-Ophthalmol
2. Chauhan DS, Marshall J. The interpretation of optical coherence tomography images of the retina. Invest Ophthalmo Vis Scil
3. Sihota R, Sony P, Gupta V, et al. Diagnostic capability of optical coherence tomography in evaluating the degree of glaucomatous retinal nerve fiber layer damage. Invest Ophthalmol Vis Sci
4. Pan Y, Arlt S, Birngruber R, Engelhardt R. Optical coherence tomography in turbid tissue: theoretical analysis and experimental results. Society of Photo-Optical Instrumentation Engineers
5. Fisher JB, Jacobs DA, Markowitz CE, et al. Relation of visual function to retinal nerve fiber layer thickness in multiple sclerosis. Ophthalmology
6. Srinivasan VJ, Wojtkowski M, Witkin AJ, et al. High-definition and 3-dimensional imaging of macular pathologies with high-speed ultrahigh-resolution optical coherence tomography. Ophthalmology