Advances in medical technology have led to early clinical trials for vision restoration therapies such as gene therapy,
1,2 stem cell transplantation,
3 and retinal prostheses.
4,5 There is now a real chance that we will soon be able to restore vision to people with profound vision loss.
The likely potential recipients of early vision restoration treatments will have visual function at a level that is very difficult to accurately assess.
6,7 Historically, studies of people with such poor vision have not needed significant rigor around their measurement. However, when implementing interventions that aim to improve visual function, which at first are likely to be small gains, it becomes extremely important to develop strategies for more accurate and reliable measures of residual function. A baseline vision assessment must have the ability to determine the extent of any residual functional vision before claims can be made about improvements post intervention. In addition, these measures must be sensitive enough to detect small levels of improvement after the intervention. Traditional terms of visual acuity such as “hand movements” or “counting fingers” are completely inadequate in this setting.
In subjects with low vision, it is well accepted that subjective Goldmann kinetic perimetry gives the most accurate and repeatable results of residual visual field.
8 Full field ERG (ffERG) gives an objective quantitative measure of retinal function; however, in moderate to advanced RP, the ffERG is usually undetectable, even when subjective islands of vision can be detected on Goldman kinetic perimetry.
9 By using a common signal processing method, discrete Fourier transform (DFT) analysis, it is possible to detect some residual signal in the ffERG.
10–12 Discrete Fourier transform analysis is commonly used in mathematics, biology, and engineering fields to analyze signals and decompose those signals into component frequencies for simplified analysis. Whilst both the DFT and its analog equivalent (narrow-band filtering) have been used in patients with retinal degeneration previously,
10,13–18 neither are routinely used to quantify the ffERG responses, particularly the flicker ffERG signals, in a clinical setting. In a review of the DFT technique for visual electrophysiology by Bach and Meigen
11 in 1999, they concluded that this technique can “increase the reliability of physiologic or pathologic interpretations.”
In addition, with the availability of high-resolution spectral domain optical coherence tomography (SD-OCT), it is now possible to identify discrete layers of the retina within the living eye. This enables us to make correlations between detection of residual visual function and residual retinal layers such as the outer nuclear layer (ONL), which contains the nuclei of the photoreceptors.
The aim of this study was to accurately document residual visual function in participants with very low levels of vision, in whom the standard ffERG waveform parameters were undetectable. In this population of patients with an undetectable ffERG, we used Goldman visual field (GVF) and DFT analysis of the flicker ffERG response to assess residual function and then correlated with the structural measurement of the ONL on SD-OCT.