Careful cone counting critical for clinical care

Borrowing from modern astronomical technologies, adaptive optics (AO) offers much promise for improved ophthalmic, and particularly retinal, imaging. Its cellular-level resolution has already provided new information about the in vivo distribution of cones, retinal pigment epithelial (RPE) cells and even rod photoreceptors in health and disease, and promises new understanding of retinal capillary function and neurovascular coupling. At the heart of AO is the recently developed ability to quantify the subtle optical aberrations of the living human eye, and in real time correct these by reflecting light from precisely deformable mirrors. In the research setting, AO instruments have been available for more than a decade. Among current research instruments, there is a diversity of illumination and imaging approaches: flood (based on the optics of a standard fundus camera), confocal scanning devices (AO scanning light ophthalmoscopes) and AO-optical coherence tomography (OCT). All of these have been designed to use single or multiple wavelengths, and have various strengths and weaknesses in imaging the three-dimensional structure of the retina. Although incremental steps are being taken, commercial AO instruments have yet to become mainstream due to inherent complexities in controlling optical alignment, calibrating control loops and analysing the huge data structures delivered. The RTX-1 is the first AO device to be marketed to an audience outside of research laboratories. It is a flood-illuminated, single-wavelength device that is capable, under optimal conditions, of capturing diffraction-limited images of the smallest foveal cones. Using this device and its in-built image analysis software, Bidaut-Garnier et al. (in this issue of CEO) provide an initial characterization, in normal patients, of the variability in parafoveal cone density measurement. This exercise is an important step towards mainstream utilization of such instruments: as this report shows, obtaining high-resolution images is only the beginning. The reliability of seemingly simple analyses, such as counting cones and determining cellular densities, is influenced by inherent variations in image quality (determined by patient factors such as tear film stability, fixation and optical alignment), but also by the choices made by operators at both image acquisition and analysis stages. Users of new technology need assurance that measurements are accurate and representative of a patient’s current state of health or pathology, that results are meaningful from one day to the next, and that findings can be replicated not only by the same instrument, but also when analysing and comparing data from one imaging centre to the next. This issue’s report adds to such understanding about the confidence that one might have in any single measure of cone density, and reveals that there is more work to be done. As the presented data illustrate, an important limitation of the RTX-1 instrument is that even for normal eyes, it is not routinely capable of imaging the very central patch of foveal cones. This is demonstrated by Figs. 1c and 2 of Ref. [10], which show a decrease in the image analysis software’s report of cone density within the central 2.5°, in obvious conflict with histological findings and that of research-grade AO instruments. It would have been preferable for this central area to be removed from the cone density map in acknowledgement of the limitations of the equipment and analysis software. Nevertheless, with this caveat in mind, there is scope for useful analyses of the cone density and associated statistics to be obtained from defined regions beyond this central foveal region. Comparisons with the results of research-grade AO instruments would seem to be the next logical step. Spectral domain (SD)-OCT has become the clinical workhorse for providing high-resolution photoreceptor structural information in health and disease. For commercially available AO systems to provide a