Spectralis 7 line raster scan1/21/2024 ![]() In the acute phase of MSON, ONH swelling due to axoplasmic flow stasis in the inflamed optic nerve may be demonstrated on OCT and the elevated pRNFL measurements used to objectively quantify the swelling. The earliest use of OCT in neuro-ophthalmology was in 1999 where Parisi and colleagues reported pRNFL thinning on OCT in MS patients with previous MSON. Neuro-ophthalmic conditions Multiple Sclerosis (MS) and demyelinating optic neuritis (MSON) The importance of distinguishing artefacts from true pathological changes cannot be understated, not only to ensure optimum patient care but also to avoid invasive and costly investigations. Likewise, assessment of retinal thickness should be limited to the area encircling the parafoveal macular rim. The presence of ocular disorders and poor visual acuity may result in fixation errors and induce measurements that appear variable over time. As described earlier, patients should be monitored consistently using similar OCT methodologies. Repeatability of automated segmentation measurements has been demonstrated to vary across different OCT devices and depends on the specific scan protocol and algorithm software updates. The introduction of eye tracking and retest software has improved reproducibility of RNFL measurements. This may not be possible with severe retinal pathology. ![]() ![]() Again, manual correction should be done to improve the accuracy and reproducibility of the measurements. In some cases, particularly in eyes with ocular pathology or in scans of poor image quality, the software can fail. OCT segmentation software algorithm automatically detects the pRNFL and mGCIPL. For correct analysis, accurate placing of the measurement area should be checked and amended manually if necessary. The centre of the fovea on OCT may differ from the retinal locus of fixation and deviations of 60 ± 50 μm between fixation and the centre of the foveal avascular zone can occur. Interpretation of automated OCT measurements need to take into account potential artefacts. Often these normative databases are made up of Caucasian middle-aged subjects, and as such this must be considered when evaluating measurements from a patient not in this demographic, for example a child or a patient of different ethnicity. Secondly, all OCT measurements are compared to a normative database. Foremost, different OCT machines have different measurement protocols and so patients must be reviewed on the same machine using the same scanning protocol for accurate longitudinal comparisons to be made. The strict anatomical structure of the retina and maintenance of retinoscopic organisation with the afferent visual pathway, as described in Table 1, increases the utility of OCT in evaluating central nervous system pathology.įull size image Pitfalls to be aware of in OCT assessmentĪs with any technology, there are limitations, pitfalls and potential errors that must be considered, especially when using derived measurements in clinical decision-making. OCT imaging can capture and quantify axonal loss through measurements of retinal nerve fibre layer (RNFL) thickness, and neuronal damage through measurements of ganglion cell layer (GCL) or combined ganglion cell layer-inner plexiform layer (GCIPL) thickness. Beyond this, afferent visual pathway lesions involving the optic nerve, chiasm or tracts can lead to visible axonal loss caused by direct retrograde axonal degeneration. Lesions involving the pre-laminar area can be assessed with spectral domain (SD-OCT) and enhanced depth imaging OCT (EDI-OCT) of the ONH. OCT allows non-invasive visualisation of the anatomy of the most anterior part of the visual pathway, from retina to lamina cribrosa. Alongside advances in technology, its application in the field of neuro-ophthalmology specifically continues to expand. Optical coherence tomography (OCT), a quick and reproducible imaging technique using low coherence interferometry to produce cross-sectional images of the retina and optic nerve head (ONH), has become one of the most valuable tools employed in the assessment of ophthalmic patients.
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