Fixation Instability: A New Measure of Neurologic Dysfunction in Multiple Sclerosis

Multiple sclerosis (MS) can have myriad effects on the afferent and efferent visual systems. Research has demonstrated that even MS patients who have never experienced an acute visual event (optic neuritis, internuclear ophthalmoplegia) are likely to have loss of retinal nerve fiber layer (RNFL) and/or macular ganglion cell–inner plexiform layer (GCIPL) on optical coherence tomography (OCT).¹ It has been suggested that structural OCT measurements might be used to assess MS disease progression and to measure therapeutic response and failure²; however, acute MS exacerbations, such as optic neuritis, will induce OCT changes, such as RNFL edema, that could mask true disease progression and loss of axonal integrity. GCIPL measurements may be less affected by such acute disease-related changes, but other technical challenges, such as segmentation errors and variability, can make subtle changes difficult to discern. Other authors have suggested measuring the velocity of adducting saccades over time to assess MS status, but no commercial system exists by which to obtain such data routinely.³ In this issue of Investigative Ophthalmology and Visual Science, Mallery and colleagues⁴ analyze data gathered by the scanning laser ophthalmoscope (SLO)-based eye tracking system that is an integral part of the Spectralis (Heidelberg Engineering, Heidelberg, Germany) spectral-domain OCT platform and compare eye movements during OCT among healthy controls, MS patients without prior optic neuritis, and those with an optic neuritis history. They report that fixation instability is greater in all MS patients, and that it is highest in those with prior optic neuritis. Interestingly, fixation instability had either weak or no correlation with GCIPL or low-contrast visual acuity findings, which suggests it may assess more central or global neurologic dysfunction. A high degree of reproducibility was observed for individual subjects, suggesting longitudinal data could be collected to assess change over time. Commercial software does not yet exist to analyze the SLO tracking data, but the work presented here suggests that developing this capability may add important information to the structural OCT data now collected almost routinely in MS patients in the course of their clinical care.