Ocular fixation instabilities in motor neurone disease

Eye movements are classically felt to be spared in motor neurone disease (MND). Although a range of ocular motor disorders have been reported, no consistent pattern has been established. Disturbances of ocular fixation have been noted in MND; however, fixation has not yet been formally examined. With the recent characterization of ocular fixation using saccadic intrusion amplitude and fixation periods, we performed a cross-sectional study to examine for abnormalities of ocular fixation in non-dementing patients with MND. A total of 44 patients and 45 controls were recruited. Fixation was examined using infra-red oculography and all subjects then underwent a neuropsychological evaluation. Saccadic intrusion amplitude was found to be greater in patients compared to controls and in particular, spinal-onset patients. Saccadic intrusion amplitude in patients correlated with neuropsychological measures sensitive to lesions of the frontal lobes. This is the first study to identify abnormalities of fixation in MND and these results indicate that ocular fixation instabilities may be a marker of the sub-clinical frontal lobe dysfunction in MND. A longitudinal study to examine if saccadic intrusion amplitude deteriorates with time would be of interest as this could provide a quantifiable objective marker of disease progression.