Objective: Using an established 15-min ocular-tracking task and structural MRI scanning, we examined the ocular tracking ability in iRBD patients and its anatomical correlates.

Background: Idiopathic rapid eye movement sleep behavior disorder (iRBD) is not only a parasomnia but also a manifestation of the prodromal stages of neurodegenerative synucleinopathies. Patients with neurodegenerative synucleinopathies frequently display eye movement abnormalities. However, whether iRBD patients have eye movement abnormalities remains unknown.

Method: We tested 40 polysomnography-proven iRBD patients and 35 demographically matched healthy controls. We computed 12 oculometric measures evaluating four aspects of ocular-tracking ability (i.e., pursuit initiation, steady-state tracking, direction-tuning, and speed-tuning). A subgroup of the participants (20 iRBD patients and 20 healthy controls) also participated in structural MRI scanning. We performed surface-based morphometry and subcortical volumetric analyses to characterize the gray matter alterations in cortical and subcortical regions.

Results: We found that the ocular-tracking ability in iRBD patients was inferior to that of healthy controls in two aspects: pursuit initiation and steady-state tracking. We also found that cortical thinning in the left Para-Insular area and right visual area V4 was associated with impaired pursuit initiation in iRBD patients. To evaluate the potential predictive value of the ocular-tracking task for phenoconversion to synucleinopathies in iRBD, we had all 75 participants undergo clinical assessments for olfactory function, non-motor symptoms, and autonomic symptoms, all of which had been validated in longitudinal studies as the markers to predict phenoconversion to synucleinopathies in iRBD. We found that the prolonged pursuit latency in iRBD is correlated with olfactory loss, the earliest marker that develops prior to other prodromal markers.

Conclusion: In conclusion, the findings of the present study show that pursuit eye movement abnormalities are already detectable in iRBD patients. The impaired pursuit initiation in iRBD is associated with cortical thinning of brain areas involved in motion processing, eye movement control and attention. The ocular-tracking deficits and the associated gray matter atrophy in iRBD patients may indicate phenoconversion to synucleinopathies.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/impaired-ocular-tracking-in-idiopathic-rem-sleep-behavior-disorder-is-associated-with-gray-matter-alterations/