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The medial longitudinal fasciculus is a key structure for conjugate horizontal eye movements by relaying signals from the abducens internuclear neurons to the medial rectus subdivision of the contralateral oculomotor nucleus. Thus, lesions involving the medial longitudinal fasciculus give rise to a typical neuro-ophthalmological sign, the internuclear ophthalmoplegia, which is characterized by impaired adduction of the ipsilesional eye and dissociated abducting nystagmus of the contralateral eye during attempted contralesional horizontal gaze. In addition, medial longitudinal fasciculus lesions may produce various other ocular motor abnormalities since the medial longitudinal fasciculus also conveys the signals for the control of the vestibulo-ocular reflex and smooth pursuit. Other possible ocular motor abnormalities include spontaneous vertical-torsional nystagmus, contraversive ocular tilt reaction, and impaired vestibulo-ocular reflex, especially for the contralesional posterior canal. Recognition of the ocular motor findings observed in the medial longitudinal fasciculus syndrome would aid in detection and localization of potentially grave lesions involving the brainstem. [ABSTRACT FROM AUTHOR]

The video head impulse test (vHIT) assesses the vestibulo-ocular reflex (VOR) during a rapid high-velocity low amplitude (10°–20°) head rotation. Patients with peripheral vestibulopathy have a reduced VOR gain with corrective catch-up saccades during the head turn. There are several pitfalls, mainly technical, which may interfere with interpretation of vHIT data. In addition, intrusive eye movement disorders such as spontaneous nystagmus that affect normal eye position and tracking can affect the vHIT results. To date there has been little study of neurological saccadic eye movements that may interfere with the interpretation of vHIT data. Here, in ten patients with a range of central neurological disorders, we describe oculomotor abnormalities on vHIT in the presence of normal range VOR gain values, recorded at a tertiary vestibular neurology service. • Certain apparent catch-up saccades visible on the vHIT data may be due to the gaze-evoked nystagmus, induced by the head turn and transient eccentric positioning of the eyes. • Saccadic oscillations may affect the vHIT by appearing immediately after the head impulse and may be indistinguishable from an overt saccade if seen without clinical correlation. • Our cases highlight that oculomotor abnormalities can interfere with quantitative recording of the head impulse VOR responses and may impact the clinical diagnosis. [ABSTRACT FROM AUTHOR]