Downbeat nystagmus: a clinical and pathophysiological review

Abstract

Downbeat nystagmus (DBN) is a neuro-otological finding frequently encountered by clinicians dealing with patients with vertigo. Since DBN is a finding that should be understood because of central vestibular dysfunction, it is necessary to know how to frame it promptly to suggest the correct diagnostic-therapeutic pathway to the patient. As knowledge of its pathophysiology has progressed, the importance of this clinical sign has been increasingly understood. At the same time, clinical diagnostic knowledge has increased, and it has been recognized that this sign may occur sporadically or in association with others within defined clinical syndromes. Thus, in many cases, different therapeutic solutions have become possible. In our work, we have attempted to systematize current knowledge about the origin of this finding, the clinical presentation and current treatment options, to provide an overview that can be used at different levels, from the general practitioner to the specialist neurologist or neurotologist.

Keywords: central neurological disorders; downbeat nystagmus; downbeat nystagmus syndrome; neural integrator; vertical reflex asymmetry.

Figure 1

The upward eye response to downward head rotation is greater than the downward eye response to upward head rotation because the anterior SCC is more aligned with the sagittal plane than the posterior SCC: thus, the ampullofugal, excitatory response of the anterior SCC cupula to downward head rotation is greater than the homologous response of the posterior SCC cupula to upward head rotation. RALP, right anterior, left posterior; LARP, left anterior, right posterior; PSCC, posterior semicircular canal; ASCC, anterior semicircular canal.

Figure 2

Dashed green is the NOD circuit that modulates the A/P asymmetry of the vertical VOR and upward slow phase due to different gravitational vector effects on the macula depending on head position. In the upright position (a), the preponderance of upward eye responses is needed to counteract the effect of gravity, so peripheral asymmetry should not be inhibited. In the supine and prone positions (b), where gravity no longer opposes movement, the natural asymmetry of the VOR is no longer necessary. Therefore, the asymmetry of the vertical VOR should be inhibited. Direct NOD lesion in humans produces positional DBN. ASC, Anterior semicircular canal; PSC, Posterior semicircular canal; U/S, Utricle/Sacculus; ON, Oculomotor nuclei; A/P, Anterior/Posterior; NOD, Nodulus/Uvula; GV, gravitational vector.

Figure 3

The diagram is a schematic representation of the neural areas involved in the control of vertical eye movements and the possible locations of lesions that may result in DBN. In green are the areas directly responsible for generating an upward slow phase: ASC and dorsal Y. In black are the FL/PF inhibitory efferents to the vestibular nuclei that modulate the upward slow phase. In red are the areas destined to generate a downward slow phase: 1,2,3,4 directly. 5 (PMT) indirectly, because by exciting the F/PF, it favors the inhibition of the upward slow phase and thus the generation of a downward slow phase. Dashed green as in Figure 2. ASC, Anterior semicircular canal; PSC, Posterior semicircular canal; U/S, Utricle/Sacculus; A/P, Anterior/Posterior; GV, Gravitational vector; F/FP, Flocculus/Paraflocculus; DY, Dorsal Y cells; PMT, paramedian tract nucleus; ON, Oculomotor nuclei; OMV, Oculomotor vermis; NOD, Nodulus/Uvula; SP, Smooth Pursuit; IV VF, Floor of VI ventricle.