Head-Shaking Nystagmus - A 'Central' Pattern

𝗢𝗿𝗶𝗴𝗶𝗻𝗮𝗹 𝗗𝗲𝘀𝗰𝗿𝗶𝗽𝘁𝗶𝗼𝗻: Evaluating for nystagmus provoked by head-shaking, so-called head-shaking nystagmus (HSN), should be performed in all patients with complaints of dizziness or vertigo, regardless of the chronicity. The maneuver is performed by passively moving the head horizontally (can also be performed vertically) at 2 Hz for about 15 cycles, and can help in differentiation of peripheral versus central etiologies. Peripheral vestibular loss - e.g., in a patient with right sided vestibular neuritis, spontaneous left-beating nystagmus (LBN) is seen acutely, and over time only mild fixation removed LBN will be appreciated. Because of Ewald's second law which states that ampullopetal endolymph flow will cause greater stimulation than ampullofugal flow within the horizontal canals, head-shaking will lead to transient, but clinically meaningful, asymmetric vestibular afferents even after central compensation has occurred. With a subacute or chronic right unilateral vestibulopathy, fixation-removed LBN is seen and will increase following horizontal head-shaking. Central vestibular conditions - e.g., velocity storage mechanisms are normally present to assist in the prolongation of labyrinthine semicircular canal inputs. Since the mechanical properties of the cupula and endolymph within each semicircular canal is such that vestibular afferents would cease firing after just a few seconds of sustained rotational stimuli, a central mechanism must be in place to prolong these signals. It is mainly thought that central patterns of HSN relate to dysfunction of the central velocity storage mechanisms. Lesions causing central HSN usually involve the GABA-ergic inhibitory projections from nodulouvular regions to vestibular nucleus, which can result in the disinhibition of velocity storage. Central patterns include 1) unusually robust HSN elicited by weak head-shaking, 2) nystagmus that reverses direction after head-shaking (e.g., LBN becomes RBN), biphasic HSN, 3) strong HSN in patients without clear unilateral vestibular loss, or 4) perverted HSN (e.g., vertical nystagmus after horizontal head-shaking, usually downbeat and associated with floccular lesions). Shown here is a 70-year-old man with vague episodes of lightheadedness that were not clearly vestibular, who despite having a normal vestibulo-ocular reflex on the left side (i.e., no left sided unilateral vestibular loss), had very strong and persistent right-beating HSN. Although this is considered to be a "central" pattern of HSN, there were no cerebellar, ocular motor or vestibular abnormalities and brain MRI was normal (with particular attention to the nodulus/uvula and vestibular nuclei), so the etiology of his lightheadedness and "central" HSN remained elusive during his initial evaluation. 𝗡𝗲𝘂𝗿𝗼-𝗼𝗽𝗵𝘁𝗵𝗮𝗹𝗺𝗼𝗹𝗼𝗴𝘆 𝗮𝗻𝗱 𝗡𝗲𝘂𝗿𝗼-𝗼𝘁𝗼𝗹𝗼𝗴𝘆 𝗧𝗲𝘅𝘁𝗯𝗼𝗼𝗸 𝗟𝗲𝗴𝗲𝗻𝗱: Evaluating for nystagmus provoked by head-shaking, so-called head-shaking nystagmus (HSN), should be performed in all patients with complaints of dizziness or vertigo, regardless of the chronicity. The maneuver is performed by passively moving the head horizontally (can also be performed vertically) at 2 Hz for about 15 cycles, and can help in differentiation of peripheral versus central etiologies. Peripheral vestibular loss - e.g., in a patient with right sided vestibular neuritis, spontaneous left-beating nystagmus (LBN) is seen acutely, and over time only mild fixation removed LBN will be appreciated. Because of Ewald's second law which states that ampullopetal endolymph flow will cause greater stimulation than ampullofugal flow within the horizontal canals, head-shaking will lead to transient, but clinically meaningful, asymmetric vestibular afferents even after central compensation has occurred. With a subacute or chronic right unilateral vestibulopathy, fixation-removed LBN is seen and will increase following horizontal head-shaking. A velocity storage mechanism exists to assist in the prolongation of labyrinthine semicircular canal inputs, beyond the mechanical constraints of the labyrinthine structures. Without such a mechanism, vestibular afferents would cease firing after just a few seconds of sustained rotational stimuli given the mechanical properties of the cupula and endolymph. Therefore, a central mechanism must be in place to prolong these signals, especially during sustained rotations. It is mainly thought that central patterns of HSN relate to dysfunction of the central velocity storage mechanisms. Lesions causing central HSN usually involve the GABAergic inhibitory projections from nodulouvular regions to vestibular nucleus, which can result in the disinhibition of velocity storage. Central patterns include 1) unusually robust HSN elicited by weak head-shaking, 2) strongly biphasic HSN, 3) nystagmus that reverses direction after headshaking (e.g., LBN becomes RBN), 4) strong HSN in patients without clear unilateral vestibular loss (this patient), or 5) cross-coupled HSN (e.g., vertical nystagmus after horizontal head-shaking, usually downbeat and associated with vestibulocerebellar lesions). (Video and legend created with the assistance of Dr. Kemar Green) https://collections.lib.utah.edu/details?id=1295175. (Video and legend created with the assistance of Dr. Kemar Green)