(vv)BaranyDBN.mp4(tt)

From: Committee for the International Classification of Vestibular Disorders of the Barany Society.2.1.2.2.1.Downbeat Nystagmus. Retrieved from http://www.jvr-web.org/images/2.1.2.2.1.-Downbeat-nystagmus-A.m4v

(vv)downbeatnys.mp4(tt)

From: Strupp, M. University of Munich. Clinical Examination of the Ocular Motor and Cerebellar Ocular Motor System. Retrieved from: https://www.youtube.com/watch?v=meXAjVoQdCI

Downbeat nystagmus, one of the most common forms of acquired central nystagmus seen clinically, is a jerk nystagmus induced by slow upward drifts of the eyes followed by resetting downward fast phases. It may or may not follow Alexander's law by increasing in downward gaze².

Downbeat nystagmus is usually increased when looking sideways and looking downwards, and is also made worse by convergence. DBN will also increase when lying prone or on the patient's side.  When it is present in downward and lateral gaze but is not visible in central gaze, ophthalmoscopy or high-resolution infrared video will often reveal it to be present in central gaze as well²:
 

Video 3. Downbeat nystagmus. Video shows downbeat nystagmus, which, by definition, should be present in central gaze, in a patient with a Chiari I malformation.
1. The first video segment shows downbeat nystagmus visible in downgaze.
2. The second video segment shows a magnified view of the right eye with a trace amount of downbeat nystagmus seen in right downward gaze (best visualized by looking at tiny scleral blood vessels for vertical motion). The downbeat nystagmus is more obvious in left downward gaze. This nystagmus was not seen in central gaze position (central gaze not shown).
3. The third video segment shows high-resolution infrared video demonstrating downbeat nystagmus in down and lateral gaze. At the end of the video, the downbeat nystagmus is finally also visualized in the central position

(vv)Dbn4.mp4(tt)

From: Rucker JC. Nystagmus and Saccadic Intrusions. Continuum (Minneap Minn). 2019;25(5):1376-1400. doi:10.1212/CON.0000000000000772

If not present in central gaze fixation, even with ophthalmoscopy, DBN may be inducible in the central position after horizontal or vertical headshaking or in a supine or supine head-hanging position. DBN induced after horizontal headshaking is variably called cross-coupled nystagmus or perverted nystagmus and is a sign of central cerebellar dysfunction from a wide range of pathologies². Both downbeat and upbeat nystagmus are poorly suppressed by visual fixation³.

DBN is associated with impaired vertical smooth pursuit eye movements; impaired fixation suppression of the vestibulo-ocular reflex (VOR), gaze-evoked nystagmus, and decreased optokinetic nystagmus (OKN)  in some patients⁴.

DBN  may increase, suppress, or convert to upbeat nystagmus with convergence or with adopting a supine or prone head position (central positional nystagmus)⁵.

Pathogenesis
The purkinje cells of the floccular/parafloccular lobe are predominantly involved in downward smooth pursuit eye movements, and inhibit premotor neurons in the brainstem, including the superior vestibular nucleus. A defect of the flocculus will result in reduced release of gamma-aminobutyric acid (GABA) and thus disinhibition of the vestibular nuclei. The proposed mechanism of action is an increase in the resting activity and excitability of Purkinje cells.

Causes
The causes are degenerative disorders of the cerebellum, cerebellar ischemia, or Arnold-Chiari malformation, and other causes of paramedian lesions of the medulla oblongata.
Causes include: paraneoplastic syndromes (anti-Yo in women (gynaecological malignancies); anti-Hu, anti-GAD, anti-Ma, anti-Ta antibodies.
DBN may also be seen with Wernicke's encephalopathy and as a side effect of drug toxicity (lithium, carbamazepine, amiodarone).
DBN occurs either with structural lesions that are usually bilateral, and is seen due to lesions in the following sites:

1. Disruption of ascending vestibular pathways⁶
2. Interruption of the paramedian commissural fibers of the vestibular nuclei at the floor of the fourth ventricle (between the vestibular nuclei)⁷
3. Lesions of the flocculus, which cause disinhibition of the VOR, with vestibular tone imbalance, in the pitch plane.
4. Lesions of the pontomedullary junction: lesions of the perihypoglossal nuclei and paramedian tract cell groups.

Treatment

1. 3,4-diaminopyridine or 4-Aminopyridine restore the pathological irregular firing of Purkinje cells to synchronized firing, by blocking potassium channels^2,8. Maximum of 30 mg daily.
2. Clonazepam
3. Gabapentin
4. Baclofen
    

(vv)Downbeat.mp4(tt)

From: Wray S. Downbeat nystagmus. Retrieved from  https://collections.lib.utah.edu/details?id=188617&q=downbeat