Timothy C. Hain, MD
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|Downbeating and rightbeating nystagmus|
Downbeating nystagmus means that the eyes drift upward, and "beat" or jump downward. Examples are shown above.
Explanations that have been offered for DBN include asymmetrical smooth pursuit and abberent tone in the central pathways serving the anterior semicircular canals. Experimentally, ablation of the cerebellar flocculus in monkeys produces downbeating nystagmus. This is probably just one of many mechanisms.
DBN is classically attributed to the central nervous system (i.e. brain). The biggest single cause is the Chiari malformation. See the figure below for an illustration. Numerous large studies have been published, largely concluding that while the Chiari may contribute as many as 1/3 of cases, cerebellar degenerations, demyelinating disease, drug toxicity, neoplasia, and "idiopathic" are other common causes.
In a review of 91 patients by Yee (1989), the most frequent causes were infarction, cerebellar and spinocerebellar degeneration syndromes, MS and developmental anomalies affecting the pons and cerebellum. Toxicity from anticonvulsant drugs probably causes nystagmus in a few patients and the author of this review has seen this more frequently than the other etiologies listed above. Clinical examinations, excluding eye movement recordings, were used to localize lesions. Localizations included the cerebellum in 88% of the patients. However, localizations to structures outside of the cerebellum were made in several patients.
Wagner et al recently reviewed 117 patients with DBN (2008). Again, no cause was found in roughly 1/3 of patients, and of the remaining 62%, most were attributed to cerebellar disease. An association between bilateral vestibulopathy was reported in this series (38%), as well as polyneuropathy. The validity of the association with bilateral vestibulopathy is open to question however, as this association was made through a bedside test (head impulse test).
|Downbeating nystagmus is present primary gaze in the light, increases in the dark, and greatly increases on lateral gaze in patient with a Chiari malformation.|
Downbeating nystagmus is reported to often increase on supine positioning (Leigh and Zee, 1991), but the author has not confirmed this in his own practice. In fact, more often it seems to be increased prone (see movie). This was also the conclusion of Marti et al (2002). DBN that is greater supine might be sometimes attributable to anterior canal BPPV. DBN seems to vary with position without any clear reason -- sometimes increasing on backward, sometimes forward, and sometimes it is simply unaffected.
Cancer: Downbeating nystagmus, especially on lateral gaze, is the cardinal sign of a paraneoplastic cerebellar degeneration syndrome -- dizziness due to a cancer elsewhere in the body. In this case, the nystagmus characteristically increases on lateral gaze, and is accompanied by inability to walk. This is not the main cause of DBN, but as it is very important, one should be very alert to this possibility.
BPPV: If one confines ones attention to older persons with "matitutional" vertigo however, it does seem to be clearly related to supine positioning. In this case, it may be a peripheral nystagmus -- anterior canal BPPV, rather than a central nystagmus. This variant is only seen supine.
Superior Canal Dehiscence. We have encountered several patients with DBN on prone.
Normal variant: Small amounts of downbeating nystagmus are seen in many normal subjects when using sensitive recording methods (i.e. video ENG in the dark). If downbeating nystagmus cannot be seen during fixation, it is unlikely to reflect a serious pathology.
Migraine associated vertigo may be associated with DBN (this is an observation of the author of this page).
Downbeating nystagmus can be elicited by the head-shaking test. This is also called "perverted" head-shaking nystagmus -- a nystagmus which is not in the plane of head-stimulation. It is often associated with an underlying mechanism of migraine or cerebro-vascular disease (this is the authors unpublished opinion). "Perverted" head-shaking nystagmus has also been reported in persons with cerebellar lesions, and this may be the link between pathology and physiology.
Downbeating nystagmus is sometimes elicited by Valsalva. This is usually innocuous.
Ear disorders rarely cause DBN (at least sitting up). Congenital nystagmus is also rarely a cause of DBN.
If a previously undiscovered DBN is encountered on an ENG recording, referral to a neurologist is usually a good idea, as most of the causes of DBN are central.
Treatment of DBN is largely directed towards the underlying cause. There are a few medications that are occasionally helpful -- 3-4 DAP has been reported to be useful in DBN due to cerebellar disturbances. This is an "orphan drug" in the US.
Medications that are of general use for nystagmus may be helpful too -- benzodiazepines, gabapentin, and perhaps memantine.
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