Downbeat nystagmus: aetiology and comorbidity in 117 patients

Objectives: Downbeat nystagmus (DBN) is the most common form of
acquired involuntary ocular oscillation overriding fixation.
According to previous studies, the cause of DBN is unsolved in up
to 44% of cases. We reviewed 117 patients to establish whether
analysis of a large collective and improved diagnostic means would
reduce the number of cases with ``idiopathic DBN'' and thus change
the aetiological spectrum.Methods: The medical records of all
patients diagnosed with DBN in our Neurological Dizziness Unit
between 1992 and 2006 were reviewed. In the final analysis, only
those with documented cranial MRI were included. Their workup
comprised a detailed history, standardised neurological,
neuro-otological and neuro-ophthalmological examination, and
further laboratory tests.Results: In 62% (n = 72) of patients the
aetiology was identified (``secondary DBN''), the most frequent
causes being cerebellar degeneration (n = 23) and cerebellar
ischaemia (n = 10). In 38% (n = 45), no cause was found
(``idiopathic DBN''). A major finding was the high comorbidity of
both idiopathic and secondary DBN with bilateral vestibulopathy
(36%) and the association with polyneuropathy and cerebellar ataxia
even without cerebellar pathology on MRI.Conclusions: Idiopathic
DBN remains common despite improved diagnostic techniques. Our
findings allow the classification of ``idiopathic DBN'' into three
subgroups: ``pure'' DBN (n = 17); ``cerebellar'' DBN (ie, DBN plus
further cerebellar signs in the absence of cerebellar pathology on
MRI; n = 6); and a ``syndromatic'' form of DBN associated with at
least two of the following: bilateral vestibulopathy, cerebellar
signs and peripheral neuropathy (n = 16). The latter may be caused
by multisystem neurodegeneration.