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Structural and functional MRIs disclose cerebellar pathologies in idiopathic downbeat nystagmus

From the Department of Neurology (K.H., T.S., R.K., A.D., J.W., M.S., T.B., S.G.), Department of Neuroradiology (M.H., G.S.-A.), and Center for Sensomotor Research (T.B., S.G.), Klinikum Grosshadern, Ludwig-Maximilians University, Munich, Germany.

Address correspondence and reprint requests to Dr. Katharina Hüfner, Department of Neurology, Klinikum Grosshadern, Neurologisches Forschungshaus, Marchioninistr. 23, 81377 Munich, Germany katharina.huefner{at}med.uni-muenchen.de

Background: Neurologic disorders in which the etiology and pathogenesis are not yet understood are termed idiopathic. Downbeat nystagmus (DBN) is a frequent eye movement disorder that clinically manifests with oscillopsia and postural instability. Forty percent of patients with DBN are classified as having idiopathic DBN, because no underlying pathology can be demonstrated by conventional MRI or laboratory tests.

Methods: We evaluated gray matter brain volumes of 11 patients with idiopathic DBN and compared them to those of healthy controls using voxel-based morphometry. In a second, functional MRI experiment, patients and controls performed downward smooth pursuit eye movements (DOWN), which were then compared with straight-ahead fixation of a stationary target (MID).

Results: Small areas of localized gray matter atrophy were detected in the lateral cerebellar hemispheres (lobule VI) and ocular motor vermis of patients with idiopathic DBN, but not in the flocculus and paraflocculus. The functional imaging data, however, revealed reduced activation in the parafloccular lobule and in the ponto-medullary brainstem of the patients when they performed smooth pursuit eye movements downwards.

Conclusions: The applied specialized imaging and data analysis techniques disclosed pathologies in an idiopathic eye movement disorder. The focal atrophy found in the vermal and lateral cerebellar regions in downbeat nystagmus (DBN) may lead to deficits in smooth pursuit eye movement initiation, which in turn causes hypofunction of the parafloccular lobe, associated with DBN. Our data are in line with experiments in primates showing that ablation of the floccular and parafloccular lobes disrupts smooth pursuit and causes DBN.

Abbreviations: AEP = auditory evoked potentials; BOLD = blood oxygenation level dependent; BP = blood pressure; DBN = Downbeat nystagmus; DLPN = dorsolateral pontine nuclei; DV = dorsal vermis, ocular motor vermis; EMG and NCS = electromyography and nerve conduction studies; ENG = electronystagmography; FEF = frontal eye field; FL/PFL = floccular/parafloccular lobe; fMRI = functional MRI; FN = fastigal nucleus; HTT = head-thrust test; MST = medial superior temporal; MT = middle temporal; NRTP = nucleus reticularis tegmenti pontis; OKN = optokinetic nystagmus; OMN = ocular motor nuclei; PC = Purkinje cells; PMT = nucleus of the paramedian tract; ROI = region of interest; SEP = somatosensory evoked potentials; SLO = scanning laser ophthalmoscope; SVN = superior vestibular nucleus; SVV = subjective visual vertical; VBM = voxel-based morphometry; VES = ventricular extrasystoles; VOR = vestibulo-ocular reflex.

Supplemental data at www.neurology.org

Supported by the Deutsche Forschungsgemeinschaft (Gl 342/1-1; BR 639/6-3) and the research program FoFöLe (Reg.-Nr. 324) of the Ludwig-Maximilians University.

Disclosure: The authors report no conflicts of interest.

Received December 5, 2006. Accepted in final form April 16, 2007.

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