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Acquired pendular nystagmus: oculomotor and MRI findings.

The clinical, oculomotor and ophthalmological features of 27 patients with pendular nystagmus were studied in whom 22 also had MR imaging of the brainstem. The nystagmus was predominately horizontal in 4 patients, torsional in 5, vertical in 3 and mixed in trajectory in 8. Fifteen patients had conjugate nystagmus. Twelve patients had disconjugate nystagmus. Eight patients had INO. In 16 patients visual acuity was 6/12 or worse. Acuity and the presence of INO were unrelated to the conjugacy of the nystagmus. The MRI cuts at the medullary, pontine and midbrain levels were analysed statistically to determine the areas where there was significant (< 0.05%) overlap between areas of abnormal signal in different patients. Significant target areas for lesions causing the nystagmus were: in the pons the medial vestibular nucleus, central tegmental and paramedian tracts; in the medulla the inferior olivary nucleus, reticular formation, dorsal accessory olivary nucleus, central tegmental tracts and olivo-cerebellar fibres; in the midbrain the red nucleus and central tegmental tracts. Horizontal pendular nystagmus was preferentially associated with pontine lesions and torsional nystagmus with medullary lesions. Patients with conjugate nystagmus had a tendency to have bilateral mirror image MRI lesions (p = 0.028). The prevalence of lesions in our patients raises a possibility that more than one neuronal mechanism must be affected to produce pendular nystagmus. The inferior olive may be responsible for the rhythm of ocular oscillation. The disruption of pathways proximal to the oculomotor nuclei may determine the instability in terms of individual eye movement.