The localizing value of nystagmus in brainstem disorders

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Abstract
As causes for conjugate jerk nystagmus at the midposition of the eye. vestibular imbalance, a neural integrator deficit, smooth pursuit imbalance and a saccade generator deficit have been considered. The authors investigated anatomically brainstem lesions of patients with downbeat, upbeat, torsional and horizontal nystagmus. Although relatively common, downbeat nystagmus is only rarely seen with brainstem lesions. In these instances it is localized in midline medullary structures. Upbeat nystagmus is more often caused by a discrete brainstem lesion in the medulla, which can be as far caudal as the cranio-cervical junction. Lesions have also been found at the pontine level. In the mesencephalon, torsional nystagmus occurs with lesions to the interstitial nucleus of Cajal and the rostral interstitial nucleus of the MLF. In addition torsional nystagmus is seen after vestibular nuclei and lateral medullary lesions. Both lesion sites are also found with horizontal nystagmus. Although in some instances pathophysiology of anatomical structures can largely explain the nystagmus (e.g., torsional nystagmus) this is not the case in others. This applies particularly for the caudal medullary lesions seen with upbeat nystagmus, since the oculomotor related structures of the vestibular nuclei and the nucleus praepositus hypoglossi do not extend so farcaudally. A possible anatomical candidate are the cell groups of the paramedian tract (pmt). They do extend so far caudally, receive an input from virtually all ocular premotor structures in the brainstem, and project to the parts of the cerebellum, known to be involved in gaze-holding.