title: Downbeat nystagmus associated with damage to the medial longitudinal fasciculus of the pons: a vestibular balance control mechanism via the lower brainstem paramedian tract neurons.
abstract: The paramedian tract (PMT) neurons, a group of neurons associated with eye movement that project into the cerebellar flocculus, are present in or near the medial longitudinal fasciculus (MLF) in the paramedian region of the lower brainstem. A 66-year-old man with multiple sclerosis in whom downbeat nystagmus appeared along with right MLF syndrome due to a unilateral pontomedullary lesion is described. In light of these findings, a possible schema for the vestibular balance control mechanism circuit of the PMT neurons via the flocculus is presented. Damage to the PMT neurons impaired the elective inhibitory control mechanism of the anterior semicircular canal neural pathway by the flocculus. This resulted in the appearance of anterior semicircular canal-dominant vestibular imbalance and the formation of downbeat nystagmus. From the pathogenesis of this vertical vestibular nystagmus, the action of the PMT neurons in the vestibular eye movement neuronal pathway to maintain vestibular balance was conjectured to be as follows. PMT neurons transmit vestibular information from the anterior semicircular canals to the cerebellum, forming a cerebellum/brainstem feedback loop. Vestibular information from that loop is integrated in the cerebellum, inhibiting only the anterior semicircular canal neuronal pathway via the flocculus and controlling vestibular balance.
authors.affiliation: Department of Neurology, Institute of Clinical Medicine, University of Tsukuba, Tsukuba, Ibaraki 305-8575, Japan. nakamagoek@md.tsukuba.ac.jp
authors.forename: Kiyotaka
authors.initials: K
authors.lastname: Nakamagoe
authors.forename: Natsu
authors.initials: N
authors.lastname: Fujizuka
authors.forename: Tadachika
authors.initials: T
authors.lastname: Koganezawa
authors.forename: Tetsuto
authors.initials: T
authors.lastname: Yamaguchi
authors.forename: Akira
authors.initials: A
authors.lastname: Tamaoka
mesh_terms: D000368:Aged; D023882:Dominance, Ocular; D004558:Electric Stimulation; D005133:Eye Movements / Q000502:physiology*; D006801:Humans; D008279:Magnetic Resonance Imaging; D008297:Male; D009474:Neurons / Q000473:pathology* / Q000502:physiology; D009759:Nystagmus, Pathologic / Q000473:pathology*; D011149:Pons / Q000293:injuries / Q000473:pathology*; D014726:Vestibular Nuclei / Q000473:pathology*