[Nystagmus the diagnosis of vertigo and dizziness].

Vertigo or dizziness is primarily caused by peripheral vestibular disorders, such as benign paroxysmal positional vertigo (BPPV) and vestibular neuritis. BPPV can be diagnosed from associated positional torsional or direction-changing horizontal nystagmus and can be treated with canalith repositioning procedures. In contrast, vestibular neuritis and other acute peripheral vestibulopathies can be diagnosed from associated unidirectional horizontal nystagmus. Evaluation of nystagmus is essential for the diagnosis of peripheral vestibular disorders. Vertigo/dizziness caused by disorders in the brainstem or upper cerebellum is usually associated with other neurological signs or symptoms, such as motor palsy, sensory deficit, dysarthria, ocular motor palsy, and limb ataxia. In contrast, vertigo/dizziness caused by disorders in the lower cerebellum is not associated with these signs or symptoms; however, truncal ataxia becomes apparent in a standing position. Small lesions in the lower cerebellum can rarely cause unidirectional horizontal nystagmus directed toward the side of the lesions or direction-changing apogeotropic positional nystagmus; both types of nystagmus are enhanced when a patient lies on the non-affected side. This positional enhancement suggests that the same pathogenetic mechanism is involved in both types of nystagmus. The cerebellar lesions may disinhibit both semicircular-ocular and otolith-ocular reflexes. Semicircular-ocular reflex-dominant disinhibitions may result in the ipsilateral horizontal nystagmus, whereas otolith-ocular reflex-dominant disinhibitions may result in the direction-changing apogeotropic positional nystagmus.