DOWNBEAT NYSTAGMUS

DOWNBEAT NYSTAGMUS:Spontaneous jerk nystagmus in primary gaze that is predominantly vertical includes upbeat nystagmus
and downbeat nystagmus.Downbeat nystagmus is usually present in primary position, but is greatest when the patient looks
down (Alexander law) and to one side. On upward
gaze, the nystagmus is less pronounced or disappears
completely. Patients with downbeat nystagmus often complain of vertical oscillopsia due to retinal slip produced by the nystagmus slow phase. Downbeat nystagmus is often associated with horizontal gazeevoked nystagmus; convergence may increase, suppress, or convert the nystagmus to upbeat Nystagmus. Downbeat nystagmus may occur with cervicomedullary junction disease, midline medullary lesions, posterior midline cerebellar lesions, or diffuse cerebellar disease.Deficient drive by the
posterior semicircular canals, whose central projections cross in the floor of the fourth ventricle, has been postulated as an explanation for downbeat nystagmus.

[Gossypol]

Thankyou for making the concept clear by your video. 👍