So, yes, smoking can cause upbeat nystagmus. I think the most important thing is to see if it's only present without fixation in the dark, or if it's present with fixation. Slow velocity nystagmus without fixation probably just reflects nystagmus in dark in normal individuals. Another scenario however is to have upbeat spontaneous nystagmus that is present both with fixation and without fixation as well as being associated with other ocular motor findings e.g. lateropulsion of saccades; impairment of vertical canals on vHIT or any abnormality in the neurological exam. So isolated, mild, upbeat, nystagmus, with fixation removed probably, I wouldn't worry that much, because it might occur in normal individuals. But upbeat nystagmus with fixation, particularly when associated with other ocular motor findings should be seen as a significant finding.