SALVADOR, Brazil (Reuters Health) Oct 13 - The effect of nicotine on eye movement was the subject of two studies presented here at the XIX Brazilian Congress in Neurology. In one, Brazilian investigator Dr. Christiane B. Pereira, working with colleagues at the University of Munich, found that nicotine could induce specific types of nystagmus — or alleviate it, if it was already an underlying condition.
Dr. Pereira reported that half of fifty normal subjects developed nystagmus after smoking one cigarette containing 0.9 mg of nicotine. Using videonystagmography and a "search coil" placed on the eyeballs, the direction of the nystagmus could be accurately assessed.
Dr. Pereira found that nystagmus was in an "upbeat" direction in 30% of cases, horizontal in 37% and diagonal in 18%. Rotational nystagmus, measured using the search coil, was found in seven of eight subjects studied.
Measuring the effect of the nicotine on dizziness showed that all the subjects that developed nystagmus demonstrated significantly increased postural sway compared with no nicotine intake. In addition, Dr. Pereira reported that changing head position worsened the nystagmus in almost all cases.
The investigators were thus able to postulate that the nicotine was acting on the vestibular pathway from the semicircular canals of the inner ear as opposed to the cerebellum or the ocular muscles themselves, which Pereira noted are both intrinsically involved in balance.
"Interestingly, no 'downbeat' nystagmus was found in any of the subjects," Dr. Pereira said in an interview with Reuters Health. "Knowing that smoking causes 'upbeat' nystagmus in 30% of cases, we wondered what would happen to patients who suffer with the rare 'downbeat' form of nystagmus when they smoked a cigarette."
"We found that in patients who have 'downbeat' nystagmus caused by cerebellar lesions, nicotine made their nystagmus much worse. However, in patients who have idiopathic 'downbeat' nystagmus, the nicotine actually made the nystagmus much better, both objectively measured using videonystagmography and subjectively," she commented.
"We think that nicotine is acting by stimulating the vestibular tone of the anterior and posterior semicircular canals that respectively affect the superior and inferior rectus muscles of the eye," Dr. Pereira explained.
"In 'downbeat' nystagmus caused by cerebellar lesions, the anterior vestibular tone is already overactive. The stimulating effect of the nicotine simply makes this more pronounced and so by stimulating the superior rectus muscles even more, the nystagmus gets worse," she said. "However, in idiopathic 'downbeat' nystagmus, the posterior vestibular tone is much reduced. The stimulating effect of the nicotine serves to revert this, and by stimulating the inferior rectus muscles of the eye the nystagmus is improved."
"This may suggest an important therapeutic avenue in patients with downbeat
nystagmus that we will need to investigate further," Dr. Pereira concluded.