Neurotological follow-up studies upon Minamata disease

During 1986-87, 35 patients with organic mercury intoxication, who had been recognized as having Minamata disease by the Arbitration Committee in Niigata during 1968-78, were followed up neurotologically in Kido Hospital, Niigata, Japan. Characteristic follow-up features were served, as follows. 1. In 6 cases, spontaneous nystagmus had disappeared, but had newly appeared in 9 cases. However, positional nystagmus was markedly improved in 15 cases. 2. In OKN tests, especially in the vertical OKN test, relatively significant deterioration was observed at the follow-up examinations. 3. There was little significant change in the caloric nystagmus test and body-equilibrium tests. 4. In pure-tone audiometry, 16 out of 58 ears showed deterioration of hearing (28%), while 4 ears showed improvement (7%). In the neurotological findings there was much variation among the patients themselves. Therefore, these variable findings should be considered as being caused not by duration of the intoxication alone, but also by ageing factors in the patients.