Subacute myelo-optico-neuropathy (SMON) is a severe neurodegenerative disorder caused by poisoning due to over-dose and prolonged oral administration of clioquinol. SMON is characterized by subacute onset of sensory and motor disorders in the lower half of the body and visual impairment preceded by abdominal symptoms, such as abdominal pain and diarrhea. Despite clinical features mimicking infection or multiple sclerosis, the discovery of the green hairy tongue and the green urine in SMON patients aroused researchers' interest and thereby began solving the cause of SMON. The green color was derived from a chelate compound of clioquinol with ferric iron. A large number of SMON were observed throughout Japan, and the total number of cases reached nearly 10,000 by 1970. After the governmental ban on the use of clioquinol in September 1970, there was a dramatic disappearance of new case of SMON. However, in Japan, there are still more than 2500 SMON patients suffering from severe dysesthesia/paresthesia and ataxic paraplegia.
