Epidemiologic Studies on Subacute Myelo-Optico-Neuropathy with Abdominal Symptoms (SMON) in Ibara City, Okayama Prefecture

Abstract

Since first reported by Takasaki in 1961, increasing number of reports of the so-called subacute myelooptico-neuropathy with abdominal symptoms (SMON) has been coming from many places throughout Japan and drawing much public attention.
As the knowledge on its symptomatology and pathology advanced, it has now become recognized as an independent clinical entity and a new disease, though its etiology still remains unknown.
As of the end of 1968, SMON cases occurring in Ibara city, Okayama prefecture, western part of main land Japan, have accumulated up to 114 in number. Of them, 111 cases were able to be treated at Ibara Citizen's Hospital.
This paper describes epidemiological features of this disease in this city and some clinical observations made in this hospital.
1) Eighty two cases were able to be put under precise clinical observations in the hospital. About 70% of them were typical SMON showing signs of lateral cortico-spinal tracts and posteriol columns (mainly Goll's tracts) disturbances ensuing from abdominal symptoms. Of these typical ones, 45% were accompanying visual disturbances (12% catastrophed into blind). Remaining 30% were generally mild in their symptoms and considered to be abortive form of SMON, though they had abdominal signs and ensuing nerve disorders in the lower half of the body.
2) Incidence by year, season, sex and age:
SMON victims were 1 male (M) and 1 female (F) in 1962, 2 M 1 F in 1963, 1 M 3 F in 1964, 4 M 7 F in 1965, 3 M 6 F in 1966, 5 M 27 F in 1967, 19 M 31 F in 1968. As shown above, the number was demonstrably on increasing trend since 1967. Generally, the incidence was higher in warmer season like June to October, but considerable number was also seen in March. Looking all instances as a whole, female is predominant to male, the ratio being 2: 1, and, above all, the incidence of female in 50S was highest (720 persons per 100, 000 livings) followed by that of 40S (550 per 100, 000 livings).
When observed by year, the incidence of female in 60S was highest in 1966, that of female in 50S and 40S was predominant in 1967, and that of female in 30S was highest in 1968. It means the distribution of favorable age for this disease was shifting to younger side year by year.
3) Some other epidemiological findings:
1. There had been large scale epidemics of non-bacterial meningitis (ECHO 6 was then claimed as causative agents) in 1964, wrapping almost the whole city. Since then, in some districts of this city, the incidence was lingering, but in the other, no incidence at all. In the former districts, SMON appeared since 1965, the ensuing year of the non-bacterial meningitis epidemic. On the other hand, in the latter districts, SMON appeared first in 1966 or 1967, one or two years interval from that. Interestingly, the victims of non-bacterial meningitis were all under 16 years of age, and those of SMON were over 15.
2. The incidence was lower in districts where there was tap water supply system. Drinking water seems to have some relationship to this disease.
3. In this series, 5 instances of intra-familial multiple occurrence (11 cases) were observed. Three instances of multiple outbreaks of this diseases within small circles were also observed; one in a factory workshop, one in a high school athlete club, and one in a hospital pharmacy.
4. Three persons were found to have developed SMON two to three months after they moved to this city from other districts.
All these epidemiological facts are more or less suggestive of the infectious nature of this disease.