Epidemiology of myasthenia gravis in Finland

Abstract

The epidemiological results of an investigation of myasthenia gravis in Finland which started in the autumn of 1963 are presented. Most of the material was collected from the years 1954–1966. The total number of cases ascertained was 183 of which 4 were cases of neonatal myasthenia in complete remission. For 1956–1965 the annual incidence of myasthenia gravis in the whole of Finland was 2.27 per million and in the central hospital district of Helsinki 3.52 per million. The age distribution of all the patients included at the onset of symptoms is presented as well as the duration of the disease up to the moment of diagnosis. The prevalence of myasthenia gravis in the whole of Finland on 1 January 1964 was 102 cases (plus 3 cases of neonatal myasthenia in complete remission), of which 77 were women and 25 men. On 1 January 1968 the corresponding prevalence figure was 117 (plus 3 cases of neonatal myasthenia), 85 women and 32 men, i.e. a frequency of 25 per million. The corresponding prevalence in the Helsinki central hospital district was 42 per million. The number of established cases in the central hospital districts in the eastern region of the country was relatively much lower than in the southern and western central hospital districts. Comparisons are drawn as to the distribution of the prevalence cases with regard to place of birth, place of residence at the onset of symptoms and the place of residence at the time of prevalence. No significant changes were observed. The possible significance in the differences of regional frequency between the hospital districts is discussed. It is estimated that the actual prevalence of myasthenia gravis in Finland is 50–75 per million inhabitants. The annual mortality for the years 1957–1966 of those cases of the series in which the diagnosis was confirmed was 0.74 per million. This figure is probably lower than the actual rate. If the number of other cases suffering from myasthenia gravis according to their death certificates is also taken into account, the corresponding annual mortality figure is 0.87 per million. Of the 48 deaths in patients belonging to the series of confirmed cases, myasthenia gravis was entered on the death certificate as the primary cause of death in 35 of the cases and as a contributory cause of death in 4 of them, but was omitted entirely in 9 of them. Of all deaths, 58.5% took place during the first 3 years of the illness. The results of this epidemiological study of myasthenia gravis carried out in Finland are compared with those of similar investigations carried out in England, USA, Norway and Iceland as well as with results of less comprehensive studies reported from Canada, New Zealand, Japan, Malta and Africa. The results concerning Finland are not inconsistent with those obtained elsewhere. The causes of variations in frequency are discussed.