Posted on December 9, 2009, under Epilepsy.
The incidence of a disease means the number of new cases in a defined population (usually
100 000) in a defined period of time (usually one year).
Good figures for the incidence of new cases of epilepsy come from the population of Olmstead County in Minnesota. People in this rural part of the USA do not move around very much, and have the good fortune to be cared for by doctors at the famous Mayo Clinic. Research workers there have long had an interest in identifying all patients with epilepsy.
The incidence of new cases is highest in infancy and in old age, but new cases can occur at any age. Throughout middle life the incidence is about 40 cases per 100 000 per year. As the years go by, the risk of having had epilepsy at some time in one’s life increases in a cumulative fashion. The cumulative incidence in a population of children studied in the UK was 410 per 100 000 by the age of eleven, 600 by the age of 16, and 1000 per 100 000 by the age of 23. From the United States study cited above, the cumulative risk by age 75 was 3400 per 100 000 (3.4 per cent) for males and 2800 per 100 000 (2.8 per cent) for females. Epilepsy is thus not a rare or unusual disorder; seizures may impinge upon the lives of any one of us.
Another word used in counting cases of disease is prevalence. Here it is best to consider first another common illness which has a prolonged and steady course such as Parkinson’s disease. It is quite easy (though expensive) to do a door to door survey and count the people found to have Parkinson’s disease, as the signs of it will always be apparent. Prevalence is usually expressed per 1000. The prevalence of cases per 1000 population means that this number of people have the disease on the day of the survey. This technique is more difficult for epilepsy because of its episodic nature. Clearly, common sense dictates that if someone had a seizure during a day on which a survey day was done, they should be included, but what about someone who had many seizures in the past, but none for three years? One has to judge where to draw the line. In practice, most surveys of prevalence include people who have had more than one non-febrile seizure in the past, and are on continuing anti-epileptic drugs and/or who have had at least one seizure in the last two years. After early childhood, the prevalence is more or less constant throughout life at about seven per 1000 in developed countries, and considerably higher in developing countries.