Based upon the LR calculations in this population-based study, 1 in 26 people will develop epilepsy during their lifetime. Men have a higher risk of developing epilepsy (1 of every 21 males) than women (1 of every 28 females). This approach is more accurate than cumulative incidence, and it is better comprehended by most people who are accustomed to similar statistics provided for cancer.
Cumulative incidence has been used in etiologic studies of epilepsy and for public health purposes to determine health care needs of a population. It is useful in children, because mortality is low, and in etiologic studies. However, cumulative incidence may overestimate the risk for developing diseases with high incidence in the elderly where significant mortality represents a competing risk. This is because the cumulative incidence assumes that individuals who die before they can be observed to have the disease are assumed to have developed the disease at the same rate as those who survive.11
In contrast, LR adjusts for the competing risk of death by setting the risk for disease after death at 0, and it provides a better measure of the risk for developing epilepsy given survival to a specific age, particularly in the elderly where the risk of death due to other causes is high.12
LR can be used for individual risk prediction with the usual caveats because it is a measure based upon population estimates3
and not upon factors related to any one individual (e.g., familial life expectancy, risk factors). Even with this limitation, LR is informative about the number who will develop epilepsy, 1 in 26 people in the population, which may be easier to communicate than the percentage of people who will develop epilepsy by a given age.
The LR for epilepsy from Rochester, MN, is likely similar to that for other developed countries with comparable sociodemographics for the specific birth cohorts. However, our findings may not be applicable to other populations with markedly different death rates due to a low life expectancy, such as developing countries. Additionally, our findings will not be applicable to other populations that may have a greater incidence of epilepsy. These limitations of the LR are small, however, since the distribution of different etiologic groups may also differ in different age groups with very different expectations of life in these settings. The LR for specific epilepsy etiologies may be greater in populations where treatments of stroke, for example, have markedly improved survival thereby increasing the time period in which such individuals are at increased risk for developing epilepsy.
Our data are drawn from 1960 to 1979. This does not compromise the validity of the comparison between cumulative incidence and LR. However, with the increase in life expectancy, the LR of epilepsy should be greater than what we report, particularly for males. Between 1970 and 2005 in the United States, life expectancy has risen from 67.1 years to 75.2 years for males and from 74.7 years to 80.4 years for females.13
Using the methodology described in this article, LR has been assessed for other neurologic disorders, including stroke,3
and Parkinson disease.14
In the elderly, the LR for epilepsy is 0.31% for a 65- to 69-year-old and 1.6% to age 80 for a 50-year-old. In Framingham, MA, the LR is 14.3% for first stroke to age 85 for a 55-year- old and 4.0% for dementia to age 80 for a 65-year-old.
Our data suggest that almost 12 million individuals in the United States will develop epilepsy in their lifetime. The impact of this calculation is greatest in the elderly who have the highest incidence, an important concern given the aging population. Information obtained from the LR has several important uses. LR provides physicians with an estimate of an individual's risk for developing epilepsy over his or her remaining lifetime, a risk estimate that readily translates into numbers of people who are expected to develop epilepsy, and it provides useful information for health care planners estimating services needed for epilepsy.