Incidence and classification of new-onset epilepsy and epilepsy syndromes in children in Olmsted County, Minnesota from 1980 to 2004: a population-based study

Abstract

Purpose: To determine the incidence and classification of new-onset epilepsy, as well as the distribution of epilepsy syndromes in a population-based group of children, using the newly proposed Report of the ILAE Commission on Classification and Terminology 2005-2009.

Methods: We identified all children residing in Olmsted County, MN, 1 month through 17 years with newly diagnosed epilepsy from 1980 to 2004. For each patient, epilepsy was classified into mode of onset, etiology, and syndrome or constellation (if present). Incidence rates were calculated overall and also separately for categories of mode of onset and etiology.

Results: The adjusted incidence rate of new-onset epilepsy in children was 44.5 cases per 100,000 persons per year. Incidence rates were highest in the first year of life and diminished with age. Mode of onset was focal in 68%, generalized/bilateral in 23%, spasms in 3% and unknown in 5%. Approximately half of children had an unknown etiology for their epilepsy, and of the remainder, 78 (22%) were genetic and 101 (28%) were structural/metabolic. A specific epilepsy syndrome could be defined at initial diagnosis in 99/359 (28%) children, but only 9/359 (3%) had a defined constellation.

Conclusion: Nearly half of childhood epilepsy is of "unknown" etiology. While a small proportion of this group met criteria for a known epilepsy syndrome, 41% of all childhood epilepsy is of "unknown" cause with no clear syndrome identified. Further work is needed to define more specific etiologies for this group.

Figure 1. Incidence rates separate for boys and girls across age

Incidence rates for the first year of life are separated into two pieces (1–6 months and 6–12 months). This separation was possible by utilizing the complete enumeration of the Olmsted County population. The incidence rates for 1–6 months were 159.6 for boys, and 117.2 for girls, and 138.8 overall cases per 100,000 persons per year‥ Similarly, the rates for 6–12 months were 80.6 for boys, 93.3 for girls, and 86.8 overall cases per 100,000 persons per year. For comparability with other studies and to allow for age- and sex- adjustment to the U.S. population, the incidence rates for the complete first year of life are reported as one age category in Table 3.

Figure 2. Incidence rates separate for boys and girls across calendar year groups

Observed incidence rates in our study are shown for five calendar year periods. While the incident rates remained relatively stable in girls during the 25-year study period (Poisson rate regression trend over calendar year, p = 0.49), the incidence rates in boys exhibited an increase during the study period (Poisson rate regression, p = 0.007).