Epidemiology and Classification of Epilepsy
by Michael Wong, MD, PhD
Incidence and Prevalence (reviewed in Hauser 1997): The overall (age-adjusted) incidence of epilepsy in developed countries is estimated to be ~25-50/100,000 person-years. Cumulative (lifetime) incidence of epilepsy is ~3-4%. Overall (age-adjusted) prevalence estimates range from 2-50/1000, but are most commonly reported ~5/1000.
- In developed countries, incidence of epilepsy shows a characteristic age-dependence with the highest incidences at the extremes of life. Incidence is very high (~100-200/100,000) in neonates and infancy and falls dramatically after the first year of life and more gradually throughout childhood (Hauser et al. 1993). Incidence reaches a minimum in early adulthood (~25/100,000), but then increases during late adulthood to a second peak in the elderly (Forsgren 1990, Hauser et al. 1993).
- In some developing countries, the age-specific incidence is quite different from developed countries, with the peak incidence of epilepsy occurring in early adulthood and no increase in the elderly (Lavados et al. 1992, Rwiza et al. 1992).
Time Trends: In the Rochester study (Hauser et al. 1993), the overall incidence of epilepsy showed little change from 1935-1984. However, incidence in patients
- Overall (all ages): ~60% partial (localization-related), ~40% generalized (Hauser et al. 1993).
- In infancy/early childhood, a higher proportion of patients have generalized onset seizures. The proportion of partial and generalized seizures become equal by mid-childhood (~ 6 years)
- In early adulthood, the proportion of partial and generalized seizures remains equal, but then progressively shift toward a partial seizure predominance in mid-to-late adulthood.
- Overall (all ages): ~35% remote symptomatic, ~65% idiopathic/cryptogenic (Hauser et al. 1993).
- In children: ~20% remote symptomatic, ~50% cryptogenic, ~30% idiopathic.
- In elderly: ~55% remote symptomatic, ~45% idiopathic/cryptogenic.
- Age-specific etiologies of remote symptomatic epilepsy.
- children (0-14): congenital >> trauma = infection > CVA = tumor = degenerative.
- early adult (15-34): trauma > tumor = congenital > infection > CVA = degenerative.
- mid-adult (35-64): CVA > tumor = trauma > congenital = infection = degenerative.
- elderly (65+): CVA >> degenerative > tumor = trauma > infection = congenital.
- In children:
- Neonatal seizures, congenital malformation, FHx epilepsy, febrile seizures, head trauma, CNS infection, mental retardation, and cerebral palsy are significant risk factors for childhood epilepsy (Hauser et al. 1993, Nelson and Ellenberg 1987, Ross et al. 1980).
- Adverse pre- or perinatal events in themselves are not independent risk factors for epilepsy, when children with cerebral palsy or mental retardation are excluded.
- Immunization (e.g pertussis) is unlikely to predispose to epilepsy or other neurological diseases, but may trigger febrile seizures in predisposed patients by inducing fever (Gale et al. 1994, Shields et al. 1988).
- In adults (reviewed by Hesdorffer and Verity 1997):
- definite risk factors for epilepsy in adults include head injury (Annegers et al. 1980), CVA, CNS infection (Annegers et al. 1988), CNS malignancy, and Alzheimer’s disease (Hauser et al. 1986).
- possible risk factors include multiple sclerosis (Kinnuren and Wikstrom 1986), hypertension, left ventricular hypertrophy, embolic
stroke risk factors, other dementias besides Alzheimer’s (Hersdorffer et al. 1996), depression (Hersdorffer et al. 1994), alcohol
abuse (Ng et al. 1988), and illicit drug use (Ng et al. 1990).
CLASSIFICATION OF EPILEPSY
- In 1981, a Commission of the International League Against Epilepsy (ILAE) proposed the now-standard revised classification of epileptic seizures, with the major division between partial and generalized-onset seizures (Commission of ILAE 1981).
- In 1989, the ILAE revised classification of epilepsies and epileptic syndromes was published, with the major categorizations based first on distinguishing partial (localization-related) vs generalized epilepsy and second on idiopathic, cryptogenic, or symptomatic etiology (Commission of ILAE 1989).
Practical Application of the ILAE classification
- Reliability: Several studies have found that >95% of epileptic seizures and syndromes in children and adults could be reliably classified according to the ILAE system (Berg et al. 1999, 2000, Loiseau et al. 1991, Manford et al. 1992, Senanayake 1993).
- Specificity: However, a large proportion (30-66%) of epileptic syndromes end up being classified in relatively non-specific categories, such as “cryptogenic localization-related epilepsies” and “epilepsies without unequivocal generalized or focal features” (Berg et al. 1999, 2000, Loiseau et al. 1991, Manford et al. 1992), which, some authors argue (Manford et al. 1992), contain minimal useful information from a pathophysiological, therapeutic, or prognostic standpoint.
- Application to Infants: The reliability of the ILAE classification of epileptic seizures in infants has also been questioned. One study found poor interobserver reliability in classifying seizures as partial or generalized based on visual observation of the seizure alone (kappa = 0.14-0.30), and moderate to substantial interobserver reliability based on assessment of ictal EEGs (0.54-0.79) (Nordli et al. 1997).
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