Estimating the Absolute Risk of Epilepsy After Traumatic Brain Injury
There are significant risks of epilepsy following TBI, especially following severe TBI,[8,14] and even mild TBI (concussion) seems to carry an increased long-term relative risk of epilepsy. This relative risk may remain increased more than 10 years after injury.[14] Therefore, it is important to be aware of symptoms that may represent epileptic seizures in persons with TBI. Pilots and truck drivers have occupations where there is a special need to be aware of the risk of epilepsy following TBI.[17] Even though the relative risk of epilepsy after brain injury may be increased compared with the population without, it is important to estimate the absolute risk of posttraumatic epilepsy among these patients.[8,14]
Clinical case series and population-based studies estimate cumulative incidence of epilepsy after TBI. The probability of developing late posttraumatic seizures in the entire sample of 480 persons with TBI in the United States was 13.8% at 24 months.[18] The cumulative risk of epilepsy among 2826 persons with TBI in China was 5.0% after 3-year follow-up.[19] The 30-year cumulative incidence was 2.1% for mild TBI, 4.2% for moderate TBI, and 16.7% for severe TBI in a cohort from the United States.[8] The cumulative probability of epilepsy among 137 persons with TBI in Italy was 10% at 6 months and 17% at 12 months.[20] The cumulative incidence of epilepsy among 3,093 TBI patients in China was 9.8% at 24-month follow-up.[21] Among 275 patients in England followed for a minimum of 4 years, 28 persons (10%) developed epilepsy.[22] The absolute risk may be even higher in institutionalized persons (25% after 12-year follow-up)[23] and combat brain injury (42% after 30–35-year follow-up).[24]
Although different in a brain injury population, disease stratification and follow up, these studies indicate that the risk of epilepsy is very high the first years after diagnosis, and that the cumulative incidence continues to increase with length of follow-up. To study this in further detail, we used data from the study by Annegers,[8] and calculated the cumulative probability of unprovoked seizures following brain injury by severity. We estimated the risk of epilepsy year < 1, year 1 to 4, year 5 to 9, and year 10 to 30 after brain injury by dividing the number of new-onset seizures in the time interval with the number of persons with brain injury alive without epilepsy at the start of follow up of the individual time intervals.[8] Fig. 5 shows their cumulative risk of epilepsy. However, although the cumulative probability of epilepsy continues to increase, the yearly probability of posttraumatic epilepsy decreases sharply the first years after brain injury. We calculated the average yearly absolute risk of epilepsy by dividing the number of new-onset epilepsy with the number of persons at risk at the start of the time interval divided by the number for years in the time interval (Fig. 6). Although the absolute risk of posttraumatic epilepsy is high the first year after diagnosis—especially following severe brain injury—this absolute risk decreases sharply and by 5 years of follow-up the yearly absolute risk is below 1% for all types of brain injury including severe brain injury (Fig. 6).
Figure 5.
Cumulative probability of unprovoked seizure, adapted from Annegers JF et al.8
Figure 6.
Yearly absolute probability of unprovoked seizures after brain injury, adapted from Annegers JF et al.8
Semin Neurol. 2015;35(3):218-222. © 2015 Thieme Medical Publishers
