The overall risk of early seizures varies from 2% to 15% (16,30,31); that of posttraumatic epilepsy (late seizures) is approximately 5% to 7% (24,26,28). The most important risk factor for either is the severity of the injury (7,13,32), and the greater the severity, the higher the risk of posttraumatic seizures. Stratifying seizure frequency by severity of head injury has been difficult, because definitions of mild, moderate, and severe vary in the published literature. The duration of posttraumatic amnesia, the nature of the brain injury, and the Glasgow Coma Scale have been used to grade severity of injury. Correlating with severity and increasing the seizure risk are prolonged coma or posttraumatic amnesia (longer than 24 hours), brain contusion, intracranial hematoma, depressed skull fracture, dural penetration, and, to a lesser extent, linear skull fractures (12,33,34). Risk also increases when more than one factor is present (35).

Early seizures accompany fewer than 5% of mild or moderate head injuries (16,18,26,30) and often indicate other neurologic or systemic abnormalities, especially in an otherwise mild injury (18,36). The incidence increases to 30% with subdural or intracerebral hematoma, depressed skull fracture, penetrating brain injury, or cortical contusion (19,37). Seizures after the first hour usually imply severe head injury and focal intracranial pathologic lesions, such as hemorrhage or skull fracture.

Late posttraumatic seizures are common with more severe head injury. In the series reported by Annegers and Coan (38), the risk of posttraumatic epilepsy 5 years after closed head injury was 1.5% after mild trauma, similar to the risk for the general population without such injury; 2.9% after moderate damage; and 17.2% after severe insult (mild trauma was defined as coma or amnesia lasting less than 30 minutes, no skull fracture; moderate as coma or amnesia lasting between 30 minutes and 24 hours, skull fracture, but no contusion or intracranial hemorrhage; and severe trauma as coma or amnesia lasting more than 24 hours, and/or brain contusion or intracranial hemorrhage). Temkin (37) reported that the risk of late seizures increases by 400 times of that expected in a general population in the presence of early seizures, coma for more than a week, nonelevated depressed skull fracture, dural penetration injury, one nonreactive pupil, and subdural or intracerebral hematoma. In military personnel who survive high-velocity penetrating head injuries during warfare, the nearly 50% risk of posttraumatic epilepsy (7,27,29,39) is increased by the presence of a brain abscess.

The influence of age on the development of posttraumatic seizures is well documented (12,24,40,41). Children younger than age 5 years are more likely than adults to have seizures within the first hour after mild head injury (16,30,42,43), although early seizures are less predictive of late seizures than in adults (24,26). Adults older than age 65 years are highly vulnerable to severe brain damage and late posttraumatic epilepsy from any type of head injury (28,44).

Regardless of other risk factors, even a single early seizure increases the risk of late epilepsy to more than 25% in most series (12,24,34,37,45). Late seizures are more likely to begin within the first year if there has been an early seizure.

Jennett (12) defined an immediate, or impact, seizure as “a single generalized seizure … occurring within seconds of … a mild injury in an adult” and hypothesized that it did not necessarily indicate brain injury or predict later seizures. Although later investigators (46) adopted this position, universal agreement was lacking. Kollevold (21) found no difference in the risk of late epilepsy between a first seizure occurring within minutes of a head injury or during the ensuing 6 hours. McCrory and colleagues (23) report that convulsions within seconds of impact after concussive brain injury in sport do not increase the risk of long-term epilepsy. Contrary to these observations, Barry and coworkers (47) report that impact seizures may precede late epilepsy. Temkin (37) noted an increased risk for delayed early seizures (from days 1 to 7) in patients whose seizures occurred in the first hour after head trauma. Most researchers do not distinguish between immediate and early seizures as risk factors for late epilepsy.