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injuries not complicated by skull fracture and with a post-traumatic amnesia of less than 30
minutes) do not carry a significantly increased risk of the development of epilepsy, but that
more severe injuries probably do. However, a more recent population-based study from
Denmark suggests that even mild head injuries (loss of consciousness for less than 30
minutes, post-traumatic amnesia for less than 24 hours, confusion/disorientation, or focal,
transient neurological deficit) may be associated with an increased risk1. Different definitions
of a ‘mild’ head injury are the most likely explanation for the discrepancy between these
studies.

A number of factors influence the risk of epilepsy:

Missile injuries. Several series have looked at the incidence of epilepsy following missile
injuries to the head. The best estimate of the risk of epilepsy for such injuries overall would
seem to be 50%. A number of factors further influence this risk, and these are summarised in
Table 2.

Non-penetrating head injuries. This form of head injury has been widely studied but largely
in patients admitted to neurosurgical units. It must be remembered that these represent a
selected population of head-injured patients.

If seizures are going to complicate a head injury they tend to do so shortly after injury. Around
75% of patients will have their first post-traumatic seizure within a year of injury, whether
this is a missile or blunt injury. Whilst the risk of developing seizures decreases with the
passage of time there are no good grounds for differentiating between seizures that occur in
the first week and later seizures, as far as their significance is concerned. One exception is
that seizures occurring immediately after impact do not carry an adverse prognosis for
recurrent seizures.

Jennett defined early seizures as those occurring within seven days of injury2. A total of 25%
of patients with early seizures had late epilepsy, compared to 3% of patients developing late
epilepsy in the absence of early seizures. When other factors contributing to the risk of late
post-traumatic epilepsy were excluded, i.e. depressed fracture or haematoma, late epilepsy
occurred in only 1.2% in the absence of early seizures, but in 51% of patients in whom early
seizures occurred.

The other factors which clearly contribute to the risk of late epilepsy are the presence of an
acute intracranial haematoma (31% risk) and depressed skull fracture (15% risk). In patients
without these features longer periods of post-traumatic amnesia increase the risk of epilepsy.

The risk of epilepsy shortly after traumatic brain injury is high, but how long this high risk
lasts is unknown. In a large population-based study in Denmark, it was found that the risk of
epilepsy was increased after a mild brain injury (RR 2.22, 95% CI 2.072.38), severe brain
injury (7.40, 6.168.89), and skull fracture (2.17, 1.732.71). The risk continued to be
increased more than 10 years later in each group. Interestingly, patients with a family history
of epilepsy had a notably high risk of epilepsy after mild (5.75, 4.567.27) and severe brain
injury (10.09, 4.2024.26). It appears therefore that even mild head injuries, particularly in
susceptible individuals, are associated with a greater long-term risk of developing epilepsy
compared to the general population1.