[1][2] When related to haemorrhage, the lucid interval occurs after the patient is knocked out by the initial concussive force of the trauma and then temporarily recovers, before lapsing into unconsciousness again when bleeding causes the haematoma to expand past the extent for which the body can compensate.
[3] Thereafter there is rapid decline as the blood collects within the skull, causing a rise in intracranial pressure, which damages brain tissue.
In addition, some patients may develop "pseudoaneurysms" after trauma which can eventually burst and bleed, a factor which might account for the delay in loss of consciousness.
Delayed cerebral edema, a very serious and potentially fatal condition in which the brain swells dramatically, may follow a lucid interval that occurs after a minor head trauma.
[5] Lucid intervals may also occur in conditions other than traumatic brain injury, such as heat stroke[6] and the postictal phase after a seizure in epileptic patients.