In emergency medicine, a lucid interval is a temporary improvement in a patient’s condition after a traumatic brain injury, after which the condition deteriorates. A lucid interval is especially indicative of an epidural hematoma. After the injury, the patient is momentarily dazed or knocked out, and then becomes relatively lucid for a period of time which can last minutes or hours. 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.^[1]

Because a patient may have a lucid interval, any head trauma should be regarded as a medical emergency and warrants emergency medical treatment even if the patient is conscious.

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.^[2]

Lucid intervals may also occur in conditions other than traumatic brain injury, such as heat stroke^[3] and the postictal phase after a seizure in epileptic patients.^[4]

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