Lucid Interval

Lucid interval (from Latin lucidus — bright, clear and intervallum — pause, distance) — this is a classic and highly insidious clinical phenomenon, characteristic of severe traumatic brain injury.

It represents a period of temporary, complete or partial clearing of consciousness following its initial loss at the moment of impact, followed by a catastrophic deterioration of the patient’s condition.

Etiology and pathophysiology

This phenomenon is most typical for acute epidural hematoma (rupture of the middle meningeal artery). Pathogenesis consists of two parallel processes. At the moment of a strong head impact, the individual sustains a primary injury in the area of the reticular formation, resulting in instantaneous loss of consciousness.

After a short interval, the patient regains consciousness, becomes oriented in space, and feels relatively well. This marks the beginning of the lucid interval. However, during this time, blood continues to leak from the damaged vessels, forming a hematoma. The brain compensates for this compression for a while by reducing the volume of cerebrospinal fluid and venous blood inside the skull.

Clinical significance

The duration of the lucid interval can range from several tens of minutes to twenty-four hours. Its main danger lies in the patient and caregivers underestimating the severity of the injury. The victim often refuses hospitalization, believing they ‘just have a bruise’.

As soon as the skull’s reserved spaces are depleted, compensation abruptly fails. Intracranial pressure increases exponentially. The patient, amidst clear consciousness, suddenly experiences an intense headache, repeated vomiting, becomes aggressive, and then falls into a deep coma. The pupil dilates on the side of the impact, and paralysis of the opposite side of the body occurs. Recognition of the lucid interval in the history (as reported by witnesses) is an absolute signal to the emergency physician for the presence of increased intracranial pressure against the background of an intracranial hemorrhage.