Explain loss of consciousness in concussion

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concussion loss of consciousness mechanism pathophysiology

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Loss of Consciousness in Concussion

Definition

Concussion is a trauma-induced alteration in mental status that may or may not involve loss of consciousness (LOC). LOC, when it occurs, is actually the most dramatic but not the most consistent feature - amnesia and confusion are the true hallmarks. Notably, 90% of concussions do not involve LOC at all.

Adams and Victor's Principles of Neurology, 12th Ed., p. 894
Miller's Review of Orthopaedics, 9th Ed.

Mechanism of LOC

1. Rotational Forces - the Core Explanation

The widely accepted mechanism centers on rotational (angular) acceleration of the brain within the skull. When the head is struck or suddenly decelerated, the brain - partly tethered but suspended - lags behind the skull due to inertia, then rotates in an arc centered at the point of tethering in the high midbrain/mesodiencephalic junction.

This was first rigorously demonstrated by Denny-Brown and Russell (1941): a concussion required a freely moving head at the moment of impact. Preventing head movement with the same force failed to produce concussion. Ommaya and Gennarelli (1974) confirmed this by photographing brain movement through a transparent calvarium.

The torque generated at the level of the upper reticular (ascending arousal) formation disrupts its function in maintaining wakefulness - directly causing LOC.

Adams and Victor, p. 894
Plum and Posner's Diagnosis and Treatment of Stupor and Coma, p. 315

2. Direction of Acceleration Matters

Gennarelli's experiments in monkeys showed a clear difference:

Sagittal plane acceleration: typically brief LOC
Lateral direction acceleration: prolonged and severe coma

This explains why the duration of LOC varies so widely, and is one reason why brief vs. prolonged unconsciousness represents a quantitative, not qualitative, difference (as Symonds argued).

Plum and Posner, p. 315

3. Neurophysiology - What Happens at the Cellular Level

At the moment of impact, the brain undergoes abrupt neuronal depolarization and excessive release of excitatory neurotransmitters:

Glutamate surge - the dominant initial event
Potassium efflux from cells
Calcium influx into cells (triggers downstream metabolic cascade)
Cortical spreading depression - waves of ionic disruption that spread across the cortex, temporarily suppressing neuronal activity
Altered cerebral blood flow and glucose metabolism

These changes collectively impair neuronal and axonal function throughout the arousal network, producing the transient LOC.

Plum and Posner, p. 315

4. Reticular Formation Disruption

Foltz and Schmidt (1956) demonstrated in concussed monkeys that:

Sensory transmission through lemniscal pathways was unaltered
The effect of sensory input in activating the reticular formation was blocked
Electrical activity of the medial reticular formation was depressed more severely and for longer than the cerebral cortex itself

This confirms the reticular activating system (ARAS) as the anatomical site of concussive LOC, not the cortex directly.

Adams and Victor, p. 895

Brief vs. Prolonged LOC

Feature	Brief LOC	Prolonged LOC
Duration	Seconds to minutes	Hours or more
Mechanism	Transient shearing of ARAS at mesodiencephalic junction	Diffuse axonal injury (DAI) + mechanical injury
Imaging	Usually normal CT/MRI	May show axonal retraction balls, corpus callosum or dorsolateral mesopontine hemorrhage on MRI
Pathology	Functional/ionic	Structural - DAI, axonal retraction balls, microglial clusters

Prolonged unconsciousness (as seen in diffuse axonal injury) is associated with:

Hemorrhagic injury to the corpus callosum (flung against the falx)
Injury to the dorsal surface of the mesopontine junction (hammered against the free edge of the tentorium)
This pattern was confirmed in 45 human autopsy cases by Adams et al.
Plum and Posner, p. 315-316

Clinical Features of LOC in Concussion

In its fullest form (Adams & Victor's description):

Immediate abolition of consciousness at the moment of impact
Suppression of postural reflexes (collapse if standing)
Transient respiratory arrest
Brief bradycardia followed by fall in blood pressure (after a momentary rise)
Extensor plantar responses (Babinski sign) during and just after unconsciousness
Concussive convulsions - brief tonic extension of limbs, clonic movements lasting ~20 seconds; not a predictor of later epilepsy
Retrograde amnesia - inability to recall events just before the impact
Post-traumatic (anterograde) amnesia - memory gap extending until the patient can form consecutive new memories; its duration is the best clinical index of injury severity

Recovery occurs in stages: stirring → eye opening → return of reflexes → responding to commands → gradual re-engagement with environment. During the recovery phase patients may appear to converse normally but retain no memory of it, closely resembling transient global amnesia.

Adams and Victor, p. 895

Special Case: Blast Injury

Concussion from blast does not fit the rotational model neatly. The prevailing hypothesis is that a pressure shock wave travels directly through the brain, disrupting neural function throughout the cerebral hemispheres and/or the midbrain reticular formation. This remains an active area of research in military medicine.

Adams and Victor, p. 895

Key Takeaway

LOC in concussion is primarily caused by rotational shearing forces disrupting the ascending reticular activating system at the mesodiencephalic junction, compounded by a neurochemical cascade (glutamate release, ionic shifts, spreading depression). LOC is neither necessary nor sufficient for the diagnosis of concussion, and its duration reflects the severity of ARAS disruption - ranging from pure functional/ionic disruption (brief LOC, normal imaging) to structural diffuse axonal injury (prolonged coma, abnormal MRI).

Sources:

Plum and Posner's Diagnosis and Treatment of Stupor and Coma, pp. 315-316
Adams and Victor's Principles of Neurology, 12th Ed., pp. 894-895
Miller's Review of Orthopaedics, 9th Ed.
Recent review: Mohanty M & Torres AR, "Pathophysiological pathways in mild traumatic brain injury," Semin Pediatr Neurol 2026 Apr [PMID: 41962995]

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