According to American Academy of Neurology, concussion is a biomechanically induced clinical syndrome related to alterations in brain function.

The injury can be caused by a direct blow to the head, face, or neck or by a blow elsewhere to the body with force transmitted to the head. Loss of consciousness occurs in only 8%-19% of concussions and is not a defining feature.

Concussion symptoms typically resolve within 7-10 days, but a minority of patients report symptoms that persist for months, even years. Children may take longer to recover than adults. Most cases of concussion are referred to as mild traumatic brain injury (mTBI) and are associated with Glasgow Coma Scale scores 13-15. Loss of consciousness, if any, is present for less than 30 minutes, and posttraumatic amnesia (if present) is present for less than 24 hours.

It is estimated that 1.6-3.8 million sport-related TBIs occur in the United States each year. Concussions result in more than 100,000 emergency department visits each year for school-aged children in the United States. The majority of concussions are mild cases accompanied by negative findings from MRI and CT studies. This means that in the majority of cases no changes or abnormalities will be discovered following MRI or CT scan. It is unclear whether age or level of competition affects concussion risk, but concussion rates tend to be higher in games than in practices.

Concussion is a functional, rather than a structural brain injury. Studies have demonstrated that it is the biomechanics of acceleration-deceleration forces on the head that can induce concussion, even if there was no direct impact to the head or skull.

Neurometabolic cascade that follows a concussion includes disrupted neuronal membranes, efflux of potassium, and release of glutamate, a neurotransmitter with excitatory function. Membrane ion pumps use adenosine triphosphate (ATP) to restore the normal cellular membrane potentials. The increase in cellular adenosine triphosphate metabolism following injury can produce a relative energy deficit, with a consequent depression of neuronal activity. Changes in neurometabolic state also include accumulation of calcium ions inside the cell, free radical production, impaired glucose metabolism, and abnormal axonal transport. There may also be changes in the blood flow in the brain which further worsens the energy crisis. Clinical features of concussion that represent the underlying energy crisis in the brain include headaches, dizziness, irritability, and impaired concentration and memory. Concussive injury to the developing brain may be associated with greater risk of long-term functional impairments because of changes in neuronal plasticity and insufficient myelination of axons in the brain. 

After a mild traumatic brain injury, there is a period of time when vulnerability to a subsequent concussion is high, this temporal window resolves during 7-10 days. Animal models suggest that the negative impact of repeat concussion diminishes 7 days after initial TBI. Studies of children ages 11-15 years demonstrated that cerebral blood flow values  improved over time; 27% matched the control values by 14 days after the injury, 64% after 30 days. 

American Academy of Neurology emphasizes that helmets can be life-saving.  

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