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Educational Information About Traumatic Brain Injury

Mild Traumatic Brain Injury

Mild traumatic brain injury and concussion are not exactly the same things.

"Mild" is a subjective term, but in the medical world, there are criteria that are accepted as reasonably objective measures of “mild.”

The Glasgow Coma Scale is a simple, but reliable clinical measure of responsiveness that is used at an accident scene, in the emergency department or in the intensive care unit (ICU) to define the severity of a patient’s injury. The score ranges from 3, which means the patient is in a deep coma, to 15, which means the patient is alert and oriented. There are 4 levels of injury severity by initial GCS: very severe (3-5), severe (6-8), moderate (9-12) and mild (13-15). In large studies focusing on the eventual recovery of brain-injured patients, the GCS level has been a roughly accurate predictor of recovery.

The American Congress of Rehabilitation Medicine (ACRM) devised a more specific set of criteria to define “mild” that takes other factors into account: how long the patient was unconscious, how long the patient was confused or experienced memory loss and the type of brain injury seen on CT scan. With these factors included, a “mild” injury is defined by unconsciousness for no more than one half hour, GCS 13-15, confusion or memory loss for up to 24 hours and no structural lesions on brain images such as CT scans.

To a non-medical person, some of these ACRM criteria probably do not seem very “mild,” but research has shown that they are roughly accurate predictors of long-term recovery.


Concussion is more along the lines of what most of us would consider a “mild” injury. Concussion is often used as a synonym for “mild traumatic brain injury,” but the medical community uses this term only for injuries at the most mild end of the mild traumatic brain injury spectrum. Concussion also has very specific definitions in the medical world. Many medical organizations have defined conscussion with clinical guidelines: CDC, NIH, American College of Rehabilitation Medicine, American Academy of Neurology and others. All of these definitions include a brief period of loss or alteraion of consciousness and/or a brief lapse of memory following injury. 

There is currently no medical test that confirms or measures concussion. This may be available in the near future. 

There have been international conferences on concussion at which professionals have tried to provide a more clinically precise and useful definition. Many of these have focused on concussion in sports, perhaps because they are more easily identified, and thus, evaluated. Here is some information cited directly from the 3rd International Conference on Concussion in Sport (2008). The document produced by at this conference offers very clear definitions and guidelines.

“Concussion is a complex pathophysiological process affecting the brain, induced by traumatic biomechanical forces … Concussion typically results in the rapid onset of short-lived impairment of neurological function that resolves spontaneously …a graded set of clinical symptoms that may or may not involve loss of consciousness.”

The implications of this definition provide clinical guidance:

  • Concussion “affects the brain” not just the head; bleeding scalps and hurting heads are not concussions.
  • People recover from concussions 
  • Only in a small percentage of cases are post-concussive symptoms prolonged

The conference report endorsed the conclusion that the symptoms of concussion fall into one of three general areas: Somatic (physical), cognitive (brain function), and emotional (psychological). Thinking of a patient’s symptoms in this manner can guide treatment better than collapsing all symptoms into “post concussion syndrome."

Moderate to Severe Brain Injury

In the acute phase after injury, severity is defined by the Glasgow Coma Scale: very severe (3-5), severe (6-8), moderate (9-12). Patients with a GCS of 9-12 may be lethargic and slow to respond but they are often confused, disoriented, even agitated or combative, and unable to follow commands or requests. Patients with a GCS of 3-8 are not responsive except perhaps to pain – what a layperson would consider to be coma.

With higher speeds at contact, higher rotation in falls and greater distance of falls, there can be increasing severity of diffuse axonal injury (DAI). This is the primary contributor to initial severity on the Glascow Coma Scale. Because of several structural and mechanical factors of the brain, DAI may be primarily in the frontal and temporal white matter and the corpus callosum.

Coma in severe injury may be prolonged. Emergence from coma will almost always follow a path of low arousal to  agitation to confusion to orientation but still impaired cognition, and finally slow improvement in cognition. There are several clinical scales that physicians and therapists use to follow this improvement. Although the focus of recovery is on cognitive and behavioral problems, more severe diffuse axonal injury can also cause substantial problems with movement, speech and walking.

Subdural or epidural hemorrhages (bleeding) are outside the brain and do not directly cause brain damage. If large, either may cause brain damage by compression (herniation) and contribute to early severity. Epidural hematomas, in particular, may accumulate very quickly and cause death if removal is delayed when they are large. Once removed, however, there need not be any residual brain damage from these hemorrhages.

Focal cortical contusions (FCC) are bruises on the surface of the brain. These can be associated with “complicated” mild injuries. They can be caused by direct trauma often associated with skull fracture and subdural hematoma (bruising). In this case, the focal cortical contusions might be located anywhere over the surface of the brain. They can also be caused by momentum of the brain against the inner surface of the skull. For a combination of structural reasons, these focal cortical contusions are almost always on the under surface of the frontal lobes, on the most anterior (front) surface of the frontal lobes, or on the anterior surface of the temporal lobes.

Like stroke, which is another cause of injury to specific parts of the brain, focal cortical contusions do not cause coma. They do produce impairments based on the functions of the damaged parts of the brain. When there is little diffuse axonal injury but significant focal cortical contusions, this may considered a “complicated mild” injury. It is more accurate to define the injury severity along the two distinct dimensions of injury: DAI, FCC and any complication of subdural or epidural hemorrhages.