Traumatic brain injuries may be classified as mild, moderate, or severe to indicate the initial severity of the injury. The type injury may be either diffuse or penetrating. 

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A mild brain injury may be caused by direct blows to the head, gunshot wounds, sudden shaking of the head, or force from a whiplash-type injury. Both closed and open head injuries can produce a concussion. A concussion is the most common type of traumatic brain injury and causes a diversity of symptoms.

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A concussion is:

• caused when the brain receives trauma from an impact or a sudden momentum or movement change and the blood vessels in the brain may stretch and cranial nerves may be damaged.

• A person may or may not experience a brief loss of consciousness (not exceeding 20 minutes). A person may remain conscious, but they may feel “dazed” or “punch drunk” or "out of it."

• A concussion may or may not show up on a diagnostic imaging test, such as a CAT scan.

• Skull fracture, brain bleeding, or swelling may or may not be present.

• A concussion can cause injuries resulting in permanent or temporary damage.

• It may take a few months to a few years for a concussion to fully heal.

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A moderate brain injury is one that can be caused by a contusion which is a bruise (bleeding) on the brain or caused by a bump, blow, or jolt to the head or by a penetrating injury (such as from a gunshot) to the head. This type of injury may or may not require surgery depending upon severity.

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A severe brain injury is one that may be caused by:

• A “through-and-through” injury occurs when an object enters the skull, goes through the brain, and exits the skull. Through-and-through traumatic brain injuries include the effects of penetration injuries, plus additional shearing, stretching, and rupture of brain tissue.

• Objects traveling at a low rate of speed through the skull and brain which can ricochet throughout the skull and widen the area of damage.

• A penetrating injury to the brain occurs from the impact of a bullet, knife, or other sharp object that forces hair, skin, bone, and fragments from the object into the brain.

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A severe brain injury may cause an individual to experience an unconscious state, where one appears to be in a deep sleep and cannot be aroused or respond purposefully. Assessments will typically reveal that the individual does not have any sleep or wake cycles. This results in a loss of consciousness (often referred to as a coma.) Depending on different factors and the severity of injury, an individual may remain in a coma, emerge from a coma, or experience an increased level of consciousness.