Head injuries occur when the head is directly injured by a sharp or blunt object. They can also result from the sudden acceleration or deceleration of the head, which can cause the brain to come into contact with the skull.

Head injuries are sometimes mild and have only minor repercussions for the brain. However, severe head injuries can seriously affect the brain and cause problems with speech, movement and behavior. According to the National Center for Injury Prevention and Control (NCIPC), roughly 1.4 million people sustain an internal head injury (traumatic brain injury) and 50,000 people die as a result of these injuries each year in the United States. Moreover, deaths related to head injuries account for one-third of all injury deaths, according to the NCIPC.

The brain is protected by the thick bones of the skull and a layer of tissue called the meninges that surrounds and cushions the brain and spinal cord. If the head is hit with sufficient force, the skull, meninges or brain can be damaged. Severe brain injury can occur when the brain shifts inside the skull, or when bleeding or bruising inside the skull cause pressure to build against the brain. This can disrupt the normal function of the brain and cause symptoms that range in severity from headache to permanent loss of brain function (brain death).

Following a severe head injury, the patient can become unconscious, such as during a stupor (state of impaired consciousness) or coma. In some cases, the patient may “wake up” and regain normal cognitive function. However, if the injury is too severe, the damage may be permanent and the patient may never regain full consciousness.

Scientific evidence indicates that even cases of apparently mild head injury can cause long-term illnesses later in life. This is especially likely if several head injuries are sustained over a period of time. Illnesses that can be triggered by head injuries, or that may occur earlier in people with a history of head injuries, include:

• Dementia. People who receive repeated blows to the head over a sustained period of time (e.g. professional boxers) can develop a form of dementia called dementia pugilistica. This type of dementia often occurs simultaneously with Parkinson’s disease. Another type of dementia, called post-traumatic dementia, can develop in people who experienced a head injury that resulted in a coma.
  
  
• Hypothyroidism. Brain injuries that damage the pituitary or thyroid glands can cause an imbalance of hormones in the body. Hypothyroidism (also called an underactive thyroid) can cause fatigue and weight gain, and usually requires treatment with synthetic hormones.
  
  
• Epilepsy. People who have sustained a head injury are up to two times more likely to develop epilepsy. Seizures immediately following the injury are common and generally not considered to be epileptic in origin. However, some people experience recurrent seizures as a result of the damage sustained to the brain from the injury.

Medical scientists are also finding increasing evidence indicating a link between head injuries and Alzheimer's disease and Parkinson's disease.

Head injury is more common in the following populations:

• Males aged 15 to 24
  
• Elderly people over age 75
  
• Children under age 6