Superficially, head injuries can be divided into two categories: those that damage the exterior of the head and those that damage the interior of the head. Head injuries often fall into both categories and severe internal damage can sometimes be masked by apparently mild external injuries. Alternatively, external head injuries can appear severe even when no internal injury has occurred. This is partially because the numerous blood vessels under the skin of the scalp can cause heavy bleeding, even when injuries are minor. The lump that sometimes appears after a head injury is caused by the blood vessels in the scalp leaking blood or fluid under the skin. These lumps usually disappear after a period of days or weeks as the body gradually resorbs the blood collection.

Internal head injuries are injuries that have damaged the brain. This type of injury is sometimes called traumatic brain injury and can range in severity from mild concussion to severe brain damage. Some types of traumatic brain injuries include:

• Concussion. Technically defined as a loss of consciousness following a blow to the head. Most people view a concussion as any head injury that results in dizziness, lightheadedness or temporary confusion, with or without loss of consciousness. It is caused by the brain shifting and coming into contact with the hard and rigid bone of the skull. Concussions can be mild or severe and are the most common type of head injury, especially among children.
  
  
• Skull fractures. A crack or break in the hard bone of the skull. This can happen after a heavy blow to the head. Young children may be more susceptible to skull fractures because the skull may not be completely formed. Skull fractures may be simple and linear, which means that the break runs in a line with no splintering or break in the skin. They can also be compound or depressed. Compound skull fractures may involve splintering of the bone and/or tearing of the skin. Depressed skull fractures are breaks that put pressure on or splinter into the brain. Compound and depressed fractures can cause blood clots (hematoma) and other types of brain damage.
  
  
• Contusion. Bruising of brain tissue. Contusions can occur when a depressed skull fracture puts pressure on the brain, causing the blood vessels in the brain to burst and the brain tissue to swell. Contusions can also occur when the brain moves rapidly within the skull (called contrecoup). This sometimes happens during a car accident or as a result of shaken baby syndrome. Some severe contusions may require surgical removal or repair.
  
  
• Diffuse axonal injury. Damage to nerve cells (neurons) throughout the brain. This type of brain injury can occur after the head has been severely shaken or rotated sharply. As the brain moves against the skull, the nerve tissue in the brain is damaged and the neurons are unable to communicate with each other effectively. Diffuse axonal injury often results in coma or permanent brain damage.
  

  
  

• Hematoma. Bleeding from a major blood vessel in the head. There are three layers of meninges – dura mater (outer lining), arachnoid (middle lining) and pia mater (the inner lining) – which line and protect the brain and spinal cord. The meninges contain cerebrospinal fluid and numerous blood vessels. Epidural hematoma occurs when there is bleeding between the skull and the dura. Subdural hematoma occurs when there is bleeding between the dura and the arachnoid membrane. Intracerebral hematoma occurs when there is bleeding within the brain.
  
  
• Anoxia. Lack of oxygen to the brain. This can be caused by a stroke (where the brain is deprived of oxygen-carrying blood) or an event, such as drowning or cardiac arrest, when the brain is deprived of oxygen despite adequate blood-flow. Brain cells that are not supplied with oxygen can die within minutes. Severe instances of anoxia can cause coma or brain death.
  

  
  

Another common categorization of head injuries is whether the injury is closed or open. A closed head injury occurs when the head is hit by a blunt object, for example. This type of injury can be mild or severe. Concussion is a type of closed head injury. An open head injury occurs when an object penetrates the skull and enters the brain tissue or leaves an opening in the skull that exposes the intracranial contents to the outside environment. These types of injuries are almost always severe and happen most frequently as a result of car accidents.