Amnesia is a temporary or permanent loss of memory. It usually results from an impairment of the brain brought on by a medical condition, such as a blow to the head (traumatic brain injury) or the use of certain substances (e.g., drugs, alcohol). In some relatively rare cases, psychological trauma can trigger amnesia.

Many different types of amnesia can result from a brain injury, including:

• Anterograde amnesia. Memory loss related to events that occur after damage to the brain. Anterograde amnesia involves the inability to learn and recall new information.
  
  
• Retrograde amnesia. Memory loss related to events that occurred before damage to the brain. Patients are unable to retrieve long-term memory of events and information learned prior to the brain trauma.
  
  
• Transient global amnesia. Sudden and temporary loss of the ability to store new memories of events that occur during the episode.
  

In rare cases, some patients experience memory loss as a result of dissociative disorders. These occur when people frequently escape reality by suppressing their memories or taking on alternate identities and do not involve medical trauma to the brain.

Amnesia manifests in different ways. Typically, patients forget details of past events that occurred in their lives, are unable to recall information they learned in the past or have difficulty learning and recalling new information. The extent of memory loss is usually correlated with the nature and severity of the injury or trauma that triggered the amnesia.

Patients who experience unexplained memory loss should see a physician, who will perform a complete physical and mental examination and compile a thorough medical history. Tests such as a computed axial tomography (CAT) scan and psychological testing may also be performed to help determine the cause of the amnesia.