As a symptom, memory loss can occur in many different forms. Memory loss may be temporary or permanent. It can last for brief periods of time (e.g., for several hours) or it can last for longer periods of time (e.g., days, months, years). Impairment can be partial or complete. It can involve short-term memory loss (inability to remember or store recent events), long-term memory loss (inability to remember events that occurred in the past), or both. It can also involve the inability to store new information into memory.

There are two major types of memory loss caused by disease or injury to the brain. These are:

• Anterograde amnesia. The inability to store or recall new events and information – those occurring after the moment of brain damage. Patients with anterograde amnesia typically struggle to remember day-to-day events, but have less difficulty remembering events from their childhood. It is thought that anterograde memory loss is caused by damage to the hippocampus, a structure located in the temporal lobe of the brain. The hippocampus is known as the gateway to memory. Research has shown that memories stored in the hippocampus are permanently retained. Thus, patients with damage to the hippocampus may have difficulty storing new memories.
  
  
• Retrograde amnesia. The inability to recall events and information that occurred prior to the moment of brain damage. Most cases of retrograde amnesia involve memory loss of events directly preceding the brain injury. This is often the case for people involved in automobile accidents who cannot remember the events that led to the accident. In some cases, retrograde amnesia involves the loss of all memories that occurred before brain damage, including those from childhood and memories of family and friends.

Transient global amnesia is another type of memory loss. It is believed to be related to either a transient ischemic attack (mini-stroke) or a seizure without convulsive activity. This type of memory loss is temporary and may involve both anterograde and retrograde amnesia.

A less common type of memory loss is psychogenic amnesia. This is also called dissociative amnesia because certain mental processes (e.g., memory) become separated from the rest. Psychogenic amnesia is psychological in origin. It is not caused by physical injury or disease of the brain, nor does it involve any disruption of brain functioning. Psychogenic amnesia is often the result of severe emotional trauma that may be too distressing for a person to recall. It tends to appear in the form of localized amnesia, which affects all memories during a particular period of time. This type of memory loss usually appears suddenly, is temporary, and may be resolved with therapy.

There are many different subtypes of psychogenic memory loss (e.g., localized amnesia, selective amnesia, generalized amnesia, systematized amnesia). For example, the memory loss associated with post-traumatic stress disorder (PTSD) may be considered a type of psychogenic memory loss.