Amnesia (Memory Loss)

What amnesia means

Amnesia comes from the Greek for "forgetfulness". In medicine it refers to a specific loss of episodic memory (memory for events) and sometimes factual memory, caused by damage or disruption to memory systems in the brain, particularly the hippocampus and nearby structures in the temporal lobes, and the thalamus.

People with amnesia usually know who they are, can speak and reason, and keep skills like riding a bike or playing an instrument (procedural memory). This separates amnesia from delirium, in which attention and awareness are disturbed, and from dementia, in which many thinking skills decline. The everyday forgetfulness of misplacing keys is covered under memory loss.

Types of amnesia

Causes of amnesia

Head injury

Concussion and traumatic brain injury commonly cause a gap around the event. The length of post-traumatic amnesia (the time until continuous memory returns) is one of the best indicators of how severe a brain injury is. More than 24 hours suggests a severe injury. See head injury.

Transient global amnesia (TGA)

A sudden episode, usually in people aged 50 to 70, in which the person cannot form new memories and asks the same questions over and over ("What day is it? Why are we here?"). They know who they are and act normally otherwise. It lasts 1 to 10 hours and fully resolves within 24 hours, leaving a gap for that period. Triggers can include strenuous exercise, sex, plunging into cold water, or emotional upset. It is harmless and rarely recurs, but it must be distinguished from stroke and seizures.

Stroke and TIA

Strokes affecting the thalamus or the posterior cerebral artery territory can cause sudden amnesia, usually with other symptoms such as vision loss or weakness. See transient ischemic attack.

Seizures

Temporal lobe epilepsy can cause episodes of lost time, and transient epileptic amnesia causes repeated brief amnesic episodes, often on waking. Memory is also lost for the period of any convulsive seizure.

Alcohol, drugs and thiamine deficiency

• Heavy drinking causes blackouts.
• Benzodiazepines, sleeping pills (zolpidem) and anesthetics cause gaps.
• Drugs used to spike drinks, such as GHB and flunitrazepam, cause memory loss.
• Long-term alcohol use, severe vomiting (including in pregnancy), malnutrition or weight-loss surgery can cause thiamine (vitamin B1) deficiency, leading to Wernicke encephalopathy and then Wernicke-Korsakoff syndrome with lasting anterograde amnesia and made-up stories to fill gaps (confabulation).

Brain infections and inflammation

Herpes simplex encephalitis has a predilection for the memory regions of the temporal lobes. Autoimmune encephalitis can cause memory loss with seizures and behavior changes over weeks.

Lack of oxygen

Cardiac arrest, near-drowning, carbon monoxide poisoning or a severe asthma attack can damage the hippocampus, which is very sensitive to low oxygen.

Psychological causes

Dissociative amnesia is an inability to recall important personal information, usually about a traumatic or stressful event, that is too extensive to be ordinary forgetting. It is linked to abuse, combat and disasters. A rare form, dissociative fugue, involves travel or wandering with loss of identity. See dissociative disorder and post-traumatic stress disorder.

Other causes

Brain tumors, electroconvulsive therapy, and early Alzheimer's disease (which begins with difficulty forming new memories) are other causes.

How doctors diagnose the cause

The doctor will want a witness account: what the person was doing, how they behaved, how long it lasted, and whether there was a fall, jerking, confusion or alcohol or drug use. Assessment includes:

• Neurological exam for weakness, vision loss and other signs of stroke
• Memory and cognitive testing: such as recalling words after a delay
• Brain imaging: CT for acute head injury or suspected stroke, MRI for more detail of the hippocampus and small strokes
• EEG to look for seizure activity
• Blood tests: glucose, sodium, thiamine, B12, thyroid, liver function, alcohol and drug screens
• Lumbar puncture if infection or autoimmune encephalitis is suspected

Treatment

Treatment depends on the cause:

• Thiamine given promptly, often by injection, for suspected Wernicke encephalopathy
• Antiviral treatment for herpes encephalitis, started early
• Anti-seizure medicines for epileptic amnesia
• Stroke care and prevention for vascular causes
• Rehabilitation after brain injury, including occupational and cognitive therapy
• Psychotherapy for dissociative amnesia, focusing on safety and processing trauma at the right pace

Practical strategies help people with lasting memory problems: a diary or smartphone reminders, routines, labeled cupboards, pill organizers and involving family.

Outlook

Transient global amnesia resolves fully. Memory after mild concussion usually returns except for the minutes around the injury. Dissociative amnesia often resolves, sometimes suddenly. Amnesia from Korsakoff syndrome, severe brain injury or encephalitis may improve partly over months to a year, but some loss is often permanent.

Frequently asked questions

What are the main types of amnesia?

The main types are anterograde amnesia (trouble forming new memories), retrograde amnesia (losing past memories), transient global amnesia (a temporary episode lasting hours), post-traumatic amnesia after head injury, and dissociative amnesia linked to psychological trauma.

What causes sudden memory loss?

Sudden memory loss can be caused by head injury, stroke, seizures, transient global amnesia, alcohol or sedative drugs, brain infection, low blood sugar or severe stress. Because stroke and bleeding are possible, sudden memory loss needs emergency assessment.

Can stress cause amnesia?

Yes. Severe psychological trauma or stress can cause dissociative amnesia, in which personal memories, usually of the stressful events, become inaccessible. Ordinary stress can also cause forgetfulness from poor concentration, which is not true amnesia.

Is transient global amnesia dangerous?

Transient global amnesia is usually harmless. It resolves within 24 hours, does not damage the brain and rarely recurs. Because it can look like a stroke or seizure, anyone with a first episode should be assessed in an emergency department.

Can amnesia be cured?

It depends on the cause. Amnesia from transient global amnesia, concussion, drugs and dissociation often resolves. Amnesia from brain damage, such as Korsakoff syndrome or severe encephalitis, may improve partly with treatment and rehabilitation but can be permanent.

Do people with amnesia forget who they are?

Rarely. Most people with amnesia keep their identity and general knowledge. Loss of personal identity is mainly seen in dissociative fugue, a rare psychological condition, and is far more common in films than in real life.