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Memory is defined as the storage of information in the brain. This information is stored in the form of facilitated synaptic tracts in the brain. Each time certain sensory signals pass through a specific sequence of synapses, they become facilitated. The same information can be recalled by the self-stimulation of these synapses by the signals arising within the brain. Self-stimulation is possible due to the facilitation of the synapses.
Amnesia is defined as a temporary or permanent state of decreased memory. Depending on the cause of damage, it may result in partial or complete memory loss. Amnesia can occur either due to damage to some areas of the brain or due to some substance abuse. It may also be present in some individuals at the time of birth.
In this article, we will discuss major types of amnesia, factors that can cause amnesia, the clinical features of the diseases, and its management.
Types of Amnesia
There are two main types of amnesia; anterograde amnesia (inability to form new memories) and retrograde amnesia (old memories are lost). However, various other types of amnesia have also been studies.
Following are the types of amnesia that are known to the neuroscientists today.
It is a type of amnesia in which the person is unable to create new memories. He remembers everything from the past. However, he cannot keep the record of the event that occurred after the injury to the brain.
Role of Hippocampus
Hippocampus is a part of the brain present in the medial temporal lobe. It is involved in the formation of long-term memory. It has been found that the removal of the hippocampus in some patients resulted in the inability to store new information in the long-term or intermediate memory. However, the memories stored in the brain prior to the removal of the hippocampus remained intact. Thus, it is believed that any damage or injury to the hippocampus can result in anterograde amnesia.
Anterograde amnesia can also result from some mother causes other than the hippocampal damage. These include stroke, trauma, surgery, encephalitis, alcoholism, Wernicke-Korsakoff syndrome, anoxia to the brain, etc.
This type of amnesia cannot be treated via pharmacological management. However, patients can be educated to take care while performing daily life activities and develop procedural memory. The friends and family of such patients can help them live a better life.
In this type of amnesia, the patient is unable to recall memories from the past. He can form memories of the recent events that will remain intact. However, the memories stored in the brain prior to brain damage are lost. Complete or partial memory loss can be seen depending on the extent of the damage.
When retrograde amnesia occurs, the memories of the events in the recent past are more affected as compared to the distant past. It is due to the reason that the distant memories have been recalled so many times that their memory tracks become deeply ingrained in the brain. These memories are stored in the widespread areas of the brain.
Retrograde amnesia can result from damage to different parts of the brain other than the hippocampus. It can also be due to cerebrovascular accidents, stroke, head trauma, alcohol abuse, or drug abuse, etc.
Role of Hippocampus
Although damage to the hippocampus causes anterograde amnesia, some hippocampal lesions show retrograde amnesia to a small extent along with anterograde amnesia. Studies suggest that these two types of amnesia are partially related to the hippocampus.
Both anterograde and retrograde amnesia can coexist in some patients at the same time.
It is a temporary type of amnesia characterized by episodic memory loss. One episode of retrograde memory loss may last from hours to days or even years. During the episodes of memory loss, patients are unable to recall their personal information. Recent as well as distant memories are lost.
Dissociative amnesia is an episodic type of memory disorder in which retrograde amnesia is present while anterograde amnesia is not seen. This type of amnesia does not result from neurological damage to the brain. Rather, it is due to some psychological causes. Thus, it is called psychogenic amnesia. Psychological events such as emotional stress can trigger this episodic memory disorder.
Some examples of memory loss seen in this type of amnesia are as follows:
- Patients are unable to recall information about traumatic or stressful events in the past. It is due to a psychological defense mechanism that prevents access to information related to an event that might have a psychological impact such as a disaster.
- The psychological trauma may cause a temporary fugue state. The patients may forget their identity and assume a new one.
This type of amnesia is caused by any trauma or injury to the head. The trauma may be caused due to an automobile accident, fall from a height, a strike on the head, etc.
The trauma to the head may result in anterograde amnesia, retrograde amnesia, or both. The extent of memory loss depends on the degree of trauma. It might be temporary or permanent.
Besides memory loss, other functions of the brain are often compromised. Early recovery from amnesia can indicate recovery of other brain functions.
Depending on the extent of trauma, memory loss may range from forgetting recent events to forgetting everything before the trauma. The period of recovery also depends on the degree of damage taken by the brain.
This inability is commonly seen in adults where they are unable to recall events from their early childhood. Individuals are unable to recall events that occurred from birth to four or five years of age.
Earlier, this condition was attributed to psychosexual development. However, modern scientists believe that it is due to the development of the brain at an early age. Moreover, it is also considered to be due to language development. It is believed that individuals are unable to recall events that have occurred in the pre-language phase of life.
Transient Global Amnesia
This type of amnesia is seen in middle-aged persons. The patients show a discrete and reversible loss of retrograde memory function characterized by repetitive questioning while engaging in a conversation.
It arises from abnormalities in the hippocampal area of the brain that can be visualized by certain imaging techniques such as MRI (magnetic resonance imaging). Although the clear cause of this type of amnesia is still not known, it is attributed to transient reduced cerebral blood flow, transient seizures, or migraine attacks.
The patients are unable to recall events that have occurred more than a few minutes ago. However, the memory of immediate events is preserved. The consciousness of the patients is also preserved. Memory and normal behavior often return after 4 to 6 hours.
It is a short-term amnesia caused by some drugs such as benzodiazepines. These drugs interfere with the memory formation process in the brain. The patient is unable to recall the events that occurred when he was under the influence of an amnestic drug. However, once the effect of the drug is over, the memory function is restored. No anterograde or retrograde amnesia is seen in the patient after recovery.
This practice is often done for medical purposes such as to make the patient forget about the surgery etc. However, amnestic drugs can also be used for immoral gains.
A drug called flunitrazepam causes short term memory loss for some hours. The patient is unable to recall anything that happens after taking this drug. It was abused by some people who started using it as a date rape drug. The drug was later discontinued by the FDA.
Amnesia in Korsakoff’s Syndrome
Korsakoff’s syndrome occurs in chronic alcoholics. The chronic use of alcohol causes thiamine deficiency (vitamin B1). Thiamine deficiency can also be seen in non-alcoholics due to malnutrition or poor diet.
This disease is characterized by the presence of both anterograde and retrograde amnesia. The short term memory of the patient is preserved. However, he may have difficulty in recalling past events. Immediate memory is also affected.
In addition to memory loss, some other dysfunctions of the brain are also seen in these diseases. Chronic alcohol consumption is the number one cause of this disease worldwide.
In this type of amnesia, patients forget certain parts of their memory. They may forget certain events that have occurred in their life, may forget the names of some individuals they have known earlier, may forget the location of certain places, etc.
This type of amnesia is often used for treatment purposes in psychiatry. Selective amnesia is induced by techniques such as electroconvulsive therapy (ECT) to make patients forget certain distressful events that have happened in the past.
This type of memory loss is seen in epilepsy patients. It is considered to be a response to some anti-epilepsy drugs. It is a rare condition that is found only in patients having temporal lobe epilepsy.
Causes of Amnesia
After studying the types of amnesia, you might have an idea of factors that are responsible for this memory disorder. In this section, we will go in some details and further explore these factors.
The causes of amnesia can be divided into three categories; physical trauma, psychological trauma or traumatic events, and nutrient deficiencies.
Any physical trauma to the head can result in amnesia. It involves any physical injury encountered by head due to car accidents, fall from some height, hit by some metal rod, head trauma during a fight, etc.
Head trauma that breaks skull and meninges can cause serious brain injury. This cerebral injury can result in both anterograde or retrograde amnesia.
Psychological trauma refers to the traumatic events that are distressful to a person. Different events can cause psychological trauma to a person depending on his behavior.
Some events are so distressful to mind that it prefers to forget them, instead of dealing with the stress caused.
Dissociative amnesia or psychogenic amnesia is an example of amnesia caused by psychological trauma.
The deficiency of a nutrient may also cause memory loss. For example, thiamine deficiency in Korsakoff’s syndrome causee anterograde and retrograde amnesia.
Electroconvulsive Therapy (ECT)
In this therapy, electrical seizures are given to the patients. These can result in anterograde and retrograde amnesia.
An example of amnesia caused by ECT is selective amnesia used for therapeutic purposes in psychiatry.
In this section, we will learn how amnesia affects different types of memories. We will study the clinical features seen in amnesia patients.
If the patients do not have severe anterograde amnesia, they can still learn new information, more importantly, the non-declarative one. However, patients suffering from severe anterograde amnesia fail to remember any new information in the future. They are unable to keep the memory of any event occurring in their life.
Effect of Amnesia on Declarative Information
Retrograde amnesia results in loss of declarative memory. However, the extent of memory loss depends on the severity of the diseases. Cases have been studied in which patients had a declarative memory loss but were still able to perform some declarative tasks. The part of the brain injured determines the extent of declarative memory loss.
Amnesia also has different effects on two types of declarative memory; semantic memory and episodic memory.
Effect on Semantic Memory
Semantic memory is the memory related to the facts. Retrograde amnesia results in loss of semantic memory due to damage to the medial temporal lobe.
Studies reveal that patients with anterograde amnesia can also larn some new semantic information after bearing some difficulty.
Effect on Episodic Memory
Episodic memory is related to the events of daily life. Patients with anterograde amnesia are unable to form any new episodic memory after the injury to the brain.
Effect of Amnesia on Non-Declarative Memory
Non-declarative memory is associated with skills and learning. It is acquired by practice and not recollection. Studies have shown that patients with anterograde or retrograde amnesia can form new non-declarative memories with some practice. Patients also show some improvements in learning capabilities with time.
Let us now talk about the treatment and management of amnesia patients.
Some types of amnesia recover without any therapeutic intervention. An example is a drug-induced amnesia. In this case, the patients gain normal memory after the effect of the drug is over.
However, in other cases, therapeutic intervention is necessary.
Different cognitive and occupational therapies have been designed for amnesia patients. These therapies help them improve their learning skills, and also help them to regain their lost abilities. Patients also try to retrieve some old memories and form new memory retrieval paths during these therapies.
Besides, some digital devices have been designed for amnesia patients to help them in daily life activities. These digital devices work as personal assistants to remind them of important times and events that are to come in the future. The digital assistance devices have surely increased the quality of life of amnesia patients.
Some medical treatments can also be given to cope with the underlying medical condition causing amnesia. This is true in the case of amnesia caused by Korsakoff’s syndrome.
Amnesia is a temporary or permanent memory loss that can occur due to several reasons. Amnesia is divided into different categories with each one having its own clinical manifestations.
Anterograde amnesia is a memory disorder in which the person cannot make new memories related to the events taking place after trauma. Damage to the hippocampus is seen as the most common cause of anterograde amnesia. It might also be due to stroke or physical trauma to the head.
Retrograde amnesia is a memory disorder in which a person can form new memories but is unable to recall the older ones. It also occurs due to damage to different areas of the brain.
Anterograde and retrograde amnesia can also co-exist in some patients.
Dissociative amnesia or psychogenic amnesia results in episodes of memory loss that are due to some psychological causes.
Post-traumatic amnesia is seen in patients after a trauma to the head causing both anterograde and retrograde amnesia.
In infantile amnesia, people are unable to recall events that have happened in their early childhood, most importantly in the first four years of life.
Transient global amnesia causes reversible memory loss in middle-aged patients.
Some drugs have the potential to cause short term amnesia called drug-induced amnesia. These drugs are commonly used in hospitals during surgical procedures.
Korsakoff’s syndrome and epilepsy can also cause amnesia in patients.
Selective amnesia is seen in some patients where they forget certain parts of the memory.
Causes of amnesia are divided into three categories;
- Physical trauma includes any physical damage to the head or brain as in automobile accidents, etc.
- Psychological trauma includes any emotional trauma or disturbance
- Nutritional deficiencies can result in trauma as a deficiency of Vitamin B12
Amnesia affects both declarative and non-declarative memories of a person.
Management of amnesia includes cognitive and occupational therapies, medical intervention, and the use of some digital devices to improve the quality of life.
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