Injuries to the medial temporal lobe cause deficits in antegrade memory, and often also in retrograde memory.

In this article we will analyze what amnesic syndrome is and what its main causes are, including the most common and characteristic one: Wernicke-Korsakoff syndrome, which is due to malnutrition and alcohol abuse.

What is amnesia syndrome?

The concept “amnesic syndrome” has a relatively generic character . It is used to refer to any permanent alteration of memory that occurs as a result of damage to the brain, so it can include disorders due to very different causes; however, the Wernicke-Korsakoff syndrome is the most representative.

The term “amnesic syndrome” is usually used to refer to disorders that specifically affect memory, without other cognitive deficits being present (e.g. in intelligence or language). If they are, memory problems often have a far greater clinical significance than others.

Consequently, the causes of the amnesic syndrome tend to consist of focal lesions of the medial areas of the temporal lobe , specifically in the structures of the limbic system involved in the consolidation and recovery of memories, such as the hippocampus and the amygdala, which are fundamental in spatial and emotional memory respectively.

Types of amnesia: retrograde and antegrade

This type of damage causes the affected person to have severe problems learning new information; this phenomenon is known as anterograde amnesia, and is sometimes, but not necessarily, accompanied by retrograde amnesia, which consists of forgetting memories that were coded before the brain injury.

At the clinical level, patients with amnesia syndrome may show apparently normal functioning, since their working memory is not affected, nor is their ability to make new procedural learnings. However, deficits in declarative memory interfere to a great extent in the life of these people .

Retrograde amnesia is usually more evident for recent than distant memories, although its severity depends on the injury. As for anterograde amnesia, it is now known that it is not usually as extreme as was thought years ago, since the maintenance of other mnemonic functions makes it possible to compensate for declarative deficits.

Main causes of this disorder

Any factor that can damage the medial temporal regions of the brain has the potential to cause an amnesic syndrome. The most common causes are Wernicke-Korsakoff syndrome, which is associated with excessive alcohol consumption, herpes encephalitis and anoxia, as well as strokes, bleeding and tumors in the brain.

Retrograde amnesia is also one of the most characteristic side effects of electroconvulsive therapy, which is occasionally applied in cases of major depression and bipolar disorder, among other psychological disorders. However, many authors would not consider these effects a true amnesia syndrome because of their transience.

1. Wernicke-Korsakoff syndrome

Wernicke-Korsakoff syndrome is due to deficiencies of vitamin B1, which is also called thiamine . The most frequent cause is malnutrition, especially when it is related to alcohol abuse over a long period of time. This disease consists of two phases: Wernicke’s encephalopathy and Korsakoff’s syndrome.

During the acute phase, symptoms and signs such as attention problems, confusion, temporary, spatial and personal disorientation, apathy, neuropathic pain, ataxia (general lack of motor coordination), nystagmus (involuntary movements of the pupils) and ophthalmoplegia (paralysis of the eye muscles) appear.

The term “Korsakoff’s syndrome” refers to the chronic phase of this process, in which severe alterations appear in both antegrade and retrograde memory; in the latter case, forgetfulness usually affects coded memories up to two decades before Wernicke’s encephalopathy occurred.

2. Strokes

Strokes and bleeding in the brain are two very common causes of amnesia syndrome, particularly when they occur in the anterior communicating artery, the anterior cerebral artery, or the Heubner’s artery. The involuntary fabrication of memories, a characteristic sign of amnesia syndrome , is very common in these cases.

Another similar phenomenon that is also associated with amnesia is cerebral anoxia, which consists of the interruption of the oxygen supply to this organ as a result of cardiac arrest; this can cause the destruction of cells in the hippocampus and other regions of the temporal lobe, which explains the appearance of memory problems.

3. Tumors in the ventricles

The development of tumors in areas close to the limbic system often causes amnesia syndrome . A particularly noteworthy case is that of tumours in the third ventricle, which often damage the fornix, the main connection between the thalamus and the structures involved in memory.

4. Herpetic encephalitis

Herpes encephalitis is a disease of the central nervous system caused by infection with the herpes simplex virus . It can cause permanent symptoms such as amnesia, confusion, and seizures from lesions in the hippocampus, tonsil, uncus, and other areas of the limbic system.