Anterograde Amnesia - Causes

Causes

This disorder is usually acquired in one of few ways: One cause is benzodiazepine drugs, such as midazolam, flunitrazepam, lorazepam, temazepam, nitrazepam, triazolam, and nimetazepam, which are known to have powerful amnesic effects. This has also been recorded in non-benzodiazapine sedatives which act on the same set of receptors, such as Zopiclone (Also known by brand names Imovane and Zimovane). Another cause is a traumatic brain injury in which damage is usually done to the hippocampus or surrounding cortices. It can also be caused by shock or an emotional disorder. Illness, though much rarer, can also cause anterograde amnesia if it causes encephalitis, which is the inflammation of brain tissue. For example, herpes simplex virus type I, when left untreated for over 96 hours, may lead to permanent damage in hippocampal regions and a permanently reduced or eliminated ability to encode new explicit memory (also known as declarative memory), which consists of two main subdivisions: episodic memory and semantic memory. If the damage due to encephalitis is over a certain threshold, encoding new episodic and/or semantic memory becomes impossible for the patient, leading to anterograde amnesia. Patients suffering from anterograde amnesia may have either episodic, semantic, or both types of explicit memory impaired for events after the trauma that caused the amnesia. This suggests memory consolidation for different types of memory takes place in different regions of the brain. Despite this, current knowledge on human memory is still insufficient to “map out” the wiring of a human brain to discover which parts of which lobe are responsible for the various episodic and semantic knowledge within a person’s memory.

Amnesia is seen in patients who, for the reason of preventing another more serious disorder, have parts of their brain known to be involved in memory circuits removed, the most notable of which is known as the medial temporal lobe (MTL) memory system, described below. Patients with seizures originating in the MTL may have either side or both structures removed (there is one structure per hemisphere). In addition, patients with tumors who undergo surgery will often sustain damage to these structures, as is described in a case below. Damage to any part of this system, including the hippocampus and surrounding cortices, results in amnesic syndromes. This is why people who suffer from strokes have a chance of developing cognitive deficits that result in anterograde amnesia, since strokes can involve the temporal lobe and the temporal cortex, and the temporal cortex houses the hippocampus.