When people of think of amnesia, they usually first think of the mysterious stranger in a soap opera, who shows up in town after an accident or traumatic experience, having forgotten their own identity, and the efforts that ensue to uncover the missing information.
Actually, in medical practice this type of “soap opera” amnesia is psychogenic, a so-called psychogenic fugue state, very different from the amnesia seen with organic neurologic disorders like head trauma or stroke.
The character Dory in the 2003 Pixar Movie Finding Nemo is a better representation of organic neurologic amnesia – she has anteriorgrade amnesia and cannot retain new information, but remembers her name and other details of her own identity.
It is thought that new memories are formed by a structure of the brain known as the Papez ciruit or limbic system, which includes the hippocampus, (subiculum), fornix, mammillary bodies, anterior thalamic nucleus, cingulum, and entorhinal cortex.
Any lesion or process which interrupts this circuit will prevent the formation of new memories, leading to anteriograde amnesia.
Temporary Anteriograde amnesia can be caused by:
zzzzhead trauma – post-traumatic amnesia or PTA,
zzzzand from Transient Global Amnesia (TGA).
TGA is a sudden onset of temporary anteriograde amnesia usually lasting lasting 4-12 hours. It is often triggered by an emotional event or sexual intercourse. During the episode, the affected patient is alert and lucid, cognizant of their own identity, but appears perplexed and may ask the same questions repeatedly. The exact cause is unknown. The recurrence rate is low.
More permanent anteriograde amnesia can be caused by:
zzzzbrain damage resulting from herpes encephalitis,
zzzzor from hemorrhage into the mamillary bodies – “Korsakoff’s psychosis”.
Korsakoff’s psychosis is caused by thiamine deficiency, usually related to chronic alcoholism. Affected patients have permanent anteriograde amnesia, and so they live in the past, and confabulate (make up details) to fill in the gaps in their memory.