Hypergraphia is an overwhelming urge to write. It is not itself a disorder, but can be associated with temporal lobe changes in epilepsy and mania.
…Several different regions of the brain govern the act of writing. The physical movement of the hand is controlled by the cerebral cortex which comprises part of the outer layer of the brain. The drive to write, on the other hand, is controlled by the limbic system, a ring-shaped cluster of cells deeply buried in the cortex which governs emotion, affiliated instincts and inspiration and is said to regulate the human being’s need for communication. Words and ideas are cognized and understood by the temporal lobes behind the ears, and these temporal lobes are connected to the limbic system. Ideas are organized and edited in the frontal lobe of the brain. Temporal lobe lesions cause temporal lobe epilepsy, however it is also known to run in families. Hypergraphia is not a frequent manifestation of temporal lobe epilepsy.
As of current, hypergraphia is understood to be triggered by changes in brainwave activity in the temporal lobe.