Parkinson's disease is a progressive neurological disorder that results from degeneration of neurons in a region of the brain that controls movement. This degeneration creates a shortage of the brain signalling chemical (neurotransmitter) known as dopamine, causing the movement impairments that characterise the disease. Parkinson's disease was first formally described in "An Essay on the Shaking Palsy", published in 1817 by a London physician named James Parkinson, but it has probably existed for many thousands of years. There is no cure for Parkinson's disease. Many patients are only mildly affected and need no treatment for several years after the initial diagnosis. When symptoms grow severe, doctors usually prescribe levodopa (L-dopa), which helps replace the brain's dopamine. Sometimes doctors prescribe other drugs that affect dopamine levels in the brain. In patients who are very severely affected, a kind of brain surgery known as pallidotomy has reportedly been effective in reducing symptoms.