Parkinson’s disease is a neurological disease that causes symptoms that affect the body’s movement (motor symptoms), including tremors, rigid muscles and an overall slowdown in movement. It primarily affects people who are middle-aged and older. Parkinson’s disease affects approximately 1 million people in the United States and 6 million people worldwide.
While the underlying cause of Parkinson’s disease in most patients is unknown, we do know that the disease occurs when there is a loss of neurons in the part of the brain that produces dopamine. These neurons rely on an enzyme known as aromatic L-amino acid decarboxylase (AADC) to make dopamine from its precursor, levodopa. As Parkinson’s disease progresses and more neurons are lost, AADC levels drop, and this limits the brain’s capacity to make dopamine.
Declining levels of dopamine lead to the motor symptoms that are most often associated with Parkinson’s disease, including tremors, slow movement or loss of movement, rigidity and postural instability, which is when individuals have difficulty keeping their bodies balanced (often resulting in a them leaning forward, backwards or to the side). Motor symptoms among people with advanced Parkinson’s disease include falling, freezing (the temporary inability to move), and difficulty with speech and swallowing, with patients often requiring the daily assistance of a caregiver.
There are currently no therapies that effectively slow or reverse the progression of Parkinson’s disease. Levodopa, which was discovered more than 40 years ago, is usually effective in people in the early stages of the Parkinson’s disease, but they often become less responsive to it as the disease progresses and require more levodopa to manage their symptoms. Up to 15 percent of people with Parkinson’s disease (or approximately 100,000 patients in the United States) have motor symptoms that are not well-controlled with levodopa.