Features of Parkinson's disease include tremor, slow movement (bradykinesia), and rigid muscles (rigidity). These conditions may be treated differently than Parkinson's disease. Parkinson's-plus syndromes are a group of disorders characterized by the degeneration of nerve cells in different parts of the brain. They include progressive supranuclear palsy (PSP), corticobasal degeneration (CBD), and multiple system atrophy (MSA), among others. These syndromes usually respond poorly to levodopa or dopamine agonists. Secondary or symptomatic parkinsonism describes the syndrome of parkinsonism when it occurs as the result of an identifiable cause. For example, certain medicines, brain tumors, strokes, infections (such as encephalitis), and toxins (such as carbon monoxide or manganese) can cause secondary parkinsonism. Experts describe symptoms and stages of the disease differently.