Parkinson's disease can be difficult to diagnose because many conditions cause features of parkinsonism (tremor, muscle stiffness, slow movement, and unsteadiness). Certain medications (such as antipsychotic medications and some medications used to treat high blood pressure) can also cause symptoms similar to those of Parkinson's disease. Conditions that may cause symptoms similar to those of Parkinson's disease include:

• Essential tremor, which causes no symptoms other than tremor. Essential tremor often runs in families and can begin before age 25.
• Parkinson's-plus syndromes such as progressive supranuclear palsy (PSP) and multiple system atrophy.
• Alzheimer's disease. Confusion and memory loss similar to that seen in the early stages of Alzheimer's do not occur in people with Parkinson's until the late stages of the disease.
• Brain injury.
• Thyroid disorders.
• Liver disease.
• Brain infection (such as encephalitis).
• Brain tumor.
• Stroke.
• Carbon monoxide poisoning.
• Heavy metal (such as manganese) poisoning.
• Psychiatric disorders.