Alzheimer’s and Parkinson’s Disease: A Breakdown of the Differences and Similarities

Alzheimer’s and Parkinson’s disease are both neurological diseases that cause progressive damage to the brain, resulting in a decline in neurocognitive function. While Parkinson’s disease is known for affecting movement and function and Alzheimer’s disease is known for affecting cognition, they are a bit more complex than that. 

Below, we provide an overview of Alzheimer’s and Parkinson’s disease with a comparison of key similarities and differences.

Dementia

Dementia is an umbrella term used to describe a group of symptoms associated with a decline in memory, reasoning, thinking, or other cognitive functions. These symptoms may include a decline in memory, poor reasoning and judgment skills, changes in thinking or problem-solving abilities, and changes in language and communication abilities.

Many of the following conditions, including Alzheimer’s and Parkinson’s disease, can cause dementia:

• Alzheimer’s disease
• Parkinson’s disease
• Lewy Body dementia
• Vascular dementia
• Frontotemporal dementia
• Mixed dementia
• Huntington’s disease
• Creutzfeldt-Jakob disease
• Posterior cortical atrophy

Alzheimer’s Disease

Alzheimer’s disease is the most common cause of dementia, accounting for approximately 60% to 80% of all dementia cases. Most commonly, Alzheimer’s disease is characterized by beta-amyloid plaques and neurofibrillary tangles resulting from abnormal tau hyperphosphorylation.

These plaques, tangles, and the loss of neuronal connections cause progressive damage to the brain, starting in parts of the brain responsible for memory (entorhinal cortex and hippocampus). As the disease progresses, this damage continues to spread to other regions of the brain, such as parts of the cerebral cortex responsible for reasoning, language, and social behavior. 

Parkinson’s Disease

Parkinson’s disease is a progressive neurodegenerative disease that is most commonly known for affecting function and movement, though it also affects cognition, particularly as the disease progresses. Parkinson’s disease primarily impacts dopaminergic, or dopamine-producing, neurons in a specific area of the brain known as the substantia nigra. Dopamine is a key neurotransmitter that transmits signals between neurons and plays a crucial role in movement and motor control.

The lack of dopamine makes it challenging for the brain to coordinate muscle movements and can also contribute to mood and cognitive issues later in the disease course. Patients with Parkinson’s disease also lose nerve endings that produce norepinephrine, a chemical messenger of the sympathetic nervous system responsible for controlling a wide range of functions in the body, such as blood pressure and heart rate.

Alzheimer’s and Parkinson’s Disease: Similarities and Differences 

Similar cognitive and functional impairments are observable in both diseases. However, different proportions, varying manifestations along the disease continuums, and different rates of occurrence set the two apart.

Generally speaking, Parkinson’s patients will show early signs of functional impairment and in later stages may experience cognitive impairment, such as memory and thinking issues, if they develop dementia. According to the National Parkinson’s Foundation, recent studies that followed Parkinson’s patients over the course of the disease estimate that 50% to 80% of people with Parkinson’s disease eventually develop dementia. Conversely, earlier symptoms of Alzheimer’s patients may be more biased towards cognitive impairment, with functional impairments observed in later stages of the disease.