As people live longer and the world population grows older, late-onset disorders such as Parkinson’s disease and Alzheimer’s disease will affect more and more individuals. Both of these diseases are neurodegenerative and typically begin late in life, so many people assume that they are related or linked in some way. Let’s take a look at how much truth there is to that idea.
First of all, the two diseases are not related but they do share some similarities. Both Alzheimer’s and Parkinson’s disease have an onset that is late in life, usually after the age of 50. Both diseases are neurodegenerative, meaning that brain cells (neurons) become damaged and die during the course of the disease. They are also both progressive, so they get worse over time. In the late stages of both diseases, the neurodegeneration can ultimately lead to dementia – a severe impairment in memory, judgment, orientation, and executive functioning. Approximately two out of every three dementia cases are caused by Alzheimer’s disease, making it by far the most common cause. Meanwhile, dementia due to Parkinson’s accounts for a much smaller portion of all dementia cases.
Despite all of these similarities, Alzheimer’s and Parkinson’s disease are completely separate illnesses with different mechanisms, symptoms, and treatments. Parkinson’s disease is primarily a movement disorder that can eventually result in memory problems and dementia in about 50% of patients. Many individuals with Parkinson’s will never have memory problems during the course of their illness. On the other hand, Alzheimer’s is primarily a memory disorder that rarely includes any type of movement impairments. While both diseases are progressive and neurodegenerative, their disparate symptoms come from differences in etiology. Parkinson’s disease results from the loss of dopamine-producing neurons in an area of the brain called the substantia nigra. Dopamine is needed in that part of the brain to control movement and coordination, and it is estimated that it takes a 60-80% loss of these dopaminergic neurons before symptoms become outwardly apparent. These symptoms include a resting tremor (shaking at rest), muscle rigidity, slowed movements, and impaired coordination. Since the primary cause for Parkinsonian symptoms is a loss of dopaminergic neurons, approved drug treatments work to increase the amount of dopamine available within the brain. Over time, as the disease progresses, the benefits of the drugs often diminish or become less consistent.