Alzheimer's disease is not the only type of dementia, two particular forms are dementia with Lewy bodies and Parkinson's disease dementia.
Lewy Body Dementia:
Lewy Body Dementia (also called Dementia with Lewy bodies) is a form of dementia that shares characteristics with both Alzheimer's and Parkinson's disease. Lewy bodies are abnormal aggregates of protein that develop inside nerve cells in Parkinson's disease (PD) and Alzheimer's disease (AD) and some other disorders.
Lewy Body Dementia can start differently in people.
Sometimes those with LBD initially have a movement disorder that looks like Parkinson's but later they also develop dementia symptoms. Others have a memory disorder that looks like Alzheimer's but they later develop hallucinations and other behavior problems. Over time most people with LBD develop a spectrum of problems that include great variations in attention and alertness from day to day, recurrent visual hallucinations, shuffling gait, tremors, and blank expression, along with various sleep disorders.
Lewy body dementia is not a rare disease.
It affects an estimated 1.3 million individuals and their families in the United States alone. Because LBD symptoms may closely resemble other more commonly known diseases like Alzheimer's and Parkinson's, it is currently widely under-diagnosed. Nearly 80% of people with LBD received a diagnosis for a different cognitive, movement or psychiatric disorder before ultimately learning they had Lewy body dementia.
Parkinson's Disease Dementia:
Dementia is a less common feature of Parkinson's disease.
Approximately 20% of people with Parkinson's disease will develop Parkinson's Disease Dementia (PDD). Parkinson's patients who experience hallucinations and more severe motor control problems are at risk for dementia. For those patients with Parkinson's disease who go on to develop dementia, there is usually at least a 10- to 15-year lag time between their Parkinson's diagnosis and the onset of dementia.
Indications that dementia may be caused by something other than Parkinson's disease include agitation, delusions (strongly held false beliefs), language difficulties, and early onset of memory symptoms.
Alzheimer's disease and Parkinson's disease are both common in the elderly, especially in those over 85. Therefore, patients with Parkinson's who develop dementia may develop Alzheimer's dementia as well.
The main difference between Parkinson's Disease dementia and Lewy Body dementia is a bit arbitrary.
If motor symptoms come first, by at least a year, and dementia develops later, the convention is to call it Parkinson's Disease dementia. If the motor symptoms follow the dementia symptoms, the convention is to call it Lewy Body dementia (or dementia with Lewy bodies").