By current estimates, 1.4 million Americans are affected with Lewy Body Dementia (LBD). Robin Williams, the famous actor and comedian, was apparently among the people affected. Though he died by suicide, William's autopsy revealed the presence of the underlying disease.
Lewy Body Dementia is the second most common form of dementia after Alzheimer's, but it's also frequently misdiagnosed for Parkinson's or Alzheimer's. All three diseases share a common pathological etiology: the abnormal aggregation of alpha-synuclein proteins in the brain, the Lewy bodies. Though present in healthy individuals, the Lewy bodies become toxic when they form clusters in the brain, interfering with normal activities. The process by which the Lewy bodies form is still not fully known, but mutations in the SCNA gene are suspected to contribute to pathology.
In Parkinson's, the Lewy bodies are found in the brain stem and the Substantia Nigra, both brain locations that are responsible for muscle movement. Thus Parkinson's patients are afflicted with the characteristic muscle stiffness, tremors, and loss of coordination. In LBD, the protein clumps are found throughout the cerebral cortex, including the substantia nigra, which can bring on Parkinson's-like symptoms. Because the Lewy bodies are distributed throughout the brain, patients can suffer from additional symptoms, including hallucinations, loss of spatial reasoning, sleep irregularity, and cognitive and memory impairment.
According to Williams' widow, he did not suffer from any of the advanced symptoms of LBD at the time of his death. However, the early stages of LBD likely worsened his cognitive state.