Lewy body dementia, also known as LBD, is the second most common type of dementia after Alzheimer's disease. Protein deposits called Lewy bodies develop in nerve cells in the brain. The protein deposits affect brain regions involved in thinking, memory and movement. There are two forms of LBD that cause cognitive symptoms - dementia with Lewy bodies and Parkinson's disease dementia. Lewy body dementia causes a decline in mental abilities that gradually gets worse over time. People with LBD might see things that aren't there, known as visual hallucinations. They also may have changes in alertness and attention. People with Lewy body dementia commonly have symptoms of Parkinson's disease. These symptoms might include rigid muscles, slow movement, trouble walking and tremors.