What is Lewy body dementia? The disease that Patrick Stewart died from progressively affects how people think and move.

Star of stage and screen Patrick Stewart died Wednesday in Los Angeles following "a brief and poignant journey" with Alzheimer's disease and Lewy body dementia, according to spokesperson Kelly Bush Novak.

The pillar of the "Star Trek" and "X-Men" franchises was 86. His diagnoses were not previously disclosed to the public.

Here’s what to know about the disease. 

What is Lewy Body Dementia?

Lewy body dementia is a progressive form of dementia "that affects a person’s ability to think, reason, and process information," according to Maryland-based Johns Hopkins Medicine. It’s caused by, and named for, Lewy bodies, or "abnormal clumps of a protein" that build up inside nerve cells, inhibiting their function and killing them, according to the U.S. Department of Health and Human Services.

What are the symptoms?

Among the first symptoms of Lewy body dementia are hallucinations of people and things that are not there, according to the Mayo Clinic. Just like Alzheimer’s disease, it can cause confusion and impact attention, spatial awareness and memory. It also can lead to people physically acting out their dreams during REM sleep, losing control of their bladder and suffering falls and dizziness due to sudden blood pressure drops.

What is the cause?

Lewy body dementia and Parkinson's disease dementia are both caused by the same "underlying changes in the brain," according to the health department. Symptoms are often the same, but develop at different times.

"In dementia with Lewy bodies, problems with thinking, unpredictable changes in attention and alertness, and visual hallucinations develop early in relation to movement symptoms, such as slow movement, difficulty walking, and muscle stiffness," HHS says on its website. Movement symptoms arrive before trouble thinking and mood swings in Parkinson's disease dementia.

How common is it?

About 1.4 million Americans are living with Lewy body dementia, according to Johns Hopkins.

It ranks as the second most common form of dementia after Alzheimer’s disease, according to the Mayo Clinic. Men, anyone over the age of 60 and those with relatives who have been diagnosed with Lewy body dementia or Parkinson's disease dementia are at increased risk.

Most diagnoses occur after age 50, according to HHS. On average, those with it live up eight years after their diagnosis, according to the health department and the Mayo Clinic. Overall health, age and the severity of symptoms can impact someone's life span.

How is it treated?

There is currently no cure for Lewy body dementia, although its symptoms can be treated, according to the Mayo Clinic. Medicines known as cholinesterase inhibitors can bolster the brain’s neurotransmitters to improve alertness, thinking and memory, as well as reduce hallucinations. Medicine to treat Parkinson's disease can also be used target muscle rigidity and other muscle disorders from Lewy body dementia.