Patrick Stewart died after a 'brief and poignant journey' with Lewy body dementia and Alzheimer's. What to know about 2 of the most common causes of memory loss.

Patrick Stewart has died after "a brief and poignant journey" with Lewy body dementia and Alzheimer's disease, according to an Oct. 29 post on the Star Trek and Avengers star's X account . Stewart was 86.

Stewart's diagnosis was never publicly announced prior to his death, but the post on his X account said that his wife, Sunny, had been caring for him. In his final days, Stewart was "held in the warm embrace of family and friends, surrounded by live music, prayer, tears, laughter, and the kind of profound emotion that he could inspire after a lifetime of devotion to us all." The actor was described as "peaceful" and at ease in his final moments.

While Alzheimer's disease is the most common cause of dementia — accounting for an estimated 60% to 80% of all cases in the U.S . — Lewy bodies are the cause of only 4% to 16% of dementias. But they often go hand-in-hand. Lewy body dementia's rates are also thought to be underestimated. Despite its low profile, several celebrities have had the disease, including comedian Robin Williams , CNN founder Ted Turner, Great White frontman Jack Russell and Golden Girls star Estelle Getty .

Lewy body dementia is one of the most common forms of dementia. It typically begins in people aged 50 or older, according to the National Institute on Aging (NIA).

Lewy body dementia is linked to abnormal deposits of protein in the brain known as Lewy bodies, the NIA explains. "The presence of Lewy bodies disrupts the normal functioning of brain cells and impairs communication between them," Dr. Verna Porter , neurologist and director of the Dementia, Alzheimer's Disease and Neurocognitive Disorders at Pacific Neuroscience Institute in Santa Monica, Calif., tells Yahoo. That can lead to problems with thinking, movement, behavior and mood, she says. Lewy body dementia is a progressive disease with a survival rate of five to eight years for most people. In the early stages, people with Lewy body dementia can function fairly normally, but as the disease progresses, they need more assistance and eventually depend entirely on others for care, according to the NIA.

There are two types of Lewy body dementia: dementia with Lewy bodies and Parkinson's disease dementia.

How does it differ from Alzheimer's disease?

The two diseases also have different underlying causes, according to the Lewy Body Dementia Resource Center. Rather than Lewy body deposits, Alzheimer's disease is thought to be caused by a combination of abnormal plaque buildup on the outside of brain cells and tangles of a protein that form inside these cells. Both can damage and kill brain cells.

"Lewy body dementia is different from other forms of dementia based on the type of changes in memory and behavior that can be seen," Dr. Amit Sachdev, medical director in the Department of Neurology at Michigan State University, tells Yahoo Life. "This means that the family will see changes that are more obvious earlier in the disease than would be seen in Alzheimer's, vascular or other forms of dementia."

In patients with Alzheimer's, memory loss is often the first sign. They may forget recent conversations or family members' names, or they may repeat themselves. Those with Lewy body dementia may have better recall, but struggle to keep their thoughts organized. They're also more prone to hallucinations.

People are typically diagnosed with Alzheimer's later in life — around age 65 — compared to Lewy body dementia. But their life expectancy, according to the Mayo Clinic , is more variable, ranging from three to 11 years. Some people live for as long as 20 years after an Alzheimer's diagnosis.

Although the two diseases can look different, it's not uncommon for someone to have both. In fact, about half of patients with Alzheimer's disease have additional dementia-associated brain changes, and Lewy bodies are the second most common type, according to the Alzheimer's Association.

Lewy body dementia can cause various symptoms that may impact a person's cognition, movement, sleep and behavior. According to the NIA, those can include:

Changes in concentration, attention and alertness

"What sets Lewy body disease apart from other causes of dementia is the prevalence of hallucinations and delusions, as well as a sleep disorder called REM sleep behavior disorder," Porter says. "The hallucinations of Lewy body disease are typically visual. They may begin with a sense of a presence or shadow, typically in the periphery."

People with Lewy body disease may also develop delusions, Porter says. "Examples can include paranoid delusions such as a spouse's infidelity, people stealing from them and delusions of reference, meaning a strong feeling that a person or a place is not what they seem," she says.

There is no one test to diagnose Lewy body dementia. However, a specialist like a neurologist will usually conduct physical and neurological exams; do blood tests and brain imaging studies to rule out other conditions that could cause similar symptoms; and administer neuropsychological tests to evaluate memory and other cognitive functions, according to the U.S. National Library of Medicine . "An MRI can help in excluding other diseases," Sachdev says.

Because Parkinson's disease and Alzheimer's disease can cause similar symptoms, Lewy body dementia can be hard to diagnose, Sachdev says. Dr. Douglas Scharre, the director of the Center for Cognitive and Memory Disorders at Ohio State University's Wexner Medical Center, agrees, telling Yahoo Life: "Lewy body dementia is often missed, because a lot of physicians have never heard of it or don't even know what the symptoms are. Usually people have Parkinsonian symptoms — they're stiff and have balance issues."

It can take months or years of experiencing symptoms before getting a Lewy body dementia diagnosis, and it's frequently mistaken for psychiatric disorders such as depression — or missed altogether.

Robin Williams, in fact, never received a diagnosis. An autopsy revealed the hallmarks of the disease in his brain. But after his death, his wife, Susan Schneider Williams, told Today that both she and her husband knew something was wrong. She described Williams telling her he wanted to "reboot his brain."

Like Alzheimer's, there is no cure for Lewy body dementia. "Instead, we need to try to manage the symptoms," Sachdev says. Treatment may involve physical therapy, occupational therapy, mental health counseling and medications to help control cognitive, movement and psychiatric issues, according to the NIA.

"We're now looking at new treatments to help — those are in clinical trials," Scharre says. "We're making progress. We're having medications that may help reduce some of those toxic proteins as opposed to just helping with symptoms."

In lieu of flowers, Stewart's posthumous X post asked that memorial contributions be made to the Johns Hopkins Alzheimer's Disease Research Center and the Alzheimer's Foundation of America Caregiver Resources.