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Alzheimer’s disease and dementia are two diseases that many Americans are all too familiar with, but there is another dementia that plagued the late comedian Robin Williams.

It might be “the most common disease you’ve never heard of,” said Dr. James Galvin, a professor of neurology and director of the Lewy Body Dementia Research Center of Excellence at the University of Miami’s Miller School of Medicine.

Williams had Lewy body dementia, which his family learned only after his death. It’s often misdiagnosed as Alzheimer’s or Parkinson’s disease due to its early similarity to those other neurodegenerative diseases.

That unfortunate misconception is one focus of “Robin’s Wish,” a documentary releasing September 1 about the final days of Williams, before he died by suicide in 2014.

Affecting about 1.4 million Americans, Lewy body dementias — which include Lewy body dementia and Parkinson’s disease dementia — are the second most common form of dementia after Alzheimer’s disease, according to the Lewy Body Dementia Association.

Dementia is a disorder of mental processes characterized by memory disorders, personality changes and impaired reasoning due to brain disease or injury.

Lewy body dementia is associated with an accumulation of a protein called alpha-synuclein that “builds up and deposits inside of cells and some classic areas in the brain,” said Dr. Ford Vox, medical director of the Disorders of Consciousness Program at the Shepherd Center in Atlanta and a contributor for CNN. Parkinson’s disease dementia, the other Lewy body dementia, starts as a movement disorder but progresses to include dementia and mood and behavioral changes.

When working properly, alpha-synuclein — which is typically present in the brain and in small amounts in the heart, muscle and other tissues — might play a role in regulating neurotransmitters. But when this protein aggregates and forms masses (called Lewy bodies) within the brain, the consequences are severe.

The most common symptoms of LBD include impaired thinking, fluctuations in attention, problems with movement, visual hallucinations, sleep disorders, behavioral and mood issues and changes in bodily functions such as the ability to control urinating.

Over time, people with LBD lose “layer upon layer of … that life that you’ve built,” said Angela Taylor, the senior director of research and advocacy at the Lewy Body Dementia Association.

That’s what happened to Williams, who had been diagnosed with Parkinson’s disease in 2013. It wasn’t until his autopsy that his widow, Susan Schneider Williams, learned he actually had LBD. The film highlights how the disease devastated Williams’ health.

“My husband had unknowingly been battling a deadly disease,” Schneider Williams said in the documentary’s trailer. “Nearly every region of his brain was under attack. He experienced himself disintegrating.”

CNN founder Ted Turner is also battling the disease, as he revealed in a 2018 interview.

Experiences of dysfunction and ambiguity are common for many patients and their families. Here is what the disease really is, why it’s difficult to identify and how it damages people’s lives.

An elusive and insidious disease

The first published cases of Lewy body dementia occurred in the mid-1960s, but it took two decades for the disorder to be recognized by medical researchers.

“In the 1980s, as the molecular understanding of Alzheimer’s improved, it became clear that a bunch of these people didn’t seem to fit that (diagnosis),” Galvin said.

For example, patients with LBD had visual hallucinations when most Alzheimer’s patients didn’t. They also had more parkinsonism — the signs and symptoms of Parkinson’s disease, which include slowness, stiffness, tremors and imbalance — than Alzheimer’s patients.

“It wasn’t until the mid-’90s when a large group of people (the Dementia with Lewy Bodies Consortium) got together and coined the phrase ‘dementia with Lewy bodies’ and started to write diagnostic criteria that could be applied,” Galvin added. “And that really changed the game because once you have criteria, then people can start to be better classified.”

Williams' autopsy revealed the widely distributed presence of Lewy bodies in his brain.

Aside from the association with Lewy bodies — those abnormal accumulations of the protein alpha-synuclein in the brain — the exact cause of LBD is unknown.

Potential, but rare, genetic factors include higher levels of mutations of certain enzymes and a family gene that might make someone predisposed to the disease.

Sorting through the symptoms

Some patients exhibit movement disorders that doctors first diagnose as Parkinson’s disease. If those patients later develop dementia, they would then be diagnosed with Parkinson’s disease dementia.

Others may begin with cognitive or memory disorders mistaken for Alzheimer’s disease. More specific changes in their cognitive function over time can lead to the diagnosis “dementia with Lewy bodies.”

Rarely will some individuals first show neuropsychiatric symptoms, which can include hallucinations, behavioral problems and difficulty with mental activities. When those appear simultaneously, that can prompt an initial diagnosis of LBD.

To specifically and accurately diagnose a person with LBD, doctors have to ask the right questions about his symptoms, Vox said.

Delusions for Alzheimer’s patients might occur late in the disease and be ill-formed, appearing as paranoia or mistrust — such as thinking a spouse is cheating. For LBD patients, delusions happen earlier and are well-formed misidentifications, such as looking at a loved one and thinking she has been replaced by an identical impostor.

“The more detailed the assessment,” Galvin said, “the easier it is to separate out the conditions.”

Because Lewy body proteins can’t be tested like Alzheimer’s proteins can, cases of LBD are often diagnosed during hospitalization for something else, Vox said. Or diagnosis can happen postmortem, when the family requests an autopsy for closure, to gain more context for any doubts or to donate the brain for research, Taylor said.

Transforming mental, cognitive and physical health

The symptoms may first hinder a person’s ability to work, Taylor said. Then they can disrupt their ability to drive; manage their affairs and health; be socially active; dress themselves; and shower. A person might also become unable to control involuntary behaviors, Galvin said, resulting in constipation, drooling, low blood pressure or the inability to control urine or bowel movements.

A person’s inability to visually perceive the spatial relationships of objects can lead to car accidents or injuries. People with LBD can experience anxiety, depression and REM sleep disorder — in which people lose the muscle paralysis that normally occurs during deep stages of sleep and physically (and sometimes violently) act out their dreams. Once a person is finally diagnosed, the life expectancy is about four to five years, Vox said.

“They’re losing the essence of who they are slowly over time,” Taylor said. “That’s a journey that is a very difficult one.”

Research to improve diagnoses and treatments is underway, but there are currently no treatments for Lewy body dementia specifically. Most patients are treated with medications for Alzheimer’s or Parkinson’s disease, since the symptoms of LBD are similar. However, treating the various symptoms of LBD with medications not fine-tuned for the condition can be a “real art” and quickly fill up a patient’s medicine cabinet, Vox said.

“You have to weigh costs-benefits of treating this versus that, or get double effects of this medication and a little bit of that as well,” he said.

Newfound challenges for patients and families

Lewy body dementia can be a harrowing experience for both patients and their families.

Getting a diagnosis can be a matter of months- to yearslong “doctor shopping,” Galvin said.

Executive dysfunction can lead to behaviors that family members initially perceive as bad judgments. Delusions can make them frustrated and fearful.

“As a caregiver, I think one of the challenges is recognizing that we cannot use the same skills and interpersonal dynamics that we came to rely on in our relationship with the person with LBD,” Taylor said.

“We have to develop new ones because you cannot reason with somebody who is having a hallucination or delusion. Sometimes you have to more step into their reality and empathize (and) learn a new way to offer assistance without them feeling like they’re being treated like a child.”

Living with Lewy body disease

It’s possible that the same healthy diet, sleep and exercise routines that have been found to mitigate symptoms of Alzheimer’s and Parkinson’s diseases might also help people with LBD.

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“So exercise is very important in Lewy body dementia, too,” Taylor said, “because it’s biologically related to Parkinson’s disease and shares a lot of the same symptoms.”

For patients and families in need of support and guidance, the Lewy Body Dementia Association is equipped with such resources.

“Nobody should face LBD alone,” Taylor sasid. “Not the person with LBD and not the family caregiver. This disease doesn’t make anything really easy in life. And they shouldn’t have to go through it without a guide and a support.”