Robin Williams in June 2007 posing to promote his film “License To Wed” in Santa Monica, Calif. (Reed Saxon/AP)

It often starts with sleep troubles — vivid dreams that the sufferer will violently act out while lying in bed.

Then come the hallucinations, delusions, difficulty recognizing familiar faces. Sometimes there’s dizziness and balance problems. Just walking across a room can be a challenge.

Slowly, then swiftly, the patient will start to lose his mind. His attention span shortens, his problem-solving skills dissolve; paranoia, depression and anxiety can rack an already fragile brain.

This is Lewy body dementia (LBD), a debilitating, often deadly and yet little-understood illness that is thought to afflict more than 1 million Americans. Among them was beloved actor and comedian Robin Williams, according to his widow, and it was this disease that drove him to commit suicide last year.

[Robin Williams’s widow talks about the actor’s final days: ‘Lewy body dementia killed Robin’]

Though it was widely speculated that Williams had killed himself due to depression — he’d struggled with mental health problems, drug abuse and alcoholism in the past — an autopsy released last fall found abnormal clumps of protein in the actor’s brain, a tell-tale sign of LBD (also called dementia with Lewy bodies).

Though Williams had been diagnosed with Parkinson’s disease in the period before his death, the diagnosis didn’t explain his full, frightening range of symptoms: the crippling anxiety attacks, the strange delusions, the feeling that his mind was slipping from his grasp. Like so many LBD patients, Williams knew something was wrong without knowing what it was, Susan Williams said in an interview with “Good Morning America” Tuesday. That’s part of what makes the disease so difficult to bear.

“Lewy body dementia killed Robin, it’s what took his life,” Susan Williams insisted.

The wide-ranging and devastating symptoms of LBD are caused by the protein clumps, called Lewy bodies, that were found in Williams’ brain. Though neurologists have known about these bodies for more than a century, much about them remains a mystery. They’re made mostly of a protein called alpha-synuclein, which accumulates at the ends of neurons (brain cells) where they connect with synapses (the junctions that transmit signals between neurons). Alpha-synuclein’s role in the brain is murky — perhaps something to do with releasing chemicals in the brain? — but under normal circumstances, its presence seems to be a good thing.

[Caring for a mother with Lewy body dementia]

The protein becomes problematic when it starts building up in clumps: Lewy bodies. No one knows what causes this — most people with LBD have no family history of the disease, according to the Alzheimer’s Association — and there’s no way of treating it.

According to the National Institutes of Health, LBD is the third most common cause of dementia, after Alzheimer’s and vascular disease, and is responsible for 20 percent of dementia cases worldwide. But it’s rarely doctors’ initial diagnosis. Typically, doctors aren’t even aware that Lewy bodies are present until after a patient has died and they can examine his or her brain. That’s part of why LBD is so often diagnosed as Parkinson’s, Alzheimer’s or some other form of dementia.

“It’s the most common disease you’ve never heard of,” Dr. James Galvin, a neurology and psychiatry professor at Florida Atlantic University told US News and World Report in September.

While doctors struggle with diagnosis, the Lewy bodies are slowly destroying the patient’s brain cells. As an area of the brain shuts down, a corresponding symptom emerges, according to the NIH: loss of function in the cerebral cortex leads to problems with information processing, thought and language; a breakdown in the limbic system messes with emotions; the degradation of the hippocampus inhibits formation of memories.

It’s an “endless parade of symptoms,” Susan Williams said, “and not all of them would raise their head at once. It was like playing whack-a-mole.”

Lewy bodies are also responsible for Parkinson’s disease, the progressive motor disorder that causes tremors and coordination difficulties. And although people with LBD suffer a whole array of cognitive problems, including hallucinations and rapidly-developing dementia, that don’t typically appear in Parkinson’s patients, many LBD patients are initially diagnosed with Parkinson’s. (It is possible for patients to suffer both illnesses at the same time; LBD can also co-exist with Alzheimer’s.)

Misdiagnoses make LBD more difficult to endure. For one thing, the medications often prescribed for Alzheimer’s and Parkinson’s can be ineffective for LBD symptoms, or even exacerbate them. And then there’s the uncertainty of it all — knowing that something more is wrong but not being able to name it.

“We’re chasing and there’s no answers,” Susan Williams told GMA, recalling how difficult it was to understand her husband’s initial symptoms.

Williams is one of several celebrities to be diagnosed with the little-known disease. It was the cause of death listed in obituaries for legendary DJ Casey Kasem, who died last summer. Like Williams, Kasem was initially diagnosed with Parkinson’s, but the diagnosis changed as he began to deteriorate mentally as well as physically.

Stan Mikita, a star Chicago Blackhawks player of the 1960s and ’70s, was also diagnosed with the LBD in January. Now he lives in a Chicago-area facility, unable to remember his hockey career or much else about his life.

“His mind is completely gone,” his wife told the Chicago Tribune. “I don’t like to use that term, but there’s no other way to describe it.”

Mikita’s daughter, Jane, told the Tribune that she wanted to talk about her father’s illness because “we’re not the only ones going through this. And if we’re able to give a description of what he’s going through and there is someone else out there who needs help, that’s how my dad was. He always wanted to help someone else.”

Patients with LBD usually die within five to seven years after the initial onset of symptoms, though they can live for as many as two decades or deteriorate in as few as two, according to the NIH.

In her GMA interview, Susan Williams said that her husband was supposed to undergo neurocognitive testing in late August. A week before his appointment, on Aug. 11, 2014, Robin Williams hanged himself with a belt.

“Was this Robin’s way of taking control back?” ABC’s Amy Robach asked.

“In my opinion, oh, yeah,” Williams responded. “I think he was just saying, ‘no.’ And I don’t blame him one bit.”