For years, things just didn’t seem right. Doctor visits back to back. Her pancreas stopped working. Her gallbladder had to be removed. Her legs became rigid and painful. She dropped 20 pounds. A misdiagnosis, and then a correction to Parkinson’s disease. For a few years, she was better. But then her mind started playing up. Papers she had written no longer made sense. Colleagues whispered behind her back, saying she was a shadow of the person she once was. It was devastating, but in her heart, she says, she knew they were right.

Finally, the correct diagnosis: Kathleen Anduze, then 51, was told she had dementia with Lewy bodies, a neurodegenerative disease for which there is no cure, and a lifespan of five to eight years post-diagnosis. Symptoms of the disease leave patients drifting in and out of awareness, as their minds become less and less dependable.

As the disease progresses, Kathleen will bear witness to the gradual loss of her own mind.

Kathleen Anduze, 55, keeps medication and a reminder note in a bathroom cabinet in her Pleasant Valley, New York, home. Kathleen was diagnosed with Lewy body disease, a form of progressive, degenerative dementia, at age 51. Photograph: Ali Smith for The Guardian

Today, the 55-year-old former professional horseback rider and nurse who once directed medical centers, wrote policy papers and spoke at conferences around the country rarely leaves the house. She relies on her family for practical and mental support.

Visual hallucinations, delusions and nightmares plague her life. Her loved ones offer all the help they can as they watch their once dynamic matriarch’s condition worsen.

“My executive functioning has gone,” Kathleen tells me during my visit with her and her family, at their home in upstate New York last week. “I cannot mentally think ahead. I no longer understand the order of anything. Which room shall I go to? Shall I put on my clothes first? Where do I start? It’s overwhelming. Sometimes I cannot move.”

You may never have heard of it until Robin Williams’ wife Susan Williams came out and spoke about the disease in an interview with ABC last week, but dementia with Lewy bodies is surprisingly widespread.

According to the Lewy Body Dementia Association, that based their “conservative” estimate on numbers gathered from autopsy studies of individuals with dementia, an extraordinary 1.4 million Americans have the disease either all by itself, or in combination with something else. (This compares to five million Americans currently living with Alzheimer’s disease, according to the the Alzheimer’s Association, who also say that one in three seniors die with a form of dementia).

Kathleen, pictured here with her dog Tessie, reflects on her battle with Lewy body disease in her bedroom.

“It is the most common disease no one has ever heard of,” Michael Koehler, the president of LBDA jokes a little tragically. Depending on whom you speak to, it is either the second or the third most common form of dementia. Part of the mission of the LBDA, which has a meager staff of nine, is raising awareness around the disease, both among the general population and among medical practitioners. The disease is still very often not caught or misdiagnosed – generally as either being Alzheimer’s or Parkinson’s.

But dementia with Lewy bodies is a beast of its own, even if each patient presents its own version of symptoms – generally a combination of “Parkinsonism” (rigidity of limbs, tremors, difficulty in moving) in the early stages, and cognitive issues as the disease takes hold. Typical symptoms include fluctuations in alertness, visual hallucinations and paranoia, but there is no one rule, nor one “typical” case.

•••

What does a good day look like? I ask Kathleen.

“There used to be good days, now there are just good moments,” she responds. “A good day is when I manage to smile. Or when I feel like I am my former self for just a moment.”

How about a bad day?

Kathleen loses her composure and breaks down in tears, but she pushes through. Through ongoing tears, distressing words come out.

“My bad days are when I understand why people support assisted suicide. When I question why I am putting my family through this.”

Kathleen Anduze’s son Danny listens to her talk about her battle with Lewy body disease at their kitchen table.

I have caught Kathleen during her few hours of lucidity – which I was told before the train ride to visit her and her family in a beautiful, quasi-rural part of New York state was no guarantee. On Thursday and Friday, she couldn’t move from bed, I am told.

But today is Sunday, and Kathleen seems to have put every inch of willpower in her body towards speaking to me, which she does for hours, from the heart, taking few breaks and only interrupting herself to offer me food and drink.

When we meet, Kathleen is immaculately turned out: perfect blonde with purple-streaked hair (the dementia awareness color), a hint of makeup and matching top and trousers. For better or worse, I am told later, Kathleen has always taken great pride in her appearance.

Sitting around her is her family. Dave, her husband of two decades, an elevator installer, Katie, her 36-year-old daughter from her first marriage who is a Spanish teacher, and Danny, her 17-year-old son who returned from his shift at the local deli late morning and changed into a Ralph Lauren shirt.

“I want to try and explain something to you,” Kathleen says. She turns and looks at her husband, Dave. Her voice is already breaking from what she is about to explain. “I have lost my imagination,” she says.

“Do you understand what that means? When you lose your imagination, you lose everything. You lose your fantasies, you lose your intimate relations. I can no longer be with my husband.”

Her voice breaks.

Later, her husband Dave brings down photos of their younger days together. One photo, of their wedding day, shows them both on horseback. She is in white with a white cowboy hat, and he is in black, with a black hat. They are a figure of somewhat rebellious, life-embracing joy and vitality.

Kathleen’s husband, Dave, holds a framed photo from their wedding day in 1997.

But today, Kathleen lives a reclusive life. She panics in restaurants. She will unexpectedly break out in fits of screaming and crying. She “loses it”. Sometimes, she thinks someone wants to kill her. Episodes happen at home too. She recently thought the porch was on fire. It was not.

With that comes guilt, she says, for what she is imposing on her family. She holds a sense of apathy she cannot kick, she says, all of which contributes to depression.

“The depression is part of this illness to a very large extent. I have so much guilt,” Kathleen, who Skypes with her psychiatrist twice a month, says.

Kathleen and her husband Dave.

There are silver linings, if you can call them that. Since her correct diagnosis, she is getting medication to address some of her symptoms. She wears a patch, Exelon, that helps relieve the cognitive difficulties. If only Robin Williams had realized what he had, she offers, the patch could have helped him too.

As she speaks, her family has remained seated around her and hardly left her side. For more than four hours, there isn’t a hint of loss of patience. Danny, her 17-year-old, gets up once or twice to go to the bathroom, but rushes back. He hopes to go to Cornell next year to study astronomy and physics. Last year, when his mother accompanied him to see his high school guidance counsellor, she had an episode.

“I came undone,” she says, explaining she thought the guidance counsellor wanted to choke and kill her. She hid in the school’s bathrooms and called her daughter Katie in a panic.

After that, there were no more stressful outings. “There is no need to put her through that,” Dave explains.

His job – a union job – is the reason Kathleen is able to afford treatment. His health insurance covers her. But finances have been tough. In the space of five years, they have gone from two incomes to one.

For all the support they continue to provide, Kathleen’s family is also open and honest about the effect of the illness on their own lives.

Kathleen’s daughter Katie listens to her talk about her disease.

Her daughter Katie later admits the earlier than expected reversal in caregiving roles has not been easy. They all say they are happy to be around, but admit it can be exhausting and has led to sacrifices in their everyday lives.

Katie explains grasping the idea that a loved one is going to die is hard, but it is something you eventually have to accept. But then, as you slowly watch your loved one’s illness progress and the symptoms take hold, the most painful thing is witnessing the “everyday indignities”, the loss of face.

“Every single week, there is something. It’s constant. If it’s not mental, it’s physical.”

The heartbreaking irony is no one seems more aware of her descent into mental chaos than Kathleen herself.

“She is so intelligent, she looks everything up,” Dave says.

“What’s most disgusting is her awareness of the illness,” Katie adds. “I understand why they call dementia ‘the long goodbye’.”