Ruth Ketcham was convinced that mice had invaded her pristine East Northport home. She had seen them darting and scurrying.

The possibility of rodents worried Ketcham’s daughter, Jody Wade.

“My mother told me we needed to call an exterminator,” said Wade, who lives in nearby Northport.

Other types of sightings — all in her home — were being reported by Ketcham, starting in 2013, about a year after her Parkinson’s disease diagnosis. Wade recalls checking everywhere for the creatures — there wasn’t the slightest trace.

Ultimately, Ketcham’s doctor made a second diagnosis: Parkinson’s disease psychosis, a common complication barely known outside of families coping with it or doctors treating it. For decades, Parkinson’s-associated hallucinations and delusions forced many families to institutionalize loved ones.

Until April, there had been no way to effectively treat the psychosis short of tranquilizers and other potent antipsychotic drugs, none of them specifically designed for the problem — or the elderly, the chief population experiencing Parkinson’s psychosis.

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The U.S. Food and Drug Administration, however, fast-tracked the approval of a medication called Nuplazid that banishes hallucinations and delusions.

Ketcham was part the drug’s overall scientific story, having joined a clinical trial before the medication’s approval. A nonagenarian — now 92 — and pioneering patient, Ketcham’s hope was to spare others from the hallucinations that for a while had become powerful and dominant forces in her life.

Hallucinations and/or delusions affect roughly half of all people who develop Parkinson’s disease, doctors say.

Exactly how the drug works in the brain, nevertheless, remains a mystery.

“It’s a unique drug but the precise mechanism of action is not known,” said Dr. David J. Dickoff, a clinical investigator of the medication and a clinical professor with The Mount Sinai Hospital in Manhattan.

Nuplazid’s approval was based on a pivotal six-week trial of 200 patients. But 1,200 trial participants, including Ketcham, were among those in clinical studies that helped demonstrate the drug’s effectiveness.

“My mother got a plain bottle of pills and she took the medication faithfully,” Wade said. “And in about three to four weeks, she was reporting hardly any hallucinations. That’s how we knew that she didn’t get the placebo.”

Parkinson’s disease is a progressive, debilitating disorder of the nervous system typified by tremors that affect the hands, arms, legs, jaw and face. The disease also causes muscular rigidity, which slows movement and impairs balance. There is no cure.

The disorder is caused by the irrevocable loss of dopamine-producing brain cells. In a healthy brain these cells aid the transmission of nerve signals involved in movement, such as walking, standing or feeding oneself.

As it turns out, cells connected with hallucinations and delusions are totally unrelated to the dopamine system, said Dickoff, who has a private practice in Yonkers.

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Hallucinations and delusions emerge as a result of disturbances among cells in the serotonin family, another group of brain cells.

“We know that these cells — which are called the 2-A receptors — are located in the brain stem and the frontal region of the brain,” Dickoff said. “There’s also a large concentration of them in what’s called the parietal occipital cortex. If you put your hands right behind your ears, that’s where it is.”

He differentiates hallucinations from delusions, noting that hallucinations involve seeing and hearing things that are not real. Delusions, Dickoff said, are false beliefs. One key delusion in Parkinson’s psychosis is the notion of infidelity. The problem has made caretaking difficult for those attempting to help a delusional spouse.

Dickoff added that the medication is important because it’s the first approved to treat Parkinson’s psychosis, a condition that affects about 500,000 people who have the disease.

Some doctors have called Parkinson’s psychosis a silent epidemic. About 1 million people in the United States are coping with Parkinson’s and another 50,000 are diagnosed annually, according to the Parkinson’s disease Foundation. Former U.S. Attorney General Janet Reno, 78, died of Parkinson’s disease on Nov. 7. She was diagnosed in 1995 while still in office.

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Parkinson’s generally emerges around age 60 or older, but a rare form, young-onset Parkinson’s, affects people 50 or younger, data from the Michael J. Fox Foundation for Parkinson’s Research show.

For those who develop the psychosis, hallucinations can be vivid, frightening and perceived as genuine events, Dickoff said.

Ketcham, for example, reported seeing a group of people gathering nightly at the bathroom door in her bedroom. The intruders were such frequent visitors that Ketcham recognized each of them, her daughter said.

As time wore on, Wade said her mother noticed the visitors were festooning themselves in elaborate costumes. Ketcham was so convinced the people were real, she stopped wearing a thin nightgown to bed and chose opaque pajamas, instead, hoping to protect her privacy as she slept.

“Hallucinations and delusions can be profoundly disturbing and disabling,” said Dr. Mitchell Mathis, director of psychiatry products at the FDA.

He said Nuplazid is an important treatment for people with Parkinson’s disease who experience these symptoms.

Like most new medicines, Nuplazid isn’t cheap and runs about $2,000 a month, said Dickoff, who noted that a majority of insurers cover the cost.

Wade sees it as a godsend even though hallucinations sometimes emerge when her mother hasn’t had enough fluids or sleep.

“She does have hallucinations sometimes, but not like they were,” Wade said. “We now know to look for dehydration or ask how she slept.”