No cause of death has been announced for Dianne Feinstein, the longtime California senator who struggled with apparent health problems in her final years before her death Thursday evening.

She was absent from the U.S. Senate for nearly three months earlier this year while recovering from a case of shingles that led to encephalitis, a rare complication that causes inflammation and swelling in the brain. She was briefly hospitalized in August after falling at her home

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and was often seen in public in a wheelchair.

There have been indications for years that Feinstein, 90, was struggling with memory problems, even before the recognized health crises in the last year of her life.

Several of Feinstein’s colleagues and former staffers confided to reporters that she often failed to recognize old colleagues and frequently repeated herself. In July, she appeared confused during a Senate Appropriations Committee vote and began reading a prepared statement before the committee chair prompted her to “just say yes.”

Dianne Feinstein, the first woman to represent California in the Senate, dies at age 90

Feinstein claimed she was cognitively fit to serve in her fifth full term, and neither her office nor her family ever confirmed whether she had a neurological condition.

Even if the senators’ apparent memory problems were the result of dementia, the disease almost certainly would not have been her cause of death, doctors said Friday.

In general, older adults often have a confluence of several medical problems, and saying which one “caused death” can be very difficult, said Dr. Laura Mosqueda, professor of geriatrics and family medicine at the USC Keck School of Medicine. We don’t have to look for a simple answer to an actually complicated question.

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The most common cause of dementia is Alzheimer’s disease, a progressive and incurable condition. Alzheimer’s disease can be a direct cause of death in its final stages, when the disease begins to damage parts of the brain that control essential biological functions such as breathing and swallowing.

A common cause of death for people with late-stage Alzheimer’s disease is aspiration pneumonia, an infection in the lungs caused by food or other foreign substance that passes through the trachea instead of the esophagus. This becomes a greater risk because the muscles that control swallowing and chewing become weaker in the late stages of the disease.

By this time, it is no longer possible for patients to maintain even a semblance of their previous activities.

Normally you don’t see someone functioning, walking or talking. Typically, people are bedridden at that point, said Dr. Mark Mapstone, a neuropsychologist at UC Irvine.

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Doctors quickly noted that without knowing Feinstein’s health history, it was impossible to know for certain whether she suffered from a neurological disorder.

But even though her performance in the Senate in her final years was a shock to those who knew her during her long career, she was not as ill as someone with advanced Alzheimer’s disease.

Dianne Feinstein looked quite weak and sometimes seemed a little confused during public appearances, said Dr. Michael Weiner of UC San Francisco, the principal investigator of the Alzheimer’s Disease Neuroimaging Initiative. But if she did have dementia, he said, it was definitely not in the advanced stages. It wasn’t even clear if you could say she was in a moderate phase. You can only judge based on these outward appearances.