After she returned to the Senate recently, Sen. Dianne Feinstein’s office confirmed that the California senator was hospitalized for severe neurological complications of shingles that affected her brain and face.

"While the encephalitis resolved itself shortly after she was released from the hospital in March, she continues to have complications from Ramsay Hunt syndrome," a spokesperson for Feinstein, 89, confirmed on Thursday in a statement. 

Feinstein, California’s senior Democratic senator, previously said that she did not have encephalitis, noting, "It really has never been diagnosed properly."

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The senator was diagnosed with shingles on February 26 by her physician in San Francisco, then was "briefly" hospitalized until March 6. 

She returned to her home in California to recover, her office said in a statement.

Dianne Feinstein

Senate Judiciary Committee ranking member Sen. Dianne Feinstein, D-Calif., speaks during a news conference on Oct. 22, 2020, at the Capitol in Washington. Feinstein is not the first senator to take an extended medical absence from the Senate or to face questions about her age or cognitive abilities. But the open discussion over her capacity to serve underscores how the Senate has changed in recent years. (AP Photo/J. Scott Applewhite/ File)

But when she returned to the Capitol, she appeared frail and at times confused as she navigated its halls in a wheelchair, according to multiple reports.

"It is important not to speculate on anyone’s medical diagnosis without an in-person medical evaluation," Dr. Michael S. Okun, executive director of the Norman Fixel Institute for Neurological Diseases at University of Florida Health in Gainesville, Florida, told Fox News Digital late this week.

"I’m still experiencing some side effects from the shingles virus," Feinstein said.

"However," he added, "the reports on Senator Feinstein offer us an opportunity to educate the public on the importance of recognizing and treating meningoencephalitis."  

What is Ramsay Hunt syndrome?

The syndrome is a rare neurological complication of shingles that classically occurs in adults over 60 years old, according to the National Organization for Rare Disorders (NORD). 

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It sometimes can occur in younger people — as pop singer Justin Bieber was diagnosed with the condition last year.

The virus that causes chicken pox, known as varicella zoster virus (VZV), stays "dormant" in the body after someone gets chickenpox; but later in life, it can reactivate in a nerve that controls the facial muscles, known as the facial nerve

Dianne Feinstein

Sen. Dianne Feinstein, D-Calif., departs from the U.S. Capitol Building following a vote, on Thursday, May 11, 2023, in Washington, D.C.  Feinstein attended her first hearing after fighting illness and being absent from the Senate for almost three months.  (Kent Nishimura/Los Angeles Times via Getty Images)

This leads to a constellation of symptoms because of the location of the nerve in the face. Symptoms can include paralysis of one side of the face, painful blisters near the ear and ear pain, according to the National Institutes of Health (NIH).

"Even though I’ve made significant progress and was able to return to Washington, I’m still experiencing some side effects from the shingles virus," Feinstein said in a statement on May 10.

"My doctors have advised me to work a lighter schedule as I return to the Senate."

These "temporary" side effects are affecting her vision and ability to balance.

"My doctors have advised me to work a lighter schedule as I return to the Senate," she added.

Some patients have permanent facial paralysis and hearing loss from the syndrome, NORD added.

What is encephalitis?

Three protective layers of membranes known as the meninges cover the brain and spinal cord — which make up the central nervous system — to prevent them from moving, according to Cleveland Clinic.

In meningitis, these outer protective layers surrounding the brain become inflamed — but encephalitis is when the brain gets inflamed, per Johns Hopkins Medicine’s website.

Dianne Feinstein

Sen. Dianne Feinstein is shown making her way to the Senate chambers at the U.S. Capitol on Feb. 16, 2023, in Washington, D.C. Although encephalitis due to shingles can occur in people without significant medical problems, it is more common in people with immunocompromised conditions, according to medical research.  (Kevin Dietsch/Getty Images)

Patients with meningitis often present themselves to health care professionals within hours with the classic triad of symptoms of fever, headache and neck stiffness

Encephalitis has a more subacute presentation that includes changes in mental status.

"Meningoencephalitis can be caused by a viral infection of the brain and its coverings and depending on the virus type causing the infection, the associated syndrome may also result in strokes and brain bleeding," Okun said. 

As time progresses, patients may start to have mental status changes, problems with memory, thinking and reasoning. 

"If caught early, these disorders may possibly be treatable by antiviral drug therapy," he added.

"It is important to recognize that identifying which virus is involved, VZV or herpes, can make a huge difference in treatment and in outcome."

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Although encephalitis due to shingles can occur in people without significant medical problems, it is more common in people with immunocompromised conditions, such as cancer, according to previous research. 

How does shingles cause encephalitis?

"Encephalitis [after] shingles is a rare but important complication of shingles — which is a viral infection that is a late complication of chickenpox," Dr. Aaron Glatt, chief of infectious diseases at Mount Sinai South Nassau Hospital on Long Island, New York, told Fox News Digital.

He pointed out that people cannot get shingles if they've never had chickenpox.

Dianne Feinstein, Chuck Schumer

Senate Majority Leader Chuck Schumer, D-N.Y. (at right) greets Sen. Dianne Feinstein as she returns to the U.S. Capitol on May 10, 2023 in Washington, D.C. Feinstein returned to Washington after over two months away following a hospitalization due to shingles.  (Anna Moneymaker/Getty Images)

"These viruses are usually present and in ‘hiding’ in our bodies, and can be commonly reactivated later in life, especially when there is a weakened immune system," Okun added.

When the virus moves centrally toward the brain, it can cause encephalitis.

What are the symptoms?

People who have encephalitis often have symptoms that worsen over days to weeks, per Johns Hopkins medicine’s website. 

It can be caused by infections — often due to a virus or to inflammatory conditions in which the body’s immune system attacks the brain.

A patient’s specific symptoms will vary according to the part of the brain that’s infected or the particular antibody causing the inflammation.

Encephalitis due an infectious cause starts with non-specific symptoms that can be difficult to distinguish from the flu, including a headache. 

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"It was really a bad flu," Feinstein told CNN when she went to the hospital with symptoms.

As time progresses, patients may start to have mental status changes, problems with memory, thinking and reasoning. 

Dianne Feinstein, Chuck Schumer

Sen. Dianne Feinstein, in wheelchair, as she arrives at the U.S. Capitol following a long absence due to health issues on May 10, 2023, in Washington, D.C. Feinstein was fighting a case of shingles and has been absent from the Senate for almost three months.  (Kevin Dietsch/Getty Images)

In the case of an infectious cause, a patient’s specific symptoms will vary according to the part of the brain that’s infected or the particular antibody causing the inflammation — when it’s an autoimmune etiology. 

Encephalitis secondary to shingles often presents with a patient’s mental status changing within days after developing a group of painful blisters along one or more nerve distributions, according to previous research. 

How can shingles be prevented?

"Fortunately, there are excellent vaccines that are available to prevent chickenpox," Glatt tox Fox News Digital.

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"And if you had chickenpox, as most people above 40 years old have had, taking the shingles vaccine to prevent shingles — a potentially very serious complication — is very effective."