What is clinical depression? A psychiatrist weighs in on Fetterman's hospitalization.

Sen. John Fetterman was hospitalized for clinical depression on Wednesday, weeks after being sworn in as a senator from Pennsylvania, his office said.

“Last night, Senator John Fetterman checked himself into Walter Reed National Military Medical Center to receive treatment for clinical depression,” his chief of staff, Adam Jentleson, said Thursday. “While John has experienced depression off and on throughout his life, it only became severe in recent weeks.”

On Monday, Fetterman was evaluated by the attending physician of the United States Congress, who recommended inpatient care at Walter Reed. Jentleson said Fetterman “is receiving treatment on a voluntary basis.”

“After examining John, the doctors at Walter Reed told us that John is getting the care he needs, and will soon be back to himself,” Jentleson said.

The 53-year-old Democrat was plagued by a medical emergency on the campaign trail last year, and hospitalized for more than a week after suffering from a stroke in May. Just last week, Fetterman was again hospitalized after feeling light-headed. Doctors ruled out another stroke or seizure, according to his office.

A senior aide told NBC News that Fetterman "will likely be in inpatient care for clinical depression for 'a few weeks,'" and that "it's been difficult to distinguish the stroke from the depression."

"It's hard to tell at times if Fetterman is 'not hearing you, or is he sort of crippled by his depression and social anxiety,'" the aide said.

About 1 in 6 adults will have depression at some time in their life, the Centers for Disease Control and Prevention estimates; the disease affects 16 million adults every year.

"Millions of people struggle with depression every day, often in private," President Biden tweeted to Fetterman on Friday. "Getting the care you need is brave and important. We're grateful to you for leading by example."

To better understand clinical depression, Yahoo News spoke with Dr. Grant Brenner, an assistant clinical professor in the department of psychiatry and behavioral sciences at Mount Sinai Beth Israel in New York City, and author of the book "Making Your Crazy Work for You: From Trauma and Isolation to Self-Acceptance and Love." (Some responses have been edited for length and clarity.)

Yahoo News: What is clinical depression?

Dr. Grant Brenner: Clinical depression is a psychiatric condition, which has specific diagnostic criteria. To be diagnosed with symptoms, they have to be present for at least two weeks for most of the day, and people need to have the presence of a few to several of nine different symptoms. They have to include depressed mood or loss of interest or pleasure most days of the week, with five or more of the following symptoms:

Depressed mood most of the day nearly every day;

Diminished interest in or pleasure in all or almost all activities most of the day, nearly every day;

Significant weight loss when not dieting or weight gain or decrease or increase in appetite nearly every day;

A slowing down of thoughts and a reduction of physical movement, observable by others;

Fatigue or loss of energy nearly every day;

Feelings of worthlessness or excessive or inappropriate guilt nearly every day;

Diminished ability to think or concentrate or indecisiveness nearly every day;

Recurrent thoughts of death, recurrent suicidal ideation, without a plan or a specific attempt or a plan for committing suicide.

Together, these symptoms manifesting for a certain length of time meet the criteria for clinical depression — the technical name of which is major depressive disorder. There are various specifiers, meaning different types of depression, but those are the basic criteria spelled out in the Diagnostic and Statistical Manual, or DSM-5.These symptoms as diagnostic criteria have to be present, at least five of them, most of the day or all of the day during the same two-week period to meet criteria for clinical depression — the technical name of which is major depressive disorder. There are various specifiers, meaning different types of depression, but those are the basic criteria spelled out in the Diagnostic and Statistical Manual, or DSM-5.

Is clinical depression different from what we would normally think of as depression?

The word “depression” is used by the public to mean a lot of different things. And so the term “clinical depression,” if it's being used in a medical context, I would make that synonymous with “major depressive disorder.” Otherwise, it’s not clinical depression; it could be, however, many different ways that people use the word “depression” — anything from sadness to disappointment, right? “I was depressed that I didn't get to go out with my friends last night.”

It could be a stress reaction as well. People use the word “depression” often to refer to a stress reaction.

According to his chief of staff, Fetterman “has experienced depression off and on throughout his life,” and it “only became severe in recent weeks.” What are some common triggers for depression to become more severe?

It’s not unusual for a change in job to bring on an episode of depression if that involves increased stress. Stress is definitely something that can bring on a depressive episode, as well as any kind of loss or difficulty adjusting to change. Some people do develop what's called an adjustment disorder, which can be characterized by symptoms of anxiety and depression. But usually, if someone is hospitalized, they have to have a severe reaction. The model in medicine and psychiatry is called a diathesis-stress model, and what that means is that there's an underlying vulnerability or predisposition to a problem, and then stressors will tip the person over into whatever the problem is. So that could be taking on a major new position after a very difficult campaign, which is a significant stressor.

We know that Fetterman had a stroke in May of last year. What's the relationship between depression and chronic medical conditions like stroke, for example?

Many chronic medical conditions are associated with depression and other psychiatric diseases. Any kind of medical problem as a stressor generally can predispose to depression. Certain types of strokes can be associated with a higher risk of depression depending on what area of the brain that they’re in, in terms of the anatomic region that’s affected.

In addition, it can be worth noting that people who as children experienced more stressors growing up, including abuse and neglect, are likely to have higher rates of both mental and physical problems.

How is clinical depression usually treated?

Depression is treated traditionally with medications and with psychotherapy.

It’s not unusual for people to be admitted to a hospital if they’re having more significant symptoms of depression, or if they’re going to receive treatments in the hospital that either can only be given in a hospital or are much easier to be given in a hospital. Treatments like that might go beyond medication, though sometimes the hospitalization is helpful for changing medications faster than you can if someone is an outpatient. And sometimes therapy can be given much more intensively in a controlled setting like that. In a hospital, it's also easier to get other treatments beyond medications and therapy, including things like electroconvulsive therapy or transcranial magnetic stimulation.

I also think when people are admitted to the hospital who have access to more resources, and for whom it’s important to get a much more definitive treatment in place. They may admit people to the hospital so that they can get more intensive or urgent treatment.

Without knowing more about the clinical situation, the other reasons people might be admitted to the hospital is if it were for their own safety. For example, someone who’s feeling significantly suicidal or was unable to take care of themselves might be admitted to the hospital for that reason.

A senior aide to Fetterman told NBC News the senator will likely be in inpatient care for clinical depression for "a few weeks." Is that common?

The length of stay of hospitalizations varies a great deal depending on a number of different factors. I'm assuming when a politician gets admitted to Walter Reed, it's going to be a different circumstance than when someone gets admitted to a metropolitan academic center with their insurance covering it, because, for example, insurance companies will try to keep the length of stay as short as necessary.

What are some common misconceptions about clinical depression?

Common misconceptions are that depression represents a failure of the person to have done the right thing — that they didn’t try hard enough, or that if they didn’t want to be depressed they could decide not to be depressed. Those sorts of blaming the victim and stigmatizing views, where depression is seen as a moral failure or a failure of willpower, are extremely corrosive.

What is important for people to know about clinical depression?

It’s important for people to know that clinical depression is and should be treated, and that it is possible to get very good treatment responses. Not everyone will respond to the first course of treatment right away, but if people are consistent and pursue treatment, they can get significant relief.

It’s also important for people to know that the faster they get treatment for depression the better, because the length of the episode of depression is associated with more difficulty and longer-term depression. In other words, the quicker you can get it under control, the less of a problem it’s likely to be in the future.