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Barbara Walters, right, interviews with actress Margot Kidder Sept. 6, 1996 on ABC News "20/20".
VIRGINIA SHERWOOD/ABC NEWS

Superman star Margot Kidder, who died Sunday at age 69, struggled with bipolar disorder for most of her life. What is it and how is it diagnosed and treated? Read on for a primer on the mental illness:

What is bipolar disorder?

The National Institutes of Health defines bipolar disorder as a "brain disorder that causes unusual shifts in mood, energy, activity levels, and the ability to carry out day-to-day tasks."

The disorder is characterized by its "up" and "down" phases. During an "up" phase, the patient can experience a period of elation or mania which is followed by a "down" phase in which they are severely depressed.

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What is the difference between bipolar I and bipolar II disorder?

The NIH says the two forms differ in duration of symptoms and severity. Both are different from depression because patients with (unipolar) depression do not experience manic phases.

Bipolar I: This is the more severe variety. Manic phases can last at least one week with symptoms bad enough to require hospitalization, and depressive phases can last up to two weeks. It is also possible for a patient to experience "an episode of mixed features" or "mixed mania" in which they exhibit both manic and depressive symptoms at the same time.

Bipolar II: This form, which is less acute, is the variety Carey has. While patients' depressive phases can be as bad as those seen with bipolar I disorder, they either don't experience manic phases or they are not severe enough to require immediate hospitalization.

Cyclothymic disorder: This third variety is a longer-lasting but milder form of bipolar disorder in which patients experiences mood swings lasting for a period of years.

Symptoms

A person experiencing a manic episode may:

  • Display an abundance of energy
  • Feel "wired," as if they've consumed a lot of coffee or sugar
  • Appear agitated or irritable
  • Talk rapidly and in a disjointed manner
  • Be more active than usual
  • Feel more creative 
  • Feel as though their thoughts are moving fast
  • Have trouble sleeping
  • Engage in risky behavior such as spending, gambling or reckless sex

 A person experiencing a depressive phase may:

  • Feel sad or hopeless
  • Have little or no energy
  • Have trouble sleeping or sleep too much
  • Feel like they don't enjoy anything
  • Be far less active than usual
  • Not eat or overeat
  • Be forgetful
  • Think or express feelings about death or suicide

According to the NIH, patients are more likely to seek medical attention during depressive phases than manic ones.

How is bipolar disorder diagnosed?

The patient's primary physician will likely conduct a physical examination to rule out the possibility that the manic and depressive symptoms are not being caused by some other physical illness (such as a thyroid disorder or brain cancer).

Next, they will conduct a mental health evaluation and take a family medical history or refer the patient to a psychiatrist for further screening.

According to the National Alliance on Mental Illness, the average age at which symptoms occur is 25 but they can appear during the teen years. 

There are a number of other disorders whose symptoms overlap with bipolar disorder, resulting in patients being misdiagnosed with it when they actually have anxiety disorders, attention-deficit hyperactivity disorder (ADHD), or premenstrual dysphoric disorder (PMDD), a severe form of premenstrual syndrome.

What causes bipolar disorder? Who gets it?

The NIH says no single cause has been pinpointed. Multiple factors could play a role, such as genetics and brain structure and function. 

According to the NIH website, 4.4% of American adults have experienced bipolar disorder at some point in their lives. Men (2.9%) and women (2.8%) are diagnosed at a fairly equal rate. The disorder also tends to run in families and the child of a bipolar patient is more likely to develop it than someone whose parents are not bipolar. However, that is not a guarantee they will.

The National Alliance on Mental Illness says stressful life events such as a death in the family, divorce or financial problems can trigger a patient's first manic or depressive episode.

It's also very common for bipolar people to self-medicate with alcohol or drugs. A 1990 study by the National Institute of Mental Health's Epidemiologic Catchment Area (ECA) found that 61% of people with the more severe bipolar I disorder were also diagnosed with a drug or alcohol dependency issue. Among patients with bipolar II disorder, the number was 48%. (The study also found that the only mental illness more likely to coexist with alcoholism than bipolar disorder is antisocial personality disorder.)

How is bipolar disorder treated?

Doctors often combine talk therapy with medication.

Drugs prescribed for bipolar disorder include antidepressants, mood stabilizers and atypical antipsychotics (also known as tranquilizers). Doctors note that patients may have to experiment with several medications to find the right drug or combination of drugs for them. 

The NIH cautions that bipolar patients should never go off their meds without informing their physician and that if they change medications, they should immediately report side effects so their doctor can make the necessary adjustments. 

Frequent, sustained medical attention is key because bipolar disorder is a chronic illness in which the patient will likely experience more manic and depressive phases over the course of their lives.

When did manic depression become bipolar disorder and why?

The disease got its new, more clinical name in 1980 when the American Psychiatric Association published the third edition of its Diagnostic and Statistical Manual of Mental Disorders, the reference guide used by doctors, regulatory agencies, health insurance companies and pharmaceutical companies when diagnosing, treating and providing coverage for mental illness.

The name change was intended to help doctors be more specific in their diagnoses and reduce the stigma attached to the terms "manic" and "maniac." Since then, the medical community and the media have gradually switched to calling the disease "bipolar disorder."

Are there any other celebrities who suffer from bipolar disorder?

Yes, and some of them have used their platforms to help improve life for people who are bipolar.

In April, singer Mariah Carey went public with the news that she had been diagnosed with the less-severe bipolar II form in 2001 but didn't take it seriously until much more recently. She is now in therapy and takes medication.  

"I’m hopeful we can get to a place where the stigma is lifted from people going through anything alone," Carey told People. "It can be incredibly isolating. It does not have to define you and I refuse to allow it to define me or control me.”

Singer/actress Demi Lovato, who was diagnosed in 2011, made a plea for universal health care at the 2016 Democratic National Convention, noting that there can be dire consequences when mental illness goes untreated, often due to inability to pay.

After Academy Award winner Catherine Zeta-Jones went public with her bipolar II diagnosis in 2013, she was credited with helping address the stigma attached to the disorder.

Star Wars icon Carrie Fisher, who died in 2016, was remarkably frank about her bipolar disorder. She was seen as a "bright light" by other people with the disorder and mined her experiences for laughter and tears in her books, the semibiographical 1987 novel Postcards From the Edge and 2008 memoir Wishful Drinking.

Fellow Oscar winner Patty Duke, who also died in 2016, was diagnosed as bipolar in 1982 and became a mental health advocate who spoke openly of her battle with the disease in her memoirs, 1987's Call Me Anna and 1993's Brilliant Madness: Living With Manic Depressive Illness.

“I learned that you can be down and not be out of control,” she told the women's health website EverydayHealth.com. “You can also enjoy life, be ecstatic about things but not certifiable.”