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What is Major Depressive Disorder?

By Liora Hess
Updated Mar 03, 2024
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Major depressive disorder (MDD) is also known as major depression, unipolar depression, and clinical depression. It is recognized by researchers and medical professionals as an actual, biological, medical illness. Unlike mild depression or "the blues," major depression consists of at least five significantly life-impacting symptoms lasting at least two weeks. These symptoms include the following:

  • restlessness, agitation, irritability, or physical slowing
  • dramatic change in appetite, possibly with accompanying weight gain or loss
  • suicidal ideation
  • insomnia or hypersomnia
  • withdrawal, loss of interest in activities that were formerly enjoyable (also known as anhedonia)
  • feelings of hopelessness or helplessness
  • fatigue and lack of energy
  • extreme difficulty concentrating
  • feelings of worthlessness, pessimism, self-hate, or inappropriate guilt

Major depressive disorder affects approximately 15 million adults, or 5% to 8% of the adult population. Women are twice as likely as men to be diagnosed with MDD. Additionally, women are at risk for episodes of major depression during the postpartum period.

No one factor has been found to be responsible for MDD. It is thought to be caused by an imbalance in three neurotransmitters in the brain: norepinephrine, serotonin, and dopamine. Cortisol, the hormone associated with the "fight-or-flight" response, may play a role in MDD; it has been found to be increased in many adults who suffer from acute depression. Stressful events sometimes, but not always, trigger an episode of MDD. Research also increasingly shows a genetic predisposition for major depression.

Major depression may be formally diagnosed by using a screening test, such as Beck's Depression Scale Inventory, Zung Self-Assessment Depression Scale, General Health Questionnaire (GHC), or the Center for Epidemiologic Study Depression Scale (CES-D). However, simply questioning the patient about mood or anhedonia may be as effective as longer screening tests.

Treatment of major depressive disorder may include psychotherapy or medications. It has been shown that most patients achieve the best results from a combination of psychotherapy and antidepressants. When severe depression does not respond to more conservative measures, electroconvulsive therapy (ECT) or transcranial magnetic stimulation (TMS) may be of benefit. Light therapy has also been found to help patients affected more severely during the winter months. Although the herb St. John's wort is sometimes used for depression, a large study by the National Center for Complementary and Alternative Medicine has proven that it is not effective for treating major depression.

Other potentially beneficial treatments include the restoration of a regular sleep schedule, avoidance of drugs and alcohol, maintenance of good nutrition, participation in regular exercise, and increasing social support.

The prognosis for those with major depressive disorder is generally good if they seek treatment. However, it is important to note that 15% of those who are diagnosed with MDD commit suicide, have drug or alcohol problems, have tobacco dependence, or suffer from increased physical problems and premature death. Also, approximately 60% of those who have one episode of major depression will have a second episode. Furthermore, the chance of having new episodes of depression increase with each subsequent episode.

The Health Board is dedicated to providing accurate and trustworthy information. We carefully select reputable sources and employ a rigorous fact-checking process to maintain the highest standards. To learn more about our commitment to accuracy, read our editorial process.
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