Depression and bipolar disorder are commonly referred to as mood (or affective) disorders. Major hallmarks of depression include a persistent low or sad mood, decreased or absent interest in almost all activities, loss of self-confidence, and a feeling of worthlessness. Most people with bipolar disorder (formerly known as manic-depressive illness) experience alternating episodes of both depression and mania. Mania, which can be thought of as the opposite of depression, is characterized by an elated or elevated mood, increased activity, an overblown self-image, and an exaggerated sense of self-confidence. Usually, both depression and bipolar disorder are episodic—that is, bouts of illness are separated by symptom-free periods characterized by feelings of relative well-being.
Causes of Depression and Bipolar Disorder
The exact causes of depression and bipolar disorder are not well understood, but some combination of genetic predisposition and psychological and medical factors appears to play a role in these mood disorders.
Changes in the Brain
When people get depressed, chemical changes occur in the brain, and researchers believe that these changes are linked to the symptoms of mood disorders, especially depression.
The brain is composed of distinct regions, each made up of networks of nerve cells called neurons that transmit messages throughout the nervous system. Individual neurons are separated by small gaps at each end called synaptic clefts. Chemicals called neurotransmitters bridge the synaptic clefts and pass messages from one neuron to the next. Imbalances in three particular neurotransmitters—serotonin, norepinephrine, and dopamine—appear to contribute to depression and bipolar disorder, although less is known about changes in the brain that occur during the manic phase of bipolar disorder.
One specific brain region thought to be involved in depression is the limbic system, which affects our emotional behavior. An area within this system, the hypothalamus, regulates the pituitary gland, which in turn regulates key hormones and may be involved in the hormonal imbalances sometimes associated with depression.
Genetic Factors in Depression and Bipolar Disorder
Depression is often a family affair. Scientists, for example, have identified a gene that may be linked to bipolar disorder. In addition, they have found a common genetic mutation associated with a person developing clinical depression when faced with traumatic events in his or her life.
Research also shows that when one identical twin has a mood disorder, there is about a 50% chance that the other twin, who shares the same genes, will develop the illness at some point in life. One study found that if one twin developed depression, the other twin also suffered from depression in 46% of identical twins, compared with 20% of fraternal twins (who share half of their genes, like any full siblings).
Studies have also found that adopted children whose biological parents had a mood disorder had a three times greater incidence of depressive illness than the biological children of the same adoptive parents. Finally, children whose parents and grandparents experienced moderate to severe depression are at much greater risk for developing psychiatric problems than those whose relatives were not affected, according to a three-generation study.
Genetics also plays a role in treatment, influencing an individual's response to a particular drug, for example. There is evidence that people may do better if prescribed the same antidepressant medication that has been effective for a depressed first-degree relative (a parent, sibling, or child).
Because researchers are not sure exactly which genes are linked to depression or bipolar disorder, specific genetic tests will not be available for years to come. But as with heart disease and cancer, <>it is important to know if you have a family history of depression or bipolar disorder.
Although genetic factors may make a person more susceptible to mood disorders, a trigger—such as a serious medical condition or psychological stress—is often what causes a mood disorder to emerge at a specific time.
Drugs and Alcohol Abuse
Overconsumption of alcohol and certain illegal drugs can contribute to depression and make it worse, as can withdrawal from alcohol, cocaine, or amphetamines. Drugs can cause other mental changes as well. For example, amphetamines, cocaine, and phencyclidine (PCP) can all induce mania in people with bipolar disorder.
Medical Conditions That May Cause Depression or Bipolar Disorder
Medical disorders can make people prone to depression. For example, people who have dementia-causing brain disorders, such as Alzheimer's disease or Huntington's disease, are more susceptible to depression. In addition, stroke is known to trigger depression, affecting about 25% of those who have had a stroke in the left frontal area of the brain.
Hormonal disorders, such as an underactive thyroid (hypothyroidism) and Cushing's disease (in which the adrenal gland produces too much of the stress hormone cortisol, affecting blood pressure and metabolism) also can lead to depression. A recent study also suggests that low testosterone levels may be responsible for the increase in depression in men after age 65.
Vitamin deficiencies, such as insufficient levels of folic acid, vitamin B6, or vitamin B12, also have been linked to depression. (In one study of 700 women, those with a vitamin B12 deficiency were twice as likely to be severely depressed as those without this deficiency.)
Two recent studies also suggest that obese adults may be more likely to develop a mental health disorder, and those with the metabolic syndrome (a combination of excess weight, high cholesterol, high blood glucose levels, and high blood pressure) are at increased risk for depression.
Prescription drugs also can cause mood disorders. Medications such as corticosteroids, levodopa (Parcopa and other drugs used to treat Parkinson's disease), and methylphenidate (Ritalin and others, commonly used for treating attention deficit disorder) can trigger mania in bipolar disorder. Other drugs, including some used to treat high blood pressure and cancer, have been known to cause depression.
Neurological disorders or injury to the brain from trauma or tumors also can cause behavioral changes, anxiety, and/or mood disorders.