Causes:

The causes of depression are not fully known. Most likely a combination of genetic, biologic, and environmental factors are at work.

Genetic Factors

Because depression runs in families, and has a strong genetic component, compelling evidence suggests that depression is a biologic phenomenon. Data from family, twin, adoption, and genetic studies have confirmed this. Studies have found that first-degree relatives of patients with depression are two to six times more likely to develop the problem than individuals without a family history.

Biologic Factors

Evidence supports the theory that depression has a biologic basis. The basic biologic causes of depression are strongly linked to abnormalities in the delivery of certain key neurotransmitters (chemical messengers in the brain). These neurotransmitters regulate mood and associated behaviors. Scientists hope that by identifying the gene mutations that code the regulation of these neurotransmitters, they may eventually be able to predict which patients are most likely to respond to specific antidepressant drugs.

• Serotonin. Perhaps the most important neurotransmitter in depression is serotonin. Among other functions, it is important for feelings of well-being. Imbalances in the brain’s serotonin levels can trigger depression and other mood disorders.
• Other Neurotransmitters. Other neurotransmitters possibly involved in depression include acetylcholine and catecholamines, a group of neurotransmitters that consists of dopamine, norepinephrine, and epinephrine (also called adrenaline). Corticotropin-releasing factor (CRF), which is believed to be a stress hormone and a neurotransmitter, is thought to be involved in depression and anxiety. Increased CRF concentrations appear to interact with serotonin and have been detected in patients with either depression or anxiety.

Endocrine glands release hormones into the bloodstream that are transported to various organs and tissues throughout the body. For instance, the pancreas secretes insulin, which allows the body to regulate levels of sugar in the blood. The thyroid gets instructions from the pituitary gland to secrete hormones that determine the pace of chemical activity in the body. The more hormone in the bloodstream, the faster the chemical activity; the less hormone, the slower the activity.

The degree to which these chemical messengers are disturbed is determined by other factors, such as light, structural abnormalities in the brain, sleep disorders, or genetic susceptibility. For example, researchers have identified a defect in the gene known as SERT, which regulates serotonin and has been linked to depression.

Reproductive Hormones. In women, the female hormones estrogen and progesterone most likely play a role in depression.

Depression in Women

Women, regardless of nationality or socioeconomic level, have significantly higher rates of depression than men. The causes of such higher rates appear to be a mix of biologic and cultural factors.

Social and Economic Factors. The role that work, marriage, and children play in a woman's depression is complex. Many women feel that they must be everything to everyone and at the same time feel as if they are no one at all. Such a self-image is common and should be strongly considered as a major contributor to depression in many women, particularly those who work and have small children.

Hormonal Fluctuations and Life Stages. Extreme hormonal shifts can trigger emotional swings in all women. The role of hormones in depression is not clear, however, and is mostly based on observations of depression during specific stages in female development. Female hormones undoubtedly play some role in premenstrual dysphoria, postpartum depression, and SAD. These forms of depression recede or stop after menopause.

Early Puberty. Girls who go through puberty early (reaching the midpoint at 11 years or younger) are more likely to experience depression during adolescence than girls who mature later.

Premenopause. Premenopausal women ages 20 - 45 are most susceptible to depression, with 22% of this age group reporting symptoms of major depression. Specifically, premenstrual dysphoric disorder (severe depression before a period) affects an estimated 3 - 8% of women during their reproductive years. [For more information, see In-Depth Report # 79: Premenstrual syndrome.]

Perimenopause. Depression often occurs around menopause (the perimenopausal period), when, in addition to hormonal changes, other factors such as cultural pressures favoring young women, sudden recognition of aging, and sleeplessness are involved.

Postmenopause. Once women pass into the postmenopausal period, studies suggest that average depression scores are nearly as low as those in premenopausal women. In fact, many women report that after menopause, previous bouts of depression, particularly when caused by seasonal changes or premenopausal syndrome, recede or stop completely.

Premenstrual Dysphoric Disorder. The syndrome of severe depression, irritability, and tension before menstruation is known as premenstrual dysphoric disorder (PDD or PMDD), also called late-luteal dysphoric disorder. It affects an estimated 3 - 8% of women in their reproductive years. A diagnosis of PDD depends on having five or more standard symptoms of major depression that occur during most menstrual cycles, with symptoms worsening a week or so before the menstrual period and resolving afterward. PMDD has features of both anxiety and depression disorders, although experts increasingly believe it is a distinct disorder with specific biochemical abnormalities. [For more information, see In-Depth Report #79: Premenstrual disorder.]

Depression During Pregnancy. Pregnancy is certainly an occasion of great celebration for most women most of the time. However, emotions during that time are not always straightforward, and depression is a common (although most often a temporary) companion. Prenatal depression can affect a mother's sleep, physical activity, adherence to care, and appetite.

Miscarriage. Miscarriage poses a very high risk for depression, particularly in the first month after the loss. Older women with no previous successful pregnancies and those with a history of depression are at particular risk during this time. (Despite some concern that depression increases the risk for miscarriage in the first place, there is no evidence to support this.)

Postpartum Depression. Most new mothers experience weeping, irritability, and confusion for a few days following childbirth. Such symptoms, known as the "baby blues," are not considered signs of postpartum depression unless they persist in severe form nearly every day for more than 1 - 2 weeks.

Women are most likely to develop postpartum depression and other mental disorders in the first 3 months following delivery. (The risk is highest for first-time mothers, especially in the 10 - 19 days after delivery.) Other studies have reported that 8 - 20% of women have diagnosable postpartum depression within that 3-month period. In one study, 5% of these women had suicidal thoughts.

Studies have not found any association between a higher risk for postpartum depression in women and the following:

• Educational level
• Gender of the child
• Whether or not the woman breast-feeds
• Whether or not the pregnancy was planned
• Whether the delivery was vaginal or cesarean

The rapid decline of reproductive hormones that accompany childbirth is likely to play the major role in postpartum depression in susceptible women. Fluctuating thyroid hormones can also contribute to depression. Studies suggest that women who are more sensitive to hormone fluctuations are at greater risk for postpartum depression if they have one or more of the following conditions:

• A history of prior depressive episodes
• A family history of mood disorders
• Stressful life events (such as being a new mother and having an infant with medical problems)
• Lack of social support or feeling as if it is lacking

Depression in Children and Adolescents

Depressed children often suffer in silence, and depression may be evident only from reports of problems in school. It is also often difficult for adults to believe that children can be chronically depressed. Symptoms for depression in children often differ from those in adults and may include the following:

• An inability to enjoy favorite activities
• Persistent sadness
• Increased irritability
• Complaints of physical problems, such as headaches and stomachaches
• Poor performance in school
• Persistent boredom
• Low energy
• Poor concentration
• Changes in eating and/or sleeping patterns
• A greater tendency to bully others -- anxious children are more often bullied.

Risk Factors for Depression in Children and Adolescents. Depression can occur in children of all ages, including preschoolers, although adolescents have the highest risk (about 20%). Risk factors for depression in young people include having parents, particularly mothers with depression. Early negative experiences and exposure to stress, neglect, or abuse also pose a risk for depression. Sometimes depression develops after a physical illness. In adolescents, feeling alienated from parents is a strong predictor for depression.

Outlook for Future Emotional Problems. Adolescents who have depression are at significantly higher risk for substance abuse, recurring depression, and other emotional problems (such as bipolar disorder) in adulthood.

Risk for Suicide in Adolescents. Suicide is the third most common cause of death among adolescents, and is one of the most devastating events than can happen to a family. Suicide is most commonly associated with depression in young people but it is also linked with anxiety, psychosis, substance abuse, or impulsivity. More girls attempt suicide but more boys succeed, most often because they choose guns or violent methods while girls tend to overdose, which is more treatable. Nevertheless, attempts are major risk factors for a later suicide. Any expression of suicidal intent should be treated very seriously.

The following are danger signs in young people:

• Withdrawal from friends
• Sudden decrease in school performance
• Loss of interest in activities that were previously pleasurable
• Unusual irritability
• Unusual changes in sleep or eating habits

Risk factors for suicide include a history of neglect or abuse, history of deliberate self-harm, a family member who committed suicide (nearly always one who shared a common mood disorder), access to firearms, and living in communities where there have been recent outbreaks of suicide in young people. A romantic break-up is often the trigger for a suicidal attempt in teenagers. Feeling connected with parents and family protected young people with depression in one study, regardless of gender or ethnicity.

Adolescents may fail to seek help for suicidal thoughts for the following reasons:

• They believe nothing would help
• They are reluctant to tell anyone they had problems
• They think it is a sign of weakness to seek help
• They do not know where to go

Parents should not hesitate to seek professional help for their children if they suspect they are thinking about killing themselves. This is a medical emergency and requires immediate treatment.

Behavioral therapies and antidepressants are promising treatments for preventing suicide but need study. There has been a decline in adolescent suicides over the past decade, which some experts attribute to the increased use of antidepressants in this population. However, recent evidence has indicated that antidepressants can also raise the risk for suicidality (suicidal thoughts and behavior) in some people. Children, adolescents, and young adults who are prescribed antidepressant medication should be carefully monitored by both their parents and doctor, especially during the first few months of treatment, for any worsening of depression symptoms or changes in behavior. [See Suicide Risk and Antidepressant Medications in Medication section.]

Depression in the Elderly

Although depression in the elderly is very common, the aging process itself is unlikely to be the cause in most cases. An Italian study, for example, indicated that the very old (people who lived beyond 90 years of age) were no more likely to be depressed than younger adults. (The rate was 10% in both groups.) Studies on the cause or extent of depression in the elderly are not clear.

The severity of depression in elderly patients is strongly associated with poor health and less ability to function. In one study of older adults undergoing rehabilitation, half of whom were depressed, as their function improved so did their mood.

Anyone who experiences cumulative negative life events, physical illness, the death of a loved one, impaired functioning, or loss of independence can become deeply depressed. The elderly are at highest risk for such events.

Diagnosing Depression in the Elderly. Because of the complex relationship between depression, drug interactions, and serious physical illness in the elderly, an accurate diagnosis in this group is important but not always straightforward. The characteristic symptoms of depression are not always present or readily apparent in older people:

• Some older people may be aware of their depression but believe that nothing can be done about it.
• Many elderly people who are depressed may report only physical symptoms (aches and pains) or other mood states (confusion, agitation, anxiety, and irritability) related to depression rather than depression itself.
• Often they are unable or unwilling to express their feelings or are even unaware that they are depressed.
• Their symptoms are often ignored or confused with other ailments common in the elderly, including Parkinson's or Alzheimer's disease, dementia, thyroid disorders, arthritis, stroke, cancer, heart disease, and other chronic conditions.
• Depression is also a side effect of many drugs that are commonly prescribed for the elderly. It is often very difficult, then, to determine if the patient's depression is a psychologic reaction to the illness, caused by the disease itself, or completely independent from the medical condition. Both physical and emotional conditions should be considered in making a diagnosis in older people.

Many studies suggest strong associations between even mild depression and poorer quality of life as well as a shorter lifespan.

Risk for Suicide in the Elderly. Suicide in the elderly is the third-leading cause of death related to injury. Men account for 81% of these suicides, with divorced or widowed men at highest risk.

Effects of Depression on the Ability to Function. Even mild depressive symptoms in people aged 65 and above are associated with a higher risk of becoming disabled and having a lower chance of recovery.

Heart Disease and Heart Attacks. Depression increases the severity of a heart attack and may even impair a patient's response to medication for heart disease. Although people with heart disease may certainly become depressed, this does not explain entirely the link between the two problems. Data suggest that depression itself may be a true risk factor for heart disease as well as its increased severity. A number of studies indicate that depression has biologic effects on the heart, including a higher risk for blood clotting, changes in heart rate, and impaired blood flow to the heart (particularly in response to mental stress). The more severe the depression, the more dangerous to the health, although even mild depression, including feelings of hopelessness, experienced over many years, may harm the heart, even in people with no early signs of heart disease.

Mental Decline. Depression in the elderly is associated with a decline in mental functioning, regardless of the presence of dementia. Depression may be a predictor or even a cause of Alzheimer's disease. Brain scans in the elderly, for example, have reported greater atrophy in the brains of depressed individuals than in those of nondepressed ones.