Major Depressive Disorder (MDD) is a clinical syndrome of at least five symptoms impacting mood, cognition or physical well-being that last for at least two weeks and cause impairment in functioning.
Mood symptoms include depressed, sad or irritable mood, loss of interest in usual activities, inability to experience pleasure, feelings of guilt or worthlessness, and thoughts of death or suicide. Cognitive symptoms include inability to concentrate and difficulty making decisions. Physical symptoms include fatigue, lack of energy, feeling either restless or slowed down, and changes in sleep, appetite, and activity levels.
MDD affects individuals of all ages although it is more common in adults. It is a recurring and chronic illness, frequently returning for two or more episodes that often last for two years or more. While MDD is a treatable condition, fewer than 30% of those affected by the disease reported contact with a mental health professional in the past year when surveyed as part of the National Health and Nutrition Examination Survey, 2005-2006. Barriers to treatment include failure to recognize the disease, doubts about treatment effectiveness, lack of insurance coverage as well as the general stigma associated with mental illness.
The exact cause of MDD is unknown but there are psychological, biological and environmental factors that all contribute to its development. There are three neurotransmitters thought to be involved in MDD: serotonin, norepinephrine , and dopamine. When there is an imbalance in these neurotransmitters, MDD is likely to develop. A family history of MDD is also associated with the development of the disease.
While there are many different antidepressant medications and several well-established forms of psychotherapy to treat MDD, a large percentage of patients fail to respond to treatment or suffer unacceptable side effects. Drug development efforts are focused on new mechanisms of action with the potential for improved efficacy and fewer side effects.