From Kimberly Read & Marcia Purse,
Your Guide to Bipolar Disorder.
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About.com Health's Disease and Condition content is reviewed by Steven Gans, MD

Bipolar disorder (also known as manic depression) is a disease; it is a medical condition that causes psychological problems to such a degree that daily functioning is hampered by the symptoms. The most prominent symptoms include extremes of mood known as depression and mania. These emotional extremes are usually beyond normal responses to events and often last for extended periods of time. Psychosis and suicide are also concerns for those diagnosed with this disorder.

A Layperson's Definition
One textbook definition describes bipolar disorder as "a major affective disorder in which an individual alternates between states of deep depression and extreme elation." This is misleading in that bipolar disorder is much more complicated than just alternating between depression and elation. Bipolar disorder is an illness that affects thoughts, feelings, perceptions and behavior ... even how a person feels physically.

What Causes Bipolar Disorder?
Most scientists believe that mental illnesses are caused by a combination of factorsr. In bipolar disorder, these factors are usually divided into biological and psychological causes. In plain English, the main reasons mental illness develops are physical (biological) and environmental.

Types of Bipolar Disorder
In the United States, the Diagnostic and Statistical Manual of Mental Disorders IV (also known as the DSM-IV) is the primary classification system for defining the types of bipolar disorder. This classification system organizes the mood disorders under the heading of Clinical Disorders (Axis I). The International system is the ICD-10, The International Statistical Classification of Diseases and Related Health Problems, tenth revision. The ICD-10 provides a structure of "families" or related disorders.