Causes

Causes

No single cause may ever be found for bipolar disorder. Instead, a combination of biologic, genetic, and environmental factors appears to trigger and perpetuate the chemical imbalances in the brain that shape this complex disorder. Biologic factors observed or considered in bipolar disorder, as detected by use of imaging scans and other tests, include:

• Oversecretion of cortisol, a stress hormone
• Excessive influx of calcium into brain cells
• Abnormal hyperactivity in parts of the brain associated with emotion and movement coordination
• Low activity in parts of the brain associated with concentration, attention, inhibition, and judgment.
• A superfast "biologic clock"

The so-called biologic clock is a tiny cluster of nerves called the supra chiasmatic nucleus, or SCN. The SCN is located in the center of the brain in the hypothalamus region. It regulates a person's circadian rhythm, the daily cycle of life, which influences sleeping and waking.

Biologic and Genetic Factors Shared with Other Disorders

The genetics of bipolar disorder are the most intensively studied of all psychiatric diseases. Multiple genes, involving several chromosomes, have been linked to its development. Bipolar disorder also may share these genetic factors with other disorders, including schizophrenia, epilepsy, and panic disorder. It is not clear if some of these disorders are variations of a single disease or separate disorders.

Bipolar Disorder and Schizophrenia. Researchers have been investigating whether common biologic factors are involved with schizophrenia, severe bipolar disorder, and other psychoses. Schizophrenia and bipolar disorder often show up in the same family. Researchers are identifying a number of common genetic and biologic pathways that they both share. Some examples of studies comparing biologic differences and similarities include:

• Genetic abnormalities for both diseases appear on many of the same chromosomes. Locations on chromosomes 1, 6, 8, 12, and 22 have been identified as gene regions linked to bipolar disorder.
• Pathways of the neurotransmitter dopamine appear to be important in both illnesses. (A neurotransmitter acts as a chemical messenger between nerve cells.)
• Blood levels of reelin, a protein in the brain, may be useful as markers for both schizophrenia and bipolar disorder, although levels vary between the two diseases. (Reelin is a protein that is important for information processing.)
• Elevated levels of vesicular monoamine transporter (VMAT2) have been observed in the brainstems of patients with both bipolar disorder and schizophrenia. VMAT2 is a protein in the brain that regulates the transport of important neurotransmitters. The distribution of this protein in the brain, however, differs between the two diseases.
• In one study of people with bipolar disorder, the left side of the hippocampus was significantly larger than it was on the right. In patients with schizophrenia, the hippocampus' volume was decreased. (The hippocampus is located deep in the brain and stores memory.)