Bipolar Disorder

Description

Alternative Names

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. Among the biologic factors observed or considered in bipolar disorder, as detected by use of imaging scans and other tests, are the following:

• 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 and low activity in parts of the brain associated with concentration, attention, inhibition, and judgment.
• A superfast "biologic clock" (a tiny cluster of nerves called the supra chiasmatic nucleus or SCN). The SCN is located in the hypothalamus (in the center of the brain) and 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 the following:

• Genetic abnormalities for both diseases appear on many of the same chromosomes.
• 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.)

Bipolar Disorder and Epilepsy.Neurotransmitters called gamma aminobutyric acid (GABA) and norepinephrine have been implicated in mania.

• GABA helps prevent nerve cells from over-firing.
• Norepinephrine is a hormone that involves stress.

Some research has associated similar biologic mechanisms in patients with epilepsy and bipolar disorder. As in epilepsy, the more episodes a bipolar disorder patient experiences early in the course of the disease, the more frequent and severe later episodes will be. Antiseizure agents, in fact, can play an important role in the treatment of bipolar disorder.

Panic Disorder and Bipolar Disorder.Researchers are also studying the common biologic and genetic factors between panic disorder and bipolar disorder. While specific genes have not yet been identified, some researchers studying these illnesses now believe that they may represent different forms of a shared, complex condition.

Viruses

The high rate of winter births in those who develop bipolar disorder (as well as schizophrenia) has encouraged researchers to look at infectious agents as a possible cause or trigger of these mental disorders.

Borna Virus. The Borna virus is among the infectious agents being intensively studied. This virus is known to cause serious central nervous system injuries in animals, but not in people. A few studies using sensitive blood testing, however, have detected strong evidence of the infection in psychiatric patients. Some researchers believe that the virus may cause subtle changes in the human brain (in contrast to the more dramatic inflammation seen in animals) leading to a range of mental illnesses. It should be noted, however, that other research has not supported the association. Some researchers argue that psychiatric illnesses may suppress the immune system, making some individuals more susceptible to infection by the Borna virus or other microbes.

Herpes Simplex. Another possible viral link under study is herpes simplex virus 2 (HSV-2). Adult children of mothers with HSV-2 prior to delivery may have a greater risk of developing bipolar disorder and other psychoses, according to research published in 2001.