Risk Factors contributing to Bipolar Disorder

Scientists are studying the possible causes of bipolar disorder. Most agree that there is no single cause. Rather, many factors likely act together to produce the illness or increase risk for developing it.

Genetics

Bipolar disorder tends to run in families. Some research has suggested that people with certain genes are more likely to develop bipolar disorder than others.¹⁴ Children with a parent or sibling who has bipolar disorder are much more likely to develop the illness, compared with children who do not have a family history of bipolar disorder.¹⁵ However, most children with a family history of bipolar disorder will not develop the illness.

Technological advances are improving genetic research on bipolar disorder. One example is the launch of the Bipolar Disorder Phenome Database, funded in part by NIMH. Using the database, scientists will be able to link visible signs of the disorder with the genes that may influence them.

Scientists are also studying illnesses with similar symptoms such as depression and schizophrenia to identify genetic differences that may increase a person's risk for developing bipolar disorder.^17,18,19 Finding these genetic "hotspots" may also help explain how environmental factors can increase a person's risk.

But genes are not the only risk factor for bipolar disorder. Studies of identical twins have shown that the twin of a person with bipolar illness does not always develop the disorder, despite the fact that identical twins share all of the same genes. Research suggests that factors besides genes are also at work. It is likely that many different genes and environmental factors are involved. However, scientists do not yet fully understand how these factors interact to cause bipolar disorder.

Brain structure and functioning

Brain-imaging tools, such as functional magnetic resonance imaging (fMRI) and positron emission tomography (PET), allow researchers to take pictures of the living brain at work. These tools help scientists study the brain's structure and activity.

Some imaging studies show how the brains of people with bipolar disorder may differ from the brains of healthy people or people with other mental disorders. For example, one study using MRI found that the pattern of brain development in children with bipolar disorder was similar to that in children with "multi-dimensional impairment," a disorder that causes symptoms that overlap somewhat with bipolar disorder and schizophrenia. This suggests that the pattern of brain development in the two conditions may be associated with the risk for unstable moods.

Another MRI study found that the brain's prefrontal cortex in adults with bipolar disorder tends to be smaller and function less well compared to adults who don't have bipolar disorder.The prefrontal cortex is a brain structure involved in "executive" functions such as solving problems and making decisions. This structure and its connections to other parts of the brain mature during adolescence, suggesting that abnormal development of this brain circuit may account for why the disorder tends to emerge during a person's teen years. Pinpointing brain changes in youth may help us detect illness early or offer targets for early intervention.

The connections between brain regions are important for shaping and coordinating functions such as forming memories, learning, and emotions, but scientists know little about how different parts of the human brain connect. Learning more about these connections, along with information gained from genetic studies, helps scientists better understand bipolar disorder. Scientists are working towards being able to predict which types of treatment will work most effectively.