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Causes of Bipolar Disorder

The cause or causes of Bipolar Disorder remain unknown.  The general consensus is that this condition is produced by processes in certain brain centers, including the frontal cortex, limbic system and the basal ganglia, which are known to control mood, motivation, impulse control, planning and organization, and social judgment.  However, these areas cover a very broad range of cognitive functions, and probably all psychiatric disorders are somehow linked to processes in these areas.  Little is known about what is actually happening in these areas to cause Bipolar Disorder.  It is not even clear how the treatments that are used for this disorder have their effects.  

Genes and Bipolar Disorder

What does seem certain is that genetic factors play an important role in this disorder. In one study of twins, it was shown that among monozygotic twins (identical twins who share all the same genes), if one twin has Bipolar I Disorder then there is a 50% chance that the other will have the disorder; among dizygotic twins (fraternal twins who share only half of the same genes) there is a 5% chance of the second twin having the condition [ref].  In that same study, when counting all Bipolar Disorders as well as recurrent forms of Major Depressive Disorder, monozygotic twins shared these conditions in 75% of cases, and dizygotic twins in 11% [ref]. 

These findings suggest that genetic factors account for 70-90% of what leads to a Bipolar Disorder.  However, it remains unknown as to which genes are responsible for this risk.  

Stressful Life Experiences and Bipolar Disorder

Certain experiences during one's lifetime, especially traumatic experiences in childhood, can also impact on the development of Bipolar Disorder. Individuals with Bipolar Disorder who experienced physical or sexual abuse during their childhood tend to develop the symptoms of their Bipolar Disorder earlier in life, to endure Rapid Cycling and experience more episodes of Depression or Mania during their lifetime, to attempt suicide more frequently, and to have psychosis [ref].

Stressful life events later on in life can also trigger relapses of mood episodes (Major Depressive, Hypomanic, Manic or Mixed Episode) in someone who has Bipolar Disorder. Experiencing losses and other adverse events tend to trigger Depressive Episodes, while events having to do with the attainment of life goals can trigger Manic Episodes [ref].

Adaptive Advantages of Bipolar Symptoms

All of these facts have lead some researchers to view Bipolar Disorder not so much as an illness but rather as a biological variation of common behavioral patterns that can actually be advantageous for people in certain situations [ref]. For example, the increased energy, enthusiasm and motivation and the heightened speed of thinking that are common in Hypomania and Mania may be helpful for people who are in stressful situations to meet the demands of important but difficult goals. Not surprisingly, there is evidence that Hypomania actually allows people to function at a higher level than usual [ref].

From a neuropsychological point of view, these patterns of behavior correspond to features of the Behavioral Activation System (BAS), which is one of the brain's motivational systems that exists not only in humans but also in many animals, and is responsible for organizing how we engage our environments in order to achieve a desired goal. Some researchers speculate that Mania may be the BAS operating at an overly accelerated pace, whereas Depression may represent a slowing or blunting of the BAS, where the person loses much of their interest, motivation and energy [ref]. Although experiencing a Manic or Depressive Episode tends to make it harder for people to function in most of their roles [ref], this may be the biological cost of being able to experience Hypomania at certain times.

Medical Causes of Bipolar Disorder

There are various general medical conditions that are known to mimic or cause a Bipolar mood episodes (Major Depressive, Hypomanic, Manic or Mixed Episode).  Most of these conditions are associated with various typical physical signs and symptoms, which makes them distinguishable from Bipolar Disorder. These conditions include:

  • neurological disorders such as lesions of right hemispheric frontal lobes, basal ganglia and thalamus, seizure disorders, and infections or inflammation of the brain.
  • endocrinological disorders such as hyperthyroidism, Cushing's disease and pheochromocytoma.

There are also various drugs that can cause or mimic a Bipolar mood episode (Major Depressive, Hypomanic, Manic or Mixed Episode), including:

Antidepressants, substance abuse and hypothyroidism all seem to increase the risk of Rapid Cycling in patients with Bipolar Disorder [ref, ref].

 

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Diagnosis

Course & Prevalence