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What causes Bipolar Disorder and Attention-Deficit Hyperactivity Disorder (ADHD)?
As with many mental health disorders, scientists are still somewhat uncertain of the causes of both bipolar disorder and ADHD in adults. This stems from, among other things, the complex array of factors that appear to be implicated, the wide range and variability of symptoms and the difficulty of measuring brain chemistry in living subjects.
Both disorders are, however, strongly linked to genetics. Bipolar has a heritability level around 0.7 while for ADHD it's somewhere around 0.75 to 0.9. There is no single gene that triggers either of the disorders, though; instead it is a collection of genes. There are also likely to be physical and social factors involved in the development of both disorders, such as neurotransmitter imbalance, home environment and stress triggers .
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Symptoms and Definition
Bipolar and ADHD can be confused if careful diagnosis is not carried out. There are a number of similar symptoms that manifest in both cases. Impulsive behaviour, hyperactivity, aggression, defiance of authority, motor restlessness during sleep and appearing distant or uncommunicative in phases are all possible indicators of both disorders. Because of this, it is easy to see how there may be a link between the two.
It should also be noted, however, that both disorders are behaviourly defined. While a common cold has a definite cause, there is no virus that causes ADHD or bipolar. Because of this, we are left with a situation where the disease is defined by the symptoms displayed - and because neither can be diagnosed from a single symptom and both involve complex sets of behaviours it is not surprising that there is some overlap.
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Is there a Link Between Bipolar Disorder and ADHD in Adults?
ADHD is more common in children than adults, with perhaps half of those who display symptoms in childhood continuing to suffer through adulthood. In contrast, few are diagnosed with bipolar until at least adolescence and in most cases not until the subject is fully developed – the mean age for diagnosis is 26. However, many of those whose ADHD symptoms do persist have other mental health problems, including (but not limited to) bipolar. It's estimated that around 20% of those with ADHD also suffer from bipolar.
A possible reason for the link is the effect of neurotransmitters such as dopamine. Several genes relating to the production and regulation of dopamine – DAT1, DRD4 & DRD5 – have been implicated in the onset of ADHD, while there is a growing body of evidence that cyclical surges of dopamine are related to the manic/depressive episodes in bipolar disorder. Norepinephrine is another neurotransmitter that appears to be related to both ADHD and mood disorders involving manic and depressive phases.
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To further muddy the waters, there are many researchers who claim that diagnoses of bipolar disorder and ADHD in adults are overused, while some even go so far as to claim they have no scientific basis. Both disorders are so complex with such a wide spectrum of symptoms that the labels themselves can appear quite arbitrary.
In trying to understand the link between ADHD and bipolar this can actually be a helpful point of view. Attention deficiency, periods of depression, periods of mania, sleep disruption, hyperactive tendencies and periods of low energy are just some of the symptoms that appear to be associated with neurotransmitter imbalance. Because of the complexity of our brains no two cases are likely to be the same; instead there is a spectrum of possible symptoms and ADHD and bipolar disorder are like two colours on the same rainbow.
To summarize - the link between ADHD and bipolar disorder is unclear; however they do both appear to be related to levels of certain neurotransmitters in the brain. They share a number of symptoms and are often confused and can both be seen as part of the same spectrum of mental disorder. Patients with ADHD in adulthood have around a 20% likelihood of also having bipolar disorder, however many researchers argue that both diagnoses are overused and argue for a more.
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Attitude Mag, "Is It More Than ADHD? Diagnosing & Treating Bipolar Disorder", http://www.additudemag.com/adhd/article/2511.html
BBC News, "Scientists Make Bipolar Gene Find", http://news.bbc.co.uk/1/hi/wales/6728133.stm
Edvardsen J et al. (2008) "Heritability of bipolar spectrum disorders. Unity or heterogeneity?" Journal of Affective Disorders
King's College London, "The ADHD Genetic Group", http://www.iop.kcl.ac.uk/departments/?locator=335
Medwire News, "Dopamine hypothesis for bipolar disorder supported by review," http://www.medwire-news.md/55/69080/Bipolar_Disorder/Dopamine_hypothesis_for_bipolar_disorder_supported_by_review.html