Based on my previous question about ADHD and OCD.
Essentially, ADHD and OCD are kind of opposites in the sense that ADHD people (always? sometimes?) have dopamine deficiency and OCD people (always? sometimes?) have excess dopamine.
I know this may be a stupid question or based on incorrect understanding of bipolar, but how do psychologists know that bipolar is one disorder rather than a comorbidity of mania and depression?
Short answer: Bipolar disorder is probably not composed of two comorbid illnesses, but it may be on a continuum that includes some depressive disorders.
This is a good question, though it does convey some confusion associated with this diagnosis that should be cleared up.
Bipolar symptoms:
The first confusion I think is the idea that "depression", "mania", and "bipolar" are disorders. Depression and mania are not disorders - they are symptoms. And bipolar is a "spectrum" disorder - a collection or continuum of similar diagnoses, that all feature at least one manic or hypomanic episode, and in most cases at least one depressive episode.
It is true that depression without mania is one of the key symptoms of a variety of depressive mood disorders (such as major depressive disorder, sometimes referred to as "unipolar" because of the lack of manic episodes), so it may be confused with a disorder. However, this is not the case for mania, so diagnostically bipolar disorder would not be confused with a comorbidity of 2 disorders - depression and mania - because mania is not a disorder.
Mania rarely presents without other symptoms:
Although bipolar disorder is by far the most common cause of mania, it is a key component of other psychiatric conditions (e.g., schizoaffective disorder, bipolar type; cyclothymia) and may occur secondary to neurologic or general medical conditions, or as a result of substance abuse.
Presumably, a standalone manic episode may be sufficient for a diagnosis of bipolar I disorder or a "not otherwise specified" (NOS) diagnosis, depending on the severity and other symptoms involved. So diagnostically, "mania" as a standalone disorder is actually a sub-type of bipolar.
Bipolar unitarity:
The second confusion I think is the idea that a bipolar diagnosis in some way precludes the symptoms of depression and mania from having separate causes.
Historically, bipolar disorder was not seen as 2 separate illnesses because (1) it was believed that mania and depression were mutually exclusive in time - as 2 extremes (poles) of a single mood continuum, and (2) mania rarely presents without depression. The assumption of mutual exclusivity of mania and depression was called into question by the occurrence of mixed-state episodes - a condition that features symptoms of both mania and depression simultaneously.
Ultimately, many psychiatric diagnostic criteria are encumbered by some historical precedence, common clinical presentations, how patients perceive their symptoms, and also some political influence, as diagnoses are associated with healthcare subsidization, stigma and prejudice, and explanatory power for the patient. So diagnostic criteria should not generally be confused with an understanding of common cause.
The cause of bipolar spectrum disorders is not well understood, and the question of whether unipolar and bipolar disorders are distinct, or part of a single continuum, remains a hotly debated topic. The very common co-occurrence of depressive episodes whenever manic episodes are present suggests that they may be related in some way. On the other hand, depression disorders and bipolar disorders are treated with different medications, suggesting that they may be distinct disorders - depression is typically treated with anti-depressants, while bipolar depression may be more effectively treated with mood stabilizers. The disorders have some predictors (genetic, environmental, comorbidities) in common, and some different. It's also entirely possible that both disorders incorrectly encompass several unrelated sub-types.
The term "bipolar" is used to describe someone who has both manic and depressive episodes - so it is really a name for the combination of the two conditions.
Before bipolar became the popular name for this, it was described as mixed affective state and was characterised as being dysphoric mania or agitated depression depending on whether it was mostly manic or mostly depressive.
I believe it is widely-accepted now that mania and depression are not the opposite ends of a long scale, but are independent measures.
In short, I don't believe you can answer this in general - I believe it should be considered on a case by case basis.
If you want closure / a "correct" answer, the APA (V) lists bipolar as an Axis I disorder, which suggests their view is a single illness, rather than comorbidity, using the psychological definition of comorbidity. In psychology, comorbidity would be more than one diagnosis. If you can diagnose someone as "bipolar" there would be no comorbidity.
It isn't possible to apply the medical definition of comorbidity in psychology (as of 2015) because you can't detect individual diseases in a lab at this time, so bear in mind that the use of the term is highly interpretive in terms of psychology.
I would question whether for an individual it is more beneficial to treat the two aspects of their condition separately or as a whole. For example, you may decide to only treat the depression and not attempt to suppress the elevated moods - so are you treating the individual for bipolar here, or just for depression?
I am bipolar and I see my mental difference as one unit.
One thing to remember is that the term "bipolar" is just a label for people that exhibit the mood symptoms that are observable. As most bipolar people know, there are a lot of other symptoms such as fatigue/excess energy, dizziness, confusion, short term memory issues, pain, numbness, migraines... And the fact that a lot of us experience these same symptoms leads me to believe that they are all related to a singular cause in some way.
If you think about people with MS, they all experience similar symptoms that many docs attribute to lesions in the brain and such. Since these can often be seen with an MRI, docs have now named the disease based on the cause not the symptoms.
Unfortunately, we can't see the chemicals moving around the brain or the synapses at work. So until we can, we only label the condition based on the two symptoms that cause the most disruption.
As such, these mental differences have been treated for a long time using the spaghetti-to-wall technique. Throw meds at the patient to see which ones stick. And that's how it had to be since it wasn't until recently that they've been able to use better MRI imaging techniques and such to see more of the structure of the brain.
This was how most "research" was done in the field. "We think this drug targets dopamine, so therefore if a patient responds to this drug then they must have a dopamine problem which means that bipolar in general must involve dopamine." Not the most accurate way to do research, but it was the only way possible for a very long time.
So with all this said, I believe that the symptoms originate from a singular cause. However, we are talking about the brain which is the most complex organ known to man so anything is possible.
http://www.nimh.nih.gov/health/publications/bipolar-disorder-in-adults/index.shtml
In addition, people with bipolar disorder are at higher risk for thyroid disease, migraine headaches, heart disease, diabetes, obesity, and other physical illnesses.12,13 These illnesses may cause symptoms of mania or depression, or they may be caused by some medications used to treat bipolar disorder.
The most we can say is that, in general, certain psychiatric illnesses probably involve abnormalities in specific brain chemicals; and that by using medications that affect these chemicals, we often find that patients are significantly improved.
