By asking this question, I’m betraying my rather large gap in knowledge of human anatomy and brain chemisty. But,as I understand it, neurotransmitters such as serotonin and norepinephrine lie at the heart of illnesses such as depression. So why can’t psychiatrists perform blood tests to check for abnormally low levels of these chemicals? Is it that these chemicals aren’t present in the bloodstream due to the blood-brain barrier, or is there no normative concentration of them that everyone can agree on?
In fact, why don’t all vists to psychiatrists involve neural imaging and blood tests to determine if there are in fact abnormalities present? Why did my recent visit to a psychiatrist only involve an hour-long diagnostic interview?
You hit on one of the reasons: the neurotransmitters don’t all cross the blood-brain barrier. Plus, there’s really no specific correlation to everything.
Shakester mentions depression: while the latest class of drugs that treat it (SSRIs or SNRIs) do so by making serotonin available longer, they do this nearly instantaneously. And, yet, it takes weeks or months for any real effect. Plus, many of the older drugs work in different ways. One antidepressant actually decreases the amount of serotonin available. And, some people find that these antidepressants work better.
Blame the pharmaceutical companies that want to simplify depression into a lack of serotonin. But we’re pretty sure that’s not the cause, at least, not entirely. I personally don’t even think depression is one illness, but several different ones that have similar effects on mood. Heck, some people find that anesthetics relieve the problem.
In addition to the blood-brain barrier making correlation between blood levels and brain levels more than problematic, you must also take into account that the various neurotransmitter deficiencies (as in depression), or excesses (as in schizophrenia) postulated to underlie various mental illnesses, are not distributed uniformly throughout the brain.
Illustrative examples of this concept of focal, site-specific changes in neurotransmitter levels include addictive behaviour, schizophrenia, and Parkinson’s Disease (with, of course, Parkinson’s Disease not being a “mental illness” but still a prototype for diseases arising from a localized abnormality in neurotransmitter levels).
So it seems that depression is a lot more complex and less understood than I thought.
Here’s a followup question, though, that occurred to me. If SSRIs work by keeping more serotonin in the brain longer, how is it not reckless to prescribe these medications in light of the potential for serotonin syndrome? Other medications that are prescribed to supplement a chemical that is missing from the body (e.g., the thyroid hormone) or suppress a chemical (e.g. cholesterol-lowering drugs) aren’t given to patients unless the doctor is sure that the patient is, in fact, deficient or abundant in those chemicals. It seems supplementing a brain chemical would require a similar level of caution. Hence, my question about blood tests.
Your question about blood tests is a reasonable one.
The reason they don’t give them is because there is NOT ONE SINGLE metabolic test that can effectively rule out or effectively corroborate a diagnosis of ANY of the afflications dubbed “mental illness”. In other words if we limit ourselves to what we can demonstrate to exist via direct metabolic testing, we can’t even assert that there is any such thing.
There’s a pretty high confidence level that there exists something insofar as the pattern of observable behavior is, in fact, a pattern. But all diagnoses are done by observing behaviors and comparing them to the known patterns; there’s no test for “bipolar transcripterase” in the bloodstream or “n-alkyl-schizophrenocholamine” in the bloodstream or anything like that.
I would also point out that the psychiatric profession serves a dual-pronged purpose: it not only attempts to provide direct services to sufferers who seek help for their own problems, it also serves as the explanation and justification for removing disturbing people for the benefit of the people that they disturb. Were a blood test to be designed, the blood test would become the specific diagnosis for the mental illness for which it tests, and we would most likely see two things occur:
• People whose behavior is profoundly disturbing could no longer be removed and held under the excuse that based on their disturbing behavior they “must be” schizophrenic or bipolar or etc; so either they’d have to invent new categories of “mental illness” that could be diagnosed but for which there are not, as of yet, blood tests, or society would have to acknowledge head-on that we routinely apprehend and incarcerate “disturbing” people who have not committed any crime.
• Meanwhile, as it became apparent that many people who test positive via the blood test for a mental illness are neither particularly dangerous nor too incapacitated to function in society, there would be more of a challenge to the idea that having such a diagnosis legitimates being subjected to forced treatment or any different treatment for that matter unless the behavior itself, qua behavior, makes an intervention appropriate.