For mental illness and disorders we mostly lack clear biological markers or pathology, and so we have had to make do with clinical descriptions – lists of signs and symptoms. This is very much a descriptive phase of scientific understanding.
What almost every popular article I read on the subject gets terribly wrong, however, is in characterizing this as a unique feature of psychiatry, unlike the rest of medicine. A recent Wired article, for example, writes:
In most areas of medicine, diagnoses are based on the cause of illness. Heartburn and heart attacks both cause chest pain, but they’re different diagnoses because they have different underlying causes.
At least they added the qualifier “most”, but even that is misleading.
In fact most disorders and medical illnesses begin their life as a description of signs and symptoms – a purely clinically defined entity. Scientists then investigate possible causes, with the full spectrum of success. For some illnesses we have very little idea, nothing but guesses, about the cause and pathology. In others we have a completely fleshed out model of what is happening, down to the most reductionist level.
The Wired article also notes:
What doctors now diagnose as schizophrenia may in fact be several disorders with different causes that happen to produce an overlapping set of symptoms.
True, but this is also true of many medical illnesses. ALS (Lou Gehrig’s disease, which causes progressive weakness), for example, is a clinical syndrome. We don’t know the ultimate cause, so it is entirely defined clinically. It is very likely to be multiple pathophysiological diseases with a common manifestation.
Migraine headaches are another favorite example. They are diagnosed by a list of symptoms, just like DSM diagnoses. Migraine is likely many different underlying biological entities that all manifest in a similar fashion. It is also possible that underlying biological traits manifest in some people as classic migraines, in others as a different type of headache, and in still others with no symptoms.