What is a “functional” psychiatric disorder?

I have recently between engaged in a twitter discussion with Duncan Double (a retired British psychiatrist), who insists on using the organic/functional distinction when it comes to psychiatric disorders and seems to think that this distinction justifies the claim that functional psychiatric disorders have no biological causes and have only psychosocial causes.

In order for this debate to go anywhere, I think we need to be very specific about the meaning of the terms. What exactly does “functional” mean?

For the record, I am not in favor of using the term “functional” as a general descriptor for psychiatric conditions. I think it is too conceptually muddled and has a lot of unsavory philosophical baggage. Nonetheless, I need to define it first if I am to meaningfully discuss the concept.

I think there are two broad ways of defining the term "functional".

Meaning 1 (M1): A psychiatric condition is functional if it cannot be attributed to an “independently diagnosable cerebral or systemic disease”. This definition is essentially an inverse of how Lipowski (1990) and ICD-10 defined the term 'organic'. Unfortunately, disease is a very loaded term itself and it can be defined narrowly as well as broadly. As far as I can see, a reasonable interpretation would be that “disease” here refers to some sort of lesion/abnormality in the anatomical structures (stroke, tumor, etc.) or in the biochemical and physiological processes (abnormal electrical discharge, for instance in a seizure).

Lipowski elaborates that M1 is still consistent with the assumption that “all mental disorders involve cerebral processes.” Duncan Double agrees and also states that functional mental disorders have a “cerebral substrate”. Another way that Double describes it is as “mental disorders show through the brain but not necessarily in the brain.”

M1 can be narrowed down to two further meanings:

Meaning 1A (M1A): No "independently diagnosable cerebral or systemic disease" has yet been demonstrated, but it is possible that such a disease process may be identified in the future.

Meaning 1B (M1B): The condition is not the result of an “independently diagnosable cerebral or systemic disease”. Period.

It's difficult to make the claim M1B conclusively since the diagnosis of a cerebral disease is always dependent on currently available technology. For instance, two decades ago, psychosis due to NMDA-receptor encephalitis would've been considered “functional” simply because NMDA-receptor encephalitis had not yet been discovered. In a textbook from 1893, Parkinson's disease and epilepsy were described as "functional" simply because no visible lesion had been identified at that time.

From an epistemic perspective, the only claim that can be reasonably advanced is M1A. This does not mean that all conditions described as "functional" will eventually turn out to be the result of a cerebral disease; the majority will not. Rather the intended meaning is that there will be a non-zero probability that a functional disorder in question could be the result of an undiscovered cerebral disease, and if and when such a cerebral disease is discovered, then the disorder would no longer be considered "functional".  

Meaning 2 (M2): The disorder lacks a significant biological component in its etiology. Biological factors cannot meaningfully be said to be the cause or among the causes of the condition.

M1 and M2 are clearly distinct. Disease processes only account for a small subset of biological factors and biological processes. There is also a logical asymmetry: while M2 implies M1, M1 doesn’t logically imply M2. M1 allows for the possibility that functional disorders can nonetheless be caused by biological processes.

In a way M1 and M2 can even be considered contradictory if the notion that "all mental disorders involve cerebral processes" is seen as an integral part of M1. If a condition involves cerebral processes, how can it lack a significant biological component in its etiology?

Yet, despite this apparent contradiction, many folks such as Duncan Double seem to think that M2 goes hand in hand with M1. Double has clearly stated on several occasions that mental disorders do not have biological causes and have only psychosocial causes.

Although I've not been able to get any clear elaboration from Double himself, perhaps one possibility is something along the lines of: in functional mental disorders, biological components are involved in the etiology only to the extent to which biological components can be said to be involved in the etiology of "normal" psychological processes, behaviors, emotions, etc.

Consider blushing. It's the reddening of a person's face typically due to certain intense emotions. What “causes” blushing? In Aristotelian metaphysical lingo, the “material cause” of blushing would be the physiological chain of events that leads from an emotional stimulus (such as embarrassment) to the activation of the sympathetic nervous system and vasodilation, particularly in regions of cheeks and forehead. The “efficient” cause of blushing would be the emotional stimulus itself.

Is blushing "functional" in the sense of M2? If our notion of etiology and causation includes material causes, then clearly blushing is not functional, however, if our notion of etiology and causation excludes material causes and includes only efficient cause, then we can say that biological factors are not the efficient cause of blushing, psychological factors are.

Now consider “idiopathic craniofacial erythema”, a medical condition in which severe and uncontrollable blushing happens unprovoked or is provoked by mundane events. Is idiopathic craniofacial erythema "functional" (M2)? It is hypothesized that idiopathic craniofacial erythema happens as a result of an overactive sympathetic nervous system. If this hypothesis is correct, then biological factors predominantly account for the difference between “normal blushing” and “blushing disorder”. Blushing disorder is not functional (M2), even though blushing as a normal phenomenon is.  

So, let’s articulate an expanded M2:

Meaning 2* (M2*): A condition is functional if it can be explained satisfactorily by efficient and/or material causes which are psychological/non-biological. A condition is not functional if it cannot be explained satisfactorily be a psychological efficient cause and has either a biological efficient cause or a satisfactory explanation requires biological material causes.

An important point to note is that this definitional claim requires empirical evidence. In order to call something functional, we need to have some scientific explanation of the etiology of the condition. If the etiology is unknown, we cannot call it functional (M2).

Take the example of depressive disorder, which involves depressed mood most of the day, nearly every day, and/or loss of interest or pleasure in most or all activities, nearly every day. Is this functional or not? If we follow the blushing analogy, I suppose the answer depends on whether the depressive disorder is best explained by a psychological efficient cause (excessive psychosocial stress, such that in the absence of stress there would’ve been no depression) or by a biological material cause (the brain mechanisms are unusually sensitive to average levels of stress or the brain mechanisms are triggering sadness/anhedonia even in the absence of a psychosocial trigger due to, say, alterations in serotonergic transmission). Material causes can also be psychological (cognitive errors, negative automatic thoughts), or they could be a combination of biological and psychological (excessive negative automatic thoughts leading to increased inflammatory cytokines), or they can even be simultaneously both (the same process from one perspective is biological and from another perspective is psychology… fMRI studies of cognitive tasks, for example).   

Is depressive disorder functional? It can be functional in the sense of M1A as long as there is no identifiable medical disease, but we can’t say if it is functional in the sense of M2* unless we already know the etiology. Some instances of depressive disorder may be satisfactorily explained by psychological efficient and material causes, other instances may not be. The bulk of scientific evidence suggests that efficient and material causes of major depressive disorder (as well as other major psychiatric disorders) are spread out over multiple levels such as genetic factors, neurochemical transmission, personality factors, cognition, social stressors, etc.

The important thing is that the claim “Major depression is functional (M2*)” is an empirical claim; it does not logically flow from M1. The assertion needs to be supported by scientific evidence. The characterization of mental disorders as “functional” (M2 or M2*) cannot be taken for granted and it cannot simply be assumed, and it cannot simply be defined into existence.  

Before I end, I also want to consider another statement that Double frequently makes: “The essential position of critical psychiatry is that functional mental illness should not be reduced to brain disease.”

If by functional we mean M1 and by disease we mean structural/physiological abnormality, then this statement is simply a tautology, because "functional mental illness" is precisely defined by the absence of "brain disease". If by functional we mean M2* and by disease we mean “biological causes”, then this is again a tautology.

I suspect what Double really wants to say is: “Mainstream psychiatry considers mental illness to be the result of a brain disease but in fact these disorders are functional.” If by functional he means M1, then that doesn’t really threaten a neuroscientific understanding of psychiatry because M1 allows for the possibility that functional disorders can nonetheless be caused by biological processes. If by functional he means M2, then he needs to resolve the contradiction between "mental disorders involve cerebral processes" and "mental disorders have no biological causes". If he means something like M2*, then that is an empirical claim, and he needs to present scientific evidence in support and refute the existing scientific evidence pointing to the contrary that biological causes are among the causes of psychiatric disorders.