Neurology and Psychiatry are two distinct specialties within medicine, both of which treat disorders of the brain. It's completely uncontroversial to say that neurologists treat patients with brain disorders like Alzheimer's disease and Parkinson's disease. These two diseases produce distinct patterns of neurodegeneration that are visible on brain scans. For example, Parkinson's disease (PD) is a movement disorder caused by the loss of dopamine neurons in the midbrain.
Fig. 3 (modified from Goldstein et al., 2007). Brain PET scans superimposed on MRI scans. Note decreased dopamine signal in the putamen and substantia nigra (S.N.) bilaterally in the patient.
It's also uncontroversial to say that drugs like L-DOPA and invasive neurosurgical interventions like deep brain stimulation (DBS) are used to treat PD.
On the other hand, some people will balk when you say that psychiatric illnesses like bipolar disorder and depression are brain disorders, and that drugs and DBS (in severe intractable cases) may be used to treat them. You can't always point to clear cut differences in the MRI or PET scans of psychiatric patients, as you can with PD (which is a particularly obvious example).
The diagnostic methods used in neurology and psychiatry are quite different as well. The standard neurological exam assesses sensory and motor responses (e.g., reflexes) and basic mental status. PD has sharply defined motor symptoms including tremor, rigidity, impaired balance, and slowness of movement. There are definitely cases where the symptoms of PD should be attributed to another disease (most notably Lewy body dementia)1, and other examples where neurological diagnosis is not immediately possible. But by and large, no one questions the existence of a brain disorder.
Things are different in psychiatry. Diagnosis is not based on a physical exam. Psychiatrists and psychologists give clinical interviews based on the Diagnostic and Statistical Manual (DSM-5), a handbook of mental disorders defined by a panel of experts with opinions that are not universally accepted. The update from DSM-IV to DSM-5 was highly controversial (and widely discussed).
The causes of mental disorders are not only biological, but often include important social and interpersonal factors. And their manifestations can vary across cultures.
Shortly before the release of DSM-5, the former director of NIMH (Dr. Tom Insel) famously dissed the new manual:
The strength of each of the editions of DSM has been “reliability” – each edition has ensured that clinicians use the same terms in the same ways. The weakness is its lack of validity. Unlike our definitions of ischemic heart disease, lymphoma, or AIDS, the DSM diagnoses are based on a consensus about clusters of clinical symptoms, not any objective laboratory measure.
In other words, where are the clinical tests for psychiatric disorders?
For years, NIMH has been working on an alternate classification scheme, the Research Domain Criteria (RDoC) project, which treats mental illnesses as brain disorders that should be studied according to domains of functioning (e.g., negative valence). Dimensional constructs such as acute threat (“fear”) are key, rather than categorical DSM diagnosis. RDoC has been widely discussed on this blog and elsewhere — it's the best thing since sliced bread, it's necessary but very oversold, or it's ill-advised.
What does this have to do with neurology, you might ask? In 2007, Insel called for the merger of neurology and psychiatry:
Just as research during the Decade of the Brain (1990-2000) forged the bridge between the mind and the brain, research in the current decade is helping us to understand mental illnesses as brain disorders. As a result, the distinction between disorders of neurology (e.g., Parkinson's and Alzheimer's diseases) and disorders of psychiatry (e.g., schizophrenia and depression) may turn out to be increasingly subtle. That is, the former may result from focal lesions in the brain, whereas the latter arise from abnormal activity in specific brain circuits in the absence of a detectable lesion. As we become more adept at detecting lesions that lead to abnormal function, it is even possible that the distinction between neurological and psychiatric disorders will vanish, leading to a combined discipline of clinical neuroscience.
Actually, Insel's view dates back to 2005 (Insel & Quirion, 2005)....2
Future training might begin with two post-graduate years of clinical neuroscience shared by the disciplines we now call neurology and psychiatry, followed by two or three years of specialty training in one of several sub-disciplines (ranging from peripheral neuropathies to public sector and transcultural psychiatry). This model recognizes that the clinical neurosciences have matured sufficiently to resemble internal medicine, with core training required prior to specializing.
...and was expressed earlier by Dr. Joseph P. Martin, Dean of Harvard Medical School (Martin, 2002):
Neurology and psychiatry have, for much of the past century, been separated by an artificial wall created by the divergence of their philosophical approaches and research and treatment methods. Scientific advances in recent decades have made it clear that this separation is arbitrary and counterproductive. .... Further progress in understanding brain diseases and behavior demands fuller collaboration and integration of these fields. Leaders in academic medicine and science must work to break down the barriers between disciplines.
Contemporary leaders and observers of academic medicine are not all equally ecstatic about this prospect, however. Taylor et al. (2015) are enthusiastic advocates of a move beyond “Neural Cubism”, to increased integration of neurology and psychiatry. Dr. Sheldon Benjamin agrees that greater cross-discipline training is needed, but wants the two fields to remain separate. But Dr. Jose de Leon thinks the psychiatry/neurology integration is a big mistake that revives early 20th century debates (see table below, in the footnotes).3
I think a distinction can (and should) be made between the research agenda of neuroscience and the current practice of psychiatry. Neuroscientists who work on such questions assume that mental illnesses are brain disorders and act accordingly, by studying the brain. They study animal models and brain slices and genes and humans with implanted or attached electrodes and humans in scanners. And they study the holy grail of neural circuits using DREDDs and optogenetics. This doesn't invalidate the existence of social, cultural, and interpersonal factors that affect the development and manifestation of mental illnesses. As an non-clinician, I have less to say about medical practice. I'm not grandiose enough to claim that neuroscience research (or RDoC, for that matter) will transform the practice of psychiatry (or neurology) in the near future. [Though you might think differently if you read Public Health Relevance Statements or articles in high profile journals.]
Basic researchers may not even think about the distinction between neurology and psychiatry. Is the abnormal deposition of amyloid-β peptide in Alzheimer's disease (AD) an appropriate target for treatment? Are metabotropic glutamate receptors an appropriate target in schizophrenia? These are similar questions, despite the fact that one disease is neurological and the other psychiatric. There are defined behavioral endpoints that mark treatment-related improvements in either case. It's very useful to measure a change in amyloid burden4 using florbetapir PET imaging in AD [there's nothing similar in schizophrenia], but the most important measure is cognitive improvement (or a flattening of cognitive decline).
Does Location Matter?
In response to the pro-merger cavalcade, a recent meta-analysis asked whether the entire category of neurological disorders affects different brain regions than the entire category of psychiatric disorders (Crossley et al., 2015). The answer was why yes, the two categories affect different brain areas, and for this reason neurology and psychiatry should remain separate.
I thought this was an odd question to begin with, and an even odder conclusion. It's not surprising that disorders of movement, for example, involve different brain regions than disorders of mood or disorders of thought. From my perspective, it's more interesting to look at where the two categories overlap, with an eye to specific comparisons (not global lumping). For instance, are compulsive and repetitive behaviors in OCD associated with alterations in some of the subcortical circuits implicated in movement disorders? Why yes.
But let's take a closer look at the technical details of the study.
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