If a Martian with human-like anatomy and physiology visited Earth, how would humans explain why pulmonologists treat lungs, cardiologists treat hearts, and nephrologists treat kidneys, but neurologists treat some brain conditions and psychiatrists treat other brain conditions? That’s the question Thomas J. Reilly, BSc, MBChB, of the University of Glasgow in the United Kingdom, posed in a thought experiment published in the BJPsych Bulletin.1

His point is plain: “It is difficult to rationally explain to someone with no prior frame of reference why we have the split between neurological and psychiatric illness.”

Neurology typically focuses on conditions with physical markers, such as neuropathological lesions, and psychiatry focuses on abnormal brain function determined through observable symptoms, Dr Reilly notes. However, he points out that epilepsy fell under the purview of psychiatrists until its neuropathology became clear and neurologists began treating it. Magnetic resonance imaging (MRI) and other tests are similarly starting to reveal potential biomarkers for schizophrenia and autism.

Trying to delineate the disciplines with different symptomology falls flat, too. Recently identified N-methyl-D-aspartate receptor encephalitis is “clinically indistinguishable from the first episode of schizophrenia” despite having a clear neurologic pathophysiology, Dr Reilly writes. Some symptoms overlap, such as hallucinations in psychosis and Parkinson disease, depression in mood disorders and multiple sclerosis, or a variety of symptoms in different types of dementia.

“Current classification is based on convention, tradition, and quirks of history,” Dr Reilly writes. “Nature does not respect our arbitrary categorizations and neither do our patients.”

Dr Reilly is not alone in his thoughts. Since his editorial was published in 2015, several more articles have similarly questioned the division and proposed either merging the 2 specialties or at least designing a curriculum for a third path that brings them together in neuropsychiatry or behavioral neurology.2 Although such joint programs exist throughout the world, including in the United States, they are far from the standard.

Historical Underpinnings and Global Variation

Historically, a single discipline, neuropsychiatry, dominated study and care of brain disease, particularly in the 19th century. However, neurology and psychiatry began to diverge gradually around the 1930s or, especially after the 1960s, depending on who you talk to and what country you’re talking about.3,4

Glen Elliott, PhD, MD, chief psychiatrist and medical director of Children’s Health Council in Palo Alto, California, puts the beginning of the split even earlier, largely as a result of the influence of Freud’s theories at the end of the 19th century.

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“Neurology has always been solidly in the ‘real medicine’ field, whereas psychiatrists, particularly when we were heavily into psychoanalysis,” became more distant from the rest of medicine, Dr Elliott told Neurology Advisor. “That rift has gradually begun to heal as both neurologists and psychiatrists have acquired a broader array of effective interventions and as conceptualizations about psychiatric illnesses have focused more on brain dysfunction rather than on upbringing.”

Family influences still play a major role in psychiatry, he said, particularly in his field of child psychiatry, where helping parents make certain changes to their parenting methods can be crucial to effective treatment. “But that’s not the way neurologists tend to think,” he said.

For the most part, the fields have distinct approaches, philosophies, research principles, and treatment methods to nervous system disorders.3 Psychiatrists, for example, “favored symptom descriptions over laboratory tests,” writes Agustín Ibáñez, PhD, of Favaloro University’s Institute of Cognitive and Translational Neuroscience in Buenos Aires, Argentina, and colleagues.3

Dr Elliott would agree. Neurologists seek identifiable pathology by methods such as computed tomography scans, MRI, electroencephalography, magnetic encephalograms, and other more objective tests, he said, whereas psychiatrists lack useful biomarker tests for psychiatric disorders.

“Even the neurological examination is much more precisely symptom oriented, whereas the psychiatry examination is more about mental status,” Dr Elliott said. “Neurologists have all sorts of lovely physical signs that help them locate what part of the brain might be affected and what level of the nervous system may be involved.”

In some ways, the difference between a neurologic vs a psychiatric condition is how concrete or nebulous its diagnosis is.

“Psychiatry tends to be responsible for disorders where they markedly affect behavior but there’s no clear way of identifying what’s causing it except clinical interview,” Dr Elliott told Neurology Advisor.

Historical examples, such as Dr Reilly’s epilepsy example, bear this out. When Dr Elliott was in medical school, Parkinson disease was largely considered a psychiatric disorder — until physicians had specific treatments for it and a reasonable hypothesis about its underlying mechanism. It’s not just neurology that gets disorders previously under psychiatry’s domain: the psychosis caused by tertiary syphilis was a major reason for psychiatric institutionalization until its cause was recognized as a treatable infectious disease.

“It no longer was a psychiatric issue because we had a defined cause and a specific treatment,” Dr Elliott said. Similar examples include sleep disorders such as narcolepsy and sleep apnea, which now fall under neurology or otolaryngology.

The distinctions between neurology and psychiatry vary considerably across the world, Dr Elliott notes. Italy has largely included psychiatry under neurology’s umbrella, he said. Dr Reilly, from the United Kingdom, notes in his review that conversion disorder, with its neurologically unexplained symptoms, has no neuropathologic explanation, yet is treated by neurologists.5 In the United States, however, Dr Elliott said patients with conversion disorder are referred to psychiatrists as soon as the diagnosis is made.

The United States already recognizes the overall link between the fields institutionally: board examinations are managed by the American Board of Psychiatry and Neurology. Although neurology and psychiatry board examinations remain distinct, a few of the subspecialties overlap the two.