Confirmed: Neurologic, Psychiatric Disorders Distinct

Fran Lowry

November 09, 2015

A meta-analysis examining neuroimaging studies of neurologic and psychiatric disorders concludes that they are distinct illnesses.

"The basal ganglia, insula, lateral and medial temporal cortex, and sensorimotor areas showed greater impairment in neurological disorders; whereas the medial frontal cortex, anterior and posterior cingulate, superior frontal gyrus and occipital cortex (bilateral lingual gyrus and left cuneus) showed greater impairment in psychiatric disorders," the investigators, led by Nicolas A. Crossley, PhD, from Kings College London, United Kingdom, write.

The study was published in the November issue of the British Journal of Psychiatry.

Over the past few years, the distinction between psychiatric and neurologic disorders has been called into question, the investigators note.

"It has been long known that neurological disorders can present with affective or psychotic symptoms traditionally thought to be specific to psychiatric disorders and that psychiatric disorders present motor symptoms more frequently seen in a neurology clinic," they add.

Recently, brain imaging has provided an "in vivo" window into the human brain, to reveal that both neurologic and psychiatric disorders are linked to neuroanatomic and neurofunctional changes.

Some researchers have suggested that the traditional distinction between disorders of the mind and disorders of the brain is a misconception and have called for a "radical rethinking" that would reclassify psychiatric disorders as disorders of the central nervous system.

"We acknowledge that the distinction between the fields of psychiatry and neurology involves multiple factors, ranging from social and historical to biological, and that any new classification should ultimately reflect an improvement in clinical outcomes," the investigators write. "However, it is imperative that this debate is informed by scientific evidence including the biology underpinning the two classes of disorders."

The aim of the study was to determine whether there is any neuroimaging evidence for the distinction between neurologic and psychiatric disorders.

The meta-analysis included voxel-based morphometry studies reporting decreased gray matter associated with 14 neurologic and 10 psychiatric disorders.

The authors compared the regional and network-level alterations for these two classes of disease and also estimated neuroanatomic heterogeneity within and between the two classes.

In addition to finding that the basal ganglia, insula, lateral and medial temporal cortex, and sensorimotor areas showed greater impairment in patients with neurologic disorders and that the medial frontal cortex, anterior and posterior cingulate, superior frontal gyrus, and occipital cortex showed greater impairment in patients with psychiatric disorders, the investigators also found there were structural differences between the two classes of disorders. Neurologic disorders were evident in the sensorimotor and frontoparietal networks, and the effect of psychiatric disorders was evident in the visual and default-mode networks, the researchers report.

Disorders within each class, either neurologic or psychiatric, were more similar to each other in terms of neuroanatomic alterations than disorders belonging to different classes. In psychiatric disorders, the dissimilarity was greater than in neurologic disorders.

The investigators note that the study had several limitations, including possible selection bias, the fact that the two classes of disorders were compared only in terms of gray matter volume, and the fact that the effects of age and sex in the two different disorders were not taken into account.

Notwithstanding these limitations, they conclude that they have shown some divergent neuroimaging findings in neurologic and psychiatric disorders and that this suggests that neurologic and psychiatric disorders represent two distinct entities from a neuroimaging perspective.

"Novel and Ingenious"

In an accompanying editorial, Anthony S. David, MD, and Timothy Nicholson, PhD, from King's College London, describe the study as "novel and ingenious" and suggest "that we need to continue to use every ounce of our brains to answer the question of what makes a psychiatric disorder."

"The results may be taken as a slap in the face to the distinction abolitionists, yet such individuals might take heart in them in that there is no implicit hierarchy in what emerges as the brain-based hallmark of neurological versus psychiatric conditions; they both involve the functionally interesting parts of the brain, it is just that they are, quite subtly, different," they write.

The authors and editorialists report no relevant financial relationships.

Br J Psychiatry. 2015;207:429-434, 373-374. Full text, Editorial

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