Different mental Disorders : Psychology Issues and Debates

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Different mental Disorders

Syndromes / September 28, 2022

There have been recent calls to abandon the distinction between neurological and psychiatric disorders on philosophical and moral grounds. Crossley and colleagues, in this issue, meta-analyse published structural brain imaging data and prove that they are different after all – or do they?

Background

The study by Crossley and colleagues, in this issue of the BJPsych, is a rare and therefore welcome empirical contribution to the debate, recently revived, on whether the distinction between psychiatric and neurological disorders is real and/or should be abandoned. Psychiatric disorders are not just ‘mental’ but physical too.

The ‘reality’ of the psychiatry–neurology distinction, refers to the soundness of its theoretical basis. This might be best summarised by the following rule: if a disorder in question is reliably associated with a recognisable pathological process affecting the central nervous system (CNS), then it is neurological. But that of course just begs questions about what is meant by ‘reliably’? Does this mean ‘necessary and sufficient’ to cause the condition? What is ‘recognisable pathology’? Does this refer to the microscopic or macroscopic level? What then do we make of ‘quantitative change’ in the CNS in relation to some normative standard of a given magnitude? Is that ‘pathology’?

If we take schizophrenia, there is ample evidence of quantitative regional change in the CNS thanks to the widespread use of structural magnetic resonance imaging, but only insofar as this is detectable at the group level against a control group and according to some more or less arbitrary statistical threshold. The same can be said of affective disorder but at a rather lower statistical threshold. So are these neurological disorders? Perhaps, yet we do not see much interest among our neurology colleagues to start seeing these patients or engaging in research on them. What then of Alzheimer's disease? Clearly a neurological disorder and of interest to a few neurologists, but ‘core business’ for the old age psychiatrist and a shared topic of interest for research. Hence, the discussion moves rapidly from philosophy (is the distinction real?), through biology (is there evidence of a pathological process?), to sociology, culture and prejudice (whose job is it? who gets the credit/blame?).

Crossley et al's meta-analysis

The paper by Crossley and colleagues takes a novel approach. It is a meta-analysis of studies using voxel-based morphometry (VBM) to measure changes in grey matter in neurological and psychiatric diagnostic groups. So there are studies of diseases as diverse as multiple sclerosis and Huntington's disease, Asperger syndrome, anorexia and Alzheimer's disease, schizophrenia and panic disorder, each comparing a patient group with controls and reporting statistical differences in, strictly speaking grey matter density used as a proxy for volume change, usually with, areas of reduced grey matter mapped onto a brain template. Importantly, these methods look at the whole brain and are not influenced by prior assumptions of where the abnormality might be. The authors applied some basic quality controls over the included studies and applied weightings according to the numbers of participants but were interested in the broad pattern that such large data-sets might reveal.

Now, you might question the whole enterprise. Is it not like going into a general hospital, picking patients from the cardiac ward, the renal unit and the orthopaedic department and sticking them in a scanner? Little surprise then if the results showed that abnormalities tended to centre on (although not being confined to) the heart, the kidney and the skeleton, respectively? But it is not quite the same since both psychiatry and neurology are laying claim to the same organ, the brain. Going along with the idea one might then hypothesise that neurological patients show the bulk of abnormalities in the areas of the brain associated with more ‘basic’ functions: movement, sensation and, with respect to dementia, memory, whereas psychiatry patients show differences in ‘higher’ brain regions associated with self-consciousness and identity (the frontal lobes) or emotion (the ‘limbic system’).