Some notes on nosology of mental health problems

 

The commonest form of classification used in mental health is a categorical diagnostic system such as DSM-5 or ICD10. When classifying mental disorders several attributes are important and certain presumptions and conceptual choices are made. One presumption is that mental disorder (or mental illness which makes a further presumption of similarity to physical illness) have examples that exist, that aspects of it can be discovered somehow and that the concept of mental disorder “makes sense” (Pickering, 2006: p.36).  One way it can be said to make sense is if it fulfils its purpose by achieving set aims. The term mental disorder is used here as it is common terminology but I prefer the term "condition"

Diagnostic constructs have three important attributes: reliability, utility and validity. Reliability of diagnosis usually relates to inter-rater reliability i.e. how often two different assessors agree on a diagnosis. If this rate of agreement is low then the diagnostic construct is less useful as one cannot make inferences from research as to causes, prognosis or treatment response rates or communicate effectively with colleagues. This was a recognised problem in psychiatry – as demonstrated when U.K. and U.S. psychiatrists markedly disagreed on the diagnosis of schizophrenia in the same patients (Professional Staff of the United States—United Kingdom Cross-National Project, 1974). This was addressed by the focus on increased reliability of psychiatric diagnosis since DSMIII onwards (Tyrer, 2018).

Validity of psychiatric diagnosis has many definitions but one of the most influential was that they either described a set of clinical features distinct from healthy states and from other diagnostic constructs or if based on a more “fundamental” biological level that this distinguished the diagnostic construct from others that can produce similar syndromes (Kendell & Jablensky, 2003: P.8). The same authors defined utility as “provides nontrivial information about prognosis and likely treatment outcomes, and/or testable propositions about biological and social correlates” and that it was context-dependent (Kendell & Jablensky: P.9-10). “Testable correlates” refers to utility for scientific purposes whereas the prognosis and treatment information is clinical (practice) utility. The authors concluded many psychiatric diagnostic constructs lacked validity but had utility for clinical practice.

During the process of creating DSM-5 clinical utility was stated to be a primary aim and was stated to be:

“1. Conceptualizing diagnostic entities 

2. Communicating clinical information to practitioners, patients and their families, and health care systems administrators 

3. Using diagnostic categories and criteria sets in clinical practice (including for diagnostic interviewing and differential diagnosis) 

4. Choosing effective interventions to improve clinical outcomes 

5. Predicting future clinical management needs” (First et al, 2004: P.947)

 Ideally a diagnostic construct should be based on identifying common causes but is often more descriptive of clinical features instead (such as symptoms or course) (Zachar & Kendler, 2007 :P.557-558). Essentialist diagnostic constructs name entities that exist independently of human classification and examples of which share common underlying mechanisms that produce properties such as symptoms and how they change with time (course). Many psychiatric diagnostic constructs are nominalist and created by humans for a practical purpose, for example grouping together problems on the basis of some similarity  that allows useful data to be gathered for purposes such as  clinical utility (Zachar & Kendler, 2007: P.558). These latter nominalist diagnostic constructs may be practical kinds (as opposed to the natural kinds of essentialism) and may represent a heterogeneous group with little in common (Kendler et al, 2011: P.1146). Social constructivists argue that the choices to create diagnostic constructs are driven largely by social or cultural pressures (Kendler et al, 2011: P.1145) – such as to label people because they are ‘unproductive’ according to society in order to exclude them. This means we have to be careful that classifying something as psychiatric disorder is not because of unethical societal pressures.

An alternative to either natural or purely practical kinds are mechanistic property clusters (MPC) where the similarities in superficial or external properties is because many but not all examples of a MPC share underlying multi-level causal mechanisms; MPCs have fuzzy borders and typical members like the ‘prototype’ model of mental disorders and because of similarity of causal mechanisms will be useful in making predictions such as prognosis or response to treatment. (Kendler et al, 2011: P.1146-1149). MPCs though are still a promissory note that we will discover these mechanisms and until we do we cannot say a particular diagnostic construct is a MPC.

 The criteria used to define diagnostic constructs are usually indexical (used to identify examples of the construct) not constitutive (where the criteria represent the construct’s nature itself). To clarify the difference a general medical example will be used – typical rises in cardiac muscle enzyme serum levels, ECG changes and/ or a ‘typical’ history are used to identify myocardial infarction (indexical criteria) but the myocardial infarction itself (its’ constitutive nature) is the cardiac muscle cells dying due to lack of oxygen usually caused by an obstruction to their arterial blood supply. Indexical criteria do not describe all the clinically important features of a mental disorder. The authors use the example of schizophrenia where several historically recognised important clinical features are omitted from the diagnostic criteria (Kendler et al, 2011: P. 2056).

Several conflicting choices occur in psychiatric nosology (Aftab & Ryznar, 2020: P. 3). Most psychiatric diagnostic constructs are based on descriptions of clinical features not aetiology; use categories not dimensions (for practical purposes such as cognitive economy not an assumption of categorical nature of mental disorders – see Huda (2019) P. 17-34, 45-46, 50-51, 53); can emphasise cross-sectional assessment of symptoms or the course of the disorder (these approaches can be combined with greater weight on one or the other); can use ‘operational’ criteria to identify disorders or rely on phenomenological approaches or prototype matching to typical descriptions and a final choice is whether one views mental disorder as a unitary phenomenon  with individual variation or as separate disorders (such as the example of unitary psychosis contrasting with Kraepelinian dichotomy of schizophrenia and bipolar disorder).

Due to the lack of knowledge of causative mechanisms for mental disorders, many psychiatric diagnostic constructs are descriptive and nominalist, often lack validity but also often have clinical utility and involve choices in how they are constructed.

REFERENCES

Aftab, A. and Ryznar, E., 2020. Conceptual and historical evolution of psychiatric nosology. International Review of Psychiatry, pp.1-14.

First, M.B., Pincus, H.A., Levine, J.B., Williams, J.B., Ustun, B. and Peele, R., 2004. Clinical utility as a criterion for revising psychiatric diagnoses. American Journal of Psychiatry, 161(6), pp.946-954

Huda, A.S., 2019. The Medical Model in Mental Health: An Explanation and Evaluation. Oxford University Press: Oxford, U.K.

Kendler, K.S., Zachar, P. and Craver, C., 2011. What kinds of things are psychiatric disorders?. Psychological Medicine. 41(6), pp.1143-1150

Kendell, R. and Jablensky, A., 2003. Distinguishing between the validity and utility of psychiatric diagnoses. American Journal of Psychiatry, 160(1), pp.4-12.

Kendler, K.S., 2017. DSM disorders and their criteria: how should they inter-relate?. Psychological Medicine, 47(12), pp.2054-2060.

Pickering, N., 2006. The metaphor of mental illness. Oxford University Press: Oxford, U.K.

Tindall, R., Simmons, M., Allott, K. and Hamilton, B., 2020. Disengagement Processes Within an Early Intervention Service for First-Episode Psychosis: A Longitudinal, Qualitative, Multi-Perspective Study. Frontiers in Psychiatry, 11, p.565.

Tyrer, P., 2018. Robert Spitzer's legacy: agreement is halfway to truth. BJPsych Bulletin, 42(5), pp.198-199.

Zachar, P. and Kendler, K.S., 2007. Psychiatric disorders: a conceptual taxonomy. American Journal of Psychiatry, 164(4), pp.557-565