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