Read Part One of this series here: An introduction to the classification of mental disorders

Historical interactions between the DSM and the ICD

Historically, there have been attempts to harmonise the DSM (Diagnostic and Statistical Manual of Mental Disorders) and the ICD (International Classification of Diseases).

"Are disorders in the ICD-10 that lack a DSM-V equivalent rendered diagnostically obsolete?"

Despite bearing little resemblance to one another, the first DSM (1952) was heavily influenced by its predecessor, the ICD-6 (1948). The ICD-6 (1948) was the first of its kind to include a chapter on mental disorders but was deemed a “political failure”, with few countries adopting it.

The first DSM (1952) inherited the ICD-6’s three category approach, albeit using different categories. The DSM conceptualised disorders through psychodynamic aetiology whereas the ICD-6 differentiated between organic and non-organic disorders. After the Second World War, psychodynamic theories became more widely accepted in psychiatry, with many favouring a model of mental illness that privileges the unconscious mechanisms supposedly behind thoughts, feelings, and actions. Even in the face of scepticism by more medically-orientated practitioners, psychiatrists who valued psychodynamic theory took over the APA (American Psychiatric Association), the professional body behind the DSM.

1968 saw the publication of the DSM-2 and the ICD-8, marking substantial collaboration between the APA and the WHO (World Health Organisation), the latter which produced the ICD. Text within the DSM-2 explicitly forges the link between the two manuals: “with the increasing success of the World Health Organization in promoting its uniform International Classification of Diseases, already used in many countries, the time came for psychiatrists of the United States to collaborate”.

And so, the DSM-2's and the ICD-8’s chapter on mental disorders were extremely similar, both using the categories of i) psychoses, ii) neuroses, personality disorders, and other nonpsychotic disorders, and iii) mental retardation. A key difference was the ICD-8’s reliance on category names, code numbers, and inclusion/exclusion terms to describe a disorder, whereas the DSM-2 provided somewhat comprehensive descriptions instead. 

In stark contrast to its predecessor, the DSM-3 (1980) involved little collaboration with the WHO and next to no international participation. Because of this, the DSM-3 was a world away from how the ICD-9 conceptualised mental illness. The ICD-10 and DSM-4 were conceptually similar in that the WHO and the APA worked together to reduce incongruence, especially in “meaningless” inconsistencies in wording. Whilst the ICD-11 will not come into effect until 2022, a working version is available to access online. In comparison to the DSM-5, the latest version of the ICD has had global contributions.

"Equivalent" diagnoses

Today, the current version of the DSM has a code next to each diagnosis that supposedly corresponds to a diagnosis in the ICD-10. The DSM-5's preface reads that it is "harmonized with the World Health Organization’s International Classification of Diseases (ICD), the official coding system used in the United States, so that the DSM criteria define disorders identified by ICD diagnostic names and code numbers".

But are the "equivalent" diagnoses that the DSM-5 claims to see in the ICD-10 really referring to the same construct?

In many cases, they seem to be. Whilst specific criteria may be slightly different, often both manuals conceptualise a disorder through a common phenomenology. What this means is that even taking individual differences into account, there is a common experience of living with a particular disorder described in both manuals. The DSM-V and the ICD-10 do not describe Post-Traumatic Stress Disorder in exactly the same way using the same language, but both refer to a common experience.

Is Hoarding Disorder distinct from Obsessive-Compulsive Disorder? 

There are instances in which one manual can classify a disorder with a specificity that the other cannot. For example, the DSM-5 has Hoarding Disorder (300.3) as a diagnosis. Whilst is is within the Obsessive-Compulsive and Related Disorders category, its diagnostic equivalent in the ICD-10 is Obsessive-Compulsive Disorder (F42) itself. Hoarding Disorder is not characterised by obsessions and compulsions, so why is its "equivalent"? Is Hoarding Disorder a form of Obsessive-Compulsive Disorder? Or is it a distinct diagnosis in itself? 

Is a single manic episode Bipolar I?

If someone experiences a single episode of mania, the DSM-V may assign them a Bipolar I Disorder diagnosis. However, the same presentation would not be classified as this disorder through the lense of ICD-10, but as a single manic episode. This episode would be rediagnosed as Bipolar Affective Disorder if the patient experienced another mood episode such as depression or hypomania. And so, the same presentation can result in a different diagnosis depending on which manual the diagnosing clinician is using. What could this mean for a person receiving a diagnosis? Does a diagnosis of a "fully fledged" disorder engender access to treatment that an episodic diagnosis would not? 

The NICE Guidelines (National Institute for Health and Care Excellence) for managing Bipolar Disorder draws attention to diagnostic discrepancies: "both the DSM-V and ICD-10 outline diagnostic criteria for bipolar disorder; however the two criteria sets are not identical. Crucial differences centre on the number of episodes required for a diagnosis and the distinction between bipolar I and II disorders. "

When "equivalent" diagnoses fail to match up

Binge-Eating Disorder (307.51), in its DSM-5 conceptualisation, is characterised by recurrent overeating. The ICD-10 contains a diagnosis of "Overeating associated with other psychological disturbances" (F50.4), described as "overeating due to stressful events, such as bereavement, accident, childbirth, etc. or psychogenic overeating". Whilst the DSM-5's description is significantly more descriptive than the ICD-10's single phrase, it seems that these two diagnoses are denoting the same behaviours and symptoms with the same cause: an underlying psychological process. But according to the DSM-5, Binge Eating Disorder is not "equivalent" to "Overeating associated with other psychological disturbances": it is "equivalent" to a diagnosis of “Other eating disorders” (F50.8). "Other eating disorders" includes pica, the compulsion to eat objects that are typically viewed as inedible, and psychogenic loss of appetite. Surely a disorder characterised by overeating is equivalent to a disorder characterised by... overeating? The DSM-5 does not pair another disorder to the ICD-10's "Overeating associated with other psychological disturbances". This discrepancy begs the question of whether these "equivalent" diagnoses really do refer to the same construct. The working version of the ICD-11* contains the diagnosis of Binge-Eating Disorder (6B82).

How will the non-specificity of an "Other eating disorders" diagnosis affect access to treatment for people struggling with binge eating? Will it deny them the more personalised and specialised treatment that a diagnosis of Binge-Eating Disorder would allow? 

The Schizotypal construct: personality disorder, schizophrenic spectrum disorder, or both? 

Another discrepancy is in the classification of the Schizotypal construct. The DSM-5 mentions it in two different sections, under both Schizophrenia Spectrum and Other Psychotic Disorders and Personality Disorders. In the Schizophrenia Spectrum and Other Psychotic Disorders section its mention is brief, with the "personality" part of the disorder's name demoted to brackets: "Schizotypal (Personality) Disorder". The short description cites the ICD-9's and ICD-10's categorisation of it as a schizophrenic spectrum disorder as a reason for mentioning it in this chapter, but then redirects readers to the Personality Disorders section to learn about its criteria, diagnostic features, and prevalence. This means that any meaningful information about the disorder is under Personality Disorders. And so, Schizotypal Personality Disorder (301.22) contains a lengthy description. Note the absence of brackets in its name that are found earlier in the DSM-5. 

According to the ICD-10 code in the DSM-5, the diagnosis' supposed equivalent is not a personality disorder, but a disorder of a schizophrenic nature. Despite ICD-11's* shakeup of personality disorder classification, the latest update retains Schizotypal Disorder (F21) in the category of schizophrenia spectrum disorders. 

What does this mean about the disorder's aetiology? Is it a personality disorder, or is it on the schizophrenia spectrum? Could it be both? Are the differently classified disorders referring to the same construct? If not, why does the DSM-5 forge a link between them? Does the issue lie in uncertainty about the disorder's nature, or do diagnostic manuals necessarily force disorders into a single category even if this does not reflect it accurately? What are the implications for a person diagnosed with either Schizotypal Personality Disorder or Schizotypal Disorder? Will this discrepancy between manuals confuse a patient? Are patients diagnosed using the ICD-10 told that it is classified differently in the DSM-5 and vice versa? 

Notably, it is the DSM-V that cites diagnostic equivalents in the ICD-10; the ICD-10 does not list any DSM-V codes. What does this say about diagnosis? Are disorders in the ICD-10 that lack a DSM-V equivalent rendered diagnostically obsolete? Does the DSM-V link diagnoses with the ICD-10 on a "best match" basis, or are they genuinely describing the same construct? In forging a link to the manual produced by the World Health Organisation, is the American Psychiatric Association trying to highlight the DSM-V's universal applicability? 

This is the second article in a series about the DSM vs. the ICD: the tensions, politics, & practical applications of understanding mental distress though two 'competing' diagnostic manuals.

Next article: why does having two diagnostic systems matter?


* as of this article's publication date