Classification of Mental Disorders

Introduction

A perfect clinical diagnosis would usually predict the possible underlying causative factors, the pathophysiology, clinical signs and symptoms, response to treatment, and the course of the illness. Such precise diagnoses are possible in various infectious diseases and, to an extent, in surgery. However, in general medicine and in particular in psychological medicine, this ideal concept of disease is not fully attainable. This is because of our limited knowledge about the underlying causes of mental disorders and the absence of objective, measurable standard tests analogous to blood tests or histology.

Probably, some organic brain disorders — such as Dementia of Alzheimer's Type, Lewy Body Dementia, and Vascular Dementia — can be separated clinically, but to be definitive about the diagnosis one would have to rely on brain biopsy and, more often, on autopsy results.

A specific disease process is further understood and classified by its consequences, such as the impairment, disability, and handicaps it causes. However, in psychiatry, the ultimate consequences may be very similar despite differing underlying psychological and biological processes.

In psychiatry, the concept of "disease" does not encompass all of the above. Hence the broad term "disorder" is frequently used rather than "disease."

Why Should We Classify Mental Disorders?

It is relevant and important to classify mental disorders for two very good reasons:

1. To communicate with other health professionals at a day-to-day clinical level and also to carry out meaningful scientific research studies across the world.
2. To communicate meaningfully with our patients and their families — giving a diagnosis provides a framework for understanding the illness, its prognosis, and the available treatments.

Methods of Classification

There are two principal classification systems in current use:

1. International Classification of Diseases, 10th Revision (ICD-10; WHO, 1992) — the most widely used system in the UK and other European countries. It describes an internationally accepted glossary of categories of mental disorders with key sets of clinical features. The ICD-10 is also used in coding for NHS clinical activity and commissioning purposes. The World Health Organization published ICD-11 in 2019, with member states adopting it for reporting from January 2022. The UK is in the process of transitioning; clinicians should be aware of ICD-11 terminology and its significant updates, including the addition of Complex PTSD, the reconceptualisation of personality disorder, and changes to the classification of gender incongruence.
2. Diagnostic and Statistical Manual of Mental Disorders, 5th Edition (DSM-5; American Psychiatric Association, 2013) — primarily used in the United States, Canada, Australia, New Zealand, India, and China, and widely used in research globally. It superseded the DSM-IV (APA, 1994). DSM-5 moved away from the multi-axial system of its predecessor to a single-axis dimensional approach.

The DSM-IV Multi-Axial System (Historical Context)

The DSM-IV used a multi-axial diagnostic framework with five axes. Although this system has been superseded by the non-axial DSM-5, it remains useful as a conceptual framework for presenting a comprehensive formulation:

• Axis I: Main clinical diagnosis — the principal focus of attention and treatment (e.g. major depressive episode, schizophrenia).
• Axis II: Personality type or disorder; learning disability.
• Axis III: Physical disorders or conditions.
• Axis IV: Psychosocial stressors.
• Axis V: Global Assessment of Functioning (GAF) — the patient's level of functioning over the last 12 months.

ICD-10 vs DSM: Key Differences

The ICD-10 is a single-axis system, though it is available in a multi-axial format for use in primary care and research. The ICD-10 diagnostic guidelines and criteria do not explicitly include the social consequences of a disorder within the diagnostic criteria themselves, whereas the DSM-5 (and DSM-IV before it) requires evidence of clinically significant impairment in social, occupational, or other areas of functioning as part of most diagnoses.

In research, DSM-based criteria are often preferred for their greater operational specificity, while ICD-based coding is standard for NHS clinical documentation and statistics.

Main Diagnostic Categories

In both the ICD and DSM systems, the main categories of mental disorder are as follows:

1. Dementia and Organic Mental Disorders — conditions arising from identifiable brain disease, damage, or dysfunction (e.g. Alzheimer's disease, vascular dementia, delirium).
2. Psychoactive Substance Use Disorders — including harmful use, dependence, and withdrawal states for alcohol, opioids, stimulants, cannabis, and other substances.
3. Schizophrenia and Other Psychotic Disorders — including schizophrenia, schizoaffective disorder, delusional disorder, and brief psychotic episodes.
4. Mood (Affective) Disorders — including depressive disorders (mild, moderate, severe; with or without psychotic features) and bipolar affective disorder.
5. Stress-Related, Somatoform, and Anxiety Disorders — including generalised anxiety disorder, panic disorder, phobias, obsessive-compulsive disorder (OCD; classified separately in DSM-5 and ICD-11), post-traumatic stress disorder (PTSD), adjustment disorders, and medically unexplained symptoms.
6. Behavioural Disorders Associated with Physiological Disturbances:
   • Eating Disorders (anorexia nervosa, bulimia nervosa, binge eating disorder)
   • Sleep Disorders (insomnia, hypersomnia, parasomnia)
   • Sexual and Gender Identity Disorders
7. Personality Disorders — pervasive and inflexible patterns of inner experience and behaviour that deviate markedly from cultural expectations. ICD-11 has significantly reconceptualised personality disorder as a dimensional rather than categorical construct, replacing specific named categories with a severity rating and optional trait domain specifiers.
8. Childhood and Adolescent Psychiatric Disorders — including conduct disorder, emotional disorders of childhood, and attachment disorders.
9. Intellectual Disability (Learning Disability) — characterised by significant limitations in intellectual functioning and adaptive behaviour, with onset during the developmental period.
10. Attention Deficit and Disruptive Behaviour Disorders — including ADHD (Attention Deficit Hyperactivity Disorder) and conduct disorder. DSM-5 and ICD-11 both now allow ADHD to be diagnosed in adults as well as children.

ICD-11: What Has Changed?

The ICD-11 (WHO, 2019/2022) introduces several clinically important changes:

• Complex PTSD (CPTSD) is now a separate diagnosis from PTSD, reflecting the distinctive profile of people with prolonged, repeated trauma exposure.
• Personality disorder is reconceptualised dimensionally — severity (mild, moderate, severe) is rated, with optional trait domain specifiers (negative affectivity, detachment, disinhibition, dissociality, anankastia). Specific named categories (e.g. borderline, antisocial) are largely removed, though a "borderline pattern" specifier is retained.
• Gaming disorder is included as a new diagnosis under disorders due to addictive behaviours.
• Gender incongruence has been moved out of the mental disorders chapter and into a new chapter on sexual health, reflecting the view that it is not a mental illness.
• Prolonged grief disorder is a new addition.