There has been an increased awareness in the diagnosis of mental disorders in people with intellectual disability (ID). The evidence base has demonstrated that people with ID can display the same mental disorders as the general population and that the prevalence of such disorders varies according to the methods used for their assessment and diagnosis. The diagnosis of mental disorders in people with ID is a highly complex process mostly because of the difficulty or inability of some people with ID to express their feelings and symptoms. Hence, many of the diagnostic criteria for mental disorders used in the general population needed to be modified and adapted for people with ID.
The National Association for Persons with Developmental Disabilities and Mental Health Needs (NADD) has been among the leading organizations in the U.S. and internationally in providing educational and training programs, support for research projects, and publicizing important clinical and policy issues regarding people with mental disorders and ID.
In 2007, the NADD published the Diagnostic Manual – Intellectual Disability (DM–ID; Fletcher, Loschen, Stavrakaki, & First, 2007) as a companion to the American Psychiatric Association's (APA; 2000) manual for psychiatric diagnosis (i.e., the DSM-IV-TR) to assist clinicians in making more accurate diagnoses for people with ID. Approximately ten years later, with the APA's (2013) publication of their updated manual (i.e., the DSM-5), the NADD published the Diagnostic Manual – Intellectual Disability (DM-ID 2): Textbook of Diagnosis of Mental Disorders in Persons with Intellectual Disability (DM-ID 2: Fletcher, Barnhill, & Cooper, 2017) for people with ID, having taken into consideration the changes introduced by the DSM-5.
The editors of the DM-ID 2 stated that their book is designed to provide state-of-the-art knowledge of mental disorders for people with ID. The DM-ID 2 was compiled by “a multicentered, multicultural, and multifaceted” (p. 9) collaboration of over 100 clinicians, researchers, and practitioners with expertise in mental disorders and ID who worked in 26 working groups over a period of four years. A chairperson was assigned for each working group. A summary of the main points of the DM-ID 2 has also been published separately (Fletcher, Barnhill, McCarthy, & Strydom, 2016).
The DM-ID 2 consists of 27 chapters covering the main psychiatric diagnostic categories that correspond closely to the DSM-5 classification system, with modifications to make them more applicable to people with ID. Chapter 1 describes the most commonly used assessment and diagnostic methods “to assist the reader in understanding the biopsychosocial developmental approach when conducting a psychiatric assessment” with people with ID. Chapter 2 describes “behavioral phenotypes that are associated with genetic disorders, which is intended to aid in the understanding of how a disorder's genotype affects its behavioral phenotype”. Both chapters 1 and 2 are informative and comprehensive and present an added value to the publication of the DM-ID 2.
The authors of the DM-ID-2 relied upon an expert consensus model in much of their work to overcome the poverty of existing relevant research in the field. Each chapter is based on a systematic critical review of the available literature and follows guidelines for clarity and uniformity.
The principal elements of the guidelines for the structure of each chapter include both organizational and conceptual elements (e.g., chapter summary, review of diagnostic criteria, general description of the disorder, summary of DSM-5 criteria, diagnosis in people with ID, comorbidity, application of diagnostic criteria to people with ID, etc.)
Variations of the suggested guidelines are evident in several chapters of the DM-ID 2, probably due to the different prevalence of mental disorders in people with ID. Therefore, all of the guidelines were not applicable to all mental disorders. All chapters consist of three main parts: (1) summary of the DSM-5 criteria, (2) review of the literature, and (3) suggestions for applying criteria for people with mild/moderate ID and severe/profound ID.
There are important changes in the DSM-5 reflecting developments in research of genetics and neuroimaging, and there is a revised organizational structure, taking into consideration that mental disorders do not always fit entirely within the boundaries of a single disorder, but that some symptoms involve multiple diagnostic categories. The changes in DSM-5 relevant to people with ID include a lifespan approach and the abandonment of the multi-axial method. Other important changes in the DSM-5, that have been adopted for the DM-ID-2, include the amalgamation of autistic disorder, Asperger's syndrome, and pervasive developmental disorder, into one diagnostic category - Autism Spectrum Disorder (ASD). Reactive attachment disorder, disinhibited social engagement disorder, posttraumatic stress disorder (PTSD), acute stress disorder, and adjustment disorder have been combined in the DSM-5 under the diagnostic category trauma- and stressor-related disorders that has also been adopted by the DM-ID 2. Disorders previously referred to as “dementias” are now designated as major or mild neurocognitive disorders by both the DSM-5 and DM-ID 2.
A critical change in DSM-5 is that intellectual disability (ID) is included as “a discrete syndrome” with the term Intellectual Development Disorder (IDD) within the diagnostic category of Neurodevelopmental Disorders. The DM-ID 2 adopts the term IDD (in the disability field this abbreviation stands for intellectual and developmental disabilities) but makes a central point that IDD can coexist with other mental disorders. This emphasis of the coexistence of IDD with other mental disorders is an important distinction of the DM-ID 2 from DSM-5, where IDD is frequently an exclusion criterion from the diagnosis of other mental disorders.
The inclusion of IDD as a separate category, under the neurodevelopmental disorders of the DSM-5, to affirm that it is a health condition, remains controversial (Bertelli, Salvador-Carulla, & Harris, 2016). The American Association for Intellectual and Developmental Disorders (AAIDD) defines ID as a “disability” and not as a “health condition.” The AAIDD definition states that ID is “characterized by significant limitations, both in intellectual functioning and adaptive behavior as expressed in conceptual, social, and practical adaptive skills,” and that the disability originates before age 18. (Schalock et al., 2010, p. 1).
Another important change in the DSM-5 for the diagnosis of IDD is that it shifts the emphasis from IQ scores to the necessity that the onset should occur “during the developmental period and to include both intellectual and adaptive functioning deficits in conceptual, social and practical domains” (American Psychiatric Association, 2013, p. 33).
The DM-ID 2 retains IDD as a separate diagnosis, and in Chapter 4, the complex conceptual issues involved with the diagnostic criteria of Neurodevelopmental Disorders. These issues include a number of overlapping symptoms from other, co-existing, neurodevelopmental syndromes or/and other mental disorders. DM-ID 2 allows some flexibility by stating that clinicians will have to judge how best to modify inclusion, specifiers, and exclusion criteria to apply these diagnostic criteria to people with IDD.
Additional changes in the diagnostic criteria covered in the DSM-5 and adopted by DM-ID-2 include the exclusion of Attention Deficit Hyperactivity Disorder (ADHD) from Autism Spectrum Disorder (ASD); the realignment of impulse control and disruptive behavior disorders to the category of Disruptive Mood Dysregulation Disorder (DMD) for people with affect dysregulation and ADHD (previously diagnosed as bipolar disorder) and the creation of trauma- and stressor-related disorders.
The DSM-5 diagnostic criteria for major neurocognitive disorder and mild neurocognitive disorder to aid in the diagnosis of dementia have also been modified for people with ID in the DM-ID 2, but some caution is given for the validity of mild neurocognitive disorder in people with ID.
The DM-ID 2 aims to assist in the diagnosis of mental disorders for people with ID based on adaptation and modification of the diagnostic criteria of the DSM-5. It is written in a well-organized and methodological style in a field characterized by very difficult and complex issues and concepts. Individual chapters include wide-ranging discussions based on a systematic review of the literature and the extant research evidence on mental disorders among people with ID to support the suggested adaptive diagnostic criteria for people with ID. The term IDD as used in the DSM-5, has been adopted by the DM-ID 2 throughout the publication. The process of developing the DM-ID 2 does not include field trials and does not mention the association with the APA, as was the case with the development of the DM-ID.
The acceptance of a psychiatric diagnosis and classification for those with mental disorders, including for people with IDD, serves several purposes, including “medical record keeping, data collection, retrieval and compilation of statistical information, communicating with third parties, such as insurers and governmental agencies, and is the basis for eligibility and reimbursement for psychiatric services” (Sturmey, 1999, p. 4). Standardized diagnosis can also serve as the basis for communication between different professional groups and non-professionals as well as in research contexts. Diagnosis also serves the purpose of providing a summary for multiple presenting symptoms, etiology, and prognosis, and can inform therapeutic interventions. The provision of mental health services for those with ID has undergone profound transformations since the implementation of community care programs (Bouras, Ikkos, & Craig, 2017). Accurate diagnosis is of utmost importance for planning, provision, and delivery of services, monitoring, and evaluation (Bouras, 2017).
The publication of the DM-ID 2 is an important resource toward advancing our knowledge of mental disorders in people with ID. Discussion on the relationship of problem behavior known as “challenging behavior” not included in the DSM-5, would have been helpful in the DM-ID 2. The value of the DM-ID 2 remains to be confirmed in clinical practice and research studies that apply the suggested diagnostic criteria. In the future, the publication of an “operational guide” for the DM-ID 2 will serve as a user-friendly tool facilitating diagnosis and treatment for people with ID and mental disorders.
Declaration of conflict of interest: Nick Bouras has edited books on psychiatric and behavioral disorders for people with developmental disabilities and participated once in a teleconference advising in the development of the DM-ID 2.