Definitions: Serious Mental Illnesses and Co-occurring Disorders
What is mental illness?
According to the U.S. Department of Health and Human Services, Substance Abuse and Mental
Health Services Administration (SAMHSA) and the National Institute of Mental Health, mental illness
is a term that refers collectively to all diagnosable mental disorders. Mental disorders are health
conditions characterized by alterations in thinking, mood, or behavior (or some combination thereof)
associated with distress and/or impaired functioning.28 A mental illness diagnosis is made only when
certain clusters of symptoms are present for a certain period of time, other clusters of symptoms
are not present, and the symptoms that are present cause significant distress or impairment in
social, occupational, or other areas of functioning.
Federal and state regulations apply the following classifications in determining eligibility for
publicly funded mental health treatment services:
• Serious mental illness (SMI): A term that generally applies to mental disorders that significantly
interfere with some area of social functioning (e.g., work, school, family, leisure).29
• Severe mental illness or severe and persistent mental illness (SPMI): Terms that apply to
more seriously affected individuals. This category includes schizophrenia, bipolar disorder,
severe forms of depression, panic disorder, and obsessive-compulsive disorder. These terms
are often used to describe clients with the highest levels of clinical need.30
What are co-occurring disorders?
The authors use the term co-occurring disorders to refer to substance-related and mental disorders
that are diagnosed as being present in an individual at the same time. Co-occurring disorders exist
when at least one disorder of each type can be established independently of the other and is not
simply a cluster of symptoms resulting from a single disorder.31
What are Axis I and Axis II Disorders?
Axis I disorders, as they are referred to in the Diagnostic and Statistical Manual of Mental Disorders
Fourth Edition (DSM-IV), include clinical syndromes such as depression, schizophrenia, and bipolar
disorder.32 Axis II disorders, as defined by DSM-IV, are developmental and personality disorders,
including paranoid, antisocial, and borderline personality disorders. Most mental health courts
require participants to have an Axis I diagnosis, but many mental health courts also accept
individuals who have a co-occurring Axis II disorder.
Although most mental health courts focus on individuals with serious mental illnesses, their
specific target populations are often shaped by state mental health “priority population” definitions
because these definitions affect the relative availability of treatment services that community
providers can offer and be reimbursed for by the state or federal government.