William M Klykylo

Wright State University School of Medicine

I. Definition and History of Concept II. Comorbidity and Substance Abuse

III. Comorbidity in Childhood Disorders

IV. Other Comorbidities

V. Applications of Concept Further Reading


comorbidity The co-occurence of more than one disorder in the same individual. endocrinopathy A disorder caused by or affecting the endocrine system of the body. epidemiologic Relating to the study of epidemiology or to the characteristics of a disease or physiologic phenomenon in a population as a whole rather than in individuals.


Comorbidity is defined as the co-occurence of more than one disorder in the same individual. In its broadest sense, comorbidity can include the co-occurrence of medical and psychiatric disorders, such as the dementia associated with organic conditions or the affective changes resulting from endocrinopathies. In psychiatry, comorbidity is generally taken to mean the association of diagnosable psychiatric disorders. Comorbidity is an epidemiologic phenomenon, relating to the character istics of a population, and the reported comorbidity of certain disorders in a population does not necessarily imply that they will be comorbid in any given individual. However, observations of comorbidity among populations may be extremely useful in informing the therapist's understanding of an individual patient.

The observation of comorbidity between disorders does not in and of itself demonstrate any particular type of relationship between them, least of all causality. Comorbidity can result from many factors. One disorder may represent an early manifestation of another. There may be problems of classification, in which the use of same or similar symptoms define different disorders. Detection artifacts can occur. For example, the presence of one disorder in a patient may make another condition more visible, even though it may be no more common than in a general population. Similarly, the presence of one disorder may influence the observations of clinicians and make them more likely to report the presence of another disorder.

Nonetheless, rates of co-occurrence of psychiatric disorders that are far in excess of what could be expected from chance have repeatedly been reported among many different populations. The recognition of this fact by mental health professionals has been long in coming but represents a major advance in the basic assumption of mental health care. Documentation of comorbidities have likewise led to important advances in our knowledge of mental illness and thereby our ability to provide effective, comprehensive care.

One may speculate as to why the recognition of comorbidity came later to mental health than to some other areas of medicine. Often therapists have found a unitary explanation for their patients' problems to be both of heuristic value and a source of comfort. The psy-chodynamic concepts of nuclear conflict and infantile neurosis, although valuable in many respects, may have engendered a notion that individuals have one great problem alone. The tendency in some psychotherapeutic schools to value contemporaneous observations of process over a more "medical" model of diagnosis, especially prior to DSM-III may have rendered irrelevant the concept of comorbidity, based as it is on diagnosis. The ascendance of the empiric model of DSM-III and -IV can create an epistemological problem for some therapists, in that many of the plethora of diagnoses contained in the DSMs have numerous features in common. This level of detail enhances the utility of DSM criteria as discriminators among diagnoses. However, these commonalities can also obscure the differences among conditions and thereby lead clinicians to gloss over cormorbidities. In any case therapists today are much more likely than in the past to recognize that their patients frequently most confront a variety of illnesses and problems. These may often aggravate each other and must be addressed in their multiplicity if the patient is to find meaningful symptomatic relief and functional improvement.

Therapists approach patients with a certain mindset, reflective of particular schools of therapy but also of generally accepted values, such as empathy, professional responsibility, and the importance of a conceptual framework for diagnosis and treatment. Part of this framework for any therapist today must be the possibility of comor-bid disorders in any patient. This point of view, like any other assumption, should be used to enrich and amplify one's view of the patient rather than to unnecessarily codify or stereotype that view. Most data on co-morbidity is epidemiologic, although each patient has an individual course of life. In addition, the recognition of an additional problem or disorder does not necessarily require that it be treated, especially in today's climate of focused and time-limited psychotherapies. Still, most therapists have a desire to know as much as they can about their patients' life so as to serve both the patients' needs and enhance their own satisfaction.


Therapists should be aware of many particular comor-bidities that have been documented in psychiatric re search. The most extensive documentation of comorbidity involves substance abuse and psychiatric disorders. Alcoholism and other addictions affect 20% of the population at some time in their lives, and this number is greatly increased if one also considers tobacco consumption a form of substance abuse. It is reckoned that at least one third of persons with addictions have some other co-morbid Axis I disorder. In the national comorbidity survey reported in 1988, 47% of alcoholics had a comorbid psychiatric diagnosis. This number rises even higher in psychiatric inpatient settings and among the homeless.

Anxiety and affective disorders are the most common mental illnesses other than substance disorders in the general population. Thus, it is not surprising to find a high rate of comorbidity between these conditions and substance abuse. Some authors report a particular association between the primary onset of an anxiety disorder and the subsequent development of alcoholism, but the opposite temporal relationship is also quite possible. Persons with affective disorders often are attracted to substance use and abuse in an attempt to self-medicate. The Epidemiological Catchment Area (ECA) study showed that 30% to 50% of their alcoholic patients had comorbid major depression, although this rate of occurrence decreased substantially with abstinence.

Persons with schizophrenia, especially men, are also more likely than nonill controls to have a comorbid substance abuse problem. Lifetime concurrence in this group of around 50% is frequently reported for alcohol use disorders. In 1990, a report from the Eastern Psychiatric Institute showed that, among schizophrenic patients, 47% abused alcohol, 42% abused cannabis, 25% abused stimulants, and 18% abused hallucinogens. In this population, drug dependence was associated with more hospitalizations and with more psychosocial symptoms.

Other reports have suggested an increased level of comorbidity for substance abuse among adolescents and young adults with attention deficit hyperactivity disorder. This may be particularly marked among patients with associated learning problems, family dysfunction, or social economic distress. Researchers have speculated as to whether this might result from an underlying biobehavioral predisposition, the consequence of an impulsive lifestyle and social and educational failures, or various other combinations of factors.


Comorbidity of psychiatric disorders in children occurs far in excess of chance. In the studies of this phenomenon that have been conducted in the last 15 years, rates of co-occurrent disorders in participants studied from 40% to 70% are commonly reported. Child adolescent psychiatry disorders are commonly described as externalizing, such as conduct disorder and hyperactivity, and internalizing, such as anxiety and depressive disorders. Researchers have demonstrated an especially high comorbidity within each of these categories but also a very significant comorbidity between these two areas. As early as 1982 Puig-Antich reported a significant association between conduct disorder and depression, and a frequent association between hyperactivity and depression has also been reported. At times this degree of overlap has led observers to question whether or not some of these conditions are actually separate disorders.

The most commonly diagnosed psychiatric disorder in children in the United States is attention deficit hyperactivity disorder (ADHD). Various rates of prevalence from 3% to 10% have been reported. The recognition of this disorder is influenced by many external factors, such as particular family, social, and educational expectations and the availability of mental health services. It is said that this condition is both heavily underdiagnosed and overdiagnosed. It is also frequently misdiagnosed; for example, many children described as having ADHD actually have a primary depression. Nonetheless rigorous studies of children with this condition have repeatedly disclosed the frequent occurrence of comorbid conditions. As many as two thirds of elementary school age children with ADHD have at least one other diagnosable psychiatric disorder. In the Ontario Child Health Study investigators noted that 42% of such children had comorbid conduct disorder, 17.3% had somatization disorder, and 19.3% had the broad category of emotional disorder. Another study conducted by Cohen and his coworkers reported similar findings including 23% of children with overanxious disorder, 24% with separation anxiety, and 13% with major depressive disorder.

Not surprisingly a converse comorbidity is seen among children with disruptive behavior disorders such as conduct disorder and oppositional defiant disorder. Some 50% of these children have mood disorders, and perhaps 20% or more have learning problems or learning disabilities.

Anxiety disorders in children have been traditionally underrecognized. This may be a result of early theoretical notions that regarded anxiety as mainly a symptom of neurosis that could be expected to abate incidentally in the course of psychotherapy. We have learned in recent years that, regardless of its origin, anxiety persists in children as with a life of its own, regardless of its origin. The DSM-IV notes the existence of separation anxiety disorder and generalized anxiety disorder in children. Children may also experience social phobia and panic disorder. Obsessive-compulsive disorder (OCD) is one of the anxiety disorders as is posttraumatic stress disorder (PTSD).

All of these disorders, including PTSD, can be co-morbid with each other. Clinicians who follow patients over the years meet children who may first present with a separation anxiety disorder, then go on to develop a social phobia and later in life manifest OCD. Many other sequences also occur, and any and all of these conditions can occur simultaneously. It is reported that the majority of children with the specific phobia also have a second anxiety disorder diagnosis. Two thirds of children with anxiety disorders also have depressive disorders, at least at some time in their lives. Seventeen to twenty-two percent of children with a primary anxiety disorder may have ADHD, and a comorbidity with conduct disorder and oppositional defiant disorder has also been observed. Posttraumatic stress disorder in children as well as in adults requires special consideration. This disorder in and of itself has a protean range of symptoms, which may lead the clinician to misat-tribute symptoms that might be reflective of a concurrent or preexistent second psychiatric disorder. Depression and anxiety disorders are most especially observed in the company of PTSD and may require psychiatric treatment in addition to that being already offered for trauma.

Eating disorders are regrettably common in our appearance-oriented society. It is reported that 75% of female teenagers describe themselves as fat. One to four percent of adolescent and young adult women go onto develop anorexia nervosa or bulimia and perhaps one tenth as many young males. People with anorexia have increased rates of major depressive disorder, dysthymia and OCD. Persons with bulimia also have high rates of anxiety and addictive disorders. In the experience of many clinicians, it is impossible for many patients to recover from their eating disorders without adequate treatment for these comorbid conditions.


Any of the comorbidities reported in child and adolescent psychiatric patients may also be seen in adulthood. In addition, the clinician most be mindful of other frequently reported co-current conditions. Patients with schizophrenia often have concurrent cognitive deficits and affective disorders. Suicidality is common in young males with schizophrenia especially early in their illness.

There is extensive comorbidity among the affective disorders. Many researchers believe this is a manifestation of early appearance of disorders. For example, a patient who has recurrent depressive episodes may go on in time to develop full bipolar illness. More than 40% of patients with major depression can expect to have one or more "nonmood" psychiatric disorders during their lifetimes. These include alcoholism and substance abuse, anxiety disorders, eating disorders, and certain personality disorders such as borderline personality disorder. Affective disorders are often seen in association with somatoform and conversion disorders, although firm numbers for comorbidity are difficult to come by.

Persons with panic disorder have an extremely high rate of comorbidity with other anxiety disorders such as social phobia, generalized anxiety disorder and OCD. They also have a very high rate of substance abuse. Virtually all of these people, who represent 2% to 5% of the general population, have some other associated psychiatric disorder. Social and specific phobias are also reported to be comorbid with other disorders, though this comorbidity seems less than that of panic disorder. The comorbidity of PTSD with many other conditions as reported among children, is also seen in adults.

Sixty-seven percent of persons with primary OCD can expect to have a major depression during their lifetimes, and typically 31% of them at any given time. OCD also has a demonstrated concurrence with Tourette syndrome that probably results from shared neurobiological factors. OCD patients may have a delusional component to their condition, which at times may be so intense as to resemble psychosis and even require antipsychotic medication. It is not certain however as to whether there is any increased occurrence of true psychosis among these patients. Patients with OCD do have a high rate of concurrent anxiety disorders such as social phobia, specific phobia, and panic disorder.

The comorbidity of psychiatric and medical illnesses constitutes a major raison d'être for psychiatry as a specialty and is the basis for an entire subspe-cialty, consult-liaison psychiatry. A comprehensive treatment of this area is beyond the scope of this article. The DSM-IV notes that general medical conditions can be responsible for psychotic disorders, anxiety disorders, mood disorders, sexual dysfunction, sleep disorders, catatonia, and personality changes. Medical conditions such as endocrine disorders, infectious diseases, metabolic disorders, malignancies, and neurological injuries and diseases have all been associated with mental illness. Any patient entering psychotherapy should be receiving ongoing primary medical care. A patient with a known medical illness, or whose condition suggests the possibility of medical illness, deserves thorough medical assessment. The reader should consult a comprehensive textbook for further consideration of this area.


It should be apparent that in many areas of psychiatry comorbidity is the rule rather than the exception. What is the psychotherapist to do with this information? Obviously one must be aware that a patient describing a single unitary problem may have a larger range of problems or disorders. It may be necessary for the therapist to address problems beyond those originally proposed to help the patient. Even if the patient does not wish to pursue treatment for these other areas, the ethical therapist has a responsibility to share his or her observations and to advise the patient of any difficulties the patient might be facing.

Therapists may be inclined at times to omit or minimize their discussion of comorbid factors of patients. This may be out of a desire to avoid discouraging the patient or aggravating a condition. However, in the longer term, the goals of any psychotherapy are better served by honesty accompanied by tact and sensitivity. While some patients may be daunted by an enumeration of comorbid problems, many others will be reassured by a delineation of what exactly they are facing, and may even be encouraged by what progress they have already made in the face of multiple problems.

See Also the Following Articles

Collaborative Care ■ Cancer Patients: Psychotherapy ■ Medically Ill Patient: Psychotherapy ■ Substance Dependence: Psychotherapy

Further Reading

Bucholz, K. K. (1999). Nosology and epidemiology of addictive disorders and their comorbidity. Psychiatric Clinics of North America, 22,(2) 221-239.

Goldsmith, R. J. (1999). Overview of psychiatric comorbidity, practical and theoretical considerations. Psychiatric Clinics of North America, 22,(2) 331-349.

Kay, J., Tasman, A., & Lieberman, J. A. (2000). Psychiatry: Behavioral science and clinical essentials. Philadelphia: W. B. Saunders.

Klykylo, W. M., Kay, J., & Rube, D. (1998). Clinical child psychiatry. Philadelphia: W. B. Saunders.

Offord, D. R., & Fleming, J. E. (1996). Epidemiology. In Melvin Lewis (Ed.), Child and adolescent psychiatry: A comprehensive textbook (pp. 1166-1178). Baltimore: Williams & Wilkins.

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