What is “functional impairment”? Disentangling disability from clinical significance

Abstract

The formulation of disability (or “functional impairment”) is currently not operationalized in either the ICD or the DSM. In the DSM system, making a diagnosis depends on a conjoint assessment of symptoms and functioning, whereas the ICD keeps the disability construct separate from the diagnosis of mental disorders. We need an internationally agreed conceptualization between ICD and DSM in terms of better operationalization of disease and disability components. No functioning or disability should appear as part of the threshold of the diagnosis in either system.

When “disability” was added to public health measures, which had traditionally focused on mortality, it had a “Cinderella effect” on mental disorders. These disorders had never been put on public health priority lists. However, when “disability” was entered into the equation, as was the case with the disability adjusted life years (DALYs), mental disorders ranked as high as cardiovascular and respiratory diseases, surpassing all malignancies combined, or HIV 1. Using DALYs, the Global Burden of Disease study thus revealed the true magnitude of the long underestimated impact of mental health problems, due to the disability they produce 2.

Disability in mental disorders is a well-known fact for many clinicians, policy makers and researchers, as well as caregivers and persons with mental illness. Yet the form, frequency and outcome of disabilities in mental disorders are not well-defined or studied scientifically. More-over, their use in formulating diagnoses of mental disorders is both unclear and inconsistent. The World Health Organization (WHO) and the American Psychiatric Association (APA) use the construct of disability very differently in their classification systems. Without focused attention on functioning and disabilities, the current revisions of WHO’s International Classification of Diseases (ICD) 3 and APA’s Diagnostic and Statistical Manual of Mental Disorders (DSM) 4 will perpetuate divergence in diagnosing mental disorders. This would have the potential to con-found international research and clinical care.

In this paper, we propose to define disability operationally and separate it from the disease process in the diagnosis of mental disorders in both ICD and DSM systems.

DIFFERENT FORMULATION OF DISABILITY IN ICD AND DSM

Compatibility of the ICD and the DSM was already a stated goal of the DSM-II in 1968. Since then, the two diagnostic classifications have been developed in parallel. In 1980, the DSM-III was a revolutionary development in operationalizing the diagnostic criteria for mental disorders, a quest which had been made by Stengel already in 1959 5 and was then adopted in the production of the DSM-IV and the ICD-10. While the phenomenology of mental disorders was operationally defined in line with expert consensus, the formulation of disability (or “functional impairment” in DSM parlance) was not. It was included into the “clinical significance” criterion of the DSM, leaving it open to judgment by clinicians.

As shown in Table 1, the DSM, contrary to the ICD system, makes “clinical significance” an explicit part of the criteria for establishing a diagnosis. Clinical significance has two main components: distress and “functional impairment”. Distress is expressed by the subject or his/her significant others in terms of worry, concern, suffering about the condition. Sometimes it may not be expressed or may be explicitly denied. Functional impairment refers to limitations due to the illness, as people with a disease may not carry out certain functions in their daily lives. We operationally equate the “functional impairment” concept with “disability” in the WHO’s International Classification of Functioning, Disability and Health (ICF) 6.

Table 1.

Table 1 Operationalization of diagnosis in ICD and DSM

ICD	DSM
Specific phenomenology	Specific phenomenology
Signs and symptoms	Signs and symptoms
No clinical significance required (disability operationally defined in ICF)	Clinical significance (disability and distress)
Exclusion rules	Exclusion rules

TERMINOLOGICAL DIFFERENCES AROUND THE SAME CONCEPT

The DSM term “functional impairment” is not specifically defined. It is used to mean limitations in the social and occupational spheres of life. The DSM-IV-TR also refers to “other important areas of functioning”, but does not identify them.

The ICF does not use the term “functional impairment”. In this classification, the term “functioning” is a neutral one, encompassing all body functions, activities and involvement in life situations. The term “disability” means the decrements to these functions, which are known at the body level as impairments, at the person level as activity limitations, and at the societal level as participation restriction.

The DSM’s use of “functional impairment” can be taken to mean ICF’s “disability” largely, or activity limitations, narrowly. The DSM’s social functioning would include ICF’s interpersonal interactions and relationships, but may also include some of the items concerning participation in community, social and civic life. The DSM’s occupational functioning would include the activities listed under the ICF’s categories of work and employment.

To avoid a confusion, it is useful to note that the impairment of mental functions in the ICF generally corresponds to what is known as signs and symptoms of mental disorders (e.g., consciousness, orientation, energy, sleep, attention, memory, emotions).

THE DIFFERENT ASPECTS OF FUNCTIONING IN THE DSM

There are three major ways in which decrements in functioning are used in the DSM-IV-TR. The first is called “functional impairment”, which is described as dysfunction in social and occupational spheres of life, as noted above. Functional impairment is used as a criterion which must be fulfilled in order to render a diagnosis. Although never stated directly, the functional criterion in the DSM implies that a mental disorder must be associated with either distress or disability. As such, it helps establish the “threshold for the diagnosis of a disorder” 4. No guidance is given as to determining the level of disability that would constitute the contribution to the threshold for a diagnosis. It is left open to the clinical judgment of the user, which defies the basic operational approach of the DSM.

The second way functions are used is to determine the level of severity of the diagnosed disorder. The three levels of severity (mild, moderate and severe) include both symptoms and “impairments in social and occupational functioning”. Determining the level of severity is a clinical judgment. For example, the DSM-IV-TR’s guidance for the “mild” and “severe” includes either “few” or “many” symptoms above the required number and either “minor” or “marked” impairments in social or occupational functioning. “Moderate” is in between. The criteria for mood disorders are somewhat more explicit. Anchors are indicated for mild and severe disability in major depressive episode. Mild disability is addressed as “mild disability or the capacity to function normally but with substantial and unusual effort” 4. Severe disability is characterized as “clear-cut, observable disability (e.g., inability to work or care for children)” 4. The criteria for other mood disorders note different areas of disability, such as social activities and need for supervision, where the amount of supervision provides an anchor for severity 4.

The third way functions are used is to plan treatment, track clinical progress and predict treatment outcome. The Global Assessment of Functioning (GAF) is a 100-point scale used to rate both symptoms (i.e., part of the disease construct) and psychological, social or occupational functioning (i.e., part of the disability construct). Thus, in the construction of the GAF, the constructs of disease and disability are con-founded in each other. This entanglement does not allow a separate operational measurement of disability.

In summary, in the DSM system, making a diagnosis (and determining its level of severity) depends on a conjoint assessment of symptoms and functioning. These constructs are never assessed separately.

FUNCTIONING IN THE ICD

The ICD chapter V keeps the disability construct separate from the diagnosis of mental disorders. Disability is a discrete phenomenon that is evaluated separately in a different classification scheme, the ICF, as a complementing member of the WHO family of classifications. The ICF’s information on functioning and disability enriches the diagnostic information in the ICD, providing a broader, more meaningful picture of the patient’s health, which can be used for better management decisions. This separate assessment also allows studying the association between the disorder and disability by scientific methods.

Nevertheless, difficulties in a person’s functioning are occasionally included in the ICD classification of mental disorders. For example, decrements in functioning, such as poor self-care and social performance, are included as part of the description of negative symptoms in residual schizophrenia (F20.5). In this context, it is useful to note that ICD revision efforts will specifically review the diagnostic criteria to cleanly separate disease and disability constructs.

HOW TO DIFFERENTIATE DISABILITY AND SEVERITY?

The issue of disability is confounded by the definition of severity in mental disorders. Usually there is a positive correlation between the severity of an illness and the consequent disability; hence it is easy to fall in this trap. Unless one takes conceptual safeguards to differentiate severity of a mental disorder from the functional limitations that may result, it is not possible to study the interaction between the two. Theoretically, the severity of an illness is dependent on its development, spread, or the depth of dysfunction it causes in body systems. Disability is an outcome of the underlying disease in a given environment, concerning what people can do in terms of activities. For example, the severity of tuberculosis depends on factors such as the virulence of the bacteria, or the spread of the disease in the body, whereas disability depends on whether the patient with tuberculosis can work, go to school or carry out other daily activities.

The severity of a mental disorder is not always clearly and operationally defined in DSM, and is unfortunately confounded with a combination of the symptomatic constellation of the disorder and the limitations in social or occupational functioning. For example, the DSM-IV-TR explicitly states that the level of severity of a major depressive disorder or bipolar I disorder should be coded in the fifth digit. The three levels of severity defined in the DSM-IV-TR, as noted above, are: mild (few, if any, symptoms in excess of those required to make the diagnosis are present, and symptoms result in no more than minor impairment in social or occupational functioning); moderate (symptoms or functional impairment between “mild” and “severe” are present); severe (many symptoms in excess of those required to make the diagnosis, or several symptoms that are particularly severe, are present, or the symptoms result in marked impairment in social or occupational functioning) 4.

If one aims to apply a similar disease construct to both mental and physical diseases, other ways of formulating the severity of mental disorder should be explored. For example, the severity of a physical disease or disorder can be conceived in different ways: a) various thresholds on an indicator (such as mild, moderate or severe hypertension in terms of blood pressure levels); b) staging of the progress or dissemination of a disease (e.g., stage 1, 2, 3 of syphilis; classification of tumors according to the stage of their development); c) degree of complications (such as in latent, manifest and complicated diabetes mellitus).

There may be other or mixed models of severity of a disease. However, functional consequences in terms of what a patient has difficulty to do is a different construct from the severity of the disease, and has to be evaluated separately. Severe forms of diseases usually cause more disability; however, disability emerges from an interaction between the person and the environment. Depending on the context, there may be no disability in a severe disease or some disability in very mild forms of mental disorders. To address this confounding relationship, distinct constructs of disorder/disease and disability have to be operationalized 7, 9.

TOWARDS A PROPOSAL FOR A HARMONIZED DISABILITY FORMULATION IN ICD AND DSM

Currently two basic problems exist that require a solution: a) severity of symptoms (to assess the severity of many symptoms, the DSM calls for rating of functioning in a combined fashion; the ICD system, instead, does not call for ratings of functioning or disability to assess symptomatic severity); b) clinical significance of syndromes (the DSM calls for associated disability − functional impairment − as a requirement for the diagnosis of mental disorder; the ICD does not have this criterion and leaves this area to the ICF, which describes how functioning can be rated using qualifiers that connote the degree of the problem).

The DSM-V and the ICD-11 can be made compatible by allowing a separate operational assessment of disability through the DSM’s GAF scale and the ICF-linked assessment instruments, such as the WHO Disability Assessment Schedule (WHODAS, 10).

The key question is how to operationalize the ICF constructs in a succinct and clinically relevant way. Several assessment tools based on this classification system may help in identifying key areas of functioning. For example, the ICF Checklist provides the basis for a clinical assessment tool, covering the areas of cognition, communication, mobility, self-care, interpersonal relations, domestic and occupational life activities, and community, social and civic life. When these areas are coded as “present”, decrements can either be rated as mild, moderate, severe, or the total number of items can be summed, but the scale does not necessarily yield a cardinal measure of disability. Standardized metric information is needed, and can be gleaned from research conducted with the WHODAS on the population of individuals with mental disorders. A review of clinical experience and research using the ICF Checklist can support a revised set of items and assessment methodology.

As evident in Table 2, the ICF Checklist includes all domains of function/disability of the GAF scale. In the ICF Checklist, the nominal code is 0 for no difficulty, 1 for mild difficulty, 2 for moderate, 3 for severe, and 4 for complete. On a scale of 0 to 100, 0 to 4 percent is interpreted as no problem, 5 to 24 percent is a mild problem, 25 to 49 percent is a moderate problem, 50 to 95 percent is severe, and 95 to 100 is total or complete problem. The scale is calibrated in the opposite direction of the GAF scale, in which 91 to 100 is superior functioning. The GAF scale decile system is not translated into levels of severity such as mild, moderate or severe. The only WHODAS domain that is not consistent with the activities section of ICF Checklist queries about cognition. The items within this WHODAS domain would be assessed by the clinician as part of identifying symptoms of the disorders.

Table 2.

Table 2 Domains of functioning and disability in APA and WHO assessment tools

DSM –IV GAF Scale	ICF Checklist of Activities	WHODAS domains
		Understanding and communicating with the world (cognition)
	Learning and applying knowledge
	General tasks and demands
	Communication
	Mobility	Moving and getting around
	Self care	Self care
Social functioning	Interpersonal interactions and relationships	Getting along with people
	Domestic life	Life activities:
		Domestic responsibilities, work, leisure
School functioning	Major life areas
Occupational functioning	Education
	Employment
	Community, social and civic life	Participation in society

CONCLUSIONS

We need an internationally agreed conceptualization between ICD and DSM in terms of better operationalization of disease and disability components. This can be achieved by starting to use ICF domains in an operational way. In this way, thresholds for each domain of functioning could be better defined.

No functioning or disability should appear as part of the threshold of the diagnosis in either system. A separate rating of the disorder severity (i.e., mild, moderate, or severe), after a diagnosis has been made, would rely on an assessment of the development of the disease, its spread, continuity or any measure independent of disability parameters, so as to avoid co-linearity.

To put mental health in parity with the rest of health care, and integrate mental health to general health information systems, the classifications in mental health cannot afford to continue separate lines of development and should include common models and elements, including common terminology and ontology about signs, symptoms, functioning and other entities. This will create better scientific research which will lead to better assessment of outcomes and comparisons of effectiveness of health interventions.