Measurement challenges with cerebral palsy

The measurement of outcomes in children with cerebral palsy presents an array of challenges. The issues of concern for children with CP are wide-ranging, chronic, difficult to quantify, resistant to change, and more reflective of disability than illness. These parameters lead to frustration for the clinician or researcher who wishes to evaluate the status of a child or youth with CP.

CP impacts a breadth of arenas. Any person with CP, parent, or medical professional who cares for children with CP will attest that CP touches almost every aspect of life. CP causes changes in basic body functioning, such as strength, coordination, and muscle tone. CP results in difficulty with functional tasks, such as walking, swallowing, accomplishing self-care, and communicating. In turn, these deficits contribute to decreased involvement in community, school, and family activities. CP has some impact on quality of life and definitely creates additional stressors for caregivers. These facts illustrate the many potential problem areas to understand and to measure in CP. The sheer number of possible targets for measurement makes it hard to describe CP in a comprehensive and accurate fashion.

CP is a diverse diagnosis with substantial variation in impairments and severity. Although CP may manifest with marked impairments and myriad secondary impacts, some individuals with CP have fairly modest disability and few daily effects. CP is highly associated with other conditions, including cognitive impairment and epilepsy, that create their own issues. Because of the variability in symptoms, sequelae, and comorbidities, any 2 children with CP may look very different from each other. Understanding how those 2 different children are doing may mean using 2 entirely different evaluations that are targeted to the issues affecting each child. These differences create a need for flexibility in measurement strategies so as to address each child with CP in a meaningful way.

Another challenge is the long-term nature of CP. CP is not an acute illness. Children and youth with CP will always have CP, but their CP may affect them differently over time. It is of paramount importance to understand the long-term outcomes of CP and how interventions affect outcomes many years later. Studying outcomes in a longitudinal fashion for years, rather than weeks or months, is expensive, time-consuming, and messy because of the inability to control for extraneous factors. Nonetheless, longitudinal measurement is arguably the most important method to explore treatments and interventions for CP, as outcomes in adolescence and adulthood are of critical importance.

There are few gold standards or rubrics for measurement in CP. Many of the domains of concern are not easily quantified, and there are no established benchmarks of measurement. For example, there is no single, best, and universally accepted method to measure even how well a child with CP walks. Assessing things such as quality of movement, happiness, ease of care, and independence is a confusing and frustrating venture. Unlike more quantifiable outcomes (eg, blood pressure or birth weight), most concerns in CP do not have an established “ruler” to measure them.

One goal of outcome measurement is to assess changes that occur as a result of treatments or interventions. This is another concern in CP because so many of the available treatments do not seem to create big changes. With small treatment effects described for many interventions, at least with the available measurement tools, a question arises: Are the available outcome measures really good enough? Either the measurement tools are not adequately sensitive to change and cannot detect meaningful effects of treatment or the treatments have insufficient effect to be meritorious. Until each individual outcome measure can be evaluated for its discriminatory properties and responsiveness to change, this conundrum will remain.

A final challenge in measurement in CP is the difficulty of looking at CP with a purely medical perspective. This perspective considers CP to be a disease and people with CP to be ill. It suggests that CP is something that has clear biologic markers and that medical interventions can or should be used to alter the underlying disease process. These ideas translate into a measurement paradigm that includes an assumption that the basic processes of CP should be targeted with treatments and that improving the physiologic parameters of CP will result in improvements in the outcomes of interest. This is how things work for most health conditions. In the case of CP, however, this approach is not entirely satisfactory. First of all, many people with CP are not ill and are best described as having a disability rather than poor health. When viewed as a chronic disability, CP is not something that is readily amenable to direct treatment. Furthermore, the degree of impairment in the more “basic” issues of CP (eg, spasticity or strength) does not directly translate into functional (dis)abilities or more “higher-level” issues, such as quality of life. Thus, some of the usual assumptions about health-related outcome measurement are not fully applicable in CP. Understanding issues such as change or the importance of individual preferences and goals in CP becomes even more salient when CP is viewed as a disability.

Current outcome assessment in CP

In the last 10 years, perspectives on outcome assessment in cerebral palsy have been influenced and shaped by the World Health Organization’s International Classification of Functioning, Disability, and Health (ICF) as a guiding principle. This seminal work describes and codifies a unifying means of understanding health status. The 2001 version for adults was followed in 2007 by an ICF for children and youth. The ICF is a framework that captures the breadth of issues created by CP and the many arenas of impact. In brief, the ICF considers that each person’s function, disability, and health are interdependent and are modified by environmental and personal factors. Thus, the ICF provides descriptions in 3 major domains of body function, body structure, and activities and participation (execution of tasks and activities and involvement in a life situation). These domains are further classified with contextual factors, either personal or environmental. ICF domains have been used to understand and describe the many impacts of CP for individuals and allow for categorization of various CP outcome measures by the domain that is being assessed.

Use of the ICF in CP outcome measurement might best be understood with an example. Consider the use of botulinum toxin in a child with diparetic CP and an abnormal gait. A range of concepts across the ICF spectrum might be altered by the use of botulinum toxin, and each of these concepts could be assessed using different outcome measures or assessment tools. In the domain of body structure, botulinum toxin might have no effects, but it might change muscle structure or create cortical reorganization that could be detected with muscle biopsy or functional magnetic resonance imaging (fMRI). In the domain of body function, changes in spasticity and strength could be measured with the Tardieu or Ashworth scales and dynamometry. Activity and participation is a single domain but may best be considered in 2 parts. Activity for a child with spastic diparesis might be altered with botulinum toxin by changing gross motor skills or gait pattern, which could be assessed with the Gross Motor Function Measure or an observational gait scale. Participation realms would include playing on a sports team or attending more social events, which might be measured with the Children’s Assessment of Participation and Enjoyment. Environmental contextual factors, unlikely to be changed by botulinum toxin treatment, would include things such as accessible facilities and transportation to medical appointments. This example demonstrates the use of the ICF in capturing the range of issues for children with CP.

The attention to these concepts has resulted in greater understanding of the relationships among the domains of the ICF in CP. Most notably, recent work has demonstrated that there are no fixed relationships among the domains of the ICF. Although improvement at the level of body function and activity may take place (eg, decreased spasticity and better gross motor function), this does not necessarily mean improved participation or family satisfaction. Furthermore, severity of impairment or disability is not directly correlated with quality of life. These findings have been best interpreted in the light of the ICF.

Choosing the most appropriate outcome measures or assessment tools in clinical research or the clinical care of CP is challenging. With a vast array of areas of interest, many tools have been developed and are being used. Many outcome measures in use are fairly specific for 1 domain of the ICF, whereas others may span 2 or more domains. Some measures have been carefully designed, validated, and found to be reliable, whereas others were developed casually and have not been evaluated for their performance. Clinicians and researchers must think critically about the available tests, studies, questionnaires, evaluations, interviews, and technologies, and they must consider the patient or subject, the situation, and the question at hand before making selections.

When considering outcome measures, it is necessary to evaluate the psychometric performance of each measure. The best measure is one that addresses the domain of concern, is validated and reliable, can be applied readily, and is responsive to change. For many measures, some of these criteria are not yet fulfilled. Validity and reliability are key concepts in outcome measurement that demonstrate that the measure truly assesses the concepts of interest and that the assessments are accurate and repeatable. Many measures in use for children with CP have been evaluated for validity and reliability in at least a preliminary fashion. Even so, caution must be paid to using each instrument in the manner it was intended, which means administering each measure precisely as the developers instruct, including avoiding the use of tool subscales if the subscales have not been demonstrated to stand alone, using the measure in intended populations, and administering the measure as recommended. Some measures are best described as classifications. For example, the Gross Motor Function Classification System (GMFCS) delineates 5 strata of motor functioning in children and youth with CP. It is convenient and widely used to describe severity of impairment in CP, but it has not been demonstrated to be useful to detect changes with time or after interventions, and it was not designed for this purpose. The ability to pick up differences with time is called responsiveness or sensitivity to change. This concept is not well studied for most CP outcome measures. Some measures require special equipment, trained assessors, or lengthy periods of administration; these measures may be less attractive to use because of the costs or inconveniences associated with them.

An additional concern for choosing outcome measures in CP research is the need for appropriate study design. The desire for greater information about clinical prognosis, natural history, and effects of intervention is great, and many researchers strive to provide answers. Unfortunately, if studies are not designed well, much effort and many resources may be expended without yielding useful evidence to advance knowledge. In CP research, many studies lack basic design elements, such as defining a primary outcome measure (the main thing that the study intends to evaluate for change) or power calculations (to assure that the study has the right number of subjects to answer the research question). Many studies do not create high-level evidence because they do not use high-quality design features, including randomization, blinding, allocation concealment, prospective recruitment, and adequate follow-up periods. Thus, without care in study design, the selection of the perfect outcome measure does not assure that a study is of value.

Current outcome assessment in CP

In the last 10 years, perspectives on outcome assessment in cerebral palsy have been influenced and shaped by the World Health Organization’s International Classification of Functioning, Disability, and Health (ICF) as a guiding principle. This seminal work describes and codifies a unifying means of understanding health status. The 2001 version for adults was followed in 2007 by an ICF for children and youth. The ICF is a framework that captures the breadth of issues created by CP and the many arenas of impact. In brief, the ICF considers that each person’s function, disability, and health are interdependent and are modified by environmental and personal factors. Thus, the ICF provides descriptions in 3 major domains of body function, body structure, and activities and participation (execution of tasks and activities and involvement in a life situation). These domains are further classified with contextual factors, either personal or environmental. ICF domains have been used to understand and describe the many impacts of CP for individuals and allow for categorization of various CP outcome measures by the domain that is being assessed.

Use of the ICF in CP outcome measurement might best be understood with an example. Consider the use of botulinum toxin in a child with diparetic CP and an abnormal gait. A range of concepts across the ICF spectrum might be altered by the use of botulinum toxin, and each of these concepts could be assessed using different outcome measures or assessment tools. In the domain of body structure, botulinum toxin might have no effects, but it might change muscle structure or create cortical reorganization that could be detected with muscle biopsy or functional magnetic resonance imaging (fMRI). In the domain of body function, changes in spasticity and strength could be measured with the Tardieu or Ashworth scales and dynamometry. Activity and participation is a single domain but may best be considered in 2 parts. Activity for a child with spastic diparesis might be altered with botulinum toxin by changing gross motor skills or gait pattern, which could be assessed with the Gross Motor Function Measure or an observational gait scale. Participation realms would include playing on a sports team or attending more social events, which might be measured with the Children’s Assessment of Participation and Enjoyment. Environmental contextual factors, unlikely to be changed by botulinum toxin treatment, would include things such as accessible facilities and transportation to medical appointments. This example demonstrates the use of the ICF in capturing the range of issues for children with CP.

The attention to these concepts has resulted in greater understanding of the relationships among the domains of the ICF in CP. Most notably, recent work has demonstrated that there are no fixed relationships among the domains of the ICF. Although improvement at the level of body function and activity may take place (eg, decreased spasticity and better gross motor function), this does not necessarily mean improved participation or family satisfaction. Furthermore, severity of impairment or disability is not directly correlated with quality of life. These findings have been best interpreted in the light of the ICF.

Choosing the most appropriate outcome measures or assessment tools in clinical research or the clinical care of CP is challenging. With a vast array of areas of interest, many tools have been developed and are being used. Many outcome measures in use are fairly specific for 1 domain of the ICF, whereas others may span 2 or more domains. Some measures have been carefully designed, validated, and found to be reliable, whereas others were developed casually and have not been evaluated for their performance. Clinicians and researchers must think critically about the available tests, studies, questionnaires, evaluations, interviews, and technologies, and they must consider the patient or subject, the situation, and the question at hand before making selections.

When considering outcome measures, it is necessary to evaluate the psychometric performance of each measure. The best measure is one that addresses the domain of concern, is validated and reliable, can be applied readily, and is responsive to change. For many measures, some of these criteria are not yet fulfilled. Validity and reliability are key concepts in outcome measurement that demonstrate that the measure truly assesses the concepts of interest and that the assessments are accurate and repeatable. Many measures in use for children with CP have been evaluated for validity and reliability in at least a preliminary fashion. Even so, caution must be paid to using each instrument in the manner it was intended, which means administering each measure precisely as the developers instruct, including avoiding the use of tool subscales if the subscales have not been demonstrated to stand alone, using the measure in intended populations, and administering the measure as recommended. Some measures are best described as classifications. For example, the Gross Motor Function Classification System (GMFCS) delineates 5 strata of motor functioning in children and youth with CP. It is convenient and widely used to describe severity of impairment in CP, but it has not been demonstrated to be useful to detect changes with time or after interventions, and it was not designed for this purpose. The ability to pick up differences with time is called responsiveness or sensitivity to change. This concept is not well studied for most CP outcome measures. Some measures require special equipment, trained assessors, or lengthy periods of administration; these measures may be less attractive to use because of the costs or inconveniences associated with them.

An additional concern for choosing outcome measures in CP research is the need for appropriate study design. The desire for greater information about clinical prognosis, natural history, and effects of intervention is great, and many researchers strive to provide answers. Unfortunately, if studies are not designed well, much effort and many resources may be expended without yielding useful evidence to advance knowledge. In CP research, many studies lack basic design elements, such as defining a primary outcome measure (the main thing that the study intends to evaluate for change) or power calculations (to assure that the study has the right number of subjects to answer the research question). Many studies do not create high-level evidence because they do not use high-quality design features, including randomization, blinding, allocation concealment, prospective recruitment, and adequate follow-up periods. Thus, without care in study design, the selection of the perfect outcome measure does not assure that a study is of value.