chronic health conditions. While measures of physiological function fall within the realm of health conditions, not all physiological alterations have a significant health effect in terms of altering functioning (e.g., changes in the pigmentation of the hair) (Ustun et al., 2002). There is rarely a one-to-one correspondence between a diagnosis and its translation into a level of impairment in daily life. Two individuals with the same abnormal laboratory value or radiological finding may have very different life experiences, with one unaffected or only mildly affected in his or her daily life and the other experiencing major dysfunction.

In addition, the functional expression of a condition is heavily influenced by the various ways in which it is modified by other factors intrinsic to the child (e.g., personality, comorbid conditions, genetic endowment) and the environment. For example, a child who lives in a one-story house will experience fewer limitations in function when experiencing joint or muscle disease than one living in a five-story walk-up apartment.

Similarly, the level of compensatory mechanisms and treatments available (e.g., durable medical equipment, implants, medications) will change the expression of the condition in terms of the child’s functioning. Measures of functioning also have great meaning because of their implications for caregiving, dependence, and the ability to participate in social roles. Another advantage of these measures is that they permit a look at the effects of multiple conditions as well as the effects of the conditions and their treatments, including side-effects. They provide a common measure for assessing the health of children across conditions (Stein et al., 1987; Stein and Jessop, 1990).

Limitations in mobility, usual activities, or full participation in school are important aspects of physical function. Psychological function includes a wide range of functions, both cognitive (e.g., alertness, confusion, problem-solving ability, receptive language ability) and emotional (e.g., affect, mood, temperament). Given the profound developmental growth that takes place over the life course of a child, these subdomains constantly change, which creates significant challenges for accurate measurement of psychological deficits. For example, a specific disease can cause a child to regress in emotional or cognitive capacity. However, without repeated measurement of that child’s function, it would be difficult to determine whether a regression had taken place.

Social functioning refers to limitations imposed on children in the realm of their usual activities and relationships. For a young child, the subdomains of social role function include the ability to engage in ordinary play and the ability to attend school and participate in all school-related activities. Social functioning also includes measures of social integration and social connection, including the ability to make and keep friends and to play a supportive or instrumental role in the lives of others. Culture contributes in major ways to the definition of appropriate social functioning, and thus this domain may have different meaning for different subgroups.

Alterations in function include physical, cognitive, emotional, and social



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