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LETTER |
The authors are with the MPH Program in Aging and Public Health, Mailman School of Public Health, Columbia University, New York, NY.
Correspondence: Requests for reprints should be sent to Steven M. Albert, PhD, MSc, Gertrude H. Sergievsky Center, PH-19, 630 W 168th St, New York, NY 10032 (e-mail: sma10{at}columbia.edu).
We appreciate the comments of Crews and Smith, who suggest that disability is best understood using the World Health Organization’s International Classification of Functioning, Disability and Health (ICF).1 This model stresses "activity limitation" and "participation restriction," rather than disability, and explicitly includes environmental factors in assessing the impact of health conditions. Crews and Smith argue that this model helps "avoid any suggestion that disability is a negative, undesirable end state, and, by implication, a circumstance less amenable to public health intervention."
Our model (which we could not present fully in our August 2002 editorial) differs from the ICF approach in asserting a strict temporal and causal sequence. Pathology (e.g., sarcopenia) leads first to impairment (e.g., lower-extremity weakness evident in manual muscle testing); when lower-extremity weakness crosses some threshold, functional limitation becomes evident, measurable perhaps in gait speeds below age- and gender-appropriate norms. When gait speed, in turn, drops below the minimum speed required to cross at a signaled intersection, a person is likely to report difficulty or a need for help crossing the street, that is, disability.2 In our editorial, we suggested that environmental factors affect this sequence. For example, changing the timing of traffic lights or even changing the time one goes out to do errands might prevent functional limitation from becoming disability. Rehabilitation or exercise to promote leg strength, or use of a motorized scooter, could break the link between impairment and functional limitation.
One advantage of the model we propose is the solid tradition of measurement behind it. For example, even in people who do not report mobility problems, weakness in the lower extremities predicts incident disability in the activities of daily living (ADL).3 People who do not report difficulty in ADL but report they have changed the way they perform these tasks have slower gait speeds and poorer grip strength.4 This approach thus identifies key points for intervention in the pathways to disability.
By making disability an outcome, does our approach minimize the experience of people who use personal assistance or assistive technologies to perform daily activities? We do not think so. We recently interviewed a 92-year-old woman who used a walker, required personal assistance for ADL 24 hours a day, and took 10 different medicines for 6 chronic conditions. Her ADL dependence was complete, yet she scored quite high on measures of activity and social participation, and she considered every day quite satisfying and interesting. By any account this is successful aging,5 yet she also sought ways to reduce her ADL dependency. For understanding her need for assistance, the etiology of this need, and potential points for intervention, we would argue that it is useful to model disability explicitly, even if it is narrowly defined.
Finally, we wholeheartedly agree that "aging and disability need to be modeled together." This may be the key challenge for public health and the second 50 years of life.
References
1. International Classification of Functioning, Disability and Health. Geneva, Switzerland: World Health Organization; 2001.
2. Verbrugge LM, Jette AM. The disablement process. Soc Sci Med.1994;38:1–14.
3. Guralnik JM, Ferrucci L, Simonsick EM, et al. Lower-extremity function in persons over the age of 70 years as a predictor of subsequent disability. N Engl J Med.1995;332:556–561.
4. Fried LP, Bandeen-Roche K, Williamson JD, et al. Functional decline in older adults: expanding methods of ascertainment. J Gerontol A Biol Sci Med Sci.1996;51:M206–M214.
5. Strawbridge WJ, Wallhagen MI, Cohen RD. Successful aging and well-being: self-rated compared with Rowe and Kahn. Gerontologist.2002;42:727–733.
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