This study presents new information that improves our understanding of the pathway from musculoskeletal pain to severe mobility disability in older women. The progression of mobility disability was not mediated by either psychological symptoms or measures of physical impairments and functional limitations. Widespread pain, reported by 1 in 5 women without mobility difficulty at baseline, was the only category of musculoskeletal pain that consistently and independently predicted onset or worsening of severe difficulty with walking or stair climbing. Moderate lower extremity pain was a weak predictor of worsening difficulty with stair climbing but not with walking. Widespread pain, a common problem among older women, leads to disability in a process that does not follow the well-tested Nagi model of disablement.
In this study of disabled older women, we showed that the pathway from widespread pain to mobility difficulty is likely a direct relationship. This is consistent with our earlier cross-sectional research on the relationship between back pain and disability (Leveille et al., 1999
). There is a traditional belief that pain leads to reduced physical activity and subsequent muscle weakness leading, ultimately, to disablement. Although eligibility for the WHAS required that participants had at least some disability at baseline, we did not find evidence that women in the WHAS who had musculoskeletal pain were less physically active or weaker than other women, nor did they have more gait or balance problems than other women in the WHAS. Our findings address the progression of disability
rather than incident disability in women with no other impairments and disability. Thus, we do not know whether the pathway from pain to disablement among women without disability is mediated by physical activity and muscle weakness.
In reference to the Nagi disablement pathway, pain is a symptom of pathology and may be considered an impairment on the pathway from pathology to physical impairment to functional limitation and finally to disability (Nagi, 1976
). Our research suggests that widespread musculoskeletal pain, regardless of impairments such as muscle strength or gait speed, may be sufficient to cause disability. In fact, our baseline cross-sectional analysis showed the high prevalence of difficulty among women with widespread pain compared to those with none or less pain, suggesting that the pain-difficulty relationship often may be concurrent rather than sequential in many persons. In other words, having musculoskeletal pain creates immediate difficulty with mobility. For this study, we were interested in determining potential mediators of the longitudinal pain-disability relationship and the findings showed that 2 key sets of factors, physical performance and psychological factors, that often explain progression of disability had little to do with the pain-disability relationship in this population. We know from other WHAS analyses that pain status persists over time and it is likely that our findings reflect the long term effects of living with chronic pain, or also, the progression of underlying chronic diseases that may contribute to chronic pain. It is also possible that other age-related declines may limit compensatory ability to continue to function in the face of chronic pain. Further studies are needed to evaluate other possible mechanisms by which chronic pain leads to progression of disability.
Why is this discussion of the pathway from pain to disability of interest? The answer will inform both clinical and public health efforts to prevent pain-related disability. If adequate pain relief, including better management of underlying musculoskeletal pathology, is sufficient to prevent or treat pain-related disability, then it is possible that a substantial portion of disability in the older population could be resolved through more aggressive pain management approaches. Alleviation of pain may be sufficient to restore functioning in a major segment of the population that experiences chronic undertreated musculoskeletal pain. More frail persons or those who have had chronic activity restrictions related to pain may require additional physical rehabilitation.
In clinical practice and research, pain and pain-related disability are sometimes attributed to psychological factors in the absence of other explanations for the physical symptoms. In this study, we did not find that depression or anxiety accounted for the disabling consequences of widespread musculoskeletal pain. Although we know that pain and depression are related in the general adult population (Magni et al., 1990
) and among elders in the WHAS and in other cohorts (Landi et al., 2005
), research has not demonstrated that depression explains pain-related disability in older adults. Hughes and colleagues studied subgroups of older adults in Chicago and found that both arthritic pain and psychological status (depression and anxiety) were independent predictors of general functional decline after 2 years but that pain had a somewhat stronger impact on function (Hughes et al., 1994
). Ours may be the first population-based study examining the pathway from musculoskeletal pain to mobility disability in older adults. Studies in non-disabled elder cohorts using comprehensive pain assessments are needed to determine whether the pain-disability relationship is as direct as our findings indicate.
A serious limitation in studies of pain and disability in the elderly is the lack of a consistent approach to assessing and classifying pain. We have found that the ACR classification of widespread pain provides a sound method for classifying the most disabling pain that causes quantitatively different risks for older persons compared to lower extremity pain or mild musculoskeletal pain in 1 joint area. Prior studies have shown that location and severity of pain are key pain characteristics that are associated with disability (Lichtenstein et al., 1998
; Scudds and Robertson, 2000
). In addition, more severe joint pain is associated with multisite pain (Leveille et al., 1998
; Vogt et al., 2003
; Croft et al., 2005
) and multisite pain is more disabling than other pain (Hopman-Rock et al., 1997
; Leveille et al., 2001
). In fact, multisite pain is so common among older adults that studies of the disabling impact of single sites of musculoskeletal pain in the elderly may lead to inconclusive or questionable findings if multisite pain, particularly widespread pain, is not addressed in the analyses.
We also examined the potential impact of daily analgesic use on the risk for disability due to pain. As with the other potential mediators, we did not find that analgesic use altered the likelihood for disability related to pain. Again, this is a potential mediator that has rarely been studied. Further observational studies are needed. However, the optimal examination of the role of analgesics is through use of randomized trials to determine which approaches to pain relief will lead to the greatest functional improvements.
The limitations of the present analysis have to do with the composition of the study cohort, women who already had some disabilities at baseline. This group was more at risk for new or worsening disability and their experience may not reflect the risks of a more general older population. The study population was representative of the many disabled women who live in the community, one-third of all older women, and these findings can only be generalized to similar populations. Studies in the general population of older adults are needed to better understand the development of pain-related disability. It is unknown whether the pathway from pain to mobility difficulty would be the same in older men. Older women report more pain and have more disability than older men (Leveille et al., 2005
The causes of musculoskeletal pain are numerous in older adults, even though unexplained pain is frequently attributed to arthritis. Previous analyses of the WHAS suggest that polyarticular osteoarthritis may explain fewer than half the cases of widespread pain in this cohort (Leveille et al., 2002
). Clinical information to determine non-articular explanations for widespread musculoskeletal pain, such as fibromyalgia and myofascial pain, was unavailable in this study. Though widespread pain, leg pain and other pain are classified as separate entities for this research, clinical experience suggests that persistent widespread pain may represent the progression and accumulation of segmental pain problems. Though pain may fluctuate in location and severity, widespread pain may be an endpoint through which single site chronic pain problems may progress. By clarifying the disabling impact of widespread pain, our research provides an important foundation for further efforts to examine chronic widespread pain in the elderly.
In conclusion, our findings suggest that musculoskeletal pain, particularly widespread pain, is a unique domain as a cause of disablement, independent of the usual pathway to disability via physical impairments and functional limitations. Contrary to usual assumptions about pain and depression, psychological factors did not explain the strong relationship between pain and disability in older women. Research is needed to better understand the development of pain-related disability in order to determine optimum approaches to prevent and treat mobility disability in older persons with persistent pain.