Stance perturbation-evoked potentials in old people with poor gait and balance
Introduction
Balance and gait dysfunction resulting in impaired mobility or falls is, after dementia, the most frequent cause of lost independence and diminished quality of life in old people (Masdeu et al., 1997). The prevalence of gait dysfunction in old people increases rapidly in the decade after age 65 and affects almost half of those over age 85 (Odenheimer et al., 1994). Obvious reasons for impaired mobility include deconditioning, arthritis, or medical diseases that compromise the function of peripheral nerve and muscle. However, most causes are multifaceted, and many cases have disease of the central nervous system. Despite the prevalence and impact of this problem, and the known relevance of central nervous system diseases, little is known about central mechanisms responsible for impaired balance and mobility.
Specific diseases of the central nervous system, such as Parkinson's disease, cerebellar degeneration, or stroke, can produce selective dysfunction of the sensory-motor control system but only explain a fraction of mobility impairment of presumed central origin (Sudarsky and Ronthal, 1983). The remaining gait disorders present as syndromes ascribed to dysfunction of the frontal lobes or associated subcortical structures (Nutt et al., 1993). By analogy with Binswanger's disease (Thompson and Marsden, 1987, Caplan, 1995), it is possible that this dysfunction is caused by microangiopathic lesions of the white matter (Van Swieten et al., 1991, Roman, 1996). Modern brain imaging methods have found white matter lesions that increase with age and correlate with gait and balance impairment (Masdeu et al., 1989, Baloh et al., 1995, Briley et al., 1997, Tell et al., 1998). In addition, there is increasing evidence that normal aging is accompanied by loss of white matter volume relative to gray matter volume (Guttmann et al., 1998). In light of these changes in white matter structure, we felt it important to determine if age-related changes in white matter function were associated with mobility impairment. We probed white matter function with evoked potentials.
Evoked electroencephalographic potentials are sensitive to white matter disease and have been used in several studies of gait control mechanisms (Dietz et al., 1984, Dietz et al., 1985b, Berger et al., 1987). Perturbations of stance or gait that mimic real-world situations associated with falls can evoke prominent multi-component cortical potentials. These evoked potentials have been used to infer the characteristics of supraspinal reflex mechanisms activated to prevent falling (Dietz et al., 1985a). In the present study we measured and compared stance perturbation evoked potentials in young and old people, reasoning that latency delays would appear in the responses of old people at risk for falls if white matter dysfunction was present. We prospectively stratified subjects by age and by a test of physical performance known to predict future disability. We found that old people had a delayed and bifid response. In a subgroup of old subjects with reduced mobility, the interpeak interval of the bifid response was prolonged. This subgroup also had reduced performance during a quantitative test of balance generally used to test sensory integration. We propose that delayed sensory conduction plays a role in maladaptive motor responses that can lead to falls and injury.
Section snippets
Methods
All components of the study were approved by the Institutional Review Board of the University of Connecticut Health Center. Informed consent was obtained from each subject.
Results
Recruitment resulted in the enrollment of 41 subjects, 8 young and 33 old. Physical performance testing classified the 33 old subjects as 13 old-mobile and 20 old-frail. The characteristics of the study population are summarized in Table 1. All indices of gait and balance were better in young subjects than old subjects, as shown in Table 2. Except for the index of vertical force response, indices of gait and balance were significantly worse in old-frail subjects than old-mobile subjects.
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Discussion
A large scalp potential was evoked when stance was perturbed by sudden translation of the weight-bearing surface. In old subjects the small initial vertex-positive portion of the response was delayed. In young subjects the larger subsequent vertex-negative portion of the response was monophasic. In subjects over the age of 70 years this negative component was smaller and bifid. Stratification of the old subjects by their physical performance was effective, as indicated by the subsequent group
Acknowledgements
The authors thank Julie Schmidt for coordinating the study, Charles B. Hall for suggesting statistical methods, Lance O. Bauer for reviewing the manuscript, and Kevin Gaski for technical assistance.
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