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SLIPPERY SLOPES

Division of Aging and Geriatric Psychiatry The University of Texas Health Sciences Center at San Antonio an Antonio, Texas

Address correspondence to Donald R. Royall, MD, Department of Psychiatry, The University of Texas Health Sciences Center at San Antonio, 7703 Floyd Curl Dr., Mail Code 7792, San Antonio, TX 78229-3900. E-mail: royall{at}uthscsa.edu

To the Editor:

Atkinson and colleagues (1) recently found significant associations between baseline cognition and the longitudinal rate of change in gait speed in a large sample of community-dwelling older adults. Two measures of executive control function (ECF) were employed: CLOX1 (2) and the EXIT15, a shortened form of the Executive Interview (EXIT25) (3). The association between baseline CLOX1 performance and change in gait speed was attenuated in a fully adjusted model, while the EXIT15's lost significance. Baseline depressive symptoms appeared most responsible for this effect.

It is important to remember that baseline intercepts can contribute independently of slopes to multivariate models of longitudinal change. Thus, the slope of change in Instrumental Activities of Daily Living (IADL) is independently associated with both baseline EXIT25 performance and the rate of change in EXIT25 scores (4,5). Thus, 1) change in EXIT15 scores may yet be significantly associated with change in gait speed, independently of the covariates included in Atkinson and colleagues' models, and 2) covariates such as depressive symptoms should be considered as potential mediators of the EXIT15's unadjusted association with the rate of change in gait speed. The effect of any such mediator may itself be mediated through effects on the longitudinal rate of change in EXIT15 scores. In other words, the covariates that appear to mediate cognition's association with changes in gait may merely indicate the set of conditions responsible for longitudinal changes in cognition that more directly influence gait speed.

The fact that depressive symptoms explain so much of the EXIT15's association is good news, in the sense that proper diagnosis and treatment may improve future physical performance. Conversely, the survival of CLOX1 and global cognition as predictors in fully adjusted models suggests that the covariate set does not adequately describe the conditions that potentially mediate their associations. Regardless, it may yet be possible for the slope of change in ECF to mediate the association between either baseline general cognition or depressive symptoms and change in gait speed, just as change in EXIT25 scores appears to mediate the association between memory and the rate of change in IADL (5).

References

1. Atkinson HH, Rosano C, Simonsick EM, Williamson JD, Davis C, Ambrosius WT, Rapp SR, Cesari M, Newman AB, Harris TB, Rubin SM, Yaffe K, Satterfield S. Kritchevsky SB; for the Health ABC study. Cognitive function, gait speed decline, and comorbidities: the health, aging and body composition study. J Gerontol A Biol Sci Med Sci. 2007;62A:844-850.[Abstract/Free Full Text]
2. Royall DR, Cordes J, Polk MJ. CLOX: an executive clock-drawing task. J Neurol Neurosurg Psychiatr. 1998;64:588-594.[Abstract/Free Full Text]
3. Royall DR, Mahurin RK, Gray K. Bedside assessment of executive cognitive impairment: The Executive Interview (EXIT). J Am Geriatr Soc. 1992;40:1221-1226.[Medline]
4. Royall DR, Palmer R, Chiodo LK, Polk, MJ. Declining executive control in normal aging predicts change in functional status: the Freedom House Study. J Am Geriatr Soc. 2004;52:346-352.[Medline]
5. Royall DR, Palmer R, Chiodo, LK, Polk, MJ. Executive control mediates memory's association with change in functional status: the Freedom House Study. J Am Geriatr Soc. 2005;53:11-17.[Medline]

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