Decrease in Upright Postural Sway from Open to Closed Eyes: Episodic Memory Impairment Matters, Too

The ability to maintain the body's center of gravity above a small base of support is defined by an upright stable posture characterized by body sways in the anterior–posterior (AP) and medial–lateral (ML) directions.1, 2 Maintaining an upright stable posture with closed eyes is challenging.3 The variation in postural sway from open to closed eyes is used as a measurement of balance impairment; a wide magnitude of variation is associated with greater postural instability.1-5 Poor lower limb proprioception and vision, and cognitive decline contribute to postural instability.4, 5 The specific effect of early memory decline on upright postural stability has been not fully examined. Gait instability has been associated with isolated episodic memory (category of declarative memory mainly related to autobiographical events that can be explicitly stated) impairment.6 It was hypothesized that episodic memory impairment would influence upright postural stability in individuals without dementia. This study aimed to determine whether episodic memory impairment was associated with large variation from open to closed eyes in static upright postural sway in individuals without dementia. Based on a cross-sectional design, 1,589 healthy community-dwelling individuals without dementia were recruited at the Health Examination Center, Lyon, France. Information was recorded on age (mean age 70.6 ± 4.9), sex (50.0% women), body mass index (BMI; mean 26.3 ± 4.2 kg/m2), number of drugs taken daily (mean 2.9 ± 2.5), and use of psychoactive drugs (benzodiazepines, antidepressants, neuroleptics; 18.7%). A four-item Geriatric Depression Scale score of 1 or greater (23.9%) was used to screen for depressive symptoms.7 Lower limb proprioception was evaluated using a graduated tuning fork placed on the tibial tuberosity.4 The mean proprioception value was obtained for the right and left lower extremities (6.3 ± 1.7/8). Distance binocular vision was measured at 5 m using a Monoyer letter chart (mean 7.1 ± 2.2/10).4 Fear of falling (FOF, 21.3%) was assessed by asking, "Are you afraid of falling?"8 History of falls (31.6%) over the past year was ascertained. Episodic memory was assessed using the short version of Mini-Mental State Examination, composed of three successive parts (free recall of 3 words, two short distractive tasks, delayed recall of 3 words). Scores ranged from 0 (worst) to 6 (best); a score of 5 or less (22.8%) designates poor episodic memory performance.9 Static upright postural sway on a firm surface was measured on a force platform (101 × 101 cm; BioRescue, Dune, Rodez, France).4 Participants were asked to maintain a barefoot standing position with eyes open for 30 seconds and closed for 30 seconds. Total upright postural sway in the AP and ML directions was recorded in millimeters. The variation from open to closed eyes was calculated using the following formula: ((AP + ML (eyes closed)) − (AP + ML (eyes open))/((AP + ML (eyes closed)) + (AP + ML (eyes open))/2)) × 100. The Lyon Sud-Est III local ethical committee approved the study. A multiple linear regression model (Table 1) was developed to examine the association between the variation from open to closed eyes in static upright postural sway (dependent variable) and participant characteristics (independent variables). Mean value of upright postural sway was 237.5 ± 207.1 mm when participants' eyes were open and 257.4 ± 228.1 mm when participants' eyes were closed. Older age (P < .001), higher BMI (P = .01), better vision (P < .001), FOF (P = .006), and history of falls (P = .01) were associated with greater postural sway from open to closed eyes, whereas greater lower-limb proprioception score (P < .001) and episodic memory impairment (P = .02) were associated with less postural sway. The main finding was that episodic memory impairment was associated with less upright postural sway when participants' eyes were closed than when their eyes were open. This finding may be interpreted as an early impairment associated with the highest levels of upright postural control. Postural instability is usually characterized by greater postural sway.1-5 These results confirm this statement in part because greater postural sway was found in participants with a history of falls and FOF, both factors usually associated with postural instability,4, 8 but less postural sway with memory impairment is an opposite unexpected motor behavior. Variability in postural sway is necessary to maintain upright posture.1-4 Therefore, this finding may be interpreted as an early dysfunction of static postural control related to episodic memory impairment. This result is in accordance with the fact that the hippocampus is involved in postural control through the activation of the "place cells" and the "head direction cells" during vestibular inputs.10 We are grateful to the participants for their cooperation. Conflict of Interest: The editor in chief has reviewed the conflict of interest checklist provided by the authors and has determined that the authors have no financial or any other kind of personal conflicts with this paper. Prof. Beauchet has served as an unpaid consultant for Ipsen Pharma company and as a board member for Gériatrie, Psychologie et Neuropsychiatrie du Vieillissement. He has no relevant financial interest in this manuscript. Author Contributions: Prof. Beauchet has full access to the study data. Beauchet, Allali: study concept and design: analysis and interpretation of data. Beauchet: acquisition of data; drafting of manuscript; statistical expertise; administrative, technical, material support. Levinoff, Allali: critical revision of manuscript for important intellectual content. Allali: study supervision. Sponsor's Role: Not applicable.

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