Charlotte L. Shupert

Components of postural dyscontrol in the elderly: A review

The concept of a generalized aging effect on a generalized balance mechanism is discussed, and an alternative, multicomponent approach to understanding the heterogeneity of postural dyscontrol in the elderly is presented. Neural sensorimotor components of normal postural control mechanisms are identified and discussed. The effects of Parkinsons disease, hemiplegia, cerebellar degeneration, peripheral vestibular loss, and other disorders on the components of postural control are summarized. Quantitative posturography is advocated to detect preclinical manifestation of multiple musculoskeletal and neuromuscular pathologies and reduced compensatory abilities in posturally unstable elderly adults.

Organization of posture controls : an analysis of sensory and mechanical constraints

We analyse two components of posture control in standing human subjects: (1) the mechanical properties which constrain the bodys ability to execute stabilizing postural movements and (2) the mechanical and neural properties which constrain the ability of the vestibular system to sense changes in body orientation. Rules are then proposed to describe the central organization of posture controls within the sensory and mechanical constraints. The organizational rules and knowledge of constraints are combined to predict the effects of selective semicircular canal and utricular otolith lesions on postural stability and the patterns of body and head movements used to maintain balance. Our analysis leads to the prediction that semicircular canal and otolith deficits destabilize patients at different frequencies, and force them to use different patterns of body and head movements. These predictions are compared to posture controls observed in patients with different types of vestibular deficits. The additional steps required to prove or disprove the theory are discussed.

Effects of unilateral loss of vestibular function on the vestibulo-ocular reflex and postural control

Long-term recovery from surgically induced unilateral loss of vestibular function was studied in 14 patients. Seven patients underwent surgical extirpation or section of the vestibular nerve, and seven patients underwent labyrinthectomy without vestibular nerve section. The vestibulo-ocular reflex (VOR) and postural control were evaluated preoperatively and monitored for up to 4 years postoperatively with use of pseudorandom rotation (combined sinusoidal frequencies from 0.009 to 1.5 Hz) and moving platform posturography. Immediately following surgery all patients showed minimal reductions in the VOR gain constant, but marked reduction in the time constant, and marked increase in slow eye velocity bias. Bias returned to normal values within about 10 days, but time constants never returned to normal values. Results of standard Romberg tests in these patients were normal throughout the preoperative and postoperative periods. However, all patients showed marked postural control abnormalities in tests of the ability to maintain balance in unusual sensory environments in the immediate postoperative period. Seventy-five percent of the patients eventually recovered normal postural control. Postural control returned to near baseline performance with a time course similar to that of the VOR bias. However, postural control also continued to improve after the recovery of VOR bias was complete.

Abnormal postural control associated with peripheral vestibular disorders

The development of a systematic approach to the diagnosis and management of ataxias of vestibular origin depends critically on the elucidation of the complex sensory and motor interactions involved in human postural control. In this paper, the results of studies of both sensory and motor control of posture in adults and children with peripheral vestibular deficits are summarized and reviewed. In studies of the sensory organization of postural control, normal subjects and patients with peripheral vestibular deficits were exposed to unreliable information from their support surface and/or visual surround during quiet stance. While normal adults and children were able to maintain balance under these conditions, the majority of children and adults with peripheral vestibular deficits showed one or both of the following abnormalities: (1)Vestibular loss patients were unable to maintain equilibrium when forced to rely on vestibular information for postural control. (2)Vestibular distortion patients were unable to select an accurate source of sensory information when exposed to sensory conflicts during quiet stance. Preliminary results of studies of motor coordination in these patients also suggest that vestibular loss patients rely almost exclusively on ankle sway to control posture, even during balance tasks which require hip movements to maintain equilibrium. In contrast, some vestibular distortion patients appear to rely on hip motions, even when not required to do so to maintain balance. The results of these studies are discussed in terms of the implications for both sensory and motor aspects of postural control in patients with ataxias of vestibular origin.

Head-trunk movement coordination in the standing posture.

The coordination of head and trunk movement during postural sway in the anterior/posterior plane was examined in three normal adults. Postural sway about the ankles or hips was elicited in two ways: (1) In free-fall sway trials, the subject passively fell forward while the feet remained in place on the support surface (ankle sway). (2) In perturbed sway trials, subjects stood on either a flat surface (ankle sway) or a narrow beam (hip sway) which was displaced backwards at the onset of each trial. In all cases, postural responses were initiated before significant horizontal head motion was recorded. For subjects swaying about the ankles, changes in neck angle followed changes in ankle angle as the effect of the postural movement was propagated up the body. Neck muscle activation for ankle sway thus appeared to be elicited by neck stretch resulting from the postural correction. These results suggest that head and body motions may be controlled independently during active postural movements for ankle sway. For subjects swaying about the hip, however, changes in neck and hip angles were coordinated to approximately stabilize the rotational position of the head, and neck and hip muscles were activated simultaneously. These results, in contrast to those for ankle sway, suggest that control of head and body motion is coordinated on a feedforward basis during hip sway.