Balance Dysfunction in Parkinson's Disease

Basic Mechanisms to Clinical Management

Martina Mancini, John G. Nutt, Fay B. Horak

• Publisher: Academic Press
• Year: 2019
• Print ISBN: 9780128138748
• eBook ISBN: 9780128138755
• Topic: Medicine
• Subtopic: Neurology

Balance Dysfunction in Parkinson's Disease: Basic Mechanisms to Clinical Management presents the most updated information on a variety of topics. Sections help clinicians evaluate the types of balance control issues, dynamic balance dysfunction during turning, and the effects of medication, deep brain stimulation, and rehabilitation intervention on balance control. This book is the first to review the four main postural control systems and how they are affected, including balance during quiet stance, reactive postural adjustments to external perturbations, anticipatory postural adjustments in preparation for voluntary movements, and dynamic balance control during walking and turning.In addition, the book's authors summarize the effects of levodopa, deep brain stimulation, and rehabilitation intervention for each balance domain. This book is recommended for anyone interested in how and why balance control is affected by PD.- Provides the first comprehensive review of research to date on balance dysfunctions in Parkinson's disease- Discusses how to translate current neuroscience research into practice regarding neural control of balance- Provides evidence on the effects of current interventions on balance control

Chapter 1

How is balance controlled by the nervous system?

Abstract

Balance (posture) control has two main goals: postural equilibrium (stability) and postural orientation. Balance control has the following properties: the musculoskeletal system, antigravity postural tone, forward internal model of the body and world, coupling between posture and movement, and cognitive control of balance. Many different brain networks are involved in control of balance. The main roles of the basal ganglia in control of balance are: energization/scaling (vigor), 2) automatization, 3) posture-movement coupling, and 4) context-dependent adaptation. The main balance domains to assess are: standing balance, postural responses, postural transitions, dynamic balance during gait, and turning. Balance can be improved with practice.

Keywords

Anticipatory postural adjustments; Balance control; Basal ganglia; Cortical areas; Gait; Postural responses; Quiet standing

A. What is balance control?

The balance (posture control) system has two main goals: (1) postural equilibrium (stability) and postural orientation (alignment and sensory orientation). Effective control of both equilibrium and orientation depends upon properties of the balance control system including the musculoskeletal system, antigravity tone, internal model, cognitive control, coupling voluntary and postural goals, and ability to improve control via motor learning. Comprehensive assessment of balance requires examining ability to accomplish a variety of balance tasks/domains that depend upon achieving the goals of equilibrium and orientation: (1) standing, (2) transitioning between postures or between posture and movement, (3) reacting to external perturbations, and (4) walking (straight or turning). This book is organized around these four postural domains to provide clinicians and researchers a framework to consider when assessing balance control in patients with Parkinson disease (PD).

Postural equilibrium (also called balance) involves control by the nervous system to resist forces acting on the body that attempt to alter desired body position (postural alignment). ¹ Whereas inanimate objects, like a stacked rock tower can maintain static equilibrium when the center of mass of each segment is lined up over its base of support, humans continuously and actively control dynamic equilibrium by moving either, or both, the body center of mass (CoM) or the base of support. The CoM is a point that represents the average position of the body's total mass. In humans, the CoM is approximately 2 cm in front of the second lumbar spinal segment while standing, but can be in front of the body when flexed at the hips. Because the body is always in motion, even when attempting to stand still, the CoM continually moves about with respect to the base of foot support, and has been called postural sway. To control the moving CoM, we push down onto the support surface with our feet to move the body center of pressure (CoP) out beyond the CoM to corral it within the base of foot support. If the CoP is ineffective in quickly controlling the CoM, people take a step or reach out to a stable object to recover equilibrium. Postural instability is determined by how fast the body CoM moves toward the boundary of its base of support and how close the downward projection of the body's CoM is to the support boundary. Thus, holding onto a stable walker can improve control of postural equilibrium by increasing the base of support, allowing the arms to provide forces to control motion of the CoM, and by supplementing somatosensory information from the arms for control of balance.

Postural orientation involves arranging body parts with respect to the environment (gravity, visual and support surface) to accomplish tasks efficiently, interpret sensory inputs, and anticipate balance disturbances. In humans, upright orientation of the trunk with respect to gravity minimizes the forces and energy required to control the body's CoM over its base of support. However, for some tasks the position of a body part in space needs to be stabilized, whereas for other tasks, one body part needs to be stabilized with respect to another. For example, when walking while carrying a glass full of water, it is important to stabilize the hand with respect to gravity to prevent spillage. In contrast, when walking while reading a book, the hand must be stabilized with respect to the head and eyes. Subjects may adopt a particular postural orientation to optimize the accuracy of sensory signals regarding body motion. For example, in activities such as windsurfing or skiing, in which the support surface is unstable, information about earth vertical is derived primarily from vestibular and visual inputs. A person often aligns his head with respect to gravitational vertical in this situation because the perception of vertical is most accurate when the head is upright and stable. ^2,3 Anticipatory alterations of habitual body orientation can minimize the effect of an anticipated postural perturbation. For example, people often lean in the direction of an anticipated external force, or flex their knees, widen their stance, and extend their arms when anticipating that stability will be compromised. Curiously, people with PD may not flexibly modify their postural orientation faced with such anticipated perturbations. ⁴

Postural equilibrium and postural orientation are different but interdependent. For example, a patient with camptocormia, or involuntary flexion of the hips while standing, can have excellent control of their body center of mass (equilibrium) for both self-initiated and external perturbations (Fig. 1.1A). In contrast, another patient can have excellent postural orientation, both in body alignment and in multisensory orientation to the world, but find themselves falling often due to poor control of postural equilibrium for self-initiated and/or external perturbations ⁵ (Fig. 1.1B, Purdue Martin, 1967, book out of print). Although postural equilibrium and orientation can be altered independently, they are also interdependent. For example, studies have shown that a flexed postural orientation of the legs and trunk compromises the ability to recover equilibrium in response to perturbations (Chapter 4).

Figure 1.1 (A) Poor postural orientation with good control of equilibrium (CoM is over base of foot support) in a patient with camptocormia and (B) Poor equilibrium control with good postural orientation in a patient with rigidity who is falling (projection of CoM to the ground is behind the feet).
(A) Adapted from St George RJ, Gurfinkel VS, Kraakevik J, Nutt JG, Horak FB. Case studies in neuroscience: a dissociation of balance and posture demonstrated by camptocormia. Journal of Neurophysiology 2017. https://doi.org/10.1152/jn.00582.2017.

B. What are the critical properties of balance control?

Underlying effective balance is a control system with specific properties that provides flexibility, resilience, and efficiency to balance control. The properties include the musculoskeletal system, antigravity muscle tone, internal model of the body and world, coupling between posture and movement, and cognitive control of balance. All of these properties can be impaired in people with PD, resulting in difficulty controlling postural equilibrium and orientation across all the tasks they attempt.

Property 1. Musculoskeletal system: Neural control is exerted via a musculoskeletal system than provides some passive, mechanical stability and orientation. For example, standing can be accomplished with only tonic activation of soleus muscles in the calves because ligaments and other elastic structures around the joints and alignment of body segment center of mass over each segment can allow for static equilibrium. However, abnormal musculoskeletal system due to reduced joint range, reduced muscle strength, unusual segmental body alignment, etc., usually results in inefficient, less effective bala...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Dedication
• About the authors
• Acknowledgments
• Introduction
• Chapter 1. How is balance controlled by the nervous system?
• Chapter 2. Why is balance so important in Parkinson disease?
• Chapter 3. How is balance during quiet stance affected by PD?
• Chapter 4. How are postural responses to external perturbations affected by PD?
• Chapter 5. How are anticipatory postural adjustments in preparation for voluntary movements affected by PD?
• Chapter 6. How is dynamic balance during walking affected by PD?
• Chapter 7. How and why is turning affected by Parkinson disease?
• Chapter 8. Is freezing of gait a balance disorder?
• Chapter 9. How should the clinician approach imbalance in PD?
• Chapter 10. Future perspectives on balance disorders in PD
• Index