Balance Control System
The balance system includes a complex array of control processes that can be grouped into two distinct, but interdependent systems1
- The gaze stabilization system maintains gaze direction of the eyes and visual acuity during activities involving active head and body movements. Gaze stabilization and the vestibulo-ocular reflex (VOR) systems are often viewed as synonymous, even though the VOR is only one component of gaze stabilization.
- The postural stabilization system keeps the body in balance while an individual stands and actively moves about in daily life.
The gaze and postural stability systems are distinct because they rely on information from different senses, motor reactions of different parts of the body, and are mediated by different brain pathways. The two systems are, however, interdependent because gaze stability is not possible unless the body on which the head and eyes ride is also stable and because accurate vision, which is dependent on gaze stability, is a critical sensory input to postural control.
Gaze Stabilization Components
Maintaining stable gaze relies on the interactions among the following components2:
- Combinations of gaze direction information from the vestibular system and vision.
- Groups of eye muscles to control lateral and vertical eye movements.
- The brain's ability to integrate the sensory and motor functions of gaze control.
The vestibulo-ocular reflex (VOR) is a fast-acting system that relies on inputs from the vestibular system to reflexively drive eye movements that are equal and opposite those of the head. The VOR does not require visual information, although the "gain" of the VOR movement, i.e. the amplitude of eye movement relative to that of the head, is influenced by a subject's state of arousal. The VOR is effective for stabilizing gaze during rapid movements and is ineffective for slow movements.
The smooth pursuit eye movement system allows the direction of gaze to smoothly follow a desired visual target during active movement. The smooth pursuit system requires the presence of a visual target but is not affected by the presence or absence of vestibular information. This system dominates gaze control during slow movements, when the VOR system is less effective.
The saccadic eye movement system can generate rapid "catch-up" eye movements when the VOR and smooth pursuit systems fail to maintain gaze on a desired visual target, or when gaze is first directed to a new visual target. Like the smooth pursuit system, saccadic movements require the presence of a visual target but are unaffected by vestibular information.
Postural Stabilization Components
Maintaining postural stability is a more complex balance process that relies on the interaction of three major components.3
- Combinations of orientation information from vision, the inner ear vestibular system, and the proprioceptive sense of contact with the support surface.
- Motor reactions coordinated among muscles of the feet, legs, and trunk.
- The brain's ability to integrate the above sensory and motor processes, and to adaptively modify these processes in response to changes in the environment.
In normal individuals, these components work together in harmony so that balance can be maintained by an "automatic" control system requiring a minimum of conscious attention. The need for voluntary control of postural stability is relatively minor in normal individuals, although this system can come into play when automatic controls are impaired.
In contrast to the gaze stabilization system in which vestibular and visual inputs work through separate, relatively independent control systems, information from the three sensory systems interact heavily within the automatic postural control system. Adaptive sensory interactions are critical to postural stabilization, because some of the information provided by vision, proprioception, and the vestibular system can be inaccurate under some environmental conditions. While rigid flat floors and fixed vertical walls provide excellent proprioceptive and visual orientation references, compliant or irregular floors and moving objects in the visual field create conflicting information that must be ignored to prevent disorientation and instability. When the head is actively moved to direct attention to objects in the environment, balance information from the vestibular system can be adversely affected.
The Complex Nature of Balance Problems
When the balance senses are impaired by pathology, the adaptive capabilities of the brain are severely compromised as the brain struggles to sort out actual sensory conflicts from those created by the pathology. The result is a wide variety of different symptoms and functional limitations among patients with similar pathologies.4, 5, 6 By over-relying on visual information, one vestibular loss patient experiences dizziness in the presence of visual motion stimuli. By not increasing reliance on visual or proprioceptive inputs, other patients with similar vestibular losses avoid the dizziness, but will become unsteady and fall with poor lighting and irregular support surfaces. Still other patients may continue to rely on the impaired vestibular information, but can only do so by rigidly fixing the head, resulting in severe neck pain and/or headaches.
When the balance system is impaired, this normally automatic process demands intense conscious effort to overcome abnormal sensations and guard against falling. This intense effort, in turn, often leads to the common secondary symptoms such as fatigue and the inability to focus attention on other cognitive tasks.
Individual differences in adaptive responses and levels of conscious effort explain why patients experience what otherwise might seem bizarre symptoms such as the world is spinning, the floor rocking, and loss of short-term memory. These differences also explain why diagnosing and treating balance disorders is such a challenge. Knowing a patient's symptoms and underlying pathology do not fully explain the problem, nor do they provide all the information necessary to identify the most effective approach to treatment. This issue is discussed further under Assessment & Treatment.
Aging and Balance Dysfunction
The affects of aging on balance deserve special mention. The risk of developing problems in one or more of the sensory, motor, or adaptive brain components of balance increases with age as the body is exposed to degenerative or infectious diseases, or the effects of injuries accumulated over a lifetime. Thus, balance problems among older adults are frequently caused by combinations of subtle degenerative, infectious, or injury processes that are individually not clinically significant. In combination, however, they can result in significant balance problems.7 Contributing factors may include a history of injuries, such as concussions, ear infections, or serious sprains or fractures. In addition, various combinations of medications, both prescription and over-the-counter, can be detrimental to the senses or the brain's adaptive capabilities and cause either temporary or permanent damage.
Some elderly individuals experiencing balance problems have obvious medical diagnosis such as diabetes, Parkinson's disease, or even a stroke that is the primary source of the problem. Whether balance disorders result from combinations of subtle problems or obvious disease, clinical studies indicate that elderly fallers are different from their healthy age-matched counterparts and require medical treatment to maintain their functional independence and quality of life.8, 9, 10