Balancing Sensory Systems
By Lisa Dress
Perception is reality--or so the old saying goes. And in many cases this holds true, but not when it comes to balance. A person's ability to walk or run across various surfaces, watch a plane zoom through the sky, or negotiate his way through a dark room, without becoming imbalanced, is dependent on an intricate process in the body.
The process of maintaining balance involves three systems in the body: the somatosensory, the vestibular and the visual. These sensory systems, which work independently but often overlap in maintaining balance, send information to the central nervous system. A condition or disease may prohibit one of the three systems from functioning properly. The result is often imbalance, said Joel A. Goebel, MD, FACS, associate professor of otolaryngology and director of the Vestibular and Ocular Motor Laboratory at Washington University School of Medicine in St. Louis.
THE ROLE of physical therapists in treating patients with imbalance due to a problem with one of the three sensory systems is to enhance the integration of the input or to affect the way the brain responds to a deficit in one (or more) of the three sensory systems through exercise, said Susan J. Herdman, PhD, PT, associate professor of orthopedics and rehabilitation at the University of Miami, in Coral Gables, FL. Although imbalance can result from other sources such as incorrect motor input or CNS damage to motor systems, this article will focus on the effect of the three systems.
The somatosensory input the brain receives from muscles and joints stems from sensory receptors called proprioceptors. These proprioceptors are sensitive to pressure and the stretching motion (in the tissue) that surrounds them. The impulses that come from the neck (which indicate which way the head is turning) and impuls-es that come from the ankles and the bottom of the feet (which indicate the movement of the body in relationship to space and the surface a person is standing) are especially important in maintaining balance, noted Dr. Goebel.
A person with diabetic neuropathy, for example, who has decreased sensation in the feet, may experience a somatosensory deficit resulting in imbalance. As part of treatment, the somatosensory system can be enhanced (depending on how acute or chronic the somatosensory deficit is) through exercises that help the person either use the damaged system more efficiently, or through fine-tuning the other two systems. Exercises that would enhance the somatosensory system, Dr. Goebel noted, would include the patient standing in his bare feet on various surfaces to re-train the brain to interpret the input received from the somatosensory system. Dr. Herdman added that patients should also experience the various surfaces with their eyes closed to eliminate visual stimuli and help them focus their attention on somatosensory and vestibular cues.
Dr. Herdman explained how the vestibular system works: It originates in the inner ear from five balance receptor sites. These sites are located in three semicircular canals (anterior, horizontal and posterior) and two sacs (the saccule and the utricle). Each semicircular canal lies roughly perpendicular to the other two. When a person rotates his head in the plane of a particular canal, the endolyphatic fluid within the canal lags behind the movement of the canal. The fluid pushes against sensory receptors (hair cells) in the canal and temporarily bends them. This bending of the hair cells in the inner ear sends impulses to the brain via the nervous system.
The mechanism is slightly different when people change their head position or move their head in a straight line, she explained. Calcium carbonate crystals produced naturally by the body make the hair cells of the saccule and utricle react to the pull of gravity or to translational movement of the head. The hair cells in the saccule and utricle send messages to the central nervous system. The weight of the crystals pushes on the hair cells which makes them respond to the pull of gravity and are extremely sensitive to which way the body and the head are moving.
WHEN BOTH inner ears are functioning properly, the vestibular system sends symmetrical messages to the brain. The primary mechanism of recovery when one inner ear is affected would be for the remaining vestibular system and the somatosensory and visual systems, to compensate for the damaged one. "This is unique to the inner ear system. The vestibular neurons in the central nervous system increase their response to head movement through a process called adaptation. Exercises can help improve the function of the remaining vestibular system," Dr. Herdman stated.
She added that patients with inner ear or vestibular problems are often hesitant to move their heads since this creates the feeling of imbalance. But this lack of movement only leads to prolonged imbalance problems. "The vestibular system will not fully recover without head movement." Stimulus is necessary, she explained, in the same respect it would be for a weight lifter. "If you want to strengthen a muscle, the person needs to exercise and to lift weights," she said.
In most instances, Dr. Herdman noted, the body will try to compensate for any deficits. "For those who lose their inner ear function, they may use visual cues more effectively. But it's not a conscious effort--the person is not even aware that it is happening." However, this "compensation," Dr. Herdman explained, is utilized more effectively by the body through exercises that help influence or improve the remaining, functioning aspects of the vestibular system.
"If the vestibular systems in both inner ears is affected there is disequilibrium, or the person senses being off balance," said Dr. Herdman. For example, a patient taking certain medications that cause damage to the vestibular system (such as the antibiotic aminoglygosides) may partially compensate for the deficit with his somatosensory and visual systems. The physical therapist can facilitate this by recommending exercises that focus on fine-tuning those two systems.
The visual system is composed of both dynamic (what a person sees when the head or the environment is in motion) and static vision. Karen Hastings told ADVANCE in a July 14 article, the vestibular ocular reflex keeps the eyes steady when the head is in motion; therefore the visual system must interact with the vestibular system in determining if the person or environment is in motion for the proper coordination of balance in the body.
"The brain integrates the input it receives about balance from the eyes and the proprioceptors, as well as all the inner ear sources, and makes adjustments in posture or eye positioning by sending outputs to muscles in various parts of the body," according to the Vestibular Disorders Association (VEDA), a not-for-profit organization, located in Portland, OR.
The body, Dr. Goebel noted, will tend to use whatever sensory input it receives from the three systems for balance purposes. For example, if a person has imbalance due to damage to the inner ear then the body may still try to use input from the vestibular system, as well as the other two sensory systems. But, if the person continues to have imbalance or "not walk with the same stride or surety" then compensation (with the remaining systems) may require additional stimulus, in the form of exercise, in the body's attempt to return to its prior level of functioning.
UNDERSTANDING the sensory systems' contribution to balance, as well as determining the precise underlying cause of the condition, is essential for designing individually based exercise programs for patients who have imbalance or vertigo. Dr. Goebel, who sees 600 to 700 patients with dizziness a year, said he regularly works with physical therapists to determine the best treatment plan. "Communication is essential for making a game plan for therapy," he said.
Rehabilitation enhances the most efficient way the central nervous system uses sensory input. It helps the brain use the remaining sensory inputs more effectively and may also enhance the utilization of the damaged systems, Dr. Goebel noted.
Dr. Herdman added that it is important, she noted, that the etiology is determined before developing a treatment plan. "For instance, there needs to be a realization that not all patients who complain of dizziness have a vestibular problem--so if the patient is treated as so, the physical therapist may be wasting time," she said.
Dr. Goebel added that rehabilitation is not useful for any fluctuating vestibular disorder (like Meniere's disease) since compensation only occurs with a stable lesion. The vertigo that is associated with Meniere's can occur at any time and last several hours; it is associated with a temporary decrease in hearing, as well as a temporary increase in noise (tinnitus).
"Certain disease processes that are non-fluctuating can be rehabilitated with a significant amount of success, but this takes great interaction between the doctor, physical therapist and specialist," Dr. Goebel noted.
ON A FINAL NOTE, Dr. Herdman said that those who do have balance problems related to the sensory system usually respond well to exercises and many are able to return to their prior level of function within a moderate time frame.
Before therapists can initiate a rehabilitation program, Dr. Goebel pointed out, they must have an accurate diagnosis from a balance specialist such as neuro-otologist or neurologist.
* For more information, contact Dr. Herdman via e-mail at Sherdman @bpei.med.miami.edu or Dr. Goebel at firstname.lastname@example.org.