How Balance Works

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Balance statistics
How balance works
The vestibular system
The visual system
The proprioceptive system
Balance

Balance statistics

• Over 90 million Americans, age 17 and older, have experienced a dizziness or balance problem.
• A majority of individuals over 70 years of age report problems of dizziness or imbalance. Balance related falls account for more than one-half of the accidental deaths in the elderly. Furthermore, in a sample of persons age 65-75, one-third reported that dizziness and imbalance degraded the quality of their lives.
• Most experts regard BPPV as the most commonly diagnosed vestibular disorder. It accounts for at least 20 percent of diagnoses made by physicians specializing in dizziness and vestibular disorders.
• In the general population (all ages), 347,000 hospital days [per year in the U.S.] are incurred because of vertiginous syndromes.
• Falls are the leading cause of injury deaths among individuals who are over 65 years of age.
• Twenty-five percent of elderly persons who sustain a hip fracture die within 1 year.
• Annually, more than 64,000 individuals over the age of 65 sustain a traumatic brain injury as a result of a fall.
• The estimated total cost for non-fatal traumatic brain injury-related hospitalizations for falls in individuals who are 65 years of age or older is more than $3,250,000,000.  Two-thirds of these costs occurred among individual who were 75 years of age or older.

How balance works

Balance is the ability to maintain an upright position.  Coordination is the capacity to move through a complex set of movements while maintaining balance.  Balance and coordination depend on the interaction of multiple systems in the body including the vestibular (inner ear), visual, and proprioception (referred to as your “touch and feel sense”) systems.  Each of these areas has the ability to sense body position and motion and they will in turn produce nerve signals that are sent to the central nervous system (brain).  Two particular areas of the central nervous system, the brainstem and cerebellum, receive and process the data obtained from the three sensory input systems.  The result of this “central processing” is a coordinated response that allows us to maintain our balance and stability in any number of challenging situations.

The vestibular system

The vestibular system constantly monitors the motion and position of our head throughout all normal daily activities.  The vestibular system is housed within the right and left inner ears.  Within each inner ear are two sets of “labyrinths” – structures that contain a series of canals and cavities – one located inside the other.  The outer osseous (bony) labyrinth is cut out of the temporal bone, the hardest bone in the human body.  This bony labyrinth is filled with a fluid called perilymph (high concentration of sodium).  The membranous labyrinth, which is found suspended inside the bony labyrinth, contains a fluid called endolymph (high concentration of potassium).

The membranous labyrinth contains the five individual organs responsible for generating information about head movement and head position.  They are the “otolithic” organs (utricle and saccule) and the “semicircular canals” (anterior, posterior and horizontal).  The otolithic organs primarily sense linear (forward, backward, sideways) movement as well as tilt of the head.  The semicircular canals primarily sense angular (rotation around a center axis) movement of the head. 

The utricle and saccule are two pouch-like areas of the membranous labyrinth located in a hollowed out space called the “vestibule”.  Although both organs are responsible for sensing linear motion, the utricle is designed to be most sensitive to horizontal motion and the saccule is most sensitive to vertical motion.  The utricle has connections to the semicircular canals as well as to the saccule.  The connection between the utricle and the saccule forms a “Y” shaped tube that then forms the “endolymphatic duct;” a sac of fluid that ends blindly in the temporal bone.

The three semicircular canals (anterior, posterior and horizontal) are aligned at right angles to one another.  Both ends of each semicircular canal attach to the utricle.  One end of each canal widens into a bulb-like area called the “ampulla”.  The non-ampulla ends of the anterior and the posterior canals fuse together before they join the utricle.  This junction is called the “common crus”.

In each of the five balance organs, there are groups of highly specific nerve endings (“hair cells”) that are capable of sensing head position and head motion (both direction and speed).  In the otolithic organs, these hair cell groups are arranged into patches called “maculae”.  In the semicircular canals, the groups are located within each of the maculae and are referred to as “cristae”.  All of the hair cells contained in the two maculae and the three cristae have a common feature in that they project up into gelatinous membranes.  The hair cells in each of the maculae are project up into a gelatinous bed called the “otolithic membrane”.  The hair cells of each cristae project up into a gelatinous, diaphragm-like structure called the “cupula”.

The top layer of the otolithic membrane in the utricle and the saccule is embedded with calcium carbonate crystals (otoconia).  The otoconia (often referred to as “ear rocks”) can often detach from the otolithic membrane and become displaced into one of the semicircular canals causing BPPV.  The presence of otoconia adds a significant amount of weight to the hair cells of the maculae, causing them to be highly sensitive to acceleration, deceleration and gravitational forces.  In other words, when you bend down or move forward, the otoconia embedded on the otolithic membrane will cause a relative “drag”, or resistance to motion, due to inertia.  This action in turn bends the hair cells that are attached to the base of the otolithic membrane, causing them to send impulses along the nerve pathways to the brain about linear (vertical or horizontal) or gravitational (tilt or lean) changes.

The cupula, located in each of the semicircular canals, attaches from the floor to the roof of the ampulla and is designed to detect rotational movement.  When the head rotates in any one direction, the endolymph fluid in the semicircular canal is exposed to an inertia-type pressure.  This relative “resistance to motion” causes the fluid inside the canals to push against the cupula.  This in turn deflects the hair cells that are attached to the base of the cupula, generating an impulse that is sent along the nerve pathways to the brain about direction and speed of head movement.

Nerve fibers from the crista and the maculae send movement-related information to brain via the superior and inferior vestibular nerves.  The superior vestibular nerve sends information from the lateral and superior semi-circular canals, and the utricle.  The inferior vestibular nerve sends information from the posterior semi-circular canal and saccule.  Within the brainstem these two branches participate in an extensive neural network involving nerves from the eyes, the cerebellum, and the positional receptors “proprioceptors” located in the feet, legs, trunk, arms, and neck.  The brain interprets this information, making modifications in eye, head and body position to maintain a fixed eye position, and erect posture.

From birth, we tend to rely heavily on vestibular cues during movement and activities.  Damage to the vestibular system, caused by a variety of illnesses or injuries, can alter the information being sent from the inner ears to the central nervous system.  This will often result in dizziness, vertigo, nausea or unsteadiness.

The visual system

The accurate use of visual cues is a critical component of balance.  Our visual system allows us to perceive our own motion and position relative to the world around us.  Errors in our perception of visual information can lead to a sensory conflict, resulting in nausea and dizziness. 

Although we are able to see, or at least perceive, objects in a large visual field (about 200 degrees), we see things most clearly focused within the fovea, which is the central 1-degree of our visual field.  Our ability to control the position and motion of this fovea is crucial to the role of the visual system in overall balance.

There are six muscles attached to each eye that work in a counterbalancing fashion to control eye movement.  Three of the most integral eye muscle (“oculomotor”) control systems are the smooth pursuit system, the saccade system and the optokinetic system.

• The smooth pursuit system moves the eyes in order to keep the fovea fixed on a single moving target.  It calculates how fast the target is moving and then moves the eyes accordingly. 
• The saccade system is responsible for repositioning the fovea from one target to another in the visual field.  It involves simultaneous movement of both eyes and is extremely fast and accurate.
• The optokinetic system holds images stable by controlling both the motion and position of the fovea during sustained head or body rotation.  It uses a combination of smooth pursuit followed by fast corrective saccade in order to maintain stability of our visual field.

The oculomotor system is controlled by the brain connections and participates mostly in “low-frequency” (<2 Hz) activities when our head does not move quickly through our environment.  During higher frequency activities (like brisk walking, jogging or running), we need to rely on more advanced, “interactive” reflexes to stabilize our visual field in response to head or body movements.  

• The vestibular-ocular reflex (VOR) uses signals from the vestibular organs to control the position of the fovea and therefore hold images stable during brief or rapid head rotations.
• The cervical ocular reflex (COR) is a poorly understood reflex that theoretically uses muscle and joint input from the neck to aid in holding images stable during neck rotations.

Disorders of the visual system (e.g. cataracts, glaucoma, macular degeneration, progressive blindness) can affect the information being sent to the central nervous system and vestibular processing centers, resulting in symptoms of dizziness and unsteadiness. 

The proprioceptive system

The proprioceptive system is by far the most complex of the sensory input systems.  Proprioception refers to the brain’s ability to know where our body is in space.  The brain gathers information from a wide range of senses and then processes this information in order to compare it with a virtual “body map” that is subconsciously stored in our memory.  The outcome is that we know where, for instance, our arms are without having to look at them.  Proprioception also allows us to locate our ear without the aid of a mirror, or successfully scratch an itch on our back.  It is possible to extend our “body map” to include the vehicle we drive.  Most experienced drivers know the width of their car to within inch.

The proprioceptive system gathers information from:

• The vestibular system regarding the position of our head, the pull of gravity, the speed and acceleration of our movements
• Our hearing system that can estimate our distance to objects that make a sound (like a car), but also from reflective sound when indoors
• Our visual system that can see the space around us, our position in this space and our proximity to objects
• Our sense of touch regarding all objects with which we are in physical contact
• Our sense of smell, a sense on which we unconsciously rely to discern direction and distance from objects and events in our environment
• Stretch-specific nerve receptors that monitor length, tension and pressure in our muscles, joints and tendons

If any of these inputs are missing or distorted, or if the internal body map is incomplete or faulty, we will have diminished proprioception and will have to place greater reliance on other senses, such as our eyes, to compensate for the weakness.  This can lead to lack of concentration, tiredness and overall frustration.

Poor proprioception is demonstrated by the person who often walks into door frames, bumps into others when walking, doesn’t quite know where the seat of a chair is when sitting down, has difficulty stepping off curbs or will cautiously walk down stairs with one foot joining the other on each step.  As a result of impaired proprioception, people will frequently complain of unsteadiness, clumsiness, frequent stumbles and fear of falling.

Balance

The three primary balance systems (vestibular, visual, and proprioception) send signals to each other as well as to the brain about head and body movements.  In most individuals, the brain studies the signals and looks for any evidence of disagreement between the three.  It will than select the most accurate signals based of reasoning and memory.  As the brain interprets these three input signals, it generates one specific response that is sent back to the muscles of the body to keep us stable, upright, and focused on the target of our interest.  Balance is best when the signals sent into and out the brain are fast and accurate.  If one of the systems involved in this process is damaged or adversely affected in any way, the other systems must work to compensate for the loss in order to keep us balanced.  Temporary loss of one of these systems can result in postural or visual instability.  For example, signals from a damaged vestibular system can result in impaired perception of body movement or position.  This could also cause visual blurring during movement due to the loss of higher level eye reflex function. Changes in visual acuity can result in a sense of disorientation that impairs upright mobility and safety.  Proprioception changes, as seen after an athletic injury or with peripheral neuropathy, affect our body awareness and the accuracy of our reactions to movement. 

The interpretation and processing of signals sent to the brain, as well as the reflexive movements produced by the brain in reaction to these signals, can be significantly disturbed by changes or fluctuations in these systems, making it difficult to maintain our balance or to feel steady and coordinated during even simple daily activities.  When these symptoms are experienced, regardless of age, it is essential to see a qualified inner ear specialist and obtain a complete evaluation (with specific balance-related testing) to determine the exact cause of the symptoms and to determine an objective treatment course of action.

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