biodex BioSway Operation Manual page 65

CONTENTS
The visual system may provide inaccurate information to the nervous system. For example, a
person sitting at a stoplight in a car may think she has started to move when the car next to her
starts to move. The visual system "goes along" with the movement of the neighboring car and
tells the brain that both cars are moving. The CNS mediates this sensory conflict by instructing
the leg to slam on the brake to stop the car from moving forward. As soon as the foot touches
the brake the somatosensory and vestibular systems realize that the car is, in fact, not moving.
For a split second, input from the visual system was given preference by the brain, even though
the information turned out to be inaccurate.
Visual input may also be inaccurate due to diseases or disorders that affect the visual system,
such as diabetic retinopathy, cataracts, macular degeneration, injuries, or stroke.
vestibular Input
The vestibular system is responsible for processing information about movement with respect to
gravity —specifically, rotation, acceleration/deceleration, and head stabilization during gait. The
vestibular system works in conjunction with the visual system to stabilize the eyes and maintain
posture during walking (vestibular-ocular reflex). Vestibular disorders cause a feeling of dizzi-
ness and unsteadiness. Vestibular dysfunction also affects the ability of the CNS to mediate
intersensory conflicts such as that in the example given above.
Somatosensory Input
Somatosensory input consists of touch and proprioception. Input from these two sensory
sources provides critical feedback to the CNS regarding positioning in space, body sway, and
changes in terrain. The sensory input from touch and proprioception allows the muscles to
make constant, automatic adjustments to maintain balance and avoid falls.
In the example where the person in the stationary car slams on the brake, only to realize
through somatosensory input that her car has not moved, the feeling that the car is moving
when it is not is an example of a visual intersensory conflict; the conflict is resolved quickly by
pressing on the brake and feeling that the car has not moved.
Sensory disorganization
The loss or disruption of sensory input in the visual, vestibular, and/or somatosensory systems
can affect balance in a number of ways. How balance is affected depends on several factors,
including the extent of the nervous system damage, the number and extent of sensory losses,
and the availability of the other senses for compensation. In many instances, more than one sen-
sory system is impaired, as in the case of a person with a peripheral neuropathy and visual
impairment (common with diabetes and stroke). But, just as an individual with impaired vision
develops a keener sense of hearing, a person with any sensory loss will attempt to compensate
by using the unaffected or less-affected senses to improve balance.
Sensory loss
The way balance is affected by loss of sensory input depends on the extent and nature of the
sensory loss. Recall that the senses most associated with balance are somatosensory (touch and
proprioception), visual, and vestibular. Of these, the somatosensory system plays the biggest
role in balance, so losses associated with peripheral neuropathies, stroke, and other neurologic
disorders can have a profound effect on balance.
A person with sensory loss (eg, bilateral lower-leg peripheral neuropathy) who does not receive
normal sensory input from the sensory receptors in the feet and ankles will attempt to compen-
sate by depending more on visual and vestibular input for balance. If there is significant sensory
loss in the feet, a person will be unable to adjust easily to changes in the support surface during
tasks such as walking on grass or uneven surfaces, or even walking in shoes with soft soles.
— C-3 — APPENDIX C