Somatosensory Input; Sensory Disorganization; Sensory Loss; Improper Sensory Selection - biodex BALANCE SYSTEM SD Operation & Service Manual

CONTENTS

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.
A person with impaired vision from a stroke or cataracts will depend less on vision and more on
touch and vestibular feedback for balance. In this case, choice of assistive device, hand railings
for touch, and proper lighting are important. A person with a visual impairment may perform
well in a clinical setting but have difficulty with balance in more complex visual situations that
demand rapid visual interpretation of multiple visual cues. For example, a person may be safe
walking in a quiet, well-lit hallway but be unable to negotiate a busy, noisy hallway filled with
people and equipment.
Vestibular damage or loss can also have a profound effect on balance and postural control.
Vestibular impairment can cause problems with gaze stabilization, including blurred vision,
problems with balance and posture, and vertigo (Shumway-Cook & Woollacott, 2001).

Improper Sensory Selection

Sensory loss may lead to inflexible or improper sensory weighting. A person may depend on
one particular sense for postural control even if that sense leads to further instability (Shumway-
Cook & Woollacott, 2001). You may notice a person walking with head down, carefully watch-
ing every step. In this case, vision is the dominant sense being used for balance. Retraining
would involve improving the use of somatosensory and vestibular input to reduce dependence
on visual input.
APPENDIX D — D-4 —