Gait and Parkinson's Disease - A Conceptual Model
Part 3 of 3
To this point this discussion has been limited to pathology which is a
consequence of a vision perception/processing impairment. However, as PD=
progresses disturbances of the autonomic nervous system result in increas=
ed
muscle tone. This increased tone results in an increase in contraction i=
n
the major flexion muscle groups and is seen as the stooped posture common=
to so many PD people (simian posture). I call this the T-factor ("T" fo=
r
increased tone and tension) whose presence comes and goes as a consequenc=
e
of the combination of environment-induced anxiety and disturbances in the=
dynamic equilibrium of various neurotransmitters (on/off cycling).
The presence of T-factor seems to be an augmentation of the fight or flig=
ht
characteristics of the autonomic nervous system and results in a much mor=
e
malignant form of akinesia and dyskinesia. When muscles undergo
isometric contraction (for example as occurs in the lower extremities whi=
le
maintaining a standing posture) the additive result of T factor tone and=
isometric muscle tone results in rigidity - which together with vision
pathology comprise the principle components of this more malignant form o=
f
akinesia and freezing. If such a subject assumes a crawling position th=
en
his arms will have posture maintaining isometric tone and the result is
rigidity in the upper extremity and an inability to take a "step" with
one's arm. The cogwheel rigidity test is a manifestation of the effects=
of T-factor when additional tone is induced in a non-weightbearing
extremity.
In the absence of T-factor, akinesia and related gait pathology are induc=
ed
by aberrant perceptions of the environment. Appropriate visual
manipulations of environmentally induced perceptions can overcome this ga=
it
pathology - the PD subject is physically capable of ambulation. It is
the perception of the inability to take a step, which effectively inhibit=
s
ambulation. However, in the presence of T-factor the subject becomes
physically incapable of advancing the lower extremity and manipulating
vision pathology will be less effective. If however, one can initiate
ambulation it very often can be sustained because the posture maintaining=
tone of lower extremity muscles is reduced as the extremities cycle throu=
gh
the various phases of gait thereby reducing one of the two components of
tone which contribute to rigidity.
One can frequently observe PD subjects overcoming a freeze by momentarily=
leaning against a wall. This results in a brief period of reduced
weightbearing - a transient reduction in tone which facilitates ambulatio=
n.
At subnormal levels of dopamine T- factor results in severe akinesia. =
While medicated the result is freezing which occurs in performance anxiet=
y
situations (trying to get to a ringing phone or crossing a street with
approaching traffic). Note that T-factor pathology is independent of
vision based pathology. It is possible to be akinetic and freeze in the
absence of T-factor i.e. vision pathology alone for example freezing in a=
doorway. It is also possible to freeze in the absence of vision patholog=
y,
i.e. T-factor alone which occurs while "on" under performance anxiety
scenarios.
Vision only T-factor only
Doorway freeze
Gait dyskinesia Performance anxiety: crossing a street
Non-weightbearing dyskinesia
Definition of Key Terms
externally cued gait (conscious walking): gait where the gait parameters
are defined by the environment
internally cued gait (automatic, unconscious walking): gait which is
pre-programmed or overlearned
freezing: vision based pathology resulting in the inability to sustain
unconscious, automatic gait in spite of the environment being compatible
with automatic gait and the subject's intention to achieve it. =
gait initiation: the inability to produce an initiating stride of adequat=
e
length to conform to the demands of the environment - a failure to meet
one's conscious expectations of stride length.
festination: a pathologic component of conscious gait which results in a
stride which is too small to result in smooth, controlled deceleration in=
the presence of environmental conditions that require a need to decelerat=
e.
conscious cessation of gait: a conscious successful neutralization of a
displaced center of gravity, can occur during normal gait or festination.=
falling: festination, which does not catch up to a displaced center of
gravity.
conscious vision: central field vision (seeing)
unconsious vision: peripherally processed motion, perceived without
conscious attention
initiating stride: a consciously taken stride, which always follows a
position where the feet are separated by less than a threshold stride
length stride.
threshold stride length: the minimal inter-pedal distance required to
access automatic gait.
sub threshold stride length: a stride where the inter-pedal distance is
less than that of a threshold stride length.
T-factor: pathologically increased muscle tone, an exaggerated response t=
o
normal tone inducing stimuli
A Brief Discussion of Gait Enabling Devices
The fundamental underlying principle of all gait enabling devices is thei=
r
ability to augment the "perceivability" or velocity of low speed optical=
flow. =
1. Central Field Cueing Device: As mentioned above and array of objects =
or
markers linearly arranged on the floor at stride length intervals serves =
as
visual cues and can enable gait in akinetic Parkinsonians. The Central
Field Cueing Device projects the virtual equivalent of these markers onto=
the real world without impairing one's ability to see the real world. Th=
is
device requires conscious attention and results in conscious type walking=
even when the environment is potentially compatible with automatic walkin=
g.
However, if the subject is capable of unconscious walking he can learn t=
o
ignore the virtual visual imagery when the environment allows automatic
walking.
2. Virtual Vection Glasses: These glasses provide a scrolling (anterior t=
o
posterior) array of lights at the extreme periphery of the eye. The ligh=
ts
are of course real but are interpreted as virtual i.e. apparent motion an=
d
provide the subject with feedback that he is walking. This data is
processed below the level of conscious attention - only the motion not th=
e
objects are "seen". It will sustain unconscious gait but cannot initiate=
or sustain conscious gait.
3. Elevated Foot Gear: This is a very different technology is based on t=
he
same principle of augmenting and accelerating optical flow. Vision
geometry suggests that elevating the eyes above their normal height will
accelerate optical flow and this can overcome freezing. (see diagram) T=
he
problem with this approach is that the subject will eventually functional=
ly
adapt to the elevated eye position and then the benefits of the approach
are lost.
A B
B > A
Optical flow per stride is that segment of the floor, which appears to pa=
ss
under foot as we advance the foot in gait. Due to vision geometry the
optical flow per stride increases as the subject's eyes elevate relative =
to
the floor. The velocity of the optical flow increases because the
increment of time required to take the stride is unchanged.
4. Auditory/Vibratory Feedback: This is a non-vision mediated approach to=
gait enablement and as such does not really address the underlying
pathology. However, this approach is useful as a means of sustaining
automatic gait by promoting threshold stride length strides assuming one =
is
functioning in an environment that is compatible with automatic gait. Th=
e
device cannot not initiate gait but can facilitate sustaining automatic
gait. The device employs the use of momentary switches in the shoes to
provide auditory or vibratory feedback during the swing phase of gait of
each extremity.
All of the above have been clinically tested in unblinded, unscientific =
,
informal trials with varying degrees of success. There are in fact
numerous other approaches to augmenting the perceived or actual velocity=
of optical flow. Many of these have been built and successfully tested
but were abandoned due to the constraints of social acceptability or cost=
of development. All of the devices are in prototype form only and are not=
currently available to the general public. However, I remain convinced =
that a modest amount of financial and engineering resources would yield=
great dividends n terms of a useful therapeutic gait enabling device.
=
