Clinical Features of Parkinson's Disease
Most of the clinical features of parkinsonism
were recognized by James Parkinson, but he incorrectly
reported that there were no adverse psychological
symptoms. In fact, there are many debilitating
psychological manifestations of parkinsonism in addition
to the obvious motor abnormalities (see Table 1). One of
the earliest and most obvious symptoms is a progressive
resting and postural tremor which is absent during
voluntary movement and when postural muscles are not
being used. Tremor is experienced by eighty-five percent
of all patients at some point (Wichmann and DeLong 1993).
The extremities (and sometimes the face, lips, and chin)
are usually involved in tremor, but for undetermined
reasons the head- neck and voice are rarely affected.
Tremor increases with frustration, anxiety, and stress,
but tremor usually completely subsides during sleep.
Normal individuals experience "physiological"
hand tremor (phasic stimulation of muscle spindles in the
arms and hands) at a rate of six-twelve Hertz. However,
the amplitude is too small (i.e. the stimulation is too
weak) to produce action potentials in muscle fibers.
According to the research of Freund and Hefter (1993), in
parkinsonism the amplitude of stimulation is often
increased enough to activate muscle fibers but for
unknown reasons the tremor occurs at a lower frequency
of four to six Hertz.
The physiogenesis of tremor is not presently
known but some evidence shows that it is related to a
decrease of dopamine in the globus pallidus. This
suggests that dopamine plays a role in decreasing the
amplitude of physiological tremor. Such a role for
dopamine is ironic in light of the known role of dopamine
in increasing movement. A possible conclusion is that
dopamine has antagonistic effects in different pathways.
Tremor usually begins unilaterally, probably
due to more progressed dopamine neuronal degeneration on
one side of the brain. Because medial, including
postural, descending motor pathways involve spinal
interneurons which project bilaterally to motoneurons,
tremor is expected to occur both contralaterally and
ipsilaterally. However, for reasons which are unknown,
unilateral tremor often first occurs on the side
contralateral to progressed neuronal degeneration. It
might be possible that the particular segment of the
motor pathway involved in tremor only projects
contralaterally, or that there is an as yet undiscovered
ipsilateral inhibition mechanism.
Two of the most common symptoms of parkinsonism
are bradykinesia and akinesia. Bradykinesia, a delay or
slowness of voluntary movements, is thought to be caused
by an impairment in the retrieval of motor programs from
the motor cortices of the cerebrum (Groves 1983).
Akinesia is defined as a delay in the onset of movement
(specifically voluntary movement, in parkinsonism).
Akinesia can most easily be overcome by visual, auditory,
or other sensory stimulation. For example, walking often
must be initiated by lifting the patient's foot from the
floor and pushing it in the desired walking direction.
A commonly-held hypothesis is that akinesia is correlated
to a decrease in the concentrations of dopamine in the
caudate nucleus and/or putamen. Recent studies by
Rascol et al. (1993) showing that inactivity of the
supplementary motor area is simultaneous with akinesia
add more support to this hypothesis (see "indirect"
pathway in Figure 3).
Muscular rigidity, another of the hallmark
clinical manifestations of parkinsonism, is usually
proportional to the velocity of movement (the Westphal
phenomenon). A diagnostic aid for parkinsonism is
Froment's sign- muscular rigidity increase with
voluntary contralateral movement. Rigidity, like tremor,
increases with stress. Lance's (1980) hypothesis that
rigidity is caused by hyperstimulation of intact spinal
mechanisms is supported by the research of Stacy and
Jankovic (1992) in which they conclude that rigidity is
the result of disinhibition (de facto stimulation) of the
internal globus pallidus, which results in increased
stimulation of intact spinal reflexes.
Many parkinsonian patients are afflicted with
postural instability. The mechanism causing postural
instability, one of the most disabling symptoms, is
unknown. A tendency toward inclination when attempting
to stand erect is the most common type of postural
instability. The result is an abnormal center of
gravity which may lead to a festinating gait- a rapid,
short-stepped walking to try to overcome the displaced
center of gravity. However, the festinating gait is
probably primarily caused by delays in the modulation of
movement. These postural problems often cause
parkinsonian patients to fall, thus injuring themselves.
One of the least understood symptoms of
parkinsonism is dementia. Compared to other symptoms
little is known about this psychological manifestation.
It is usually not obvious (it eluded James Parkinson) and
was overlooked or underestimated for many years. Stacy
and Jankovic found that up to seventy-seven percent of
parkinsonian patients experience some form of dementia
(1992). It is characterized by chronic personality
disintegration, confusion, disorientation, stupor,
deterioration of intellectual capacity and function, and
impairment of memory control, judgment, and impulses.
There are numerous hypotheses for the cause or causes of
dementia. The mechanism could be related to cortical
degeneration, impaired neurological control of attention
(Brown et al. 1988, cited in Stacy and Jankovic 1992), or
abnormal norepinephrine and acetylcholine pathways (Cash
et al. 1987, Dubois et al. 1987, cited in Stacy and
Jankovic 1992).
Clinical Feature Explanation
Resting/Postural Tremor. 4-6Hz; usually involves
extremities
Bradykinesia Delay or slowness of
voluntary movements
Akinesia Impairments in movement
initiation
Rigidity Muscle resistance increases
with vol. movement
Postural Instability Inclination when standing
Festinating Gait Rapid awkward walking
Dementia Personality disintegration,
confusion, memory loss
Motor limitations Difficulty executing
simultaneous or sequential
movements
Table 1- Clinical Features of Parkinson's Disease
