Since 1817, PD has broadened. However, its cause is still unknown, which is why it is referred to as idiopathic PD. PD is the second most common neurodegenerative disease (Alzheimer's is first) (ACE, 1999), afflicting approximately 3% of Americans over 65 years old (Ferrini et al., 2000). PD is a progressive degenerative neurological syndrome resulting in a myriad of movement disorders. Major movement problems associated with the disease include slowness of movement (bradykinesia), freezing or akinetic periods, tremors, decreased mobility related to muscular rigidity, and dyskinesis (involuntary movements) associated with anti-Parkinsonian medications (Stanley et al., 1999).
Classic signs of PD include a "masked face" that makes the client seem emotionless, a "pill-rolling" tremor of the fingers and thumb (seen at rest), a shuffling (maybe festinating) gait, hesitance in beginning any movement, and a stooped posture. Symptoms usually become evident in the mid-50s, but it is now believed that the disease processes begin earlier in life with milder symptoms that are often ignored (American Council on Exercise, 1999).
Idiopathic PD is characterized by degeneration of neurons in the substantia nigra, which is part of the basal ganglia. These neurons make the neurochemical messenger dopamine, which is partly responsible for starting a circuit of messages that coordinate normal movement (APDA, 1997). At the microscopic level, the damaged and dying neurons in the substantia nigra show a round, cellular marker called a Lewy body, which is considered the specific pathological hallmark of PD, and because of this, the disorder is sometimes called Lewy body PD, Lewy body Parkinsonism, or simply Lewy body disease (APDA, 1997).
Eighty percent or more of cerebral dopamine has to be lost before Parkinsonian symptoms develop. The conditions related to age could well be due to the fact that both the dopamine concentration and the number of cells in the substantia nigra that produce it fall steadily from birth, some 60% or so having been lost in extreme old age. Clearly, the older the person, the smaller the additional deficit in cerebral dopamine produced by whatever mechanism that is required to produce Parkinsonian symptoms (Caird, 1991).
In the 1980s, several important new pieces of information gave insight into the cause of PD. The first was the realization that inheritance, in its simplest sense, was not a major factor in the etiology of the disease. It had been assumed that genetic factors were important because at least 10% to 15% of cases had relatives similarly affected. However, with monozygotic twins, when one had PD, the other was no more likely to share the illness than were dizygotic twins (Marsden, 1990). Another finding in the 1980s was the possible link to the environment, which may cause PD. It was found that neurotoxins may selectively destroy the substantia nigra, and induce neuropathology and neurochemical changes very much like those of PD (Marsden, 1990). A more complex theory is that the development of PD may be due to a combination of exposure to an environmental toxin with an inherited inability to adequately dispose of such a toxin. Subtle differences in the activity of drug metabolizing enzymes have been claimed in PD, and may be relevant (Marsden, 1990). Furthermore, it has been seen that PD may be drug-induced. Drugs that may cause PD include heavy tranquilizers and blood pressure medications. Symptoms usually subside when drug use is terminated or reduced.
Perhaps the most noticeable and debilitating effect of PD appears in the gait of the client. Often coinciding with the shuffling gait are freezing periods where the client with PD will suddenly freeze in the middle of the gait cycle. This can prove to be very hazardous to the well being of the individual while walking across traffic or performing other important activities of daily living (ADL). Knutsson (1972) states that the intricate series of component movements characteristic of normal gait are primarily disrupted, the changes being merely of a quantitative nature, with more or less pronounced decreases in range and speed of movement. It has been seen that despite bradykinesia, the gait can be improved in clients with PD through coaching efforts and repetitive gait exercises.
In attempting to understand the reasons for the difficulty in generating appropriate stride length in PD, it is useful to look at the pathogenesis of hypokinesia (Morris et al., 1996). Cunnington et al. (1995) suggest that in hypokinesia the interaction between the basal ganglia and supplementary motor area (SMA) is disrupted during movement performance. The SMA normally prepares for a forthcoming predictable movement with a steady increase in neuronal activity during the premovement period. In subjects with PD, however, premovement activity is greatly reduced for external and predictable movements, indicating reduced SMA involvement. With disrupted basal ganglia output, the SMA is only involved in non-cued sequential movements, which must be internally determined and thus, subjects with PD rely on external cues to guide movement, bypassing defective internal control mechanisms, which operate via the SMA.
In terms of movement performance, subjects with PD show dramatically slowed movement initiation and execution times in the absence of external cues (Georgia et al., 1993). However, in the presence of external cues, movement performance in PD subjects is dramatically improved. Underlying this improvement, Cunnington et al. (1995) found that when external cues are provided, defective internal control mechanisms (operating via the SMA) may be bypassed, allowing less impaired control of movement. Findings by Morris et al. (1994) coincide with Cunnington et al. (1995) and Georgiou et al. (1993) that suggest that subjects with PD have particular difficulty with the internal regulation of stride length, even though cadence (steps per minute) control is intact and is easily modulated for a variety of conditions. Subjects with PD do have a higher cadence rate than control subjects for any given velocity. However, the increased cadence is a compensation for reduced step size (Morris et al., 1994).
The ability to generate a normal stepping pattern is not lost in PD. Normal stride length can be elicited in PD using attentional strategies and visual cues, possibly because both these methods focus attention on the criterion of stride size (Morris et al., 1996). This was further demonstrated in a study by Enzensberger et al. where they found that by using a metronome, subjects with PD were able to lengthen their strides from 50 to 56 cm, reduce the number of steps by 9, and decrease time by 5 seconds on a 40-meter walk test. Furthermore, metronome stimulation reduced the number of freezing episodes (Enzensberger et al., 1996). These studies show a large increase in functionality though improvement of gait patterns.
Another study performed by Behrman et al. (1998) also confirmed the benefit of external cues on gait of PD subjects. The external cue used was verbal instruction. This study showed that the walking patterns of patients with PD improved to specific instructions to change a series of single gait variables. Patients with PD can intentionally walk with larger steps, faster, and with increased arm swing amplitude. Furthermore, by increasing the intensity of one gait variable, for example, arm swing amplitude, other gait variables can be improved. These findings indicate that cognitive strategies have the potential to improve the overall walking patterns of patients with PD (Behrman et al., 1998).
Treatment of PD has been long awaited. The discovery of dopamine deficiency in the 1960s changed everything (Masden et al., 1990). With this discovery, Levodopa (which is the active ingredient of Sinemet® and Madopar®) eventually became the most important and most effective drug to treat the symptoms of PD (Marsden et al., 1990). Levodopa is required because dopamine itself cannot cross the blood-brain barrier. Thus, dopamine in the blood is unable to penetrate into the CNS and cannot be used to treat PD. Levodopa is easily transferred from the circulatory system into the brain where it is rapidly converted into dopamine, helping restore the flagging dopaminergic activity (Caird, 1991). Taking Levodopa restores the amount of dopamine in the substantia nigra and striatum to near normal levels and thereby reduces the symptoms in the majority of patients (APDA). However, there have been drawbacks to the introduction of Levodopa. Many patients had nausea and vomiting due to dopaminergic stimulation of the vomiting centers via the area postrema, which lies outside the blood-brain barrier. This side effect was overcome in most patients by the introduction of selective extracerebral decarboxylase inhibitors such as carbidopa in Sinemet® and benserazide in Madopar® (Marsden, 1990). Another difficulty with Levodopa has been the body's ability to become resistant to the drug. This can be overcome by taking a break from using the drug for one or two weeks and resuming at the end of the "drug holiday."
Other options are now available, such as surgery on the brain and other new treatments, which are occasionally used. The latest technique for advanced PD is electrical stimulation of the brain by an implanted device that is connected to a battery-operated generator placed under the collarbone. It is believed that the stimulator disrupts excessive electrical activity in the brain (Limousin et al., 1996).