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Figure 1. Dopaminergic, nondopaminergic and dopa-induced features in Parkinson's disease, with suggestions for corresponding treatment strategies. DOPS: Dihydroxyphenylserine. Redrawn with permission from [117] ©John Wiley & Sons.
(Figure omitted. See article PDF.)
Idiopathic Parkinson's disease (PD) is traditionally said to be characterized by a varying combination of at least two of the following core features: bradykinesia, tremor, rigidity and postural instability, although there is increasing evidence that these features are just a part of a much broader clinical complex [1]. Postural disturbances represent one of the most disabling features of the disease. They are typically a late manifestation of PD [2] and include the distinctive stooped posture, the shuffling gait disorder and progressive balance impairment. Postural instability leads to frequent falls, often with devastating consequences, such as fractures or long-term hospitalization [3].
The pathophysiology of postural instability in PD remains insufficiently understood, but is probably complex and multifactorial. It is good to realize that what patients and clinicians perceive as 'a poor balance'actually encompasses a fairly broad range of pathophysiological processes, and this makes it difficult to easily correlate any given factor to 'balance impairment'. Indeed, it is now broadly accepted that deficiencies in many of the afferent and efferent postural systems that normally contribute to balance control [4] can contribute to the complex pathophysiology underlying postural instability in PD [5]. Examples include inadequately organized automatic postural reactions, poor anticipatory postural responses, a slowing of compensatory stepping reactions, inappropriately directed protective arm movements and a defective somatosensory integration of afferent sensory information. The net result is that patients frequently fall (or nearly do so), which is defined as inadvertently landing on any lower surface (the definition usually also states that this should not be caused by an overwhelming external force).
Clinically based studies have underscored the magnitude of the problem and the impact on quality of life, but to date have failed to provide good pathophysiological insights, mainly because bedside balance tests are relatively crude and subjective techniques [5]. For example, the widely used retropulsion test of the Unified Parkinson's Disease Rating Scale (UPDRS) only provides a gross measure of overall balance disturbances in PD, but is not designed to unravel the complex underlying pathophysiology. This situation has improved with the advent of posturography,...