Postural instability in patients with Parkinson's disease. Epidemiology, pathophysiology and management.

Postural instability is one of the cardinal signs in Parkinson's disease (PD). It can be present even at diagnosis, but becomes more prevalent and worsens with disease progression. It represents one of the most disabling symptoms in the advanced stages of the disease, as it is associated with increased falls and loss of independence. Clinical and posturographic studies have contributed to significant advances in unravelling the complex pathophysiology of postural instability in patients with PD, but it still remains yet to be fully clarified, partly due to the difficulty in distinguishing between the disease process and the compensatory mechanisms, but also due to the fact that non-standardized techniques are used to measure balance and postural instability. There is increasing evidence that physical therapy, especially highly challenging balance exercises, can improve postural stability and reduce the risk of falls, although the long-term effects of physical therapy interventions on postural stability need to be explored given the progressive nature of PD. Pharmacotherapy with dopaminergic medications can provide significant improvements in postural instability in early- to mid-stage PD but the effects tend to wane with time consistent with spread of the disease process to non-dopaminergic pathways in advanced PD. Donepezil has been associated with a reduced risk of falls and methylphenidate has shown potential benefit against freezing of gait, but the results are yet to be replicated in large randomized studies. Surgical treatments, including lesioning and deep brain stimulation surgery targeting the subthalamic nucleus and the globus pallidus internus, tend to only provide modest benefit for postural instability. New surgical targets such as the pedunculopontine nucleus have emerged as a potential specific therapy for postural instability and gait disorder but remain experimental.