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The effects of deep brain stimulation and levodopa on postural sway in subjects with Parkinson's disease
  1. D J Burn
  1. Department of Neurology, Regional Neurosciences Centre, Newcastle General Hospital, Westgate Road, Newcastle upon Tyne NE4 6BE, UK
  1. Correspondence to:
 Dr D J Burn;
 d.j.burn{at}ncl.ac.uk

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Deep brain stimulation reduces postural instability in Parkinson's disease

Postural instability, leading to falls, is a common feature of neurodegenerative parkinsonian syndromes, often associated with injury and hospital admission. In a recent prospective study of falls in 109 patients with Parkinson's disease, 68% fell during a one year follow up period.1 Postural instability in Parkinson's disease is notoriously refractory to levodopa treatment, implicating the involvement of non-dopaminergic pathways. A previous report of the use of continuous bilateral high frequency stimulation of the subthalamic nucleus in Parkinson's disease suggested that marked improvement in motor disability was accompanied by significant improvement in axial symptoms, and that a synergistic effect was obtained when stimulation was used in conjunction with levodopa treatment.2

The paper by Rocchi et al (this issue, pp 267–74)3 quantifies postural sway in six subjects with Parkinson's disease, all of whom had undergone deep brain stimulation (three of the subthalamic nucleus and three of the globus pallidus, pars interna) compared with 11 elderly control subjects. The subjects with Parkinson's disease were tested in four different conditions: off both levodopa and deep brain stimulation, on deep brain stimulation, on levodopa, and on both deep brain stimulation and levodopa. This design, coupled with the use of static posturography, allowed the authors to dissect out the effects of each treatment upon a variety of sway parameters, compared with the untreated “baseline” condition. Rocchi and colleagues found that the subjects with Parkinson's disease who were off both treatments had abnormal sway in stance. Moreover, the use of levodopa actually increased postural sway abnormalities, particularly in the mediolateral direction. The authors reasonably suggest that levodopa might exert this effect by reducing postural tone without a concomitant improvement in postural control. As the risk of falling has been correlated with a large lateral sway during stance in the elderly, this implies that treatment with levodopa might actually increase the chances of falling, at least in advanced Parkinson's disease.

When deep brain stimulation was activated, postural sway lessened and in several instances returned to normal, while the combined effect of deep brain stimulation and levodopa resulted in a postural sway that was the average of the effect of each treatment individually. The dramatic benefit from deep brain stimulation suggests that this treatment can modulate non-dopaminergic pathways, as previously proposed by Bejjani and colleagues.2 Deep brain stimulation might help integrate information from the proprioceptive system, perhaps by influencing activity in brain stem structures such as the pedunculpontine nucleus, an important cholinergic sensory relay station to the thalamus.4

The results of this study must be interpreted with some caution, however. The patient group was surgically heterogeneous, while the small numbers precluded subgroup analysis to determine which, if any, of the two targets was superior in improving in sway parameters. Furthermore, the study was cross sectional, with assessments only carried out once at six months postsurgery. Preliminary evidence indicates that patients with long term deep brain stimulation of the subthalamic nucleus also develop problems with postural instability as well as freezing.5 Nevertheless, Rocchi and colleagues' elegant study provides a basis for larger prospective studies to clarify these issues. Their work also indicates that posturography can provide a safe and sensitive means of assessing therapeutic interventions for a challenging problem in Parkinson's disease.

Deep brain stimulation reduces postural instability in Parkinson's disease

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