Elsevier

Gait & Posture

Volume 12, Issue 3, December 2000, Pages 205-216
Gait & Posture

Postural instability in Parkinson's disease: a comparison with and without a concurrent task

https://doi.org/10.1016/S0966-6362(00)00076-XGet rights and content

Abstract

The purpose of this investigation was to determine the effects of dual task performance on postural instability in subjects with idiopathic Parkinson's disease (PD) compared with healthy elderly people. In particular, we aimed to divert attention to a secondary task so the full extent of balance disturbance could be revealed without compensation by attentional mechanisms. Forty-five subjects were tested: 15 PD subjects with a past history of falls; 15 PD subjects with no history of falls; and 15 unimpaired individuals. Groups were matched for age and sex and subjects with PD were tested at peak dose in the levodopa medication cycle. Each subject was tested on their ability to maintain stability in three conditions: (1) steady standing (feet apart, feet together, step stance, tandem stance, single leg stance); (2) in response to perturbations generated by self-initiated movements (arm raise test, step test); and (3) in response to an unexpected external perturbation in upright stance, the shoulder tug test. The concurrent task was verbal-cognitive and required subjects to recite the days of the week backwards. The concurrent task produced a significant deterioration in performance for the arm raise test in all groups, the step test for the PD fallers and controls and for tandem stance in the PD fallers. Ceiling effects were evident for timed tests with feet apart and feet together resulting in poor discriminative validity for these tests. The external perturbation test showed differences between the three groups for both unitask and concurrent task conditions, yet similar rates of change from unitask to dual task conditions. Because PD fallers had a more severe initial deficit than controls, deterioration placed them in that part of the balance continuum at high risk of losing equilibrium.

Introduction

Postural instability in upright stance is common in end-stage Parkinson's disease (PD) and compromises the ability to maintain balance during everyday tasks such as walking, turning and standing up from sitting. Due to an inability to adequately balance the body's centre of mass over the base of support, patients with advanced PD are predisposed to loss of equilibrium and falls. In many cases antiparkinsonian medications have little impact on balance [1] and patients must resort to the use of attentional strategies to maintain stability [2], [3]. Attentional strategies are cognitive activities such as planning out and mentally rehearsing action sequences or consciously attending to maintaining balance whilst movements are performed [2], [3]. For the purposes of the current investigation, attention is defined as intentionally directed or habitual focussing of mental activity [4].

Although ∼40% of patients with end-stage disease have falls [5] many studies have shown only minor differences between patients with PD and control subjects in the execution of postural responses. For example, Horak et al. [6] found that postural response latencies in reaction to surface displacements of a moveable force platform were similar in PD and control subjects. The pattern of distal to proximal muscle activation was normal for both groups and patients did not have a deficit in orienting to changing sensory information. In addition, Schieppati and Nardone [7] found the amplitude and distribution of tonic leg muscle EMG recordings to be normal when subjects with PD attempted to maintain upright stance with eyes open or closed. Whereas postural sway is usually increased in patients with balance disorders arising from stroke [8], [9], head injury [10] and cerebellar ataxia [11], [12], [13] it is often reduced in patients with PD [6], [14]. The problem in PD appears to be a lack of flexibility in shifting postural responses and adjusting reflex gain according to changing task demands [6], [7] rather than a disorder in the sequencing of postural responses.

One possible explanation for reduced postural sway in PD and minimal differences between PD and control subjects in muscle response latencies to perturbation is that patients use cortically mediated cognitive processes to bypass defective basal ganglia pathways in order to temporarily overcome their balance deficits. The basal ganglia play a role in controlling skilled actions that have been practised to the stage where they can be performed quickly and easily, with little thought [15]. When the basal ganglia are dysfunctional, as in PD, skilled movements are performed more slowly and with smaller amplitude than usual [16]. By deliberately focussing attention on the task while it is performed, however, many patients can increase their movement speed and amplitude to within normal values [17]. The use of attentional strategies such as this is accompanied by changes in cortical pre-movement potentials, which become more normal when patients deliberately focus on performing a task in a set way [18]. It could be suggested that patients with PD use cortical control strategies to maintain equilibrium and prevent falling. Clinically this could manifest as deliberate overconstraint of postural reflexes in order to reduce instability which would account for the reduced postural sway observed with posturography. If this is the case then diversion of attention to the performance of a complex secondary cognitive task would presumably allow the full extent of underlying balance disorders to be revealed. It would be predicted that deterioration of postural stability under dual task conditions would be far greater in patients with PD than age-matched control subjects who do not have balance disorders.

It is well-established that concurrent motor task performance in PD accentuates movement disorders during upper limb tasks [19], [20], [21], [22] and walking [17], [23] although whether this applies to postural control has not been examined in detail. Examples of concurrent motor tasks examined in subjects with PD include repetitive line drawing coupled with squeezing a sphygmomanometer bulb [20], pressing a tally counter whilst manipulating beads with tweezers [21], flexing and extending the elbow whilst squeezing a force transducer [18], doing up buttons whilst tapping the foot [22] and carrying a tray with glasses whilst walking [23]. These investigations have shown that when patients focus on one task the performance of the other deteriorates markedly, whereas control subjects show much less change. Studies on the effect of cognitive task performance on motor control in PD have also clearly shown the effects of divided attention on motor performance. For example, our previous research illustrated that when patients with PD performed a secondary cognitive task (recital of the days of the week backwards) whilst walking, their velocity and stride length reduced to a greater extent than age-matched control subjects [17]. There has only been one investigation of the effects of secondary task performance on postural stability in patients with PD. Smithson et al. [2] tested patients with PD and age matched controls in steady stance with feet apart with and without concurrent arm raises for a maximum of 30 s. Without the secondary task, control subjects and PD subjects with no history of falls could maintain stable stance for 30 s, and PD subjects with a history of falls for a mean of 29.5 s. The mean number of arm raises in the dual task condition was 14 in the control group, 13 in the PD nonfallers group and 11 in the PD fallers group. However because the duration of steady stance in the secondary task condition was not measured, the degree to which arm raises impacted on balance could not be fully determined.

The purpose of this study was to measure the interaction between dual task performance and postural stability for a range of clinical tests of balance in patients with PD. It was hypothesised that diverting attention to a secondary cognitive task would allow better discrimination of balance impairments, by preventing patients from compensating using frontal-cognitive mechanisms to bypass defective basal ganglia pathways. Our prediction was that patients with PD would show greater performance decrements from unitask to dual task conditions than control subjects. It was further predicted that patients with PD who had a past history of falls would show greater deterioration in postural stability than patients with no history of falls. Moreover it was anticipated that patients would show differences between performance of the right and left sides, given that PD is asymmetrical in its presentation [3].

Section snippets

Subjects

A total of 45 subjects were recruited for the investigation; 15 with PD and a history of falls, 15 with PD and no history of falls and 15 unimpaired age matched control subjects. There were seven men and eight women in each group. Patients were recruited from the Movement Disorders Clinic at Kingston Centre and control subjects were recruited from the Volunteer Services Unit at Kingston, other voluntary agencies, care-givers and associates of the researchers. Preliminary unitask data from a

Steady stance tests

Table 3 summarises the median values and interquartile ranges for tests of steady stance with feet apart, feet together, stride stance, tandem stance and single limb stance. The table illustrates that ceiling effects were evident in the group data for feet apart and feet together in both unitask and dual-task conditions. For the unitask condition Kruskal–Wallis analyses revealed significant differences between groups for right stride stance (χ2 (2)=8.6, p=0.0138), left stride stance (χ2

Discussion

This is the first study to examine balance in patients with Parkinson's disease with and without the performance of a concurrent verbal-cognitive task. The main findings can be summarised as follows: for both the unitask and dual task conditions in steady stance, balance was more impaired in subjects with PD, and in particular those with a history of falls, than in control subjects. The same trends were apparent for the internal perturbation and external perturbation tests. With the

Acknowledgements

We thank the participants of this study for their time and commitment, Ms Winnie Tang (PT) for her dedication as research assistant and the physiotherapists of the Aged Care Program, Southern Health Care Network for their support. We also thank Ms Robyn Tapp for her valuable advice on the strategy for statistical analysis. This project was funded by grant number 971268 from the National Health and Medical Research Council of Australia.

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