As has been described in sections 2.2.1 and 2.2.3, gait is initiated and controlled by a combination of the motor pathways and cognitive processes. Stroke will affect both motor and cognitive processes as a result of damage to the brain. In particular, the cognitive deficits that result from stroke are various as discussed in section 2.3.3. There is a growing body of evidence to support the importance of the effect of these deficits in gait abilities post-stroke. Apart from the research evidence indicating the contribution of cognitive deficits to falls, there are numerous studies that demonstrate the interaction between cognitive deficits post stroke and gait.
Mulder (Mulder et al., 2002) proposes a persuasive model of the interaction between cognitive and motor function and its central role in functional recovery following nervous system damage. In line with much of the work described in sections 2.2.1 and 2.2.3, he proposes that the two systems are closely interrelated and that measurement of one aspect alone is insufficient to characterise functional recovery. As an example, walking speed is a commonly used outcome to measure recovery or the effect of an intervention. However, use of this alone as an outcome measure will not necessarily uncover the adaptive and compensatory strategies being used while performing the walking test and hence the level of functional recovery. In post-stroke gait, in addition to conventional measures of motor performance, a measure of the concentration or mental effort required to walk may reflect the level of functional recovery i.e. the level of reliance on a conscious mode of motor control. To assess the level of functional recovery over time or the lessening need for compensation Mulder et al (2002) proposed two approaches; measurement of cognitive involvement or visual dependency.
In line with Mulder’s proposal, and as discussed earlier, dual-task protocols require division of attention between walking and a cognitive task. Thus, attentional capacities are challenged and test the assumption that if motor control of gait is operating at a normal or optimal level, then simultaneous execution of an additional task should not affect gait nor performance of the additional task (Yogev-Seligmann et al., 2008). Conversely, if attentional capacities are limited, performance of at least one of the tasks will deteriorate (Segev-Jacubovski et al., 2011). Dual-task methodology is reflective and typical of real world situations where stroke patients are required to ‘walk and talk’, take note of their surroundings and remember and follow
directions. As discussed earlier, it has been widely used to investigate the cognitive effects on gait, balance and fall risk particularly amongst elderly populations (Woollacott and Shumway-Cook, 2002, Yogev-Seligmann et al., 2008, Segev- Jacubovski et al., 2011).
There are also a growing number of studies in post-stroke gait that have used a dual- task design to assess gait. Several studies have used gait speed as an outcome, finding a deterioration in gait speed under dual-task conditions (Bowen et al., 2001, Canning et al., 2006, Hyndman et al., 2006, Plummer-D'Amato et al., 2008, Dennis et al., 2009, Pohl et al., 2011). Studies have also noted a reduction in performance of the cognitive task (Plummer-D'Amato et al., 2008, Dennis et al., 2009, Pohl et al., 2011, Hyndman et al., 2006, Kemper et al., 2006).
There are a small number of studies using other gait parameters as outcomes. The addition of a cognitive task had an adverse effect on balance with double-support time increasing in two studies (Bowen et al., 2001, Plummer-D'Amato et al., 2010). A small longitudinal study (Cockburn et al., 2003) compared stride duration initially after stroke and then 1-9 months later. Stride duration improved over time with recovery more so than cognitive performance during walking. A recent pilot study noted that paretic single limb stance time as particularly susceptible to dual-task interferences (Plummer-D'Amato and Altmann, 2012). Interestingly, in a dual-task study by Hyndman et al (2006) the participants were also assessed as fallers and non-fallers based on their fall history. Fallers coped less well with a competing cognitive task than non-fallers during walking, with a significant reduction in stride length.
The interaction between cognitive function and gait is a growing area of research in healthy, older and gait-challenged populations, including fallers and those with neurological disorders. Thus, treatments for gait disorders as a result of central neurological dysfunction may require a consideration of deficits in cognitive function when assessing effectiveness. As has been discussed, stroke can result in both cognitive and motor impairments, with the latter manifesting in some stroke survivors as foot drop gait. Thus, the effectiveness of interventions to address this could be measured taking into account co-existing cognitive impairments. This point will be revisited following the next section which reviews one of the interventions available to those with foot drop following stroke.
2.4 FES
There are a number of treatment options for foot drop as the result of stroke. Apart from physiotherapy, and the use of Botulinum Toxin (BoTox), there are two orthotic interventions, ankle foot orthoses (AFO) and functional electrical stimulation (FES), the second of which is the focus for the thesis. This section of the literature review firstly describes the most common application of FES to address foot drop and the function of each component. Secondly, the effect on the nervous system is described both at local and central levels. Finally, a comprehensive literature review of the effects of FES is provided, with a particular focus on patient-centred outcomes.