Electrical Somatosensory Stimulation of the Paretic Upper Limb in Acute Stroke Patients - a Randomized Controlled Study

1. Introduction

Globally, stroke is ranked as the second leading cause of death and the third largest cause of disease burden. Stroke is associated with tremendous physical, psychological and economical demands on patients, families, the health care system and society at large. The worldwide burden of stroke is expected to increase in the coming years. In the developed countries, this trend is primarily a consequence of the growing older population; age being a major risk factor for stroke. Upper limb paresis is one of the most frequent and persisting impairments following stroke, and represents a major obstacle to regain independence in activities of daily living (ADL). In fact, it has been estimated that about 50% of the stroke survivors will be left with a non-functional arm after completed stroke rehabilitation. In order to minimize the disease burden in stroke survivors, it is of great importance to design and implement effective rehabilitation strategies targeting the paretic upper limb.

Studies have shown that recovery of upper limb functioning (i.e. recovery of impairments and activity limitations, including skills) follows a pattern with a pronounced early post-stroke recovery and a subsequent leveling off already by 6 months post-stroke. In fact, it has been shown that 80% of patients reached a plateau within 3 weeks, and 95% reached a plateau by 9 weeks. It has also been shown that regaining hand dexterity is largely defined within the first 4 weeks after stroke , indicating a critical time window for recovery of upper limb functioning. Therefore rehabilitation efforts in the early post-stroke phase are likely decisive to maximize functional recovery. However, despite the fact that recovery is most likely in the immediate weeks after stroke, there are very few studies investigating the effect of therapeutic interventions in this time period.

Several therapeutic interventions are currently used to try to aid in the recovery of upper limb functioning. There is limited evidence that hands-on therapy, including passive joint mobilization, manual stretching of soft tissue and passive exercises, is effective. Strength training in chronic stroke may reduce motor impairments in patients with mild-moderate paresis, but without any effect on ADL-performance. Likewise, there is limited evidence for the use of mirror therapy, sensorimotor and mental training. The evidence for using orthoses and other supporting devices is inconclusive. Repetitive task-oriented practice, which is probably the widest used intervention in facilitating upper limb recovery after stroke, has demonstrated promising results mostly in chronic stroke when delivered using virtual reality systems and robots, as well as constrained-induced movement therapy. However, these approaches are patient and resource demanding in terms of hours of daily training or expensive technologies. Moreover, it remains unknown if the functional benefits persist at long-term.

Electrical stimulation (ES) is another method that has been used in facilitating the recovery of upper limb functioning following stroke. ES can induce a muscle contraction, or it can be a somatosensory stimulation below the motor threshold. Regardless the type of stimulation, there is some evidence that ES can aid in reducing motor impairments, but the questions regarding the optimal stimulation protocol (e.g. current amplitude, pulse frequency, placement of electrodes, treatment duration), long-term effect and transfer of training effect into ADL remains unanswered. Since this body of evidence is primarily based upon studies conducted on ES in chronic stroke patients, it also remains unknown to what extent ES applied in the acute phase after stroke could affect the recovery of upper limb functioning. Although the vast majority of the studies have focused on ES that induces muscle contraction, it is widely accepted that somatosensory input is required for maintaining normal motor function. Research shows that motor skills acquisition and motor performance are dependent on somatosensory input, and stroke patients with intact somatosensory function experience more satisfactory response to rehabilitation. In healthy persons, the application of electrical somatosensory stimulation (ESS) to peripheral hand nerves, forearm muscles or the whole hand elicits an increase in the cortical excitability of the representations that control the stimulated body parts, and the increased cortical excitability seems to outlast the stimulation period itself. It has been hypothesized that increasing the amount of somatosensory input may enhance the motor recovery of patients following stroke. Recent studies in acute, subacute and mostly chronic stroke patients suggest that a single 2 hours-session of ESS to the peripheral hand nerves leads to transient improvement of pinch force, movement kinematics and upper limb motor skills required for ADL-performance. The higher the current amplitude, the more prominent the effect seems to be. ESS is used in conjunction with motor training in only one of these studies. One study demonstrates that the effect of a single session of ESS is maintained 30 days after cessation of intervention. Interestingly, there is emerging evidence that multiple sessions of ESS to the peripheral hand nerves in conjunction with motor training might improve motor skills of the paretic upper limb in subacute and chronic stroke patients, and these positive results seems to outlast the intervention period. When ESS is delivered in multiple sessions, it is unclear which current amplitude is optimal in subacute stroke patients. ESS of the whole hand using glove electrodes may or may not benefit the motor recovery of the paretic upper limb in chronic stroke patients. Importantly, ESS is passive in nature, causes patients minimal discomfort, has no adverse effects, is relatively cheap and can easily be incorporated in regular practice. Therefore it is valuable to establish the effect of multiple sessions of ESS in the restoration of upper limb functioning in the acute phase of stroke. 2. Purpose of the project