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The use of electrical stimulation as a treatment technique for dysphagia has prompted many inquiries to ASHA's Health Care Services team and Ethics team. ASHA does not endorse programs, products, or procedures (see ASHA's Evaluation Questions). In keeping with the principles of evidence-based practice, ASHA members should make clinical decisions based on a review of current best evidence together with clinical expertise and client values. Christy Ludlow presented on electrical stimulation at ASHA's 2007 Health Care Conference and was invited to contribute an article to The ASHA Leader due to the high interest in this topic.
—Janet Brown, director of health care services in speech-language pathology
by Christy L. Ludlow
Clinicians need to make decisions about whether or not to incorporate electrical stimulation as part of their dysphagia therapy. This article reviews information currently available on such technologies and their application in dysphagia therapy.
What is Electrical Stimulation for Dysphagia?
Electrical stimulation is the application of current to the living body to stimulate nerves or nerve endings that are either sensory or that innervate muscles. It can be applied to the body for different purposes and in different ways. Electrical stimulation is applied to stimulate neural tissues, nerve endings that innervate muscle fibers, or sensory nerve endings. Normally a single axon from a motor neuron in the brain stem or spinal cord innervates several fibers within a muscle via neuromuscular junctions. An action potential traveling in an axon releases acetylcholine, a neurotransmitter, at the neuromuscular junction. The neurotransmitter then binds to acetylcholine receptors that activate and generate a muscle fiber contraction. Electrical current can be applied to a muscle (intra-muscular) by implanting the electrode directly into muscle tissue. This technique is largely experimental at this point in time. A less-invasive method is to apply the current transcutaneously using electrodes applied to the skin, which is called transcutaneous electrical stimulation (TES).
When TES is applied to the skin, most electrical stimulation will be to sensory nerve endings within the skin, sending action potentials to the sensory cell bodies that in turn relay signals to either the spinal cord or the brain stem and, via the thalamus, to the cortex. When stimulating muscles with TES, only those closest to the skin will have their nerve endings stimulated and may contract (shorten). Deeper muscles may be stimulated only if a higher amplitude current is used and the overlying muscles and fat are thin; however, all of the overlying muscles will be stimulated to a greater degree than the deeper muscles (see Figure 1 [PDF]). Therefore, TES applied to the neck over the larynx will stimulate nerve endings in the skin and the platysma and sternohyoid muscles, which pull the hyoid downwards towards the sternum. Such stimulation results in hyo-laryngeal lowering during videofluoroscopy (Humbert et al., 2006). When TES is applied to the submental region alone, the greatest stimulation goes to the skin and the platysma and the anterior belly of the digastric, and much less to the mylohyoid and little to the deeper genioglossus with minimal effects on the hyo-laryngeal position on videofluoroscopy (Humbert et al., 2006). When TES is applied to both the submental and throat regions simultaneously, the net result is a lowering of the hyoid in the neck (Humbert et al., 2006; Ludlow et al., 2007).
Who is a Candidate for Electrical Stimulation?
Electrical stimulation should be considered only for persons with healthy sensory and motor nerves. Persons may be candidates who have injuries to the central nervous system, for example, following a stroke, a traumatic brain injury, or excision of a brain tumor. After injury to cortical and/or subcortical pathways, cortical control may be lost or cortical inputs may be blocked, interfering with voluntary or automatic firing of the motor neurons in the brain stem for the craniofacial, laryngeal, and pharyngeal musculature during swallowing (see Figure 2 [PDF]). Electrical stimulation bypasses that block by stimulating the axons from the motor neurons to fire the muscles, bypassing the central control abnormalities.
Conversely, if the client has intact sensory inputs to the brain stem, but the transmission of the sensory inputs from the brain stem to the thalamus and/or cortex is disrupted because of brain injury, then the client can no longer perceive the sensory information being relayed to the brain stem. In this case, electrical stimulation of the sensory nerves may augment the person's sensation if there is some sensory relay to the thalamus and cortex so that the client can perceive the stimulus.
In electrical stimulation of muscles, the axons innervating muscle fibers—not the muscle fibers—are stimulated. Clients with motor neuron diseases such as amyotrophic lateral sclerosis have motor neuron death resulting in axonal die back and loss of muscle fiber innervation, blocking the effects of electrical stimulation. Similarly, in persons with peripheral injury or neuropathies, such as spinal muscular atrophy, the axons die back and the muscle fibers are no longer innervated and can't be accessed by electrical stimulation. Persons with myasthenia gravis have antibodies that block acetylcholine receptors on muscle fibers, preventing muscle fiber activation. Further, in sensory peripheral neuropathies, the sensory nerve endings are lost and can't be stimulated. Therefore, TES is not an appropriate option to use with individuals with peripheral sensory and/or motor impairments.
Use of TES for Dysphagia
The question of whether the evidence supports the use of TES for dysphagia is not a simple one to answer. Evidence needs to be gathered systematically for different types of diseases/disorders (stroke, dementia, Parkinson's disease, multiple sclerosis, head and neck cancer); various therapeutic approaches (i.e., stimulation alone or combined with swallowing treatment, exercises, or diet management); different types and severity of swallowing deficits (affecting oral propulsion, reduced hyo-laryngeal elevation or delay, limited upper esophageal sphincter opening, pharyngeal weakness); the stage of disease (acute with spontaneous recovery versus chronic post-onset or early versus late disease progression); and the type of involvement (brainstem or spinal motor neuron versus cortical or subcortical motor neurons). A particular treatment may be beneficial only for clients with certain swallowing deficits or in one therapeutic setting (acute care, rehabilitation, nursing or home care), but as of now, there is not a sufficient body of research to inform these important considerations.
The limited research on this topic includes two reports of treatment outcomes before and after the introduction of TES (Blumenfeld, Hahn, Lepage, Leonard, & Belafsky, 2006; Kiger, Brown, & Watkins, 2006). One report was a retrospective unblinded chart review comparing changes in swallow severity, and found a greater benefit in a TES group compared with the traditional dysphagia treatment (TDT) group (Blumenfeld et al., 2006). The other report compared unblinded ratings of videofluoroscopic and fiberoptic endoscopic evaluations of oral and pharyngeal swallowing (FEES) before and after either TES or TDT treatment. The TDT group experienced greater improvements in both the oral and pharyngeal phases than did the TES group (Kiger et al., 2006). As changes in dietary consistency advancement did not differ between the two groups, the authors concluded that there was no difference in outcome between the two groups (Kiger et al., 2006). Thus, these two investigations yielded inconsistent results, but because neither study included random assignment of participants to different treatment conditions, their validity is questionable.
One non-randomized trial during the post-stroke acute phase compared the application of TDT with TES to the application of thermal-tactile stimulation. Participants were assigned to groups by the principal investigator during the acute recovery phase and assessment was not randomized (Freed, Freed, Chatburn, & Christian, 2001). Both treatment groups improved; however, the TES group improved to a greater degree than the thermal-tactile group. This study did not, however, compare TDT with and without TES to determine whether TDT is enhanced by the addition of TES.
A meta-analysis combining some of the TES studies found a modest effect size of 0.6 (Carnaby-Mann & Crary, 2007); however, the effect sizes for TES with TDT versus TDT alone is unknown. Two uncontrolled reports on a series of patients, one retrospective (Shaw et al., 2007) and the other prospective (Leelamanit, Limsakul, & Geater, 2002), provide some information regarding severity of dysphagia and response to TES. The prospective study found that the more severely involved participants with post-stroke dysphagia did not improve to the same degree as the participants whose swallowing problem was idiopathic or due to aging. The retrospective case series found a relationship between participants' initial severity and subsequent improvement with TES. Mildly to moderately impaired participants showed some improvement, while participants with the most severe dysphaga did not improve with TES treatment (Shaw et al., 2007). Without a control group, it cannot be determined whether or not recovery in the mildly to moderately impaired persons was due to TES, dysphagia therapy, or spontaneous recovery.
However, the results of these two case series are in agreement with the physiological findings on the effects of TES (Humbert et al., 2006; Ludlow et al., 2007). Ludlow and colleagues (2007) concluded that participants with severe dysphagia who are unable to overcome the TES induced hyo-laryngeal descent during swallowing may not benefit from TES. That is, the risk of aspiration or penetration may be increased because the application of TES can reduce hyo-laryngeal elevation even in healthy persons with normal swallowing (Humbert et al., 2006). Therefore, it is recommended that clinicians evaluate whether or not a patient has increased risk of aspiration or penetration during swallowing when the device is turned on at an effective level during videofluoroscopy before considering using TES in therapy.
Safety Issues
The federal Food and Drug Administration (FDA) must clear medical devices for patient use. The VitalStim® TES device was cleared by the FDA after a review of the information available on possible risks. Because this is a non-invasive device to be used only by trained providers (speech-language pathologists who have completed a training course), the threshold for clearance is relatively low in contrast with implanted devices such as pacemakers.
When cleared by the FDA, certain contraindications and precautions regarding the use of TES devices must be followed by trained providers. Indications for use of any muscle stimulator device are listed in 510(k) Premarket Notification Summary [PDF]. These precautions apply to use of TES devices on patients with pacemakers or other implanted electrodes, with implanted cardioverter defibrillators (ICDs), over areas of excess adipose tissue, in close proximity to diathermy (such as ultrasound), over anesthetic regions, when active motion is expected in the region of stimulation, in persons with seizure disorders, over open wounds, and over the carotid sinus (Wijting & Freed, 2003, p. 79-80).
Considerations for Clinicians
The evidence on whether or not TES is beneficial for treatment of dysphagia is not yet conclusive. Clinicians should be guided primarily by safety and cost issues. When considering adding TES to dysphagia therapy for a particular patient, the clinician should address the following issues:
- Does the client meet the indications for the application of an electrical stimulation device?
- Does the client have a condition that might contraindicate the use of electrical stimulation?
- Does the client have a condition that would indicate that precautions should be taken in using an electrical stimulation device?
- Is there any risk that TES could interfere with the client's swallow when evaluated on videofluoroscopy?
- Does the literature suggest that the client would benefit from the use of TES?
Decisions about whether or not to incorporate surface electrical stimulation during therapy for clients with dysphagia must take into account the particular client, the contraindications for the use of such a device, precautions, and evidence available regarding the potential benefits or possible risks of using such as tool in treatment. Clearly, more systematic research is needed before clear evidence will be available regarding who benefits from the use of this particular treatment tool. Clinicians should be guided by issues of safety and precautions before incorporating such devices into their practice.
Christy L. Ludlow is a senior investigator and chief of the Laryngeal and Speech Section of the National Institute of Neurological Disorders and Stroke at the National Institutes of Health. Ludlow's research focuses on the effects of functional electrical and sensory stimulation in severe swallowing disorders. Contact her at ludlowc@ninds.nih.gov.
Disclosure: Ludlow and NIH have filed patent applications for both intramuscular implanted devices and surface vibro-tactile devices to aid persons with dysphagia. Both Ludlow and the NIH could benefit financially if such devices were to become commercially available. The preparation of this article was supported by the Division of Intramural Research of the National Institute of Neurological Disorders and Stroke.
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