On the second Tuesday of each month, SLP bloggers around the world blog about research related to the field of Speech Language Pathology. I found Research Tuesday be an exciting initiative and decided to start participating last month. I aim for this to become a monthly routine. Rachel Wynn, a fellow SLP, publishes a monthly summary of Research Tuesday blog posts on her website. You can read more about it here. Here is my contribution to the Research Tuesday initiative for January,2014.
Title: ‘Electrical Stimulation and Swallowing: How Much Do We Know?’Ianessa A. Humbert, Ph.D., CCC-SLP, Emilia Michou, Ph.D., Phoebe R. MacRae, Ph.D., & Lisa Crujido, M.S., CCC-SLP
Semin Speech Lang. 2012 August; 33(3): 203–216
I chose this particular critical literature review for two reasons; one being my interest in the use of electrical stimulation for dysphagia management (I am VitalStim certified since October 2012 and have successfully used it with many of my patients ever since) and secondly, my strong belief in the greater need and use of evidence-based-practice in the field of Speech Language Pathology.
In this particular article, the authors review the values and limitations of the published literature on the topic of e-stim for swallowing. The discussion provides a review of swallowing anatomy and physiology, the fundamentals of e-stim and information essential for the readers’ independent critique of these studies.
In 1996, Freed et al introduced and described a method of e-stim (also called neuromuscular e-stim or NMES) to the anterior neck muscles and face in humans as a means of improving swallowing. Following this publication, the U.S. Food and Drug Administration cleared VitalStim Therapy® to market external e-stim for dysphagia in 2002. Since then, numerous scientific reports by different research groups have been dedicated to understanding the effect of e-stim on normal and disordered swallowing.
Along with great interest in e-stim, great controversy has also surrounded its use. The goal of this review is to help clinicians learn the value and limitations of e-stim, through an in-depth and updated review of the literature, so that they can apply this knowledge to their clinical practice.
Overview of Swallowing
In this section, the authors discuss the different phases of swallowing (oral, pharyngeal and esophageal) in detail. They emphasize, that before any clinical use of e-stim is initiated, it is imperative that clinicians have a solid understanding of normal swallowing anatomy and physiology. They briefly discuss the difference between normal and disordered swallow function as well.
Research on e-stim as a rehabilitation method has garnered much attention for its potential to minimize dysphagic symptoms, especially for pharyngeal phase dysphagia.
E-Stim: What is it?
The goal of e-stim in patients is to augment weak muscle contractions and thereby improve purposeful movement of structures that are controlled by those muscles.
This can be achieved either transcutaneously (surface e-stim) or percutaneously (intramuscular, intrinsic, epimysial e-stim). In either case, electrical current flows from a source (an external device) that alters current intensity (amplitude), through electrodes that are in direct contact with the body. The goal for rehabilitative use is to allow electrical current to create a contraction by depolarizing the nerves that are responsible for motor innervation to a particular muscle or to particular muscle fibers.
Clinicians usually instruct patients to attempt to volitionally activate the target muscle(s) with concurrent e-stim or to remain at rest and allow stimulation to cause a contraction by electrical current alone.
Compared with percutaneous e-stim, surface e-stim is most commonly used in both clinical and research environments due to its noninvasive nature. The authors then go on to describe both these types (surface and percutaneous) of e-stim in detail. They also throw light on some considerations for clinicians while using surface e-stim on the anterior neck. You can read more about these specific topics in the original article.
The investigation of percutaneous e-stim on swallowing function is limited to a small number of studies. The majority of empirical evidence of the VitalStim Therapy System for dysphagia is based on surface electrode placements.
Studies of immediate and long-term effects in normal and patient groups
Studies of the effects of e-stim on swallowing can be separated by the length of the study (immediate vs. long-term) and the population being studied (healthy vs. patient population).
Immediate effects of e-stim (single applications)
The immediate physiological effects of e-stim have been studied at rest, during swallowing, and at sensory and sensory + motor levels:
- Muscle stimulation at rest reveals the impact of the stimulation alone, unconfounded by volitional movement, which implies which muscles in a muscle group are being targeted by stimulation with a particular electrode position.
- Muscle stimulation combined with a task can show how stimulation impacts a particular movement.
- Stimulation at the sensory level occurs when only the cutaneous afferents (sensory receptors in the skin) are being stimulated by the surface electrodes.
- Sensory + motor stimulation occurs when the stimulation intensity is increased to activate both cutaneous afferents and motor nerves for a muscle contraction.
Details of the all the studies reviewed and studied can be found in the original article in Table 1. Some of these are discussed below.
1. The immediate physiological effects of surface e-stim to the submental muscles and anterior neck was examined in 29 healthy adults. Vitalstim (e-stim) was administered to participants at rest and during swallowing at the sensory + motor level. The stimulation at rest trials were tested with 10 different electrode placements; some with electrodes positioned: (a) only above the hyoid bone; (b) only below the hyoid bone, and: (c) both above and below the hyoid bone.
Results: The results of this study showed that placements with electrodes only below the hyoid (overlying infrahyoid muscles only) and placements above and below the hyoid (overlying supra and infrahyoid muscles) depressed the hyolaryngeal complex to varying degrees. The most descent was observed with a placement that targeted both supra and infrahyoid muscles. Those electrode placements that were only above the hyoid bone (overlying suprahyoids) caused non significant or minimal anterior or superior hyoid movement among participants.
2. Closure of the vocal folds was examined with e-stim using the same 10 electrode placements as in the study described above, also at the sensory + motor level. Nasolaryngoscopy was used to image true vocal fold movement, with concurrent surface e-stim at rest.
Results: Minimal vocal fold angle change was achieved with stimulation, suggesting that surface e-stim to the submental and neck regions does not produce immediate true vocal fold adduction adequate for airway protection during swallowing.
3. Ludlow et al tested the immediate physiological effects of e-stim in a group of post-stroke patients with chronic pharyngeal dysphagia. In this patient study, at-rest trials also caused significant hyoid and laryngeal descent. However, swallowing trials with concurrent sensory-only stimulation reduced instances of penetration or aspiration in this patient group. This suggests that sensory surface e-stim during swallowing in patients might alter the motor pattern similarly to what is thought to occur with other sensory-based treatments such as thermal-tactile, taste, or vibrotactile treatments.
Long Term Effects of therapeutic regimes of e-stim: Healthy and Patient Population
Pros: Long-term studies of the effects of e-stim on swallowing are becoming increasingly prevalent and have the potential to directly contribute to clinical settings, since they provide useful information about the duration and intensity of the successful interventions as well as the follow-up effects.
Cons: On the other hand, it is challenging to draw strong conclusions from a small number of long-term studies that have focused on heterogenous patient populations or have used outcome measures that are not directly comparable to one another.
All but one of the studies reviewed included functional change in swallowing as an outcome measure. Few included physiological measures and only two studies reported either quallity of life or neurophysiological outcomes. Details of these studies and their findings can be found in the original article in Table 2. Some of these that the authors discussed in the paper have been summarized below.
1. One of the earliest controlled studies examined the long-term effects of surface e-stim in stroke patients delivered as a therapeutic regime assigned to one of two treatment groups: surface e-stim or thermal-tactile stimulation. The purpose of this study was to compare the effects of e-stim to thermal-tactile stimulation and to assess the safety of surface e-stim. The methodology of the study included different durations and frequencies of treatment blocks to patients. Measurements were obtained using fluoroscopic images of swallows of various consistencies. An SLP assigned a ‘swallow function score’ from 0 to 6.
Results: A total of 99 patients completed this study and both groups ended with a higher swallow function score. However, after 2 years, 89% of e-stim patients retained the improved swallow function score, while only 67% of thermal-tactile patients retained their improved status. The authors concluded that surface e-stim was a safe and effective treatment modality for dysphagia caused by stroke and that surface e-stim resulted in better swallowing outcomes than thermal-tactile stimulation to the posterior oral cavity (faucial pillars).
2. Gallas et al examined the neurophysiological effects of submental transcutaneous e-stim, showing no significant change in measurements of excitability of the activation of cortical areas involved in swallowing.
Only two studies in literature investigated the long term effects of e-stim on a healthy population:
3. Park et al studied the effects of effortful swallowing with concurrent e-stim below the hyoid bone in healthy adults. They reported that 20 minutes of e- stim for 2 weeks caused greater hyoid elevation, but no change in forward hyoid excursion was evident. However, results faded within 2 weeks posttreatment.
4. Like Park et al, Suiter et al found no significant difference in electromyography signal between the control group and the e-stim group after e-stim treatment in healthy adults.
In this section of the article, the authors then go on to discuss the limitations of these treatment studies regarding the long term effects of e-stim on swallowing.
Discussion and Implications
The successful rehabilitation of pharyngeal phase impairments presents a unique challenge for clinicians, and will continue to do so without a more thorough understanding of how the nervous system initiates and completes this phenomenon. This is further complicated by the fact that pharyngeal swallowing is not readily seen without imaging technology (i.e., use of VFS, FEES etc.) The article stresses that we need to improve our understanding of the effects of e-stim in specific types of disorders and specific levels of severity before we can widely apply the treatment to the general dysphagic population.
As clinicians, we must take the necessary time to determine the effectiveness of a given treatment, always keeping the clinical goal for the patient in mind. If surface e-stim is a treatment consideration, its physiological effects must be assessed with imaging, similar to other compensatory mechanisms (i.e., chin tuck) or PO trials of various consistencies. This will provide an objective baseline from which change can be measured against.
The authors re-iterate,
‘The importance of having a solid knowledge of swallowing anatomy and physiology cannot be overstated for any swallowing clinician—whether e-stim is being used or not.’
Consequences of dysphagia substantially reduce quality of life, increase the risk of medical complications and mortality, and pose a substantial cost to healthcare systems. As research advances, it is each clinician’s duty to stay abreast with new findings in the use of various dysphagnia management strategies and to be vigilant in critiquing the available data.
Together, swallowing research and clinical practice has enormous potential for developing effective management strategies for dysphagia. Progressive partnerships among highly skilled clinicians and experienced researchers are required to ascertain the full potential and limitations of e-stim and other swallowing management options, that are both known and those that are not yet discovered.
What do you think of this literature review? Please leave your comments below.