Should neuromuscular electrical stimulation be an option for dysphagia treatment?

By Hyunjoo Chung, PhD CCC-SLP

Barriers to the adoption of neuromuscular electrical stimulation (NMES) as a dysphagia treatment option were identified to be related to infrastructure issues and clinicians’ misconceptions (Shune & Moon, 2012).

It was approximately 15 years ago since NMES started being introduced as a dysphagia treatment option in graduate education. Dissemination of basic principles and results of recent investigations is limited. Shune & Moon summarized uncertainty of non-users’ understanding and misconceptions that contribute to the lack of adoption of NMES in their clinical practice. I would like to touch on some of the misconceptions that tend to be formed prior to adequate learning.

Are you ready to ask the question in the title for yourself as a clinician?: Should neuromuscular electrical stimulation be a treatment option for dysphagia? In order to answer this question, you would first need to learn the foundation of NMES, research findings/updates, clinical outcomes, contraindications, precautions, and parameters incorporated by specific programs of NMES.

In reality, many non-users might have taken a stand for NMES based on limited education unless the topic was well addressed in their graduate curriculum or clinical practices. This situation may negatively impact potential recipients of NMES for dysphagia. The prevalence of individuals with dysphagia and rising interests of patients in NMES as a dysphagia treatment option requires clinicians providing dysphagia treatment to be knowledgable of NMES.

My perspectives for NMES is that neuromuscular electrical stimulation (NMES) should not be prematurely declined or adopted as an option of modality for dysphagia rehabilitation. Right! I consider NMES as an option of dysphagia treatment modality for my clients at my private practice. So, I had to learn indications, contraindications, parameters, and evidence-based clinical outcomes. I would like to share very basic information here. Yet, your decision as a clinician on the use of NMES for your clients should be made case by case based on your training and clinical experiences.

Neuromuscular Electrical Stimulation programs for dysphagia generally incorporate similar sets of contraindications, warnings, and precautions. However, there are differences in research and evidence base among different NMES devices and programs with respect to their parameters either pre-set or adjustable, physiological change in relation to electrode placement, and clinical outcomes. Therefore, understanding of the parameters, the physiological target, and expected functional change involving NMES can help a clinician make clinical decisions towards NMES application to clients with impaired swallowing.

Here’s a list of key elements of NMES. Each element will be briefly described in the following section:

• Stimulation requires an intact peripheral nerve

• Advantages for Type II muscle fiber contraction

• Contraindications

• Precautions

• Warnings

• Electrode Placement

• Parameters

• Physiological target movement & adaptation learning

• A recent systematic review on efficacy of NMES on post-stroke swallowing function Support for future studies

• Support for future studies to investigate swallowing motor learning and cortical excitability

Figure 1. Ampcare ES, a portable two-channel electrotherapy unit with a set of electrodes connected via lead wires in the respective port.

1. Stimulation requires an intact peripheral nerve

Neuromuscular electrical stimulation is delivered via pulses flown through a lead wire to a set of small electrodes placed in the submental area or on the face. The flow of nerve action potential between transcutaneous electrodes can be induced through peripheral nerves. That is, the pathway of externally applied electrical stimulus (i.e., current) is intact peripheral nerves generating action potential required for muscles to contract.

2. Advantages for Type II muscle fiber contraction

First of all, swallowing muscles have a higher portion of type II muscle fiber types that are very fast in contraction, large in motor neuron size, and fast fatiguing. Type II muscles with large motor neurons will require stronger contraction need in order to be recruited. In consideration of patients with moderate to severe acute dysphagia s/p stroke, NMES can facilitate to maximize muscle recruitment in volitional swallowing exercises and reduce disuse atrophy of the swallowing muscles more efficiently than how the muscles are recruited only through swallowing exercises alone. That the physiological change can get linked to functional outcomes. All dysphagic clients may not require such facilitated maximization of muscle recruitment to accomplish their functional goals. Clients with moderate to severe dysphagia status post stroke who demonstrate or are predicted to have a slow progress could be a good candidate for NMES for dysphagia if cleared against contraindications.

3. Contraindications (Biber Protocol)

Each contraindication serves as a reason not to use NMES in the usual way.

• Patient with active cancer

• Patient with inability to express pain or discomfort via verbal, nonverbal, or gestural communication

• Patient with a seizure disorder controlled by medication may participate with the neurologist’s approval.

• Patient with any metal implants in proximity may need to be excluded if some sensation in the area is reported

• Patient with a cardiac pacemaker: interrogation testing by the pacemaker representative can determine if NMES will interfere with pacemaker function with the cardiologist’s approval.

• Patient with any other implanted electronic devices (e.g., cochlear implants, vagal nerve stimulation, deep brain stimulation etc.) may not be a candidate. The implanting surgeon should be contacted to discuss potential risks due to interference concerns.

4. Precautions (Biber Protocol)

A measure should be taken in advance to prevent the following happening in administering NMES:

• Skin rash or hypersensitivity at the site of electrode placement

• Sensitivity to tape

• Potential adverse effects with patients who have sensory nerve damage or reduced sensory awareness

• Skin burns with abnormal conductivity

5. Warning

• Do not apply electrical stimulation to cancerous lesions, carotid sinus, infection, fractures, etc.

• The long term effects of electrical stimulation are unknown.

So far, I have addressed indications, contraindications, precautions, warnings, and advantages of NMES for muscle contraction and less misuse atrophy. Now let’s talk about other three elements that are important for specific swallowing muscles and physiological target movement.

6. Electrode Placement 

NMES application to the submandibular muscles has been supported as a trigger of faster closure of laryngeal vestibule. NMES application was also suggested for adaption motor learning in the presence of a perturbation to laryngeal closure (i.e., anterior and superior movement of hyoid via NMES on the submandibular muscles, superior laryngeal movement, therefore anteriorly widened larynx) during volitional swallowing with NMES (Watts & Dumican, 2018). In Watts & Dumican (2018), Ampcare E-series electrodes (triangular shape) were applied to healthy participators as designed for Ampcare Effective Swallowing Protocol (ESP). This placement is similar to submandibular placement of Guardian Way but the size of Ampcare E-series electrode is larger and fits within the submental region under the chin superior to the hyoid bone.

It is my understanding that simultaneous application of NMES on both submental and laryngeal regions on the neck was found to be hyoid lowering probably because the infrahyoid (larynx lowering) muscles that are larger than and more superficial to the neck surface than the thyrohyoid (larynx elevating) muscle and submental (hyoid elevating) muscles, are more recruited, thus potentially limiting the powered contraction of hyolaryngeal elevating muscles during swallowing (Humbert et al., 2006). 

The finding of hyoid lowering cannot be considered to be a negative outcome of the use of NMES for dysphagia because it was resulted by the simultaneous application of NMES on both submental and anterior neck areas.

7. Parameters for peripheral nerve excitation

Parameters of pulsed electrical current in a particular NMES device characterize electrical stimulation delivered. I learned most from Guardian Way NMES certification course about the relations among primary parameters listed below along with practical rules.

• Phase duration (microseconds): a lower phase duration recommended to increase intensity for motor responses without discomfort; higher phase duration is more likely to stimulate pain fibers

• Frequency (Pulse rate) in Hz: the number of pulses per second, usually pre-set, capacity to modify frequency useful for radiated patients with fibrotic tissue; the higher frequency, the more fatiguing

• Intensity: adjustable in increments of 1; current output; need to be high enough for optimal motor responses; as low as possible for comfort once motor responses are achieved; lower phase duration to increase intensity without increased discomfort or pain

• Ramp Up (seconds): a controlled introductory duration to the maximum amplitude in each cycle

• Cycle On & Off Time (seconds): the length of time the electrical stimulation is on or off, 1:3 to 1:5 (more fatiguing with less off time following on time)

Protocols for different NMES devices are not identical with respect to pre-set programs as well as capacity for modification of parameters (i.e., manual selection of parameters).

8. Physiological target movement triggered as adaption learning 

Ampcare ESP program has supportive research findings. The reduction in time taken for the laryngeal vestibule to close during swallowing with NMES on the submental region based on videofluoroscopic measures was found to be a physiological effect of NMES of swallowing as an output of antagonistic movement to target laryngeal closure movement (Watts & Dumican, 2018). More details can be found at https://swallowtherapy.com/research/).

Following Ampcare ESP protocol, a clinician determines the amplitude for the most effective motor activation based on observed muscle contraction, movement, and the person’s tolerance. This protocol is effective in providing the adequate perturbation to the target laryngeal movement for the client to be potentially involved in motor learning. Patients may initially experience more difficulties to achieve a laryngeal closure in the beginning and gradually decrease the level of difficulty achieving the target movement against the resistance during the swallowing with NMES on the submandibular region. The effect of sensory levels of NMES to the neck skin has an implication for facilitated volitional learning (Ludlow et al., 2007) and possibly for treating patients with post-stroke sensory-motor deficits. 

9.  A recent systematic review on efficacy of NMES on post-stroke swallowing function (Alamer et al, 2020)

In addition to the studies with physiological target movement measures with the use of NMES, more clinical outcome studies have been conducted in the past decade. A limited number of prior reviews of a limited number of studies on NMES for dysphagia might have skewed the efficacy of NMES. Alamer et al. did an updated review of larger numbers of recent randomized controlled trials.

Their systematic review on efficacy of NMES on post-stroke swallowing function revealed that NMES was found to improve the swallowing function of post-stroke patients with dysphagia. In the review, eleven studies randomized controlled trials involving 784 post-stroke patients were analyzed in terms of outcome measures of swallowing function. Primary outcome measures used were penetration-aspiration scale (PAS), video fluoroscopy dysphagia scale (VFDS), functional dysphagia scale (FDS), standardized swallowing assessments, etc. although the selection of protocols and outcome measures varied in the studies. Among the eleven studies, ten studies with NMES only or NMES with traditional dysphagia treatment were found to be improving the swallowing function of post-stroke patients as compared to other interventions.

10. Support for future studies to investigate swallowing motor learning and cortical excitability

There has been an increase in clinical outcome studies incorporating NMES for dysphagia treatment using various NMES devices and protocols. What has been discussed in terms of physiological change and associated clinical outcomes in clinical research studies may be affected by the parameters and protocols utilized as well as the disordered populations. More studies reported the effectiveness of NMES with respect to physiological change and associated motor learning theories and/or potential motor activity in the central nervous system (Oh et al., 2007; Doeltgen et al., 2010). More studies are warranted to examine the predicted physiological change in swallowing using NMES in correlations to other physiological changes. We still need to learn about the effectiveness of NMES for swallowing function in groups with different etiologies other than post-stroke groups.

Research findings do not remain static but they rather evolve. Therefore, clinicians need to revisit and re-validate their understanding of NMES as a dysphagia treatment option based on updated and advanced findings. Why? Because clinicians would need to explain the modality to the client/family as appropriate so that they consider to accept that option as part of care plan if indicated.

NMES are used more frequently in physical medicine and rehabilitation facilities as a rehabilitative technique than in other settings. As pointed out by Shune and Moon, NMES may be first introduced to families in a commercial market or ordered by a physician in the setting of the high prevalence of dysphagia, which is 22% of the population over 50 years age (www.swallowingdisorderfoundation.com). Clinicians should be knowledgeable in this modality and accountable for a decision to be made regarding the use of NMES. At least clinicians should be able to direct patients and families to beneficial resources rather so that patients can benefit from updated research findings and clinical outcomes.

Finally, more education on NMES for dysphagia should be included in the graduate school in order for clinicians to be equipped with basic knowledge (principles of NMES and parameters of stimulation) in selecting a program and device for NMES and training themselves through certification programs. Clinicians should not be barriers to patients’ adoption of NMES in dysphagia treatment because of their misconceptions and lack of knowledge. We should be trained in order to critically evaluate commercially available devices and programs for our patients and implement NMES using a selected NMES program if indicated.

That’s right! More learning for your clients and their functional goals!

References

Humbert IA, Poletto CJ, Saxon KG, Kearney PR, Crujido L, Wright-Harp W, Payne J, Jeffries N, Sonies BC & Ludlow CL. (2006). The effect of surface electrical stimulation on hyo-laryngeal movement in normal individuals at rest and during swallowing. J. Appl Physiol. 101, 1657-1663

https://doi.org/10.1152/japplphysiol.00348.2006

Ludlow CL, Humbert I, Sean K, & Poletto C (2007). Effects of Surface Electrical Stimulation Both at Rest and During Swallowing in Chronic Pharyngeal Dysphagia. Dysphagia, 22: 1-10.

Oh BM, Kim DY, & Paik NJ (2007). Recovery of Swallowing Function Is Accompanied by the Expansion of the Cortical Map. Intern. J. Neuroscience, 117: 1215-1227.

https://www.tandfonline.com/doi/full/10.1080/00207450600936254

Doeltgen SH, Dalrymple-Alford J, Ridding MC, & Huckabee ML. (2010). Differential effects of neuromuscular electrical stimulation parameters on submental motor-evoked potentials. Neurorehabil Neural Repair 24: 519-27. https://journals.sagepub.com/doi/pdf/10.1177/1545968309360417

Shune S & Moon J (2012). Neuromuscular Electrical Stimulation in Dysphagia Management: Clinician Use and Perceived Barriers. Issues In Communication Science and Disorders, 39, 55-68.  

Watts and Dumican (2018). The effect of transcutaneous neuromuscular electrical stimulation on laryngeal vestibule closure timing in swallowing. BMC Ear, Nose and Throat Disorders 18 (5): 1-7. https://doi.org/10.1186/s12901-018-0054-3

Alamer A, Melese H, & Nigussie F (2020). Effectiveness of Neuromuscular Electrical Stimulation on Post-Stroke Dysphagia: A Systematic Review of Randomized Controlled Trials. Clinical Interventions in Aging 15, 1521-1531.

McAdoo R, Campbell R, & Polansky R. Deciphering Dysphagia with Ampcare’s Effective Swallowing Protocol (ESP): Training for the Use of Ampcare’s ESP in the Treatment of Dysphagia

Biber T. PT/ST Swallowing Disorders Program, Cleveland Clinic Hospital.

Clinical Use & Application of Guardian Way training guide.