Both novice and experienced clinicians may experience confusion, frustration, or both when faced with the inconsistent messages circulating throughout the profession regarding the use of oral motor (OM) treatments. On one hand are the myriad of OM materials and workshops as well as testimonials touting the benefit of using OM treatments; on the other hand are evidence-based practice reviews consistently warning against using these and other untested or unproven treatments.Moreover, even among OM proponents, opinions differ about the rationale for, nature of, and most appropriate application of the various techniques. Because the treatment literature does not provide compelling evidence that OM treatments in general are beneficial, much less when and how OM treatments should be applied, clinicians must rely on other means of determining whether specific strategies may benefit individual clients.
Although the rationale for adopting OM techniques varies depending on the specific techniques and the clients for whom they are selected, I believe a basic philosophy leads to the adoption of any OM treatment. That is, we know that speech and swallowing are motor behaviors (i.e., they involve movement) and that disrupted or immature movement control may interfere with speech and swallowing effectiveness. Accordingly, we might expect that alleviating underlying motor impairments or facilitating motor system development will bring about improved speech and swallowing function. For this philosophy to lead to sound clinical decision-making, clinicians must have a thorough understanding of the nature of neuromuscular impairments as well as the treatments purporting to address such impairments.
Understanding Neuromuscular Impairments
Definitions and assessments
The oral mechanism examination is the primary tool traditionally used to assess the World Health Organization (2001) International Classification of Function level of body structure and function for speech and swallowing. However, there are inconsistencies among SLPs about the nature of tasks included in an "oral mech" exam as well as how observations are interpreted. To avoid potential confusion, I will review briefly the nature of selected neuromuscular impairments along with the assessment strategies employed to identify them. I will also describe the clinical observations that signal the presence of these neuromuscular impairments (see sidebar).
Weakness. A reduction in strength and/or endurance is called weakness. Given that strength typically is defined as the ability to produce force against resistance, it follows that strength assessment must involve tasks that entail exerting force against resistance. For example, in a traditional oral mech exam, the SLP may assess tongue strength by having the client push against a tongue blade while the SLP provides resistance. Typically, the presence of weakness is noted if the client is unable to exert force to overcome some amount of resistance provided by the examiner.
Unfortunately, no standards exist regarding the "some amount" of resistance clinicians should provide; thus the clinician must judge subjectively whether the client exhibits a reduced ability to produce force. It is relevant to note that tasks requiring the client to move an articulator as far as possible in any given direction without external resistance do not measure strength per se. These tasks in fact address range of motion (ROM). Although reduced ROM may result from weakness, a number of other neuromuscular impairments also affect ROM (for example, rigidity), and thus it is best not to use ROM as the primary indication of weakness. Unfortunately, some muscle groups are not readily accessible for the application of resistance (e.g., velum, pharynx, larynx). In these cases ROM may be the only index available for assessing weakness.
Endurance is likewise assessed with resistance tasks; however, in this case the clinician notes how long the client can produce a target level of force over multiple repetitions or during a sustained contraction. The traditional oral mech exam typically does not include endurance measures for specific muscle groups. Instead, clinicians often diagnose reduced endurance if speech or swallowing performance declines over time (for example, over the course of a conversation or meal). Although this inference may well be valid, it does not provide detailed information about the specific muscle groups that are prone to fatigue.
Objective measures of strength and endurance, such as the Iowa Oral Performance Instrument (IOPI, Breakthrough, Inc.), address some of the limitations of the subjective measures described above. Even these tools, however, do not assess strength/endurance of every muscle group of interest during a speech/swallowing examination. Clinicians must thus develop their observation skills in order to note evidence of reduced ability to produce force.
Alterations in muscle tone. Muscle tone is the amount of resistance a muscle group exhibits in response to passive stretch. Thus, contrary to the popular use of the term, muscle tone is not a visual characteristic (e.g., "her triceps appeared well toned") but rather a physiologic response to changes in muscle fiber length.
Although both peripheral and central nervous system mechanisms influence muscle tone, of primary interest to this discussion is the peripheral mechanism because it is the target of most neuromuscular treatments addressing tone. Most striated muscles contain muscle spindles that sense changes in muscle fiber length. When muscle fibers are elongated, sensory endings in the muscle spindle are stimulated initiating a reflexive contraction of the muscle fiber. Known as the stretch reflex, this physiologic response contributes to control of posture and other stabilizing actions such as subconscious maintenance of jaw closure. Spasticity, which is one type of hypertonicity (excessive resistance to passive stretch), is thought to reflect overactivity of the stretch reflex.
Tone is assessed subjectively by the examiner moving the client's articulator (e.g., extending the arm) and gauging the amount of perceived resistance. In this example, stretching the bicep fibers causes a reflexive contraction resisting the passive arm extension. This type of muscle tone assessment is not included in the traditional oral mech exam for the simple reason that the orofacial musculature lacks the clear agonist and antagonist relationships characteristic of the limb musculature. That is, pulling the lips or tongue in any given direction will affect the fibers of a number of muscle groups making it difficult to judge the tone of a specific muscle group. Moreover, muscles of the velum, pharynx, and larynx are largely inaccessible for applying passive movement. Instead, many clinicians assess muscle tone inferentially. For example, the presence of a facial droop may signal hypotonicity whereas slow movements are often thought to reflect hypertonicity.
Another issue confounding the assessment of tone in the orofacial musculature is the role of muscle spindles in these muscle groups. That is, only the jaw-closing muscles demonstrate the density of muscle spindles and pattern of stretch reflexes similar to the limb musculature. The lingual, pharyngeal, and laryngeal muscles do not exhibit the typical pattern of stretch reflexes, even though muscle spindles are present in these muscle groups. Finally, the muscles of the lips and face exhibit an absence or reduced density of muscle spindles and do not demonstrate stretch reflexes. Thus, the regulation of muscle tone in these muscle groups differs significantly from tone regulation in the limbs, which has implications for the applications of selected OM treatments.
Altered sensation. Proponents of OM treatments often cite disruptions in tactile, kinesthetic, and/or proprioceptive sensation as contributing to speech and/or swallowing limitations. Unfortunately, aside from gross assessment of tactile sensation (e.g., two-point discrimination), strategies for assessing these mechanisms are largely undescribed.
Impact of Impairments
Textbook descriptions of a number of speech (e.g., dysarthria) and swallowing disorders implicate weakness and/or alterations in tone as factors contributing to the speech/swallowing symptoms. Unfortunately, even though researchers have documented deficits in strength, endurance, or both, in individuals with dysarthria and dysphagia, the precise relationship between weakness and speech/swallowing function remains unclear. For example, we know that speech and swallowing movements do not require the maximum forces produced by the articulators yet we do not know precisely how much weakness must be present before speech and/or swallowing are affected. We know even less about how disruptions in endurance, muscle tone, and sensation relate to specific speech/swallowing functions. Thus, for the present at least, clinicians must judge subjectively the extent to which neuromuscular impairments are contributing to the speech or swallowing disorder.
Understanding Oral Motor Treatments
A thorough discussion of the full range of currently available OM treatments is beyond the scope of this article. Instead, I will highlight a selected group of commonly used OM treatments. The sidebar below provides a summary of the issues that may influence a clinician's decision to apply particular OM strategies to specific speech or swallowing impairments.
Strength training is arguably the most well-known OM treatment, perhaps because clinicians easily diagnose weakness, particularly of the peripheral articulators. Many of us are familiar with the basic principles of strength training from our own experiences with fitness training. For example, we know that we need to stress our muscles beyond their normal level of exertion (overload). Moreover, as we gain strength, it is necessary to continue to intensify our training level by increasing repetitions or the amount of resistance, reflecting the principle of progression. Finally, the principle of recovery is that we are generally encouraged to exercise any given set of muscles no more often than every other day, which allows the muscle tissue to rebuild between training sessions.
An additional, and quite important, principle of strength training that most of us have practiced, even if we were unaware we were doing so, is specificity of training. Numerous studies have shown that strength gained during exercise does not readily generalize to movements unlike the exercise itself. For example, exercises using high levels of resistance do not improve endurance (the ability to move repeatedly against low resistance). Likewise, exercises moving a muscle group in one fashion (e.g., when the limb/articulator is moving in space, as during a bench press) do not result in changes in strength for movements using the muscle groups in different ways (e.g., when the limb/articulator is stable and the body moves around it, as during push-ups).
When exercise is part of an overall fitness routine, the implication of training specificity is that individuals perform a wide variety of exercises so that gains in strength are not isolated. The significance for rehabilitation, however, is that a strength-training program must use exercises that not only target the specific muscles exhibiting weakness, but also incorporate movements very similar to those used during the target function (e.g., speech and/or swallowing).
When selecting or designing a strength-training program, clinicians should incorporate these principles. Applying the principle of specificity, the SLP should select exercises that closely match the target function. Many of the classic OM exercises (e.g., pushing the tongue against resistance outside of the mouth, sucking on a straw) use movements that are dissimilar from those used during speech and thus, clinicians must evaluate carefully the appropriateness of each exercise.
Ensuring that the client exercises against resistances higher than typically used and then systematically increasing resistance addresses the principles of overload and progression. The principle of recovery is perhaps the most challenging for SLPs to apply because no data exist regarding the recovery time of the speech/swallowing musculature. It is not uncommon for SLPs to instruct clients to complete OM exercises up to five times per day as part of a home program, yet it is unlikely that recovery will occur between such frequent sessions, particularly if the principle of overload is applied appropriately. Unfortunately, until research reveals the most effective recovery period for the speech and swallowing musculature, clinicians have only the limb literature and conventional practices on which to base their recommendations regarding frequency of training.
A final issue SLPs must consider before implementing a strength-training program is the existence of contraindications. Once again, because strength training for speech and swallowing has not been well studied, we must rely on the limb literature to identify relevant contraindications. Historically, clinicians have considered the diagnosis of progressive neuromuscular diseases such as amyotrophic lateral sclerosis or multiple sclerosis a contraindication for strength training due to the expectation that clients with these conditions will not benefit from strength training and will instead experience decreased function related to exercise-induced fatigue. Clients with spasticity also are considered poor candidates for strength training in that exercise is thought to exacerbate hypertonicity.
Interestingly, recent limb studies (e.g., Ashworth, Satkunam, & Deforge, 2004; Badics, Wittmann, Rupp, Stabauer, & Zifko, 2002; Drory, Goltsman, Reznik, Mosek, & Korczyn, 2001; White & Dressendorfer, 2004) have challenged both of these conventions by demonstrating that strength training may benefit clients with spasticity and/or neuromuscular disease without increasing hypertonicity. Unfortunately, no studies have yet explored the effect of strength training in the speech/swallowing musculature on muscle tone or the cost versus benefit of strength training for clients with neuromuscular disease.
A final, and seemingly obvious, contraindication for strength training is the absence of weakness. Most speakers possess more strength than is necessary for adequate speech, making it unlikely that increasing strength of healthy muscle groups will benefit the client.
Evidence supporting the use of strength training for improving speech production is largely lacking, even for those conditions (e.g., dysarthria) where weakness is characteristic. The most promising evidence supporting strength training has come from dysphagia treatment research, but even this literature provides only the lowest levels of evidence (e.g., case studies and small controlled studies). Given this, clinicians must assess critically whether adequate rationale exists for incorporating strength training in an individual client's comprehensive treatment program.
Quick stretch, tapping, and vibration
The following OM treatments have similar therapeutic mechanisms: Both quick stretch (rapid passive ROM) and tapping stimulate the muscle spindle to elicit a stretch reflex and thus increase muscle tone. Vibration also stimulates the muscle spindle; however, the resulting contraction of the stimulated muscle is the tonic vibratory reflex. As discussed earlier, only the jaw-closing muscles demonstrate muscle spindle actions similar to those of the limb musculature. Therefore, we would not expect these treatments to increase muscle tone of the labial, lingual, velar, or other speech muscles. Moreover, most physical rehabilitation texts caution against applying vibration to the face because of the risk of skin breakdown. Given the lack of physiologic rationale for using these tone-altering OM treatments, it is not surprising that virtually no studies have examined their potential application to the management of speech/swallowing disorders and thus no evidence is available to support their use.
Both heat and cold have been shown to improve limb movement following injury or disease. Heat raises the threshold for pain, allowing clients to move farther without pain or pain-induced muscle spasms. Although heat treatment likely has limited application for neuromuscular speech and swallowing disorders, which typically are not associated with pain, the application of heat may be beneficial for clients with pain related to head and neck cancer treatments or temporomandibular joint syndrome.
Cold may also improve ROM as well as movement control by reducing spasticity. By decreasing nerve conduction velocities, cold reduces the intensity of stretch reflexes. Given the lack of stretch reflexes in most of the speech/swallowing musculature, as well as the potential for cold treatment to interfere with volitional contractions, it is unlikely that cold treatment will be a widely applicable OM treatment.
A final caution regarding both heat and cold treatment is that, due to the risk of tissue damage, they should be used only with clients who can report discomfort. As was true for the other tone-altering treatments reviewed in this article, I know of no research that has explored the application of heat or cold for managing neuromuscular impairments accompanying speech/swallowing disorders. Although a number of studies have described the use of thermal stimulation for heightening sensory responses related to triggering the pharyngeal swallow response, these effects of cold on sensory afferents are distinct from any neuromuscular effects.
Evidence-based practice mandates that clinicians select management strategies based on the highest levels of evidence available. At present, the highest levels of evidence do not include support for OM treatments in the management of speech disorders, and thus many clinicians may appropriately reject the use of these techniques.
Other clinicians may seek to contribute to the evidence base by investigating the benefits of OM treatments, the most appropriate treatment parameters (e.g., exercise frequency, timing and duration of thermal stimulation), and/or the most appropriate recip ients of these treatments (e.g., clients with peripheral vs. central nervous system lesions). These clinicians can prepare themselves to evaluate critically the potential benefits of OM treatments by gaining understanding about the unique nature of neuromuscular impairments affecting the speech/swallowing musculature, the physiologic mechanisms of neuromuscular treatments and their potential application as OM treatments, and the potential risks and contraindications of the various OM therapies.
Armed with this knowledge, clinicians and researchers will be better able to provide sound rationale for the adoption of selected OM treatments, design treatment protocols that incorporate neurophysiologic principles, and critically assess the benefits for individual clients.