Pathophysiologic Perspectives on Muscle Tension Dysphonia

It was more than 60 years ago that the concept of a physiologic linkage between the respiration, phonation, articulation, and resonation subsystems was proposed; that excess tension originating at any level of this unifi ed speech production mechanism may result in referred hypertonicity and hyperfunction throughout this linear anatomical chain of musculoskeletal structures [1,2]. Since this seminal hypothesis, there have been countless investigations on the psychodynamic and pathophysiologic processes observed in individuals with hyperfunctional speech and voice disorders [3-17]. These inquiries have spawned numerous theoretical discussions about the most effective methods of differential diagnosis and treatment of these conditions.


Introduction
It was more than 60 years ago that the concept of a physiologic linkage between the respiration, phonation, articulation, and resonation subsystems was proposed; that excess tension originating at any level of this unifi ed speech production mechanism may result in referred hypertonicity and hyperfunction throughout this linear anatomical chain of musculoskeletal structures [1,2]. Since this seminal hypothesis, there have been countless investigations on the psychodynamic and pathophysiologic processes observed in individuals with hyperfunctional speech and voice disorders [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17]. These inquiries have spawned numerous theoretical discussions about the most effective methods of differential diagnosis and treatment of these conditions. Individuals who exhibit forceful or excessive contractions of muscles that drive voice and speech production often struggle with a broad spectrum of associated abnormal signs and symptoms, including 1) harsh-shrill vocal quality, 2) limited pitch and volume control, 3) falsetto pitch breaks, 4) episodic arrests of phonation, 5) reduced maximum phonation time, 6) vocal fatigue, and 7) extrinsic laryngeal, intrinsic laryngeal, mandibular, and tongue musculature hypertension. When these abnormalities are not caused by obvious signs of phonation subsystem pathology or tissue trauma, the diagnosis of primary or non-phonotraumatic H-MTD is descriptively appropriate.

Abstract
It is diffi cult to quantify the incidence of hyperfunctional muscle tension dysphonia (H-MTD). Although voice disorders in general have been noted to affect approximately 10% of the population in the United States, up to 40% of patient visits to voice specialists are for symptoms of H-MTD. Clearly this condition is a very common vocal pathology, yet its differential diagnosis is not always straightforward. The primary purpose of this tutorial is to present a comprehensive literature review on this condition, with particular focus on alternative etiologic theories and differential diagnostic and treatment techniques. The secondary purpose of this tutorial is to increase awareness that this disorder is a complex condition with highly variable causes, features, and treatment requirements. As a consequence of recent advances in the fi eld of neurolaryngology, and for completeness, the neurological substrates of voice and speech production are also discussed in this review. This information is included to evoke a theoretical conversation about possible patho-neurologic correlates in some patients with H-MTD; especially those who do not respond favorably to standard behavioral therapy strategies. Whereas only a brief discussion of treatment options is rendered in this paper, more detailed information on this topic will be covered in a forthcoming companion tutorial. In contrast, organic or phonotraumatic H-MTD is the diagnosis ascribed to patients whose vocal hyperfunction results in the formation of benign unilateral or bilateral vocal fold lesions, such as nodules, discreet polyps, or Reinke's edema [18][19][20]. Figures 4,5, and 6 illustrate videostroboscopy fi ndings from a 58 year old female who initially presented to our voice laboratory with harsh, shrill, husky vocal quality, with associated signifi cant limitations in pitch and volume range and control. She exhibited hallmark Type-A personality characteristics, with associated aggressive voice behaviors in all social settings; she also struggled with chronic sleep apnea.
Note that Figure 4 shows an advanced degree of bilateral Reinke's edema, vocal fold erythema, and airway dimension compromise. These fi ndings easily explain the chief presenting vocal and sleep related complaints, and they are consistent with the patient's voice abuse history.  In some patients with either the non-phonotraumatic or phonotraumatic subtype of MTD, co-existing respiration and/or articulation subsystem musculature hyperfunction exacerbates the voice problem. It has been estimated that approximately 50 % of patients evaluated by otolaryngologists for voice complaints present with characteristics of hyperfunctional dysphonia [21].
The overall purpose of this paper is to render a comprehensive review of the scientifi c literature on H-MTD.
Specifi c focus will be placed on 1) the essential neurological substrates of voice and speech production, 2) differential diagnosis of pathologic variants of this vocal pathology, and 3) treatment considerations. A forthcoming companion tutorial will provide detailed descriptions of alternative treatment strategies that have been reported for this condition over the past 5 decades, along with specifi c recommendations for future clinical research investigations.
Voluntary activation and operation of the laryngeal peripheral biofeedback system, largely depend upon descending motor stimuli from the corticobulbar tracts of both frontal lobes of the brain to the motor neuron pools of the Vagus nerve within the nucleus ambiguous of the medulla; synchronous corticospinal tact stimuli are concurrently transmitted to spinal nerves along the spinal cord so that cooperative breath support can accommodate all voice activities. Together these upper motor neuron tracts comprise the pyramidal system, and they are the primary stimulants of all voluntary voice and speech musculature contractions. Parallel sensory pathways, originating in the nerves and mechanoreceptors within the musculature, joints, and mucosa of the peripheral end organs, project to the brainstem, subcortical way-stations, and parietal lobes of the brain to complete the speech mechanism sensorymotor neuroanatomic loop.

Sensory-motor feedback
Although there is little debate about the existence of these neural pathways, neuroscientists and voice specialists are not certain that acts of voluntary phonation require ongoing monitoring or regulation by this servo system. Studies have clearly demonstrated that even in the absence of sensory feedback from the larynx subjects are usually able to maintain gross motor voice ability with minimal disturbances in pitch, volume, or vocal quality control [30][31][32][33]. These results support the hypothesis that normal voice production depends more on "open-loop" feedback operations than "closed loop" principles; the former process functioning independent of sensory information from the larynx or its central nervous system coordinates. The balancing effects of the auditory feedback system have been characterized as signifi cant contributors to the preservation of near normal voice ability under laryngeal deafferentation conditions [34,35]. We would suggest that whereas sensory feedback may not normally be required to initiate and sustain voice, such continuous information from the end organ likely provides on-line input regarding the functional status of vocal fold vibrations and whether there is a need for corrective actions to achieve all target speech behaviors. Such sensory assurances or warnings may increase the overall physiologic effi ciency of contextual speech production, even if such feedback only serves as an ancillary assistant when necessary. We further propose that prolonged disturbances in or dysregulation of this volitional-refl exive neuromuscular circuit within and between the various subsystems of the speech mechanism, regardless of the associated etiology, may be responsible for motor speech diffi culties, including some variants of H-MTD. This hypothesis may be especially testable in individuals with this vocal pathology who have no history of psycho-emotional confl icts and for whom standard behavioral therapy paradigms are ineffectual. In support of this line of reasoning, several researchers have suggested, based on their clinical experiences and experimental investigations that the larynx can accomplish primary motor functions because of the indispensable feedback support it receives from its 001-0010. DOI: http://doi.org/10.17352/2455-1759.000065 sensory and autonomic properties [36][37][38]. Additionally, there is an abundant data base on patients with irritable larynx syndrome, owing to depressed or heightened neurosensory thresholds or sensitivity. This clinical population frequently presents with one or more of the following disorders: Chronic cough, paradoxical vocal fold motion abnormalities, incessant throat clearing, laryngospasms, and H-MTD [39][40][41]. These conditions are often attributable to viral upper respiratory infections, laryngopharyngeal refl ux events, chronic cigarette use or exposure, protracted use and misuse of steroidal inhalers, direct tissue trauma, and psychogenic factors. It has been suggested by these researchers that future treatment programs for this spectrum of disorders may need to target the laryngeal sensory-refl exive feedback system, at least for the subset of patients who are recalcitrant to behavioral therapies of one type or another. A condition labeled laryngeal tension-fatigue syndrome was described 15 years ago in more than 300 patients with chronic functional dysphonia due to vocal abuse and misuse [46,47]. Most of these subjects exhibited stroboscopic evidence of multiple vocal fold cover abnormalities, owing to hyperfunctional muscle tension voice behaviors, and biomechanical tissue pathologies secondary to associated phonotrauma. It is interesting to speculate whether or not these patients struggled with inherent bioenergetic weakness of the intrinsic laryngeal skeletal muscles, which in turn resulted in these pathophysiologic sequelae. Whether or not laryngeal EMG and speech aerodynamic testing would have shed additional light on this hypothesis is an interesting question, which deserves further investigation.  [53,54]). This multi-factorial neurobehavioral childhood disorder affects as many as 10% of all school-age children. It has been defi ned as a behavioral disorder of selfregulation, causally associated with dysfunction of the frontalsubcortical system and dysregulation of both dopamine and noradrenaline neurotransmission [55]. Along this line of reasoning, we suggest that for certain subsets of patients with H-MTD there may be an important role for comprehensive psychological and neurological evaluations in their diagnostic battery. We also suspect that emerging advancements in our understanding of the neurology of voice production will eventually lay the groundwork for more inclusive subspecialty EMG signals from supra and infrahyoid muscles [56]. Results of these studies did not demonstrate consistently abnormal EMG patterns during various speaking tasks in this clinical population. Use of EMG as a reliable diagnostic tool was not supported by these fi ndings. warming, vibrotactile stimulation, counterbalancing resistance exercises, tonal reduction via stretching maneuvers, tendon pressure, and tendon tapping), to suppress aberrant muscle patterns and excite normal adaptations, and 5) laryngeal sensori-motor integration therapy [32,[71][72][73][74][75][76][77]. We propose that any of these treatments, either alone or in combination with one another, may promote several important synergistic physiologic benefi ts, including 1) rebalancing central and peripheral neuromotor and neurosensory processes necessary to support relaxed and continuous vocalization efforts, 2) increasing kinesthetic and proprioceptive awareness of muscle tension levels during coordinated speech activities, 3) breaking the habitual cycle of hyperfunctional laryngeal and/or orofacial muscle behaviors, 4) jump-starting or re-setting integrative and controlled speech mechanism musculature contractions via manipulative muscle tone and strength exercises.

Treatment considerations
The foundation for these interventions and proposed mechanisms of action is constructed from an abundance of evidenced-based material within the motor speech therapy, physical therapy, occupational therapy, psychotherapy, and neurorehabilitation literature [78]. In general, all of these treatments are essentially designed to stimulate or inhibit the sensory, motor, and autonomic central and peripheral neural pathways and end organs that normally control and regulate voice and speech production. In our forthcoming companion tutorial, these approaches to management will be discussed in detail and they will augment our coverage of the most commonly employed behavioral voice therapy techniques for H-MTD.

Conclusions
The primary objective of this tutorial was to produce a comprehensive overview of the world literature on hyperfunctional voice disorders. To facilitate a deeper understanding of the multifactorial pathophysiology of these conditions, we included a brief review of the important neurophysiologic correlates of normal speech and voice production. This information was included to reinforce our understanding of the complex volitional and refl exive neuromotor and neurosensory processes that are normally required to induce phonation and regulate smooth vocal transitions during conversational speech. Within this context of neurolaryngology, the condition classifi ed as non-phonotraumatic H-MTD was reviewed in detail. We demonstrated that whereas the etiology frequently varies from patient to patient, most patients with this suspected clinical diagnosis present with predominantly pressed or strained vocal characteristics that are often accompanied by choppy breathing patterns, episodic arrests of phonation, and vocal fatigue. We also highlighted the causal relationship between psychological stress and this diagnosis in many individuals. We cautioned that failure to recognize this well-documented fact may result in faulty or incomplete diagnostic testing, inaccurate diagnoses, and poor treatment outcomes. Focus was also placed on the importance of identifying many other possible causal associations, including laryngopharyngeal refl ux, underlying laryngeal sensory-motor abnormalities, and occult end organ pathologies (eg., vocal fold paresis or sulcus formation), to ensure comprehensive knowledge of all contributing factors and the development of etiology-based treatment programs. Testing strategies, including various speaking tasks for auditory-perceptual voice appraisal, neck and laryngeal palpation, and laryngeal videostroboscopy, were briefl y described as essential techniques for differential diagnosis of this condition. The potential adjunctive diagnostic value of quantitative acoustic analyses and speech aerodynamic testing was also discussed. Although we reinforced the fact that behavioral voice therapy continues to be a mainstay of treatment for the vast majority of patients with H-MTD, we also cautioned that alternative approaches to management may be necessary for those patients who are recalcitrant to traditional treatments. Theoretical discussions were rendered relative to why this latter subset of patients may not always respond to standard therapeutic measures. Finally, we suggested that future research directions will be addressed in greater detail in a forthcoming companion tutorial on type-specifi c treatments for this multi-varied vocal pathology.