Quantifying orofacial muscle stiffness using damped oscillation

Muscle stiffness can reflect muscle tone, often presumed to be aberrant in persons with dysarthria. This exploratory study used the Myoton-3 to assess stiffness of the lateral tongue and mid-cheek in 10 participants with various neurologic disorders--primarily lower motor neuron (n = 6), primarily upper motor neuron (n = 4), and neurolsgically normal adults (n = 4). The Myoton delivered a 25-ms pulse perturbation to the surface of the structure of interest and sensed the response with an internal accelerometer. The resulting acceleration curve was used to determine frequency of oscillation and decrement of damping; stiffness was derived from the linear displacement of tissue perforce of the perturbation. Tongue stiffness was significantly lower for the LMN group than for the normal control group, consistent with the assumption that hypotonia accompanies flaccidity. Tongue stiffness did not differ for the UMN group, nor did cheek stiffness, oscillation frequency or decrement differ between any groups. These preliminary findings indicate that stiffness can be determined from the surface of the tongue and cheek, and may be indicative of low muscle tone in LMN lesions. Although methodologic challenges remain, this novel approach has the potential to quantify orofacial muscle stiffness and document potential changes in muscle tone with disease and treatment. Normal muscle tone reflects a well-functioning neuromuscular system, with balanced central nervous system (CNS) inhibition over peripheral nervous system (PNS) reflexes and tonic activity. Abnormal muscle tone underlies motor dysfunction, such that PNS impairment interrupts normal reflex loops, and CNS impairment releases inhibition to lower motor neuron pools. Thus, hypotonia accompanies flaccidity, and hypertonia accompanies spasticity. Disorders of the basal ganglia and cerebellar control circuits are associated with disrupted muscle tone as well (hypotonia in ataxia, hypertonia in hypokinesia, and hypertonia or variable tone in hyperkinesia). Disordered muscle tone is thought to underlie many types of dysarthria (Darley, Aronson, & Brown, 1975; Duffy, 2005), yet empirical data addressing the influence of muscle tone on speech production are largely lacking. Clinical methods for assessing muscle tone in the orofacial musculature are rarely used. Only two clinical assessments for muscle tone exist for clinical use in speech-language pathology, to our knowledge. Both are subjective in nature, and neither is accompanied by supporting data (Beckman, 1988; Dworkin & Culatta, 1996). Generally, they involve passively stretching or palpating the structure of interest and rating whether resistance to the perturbation is lower or higher than normal. Tone is operationally defined as passive resistance to stretch or palpation, and can be quantified as stiffness. Two instruments, currently only available for research applications in the United States, have been applied to the assessment of orofacial stiffness. One, the OroSTIFF, was designed to assess stiffness of the perioral structures (tissues around the mouth opening, including the orbicularis oris muscles) (Chu, Barlow, Kieweg, & Lee, 2010). A pneumatically operated scissor cantilever stretches the corners of the relaxed mouth laterally, and dynamic stiffness is calculated as the change in force across a series of displacements. Preliminary data support the reliability and validity of the assessment for neurologically normal adults, and the presence of increased stiffness (rigidity) in an adult with Parkinson's disease. A second instrument, the Myoton, operates according to a damped oscillation model. The Myoton delivers a brief pulse perturbation with a thin probe to the surface of the structure of interest, and senses the resulting oscillation with an accelerometer (Vain, 1995). Veldi et al. (Veldi, Vasar, Hion, Kull, & Vain, 2001; Veldi, Vasar, Hion, Vain, & Kull, 2002; Veldi, Vasar, Vain, & Kull, 2004) used the Myoton-2 to generally reveal increased stiffness and decreased elasticity of the tongue and soft palate in middle-aged adults with obstructive sleep apnea compared to adults with no symptoms (i.e., non-snorers). Stiffness was inferred from the frequency of oscillation (in Hz) in response to an 8-ms pulse perturbation, and elasticity was inferred from the logarithmic decrement of damped oscillation; both were calculated from the acceleration curve. Although the current Myoton output includes direct stiffness values (in N/m), these were not reported in the studies by Veldi et al. This preliminary study explores the utility of the Myoton for assessing orofacial muscle tone in persons with neurological disorders that can result in dysarthria or dysphagia. Purposes of this investigation were to establish standard procedures for the assessment of lateral tongue and cheek stiffness in persons with normal and disordered neurologic systems. Specifically, we sought to demonstrate that measures of tissue stiffness could be obtained and reliably repeated from the tongue and cheeks of neurologically normal individuals. Further, we explored tongue and cheek stiffness in persons with upper motor neuron (UMN) or lower motor neuron (LMN) lesions to examine whether resting muscle stiffness varies across disorders as predicted. Finally, in participants with unilateral impairments, we explored differences in tissue stiffness between the weak and normal sides. In persons with LMN damage, we expected to find lower stiffness on the affected side, and in cases of UMN lesions, we expected higher stiffness on the affected side.