Objective Sonographic Measures for Characterizing Myofascial Trigger Points Associated With Cervical Pain

Myofascial pain syndrome is a substantial health problem in the United States, believed to affect about 23 million Americans.1 Myofascial pain syndrome is a very common, complex, yet poorly understood form of neuromuscular dysfunction consisting of motor and sensory abnormalities. It is a major progenitor of nonarticular local musculoskeletal pain and tenderness that affects every age group and is commonly recognized as “muscle knots.”2 Myofascial pain syndrome is associated with numerous pain conditions, including radiculopathies, joint dysfunction, disk abnormalities, tendonitis, and many others.3 It is characterized by myofascial trigger points, which are discrete hypersensitive hard palpable nodules located within taut bands of contractured skeletal muscle. Myofascial trigger points can be found by palpation of the soft tissue performed by a trained examiner and are painful on compression. The local twitch response is an important clinical finding that confirms the presence of a trigger point. The local twitch response is a quick, localized contraction of muscle fibers produced by strumming or snapping the taut band in a direction perpendicular to the muscle fibers. Trigger points are classified clinically as active or latent.4 An active myofascial trigger point causes spontaneous pain and may often cause general motor dysfunction (stiffness and restricted range of motion). A latent myofascial trigger point often causes motor dysfunction without pain. Pain associated with latent myofascial trigger points requires firm palpation or a mechanical stimulus to elicit.5 Otherwise, latent myofascial trigger points have all of the characteristics of active myofascial trigger points, although usually to a lesser degree.3,6 Although the specific pathophysiologic basis of myofascial trigger point development and symptoms is unknown, several promising lines of scientific study (ie, histologic, neurophysiologic, biochemical, sonographic, and somatosensory) have revealed objective abnormalities.7–13 Furthermore, our group has found that differences among active, latent, and normal sites may be evaluated objectively using diagnostic sonographic techniques, such as gray scale (2-dimensional) sonography, vibration sonoelastography, and Doppler imaging.12 Most of the investigational work on myofascial pain secondary to myofascial trigger points has involved human patients because no successful animal model has been found to elucidate how these nodules arise or how to treat them in a controlled setting. Furthermore, there are very few studies describing objective and clinically applicable methods for identifying and classifying myofascial trigger points. The few that do exist have attempted to externally quantify painful regions using electrodermal properties14 and superficial soft tissue stiffness measurements.15 Although both electrical resistance and tissue stiffness significantly changed at areas where myofascial trigger points existed, neither was able to distinguish between tissue properties of active and latent trigger points. Electromyography has also been used to measure end plate noise at myofascial trigger points before and after treatment with an acupuncture needle.16 Electromyography has shown that end plate noise at trigger point sites decreased as pain decreased after acupuncture treatment. However, only active trigger points had a local twitch response during needling.16 Human in vivo microdialysis studies of the upper trapezius muscle have found that active sites have a unique biochemical milieu compared to latent sites and palpably normal muscle. Active sites have a more acidic milieu and higher levels of inflammatory mediators, neuropeptides, catecholamines, and cytokines—substances known to be associated with persistent pain states, inflammation, and sensitization.9,10 Other noninvasive methods are currently being explored to visualize and characterize myofascial trigger points via magnetic resonance elastography.17 Magnetic resonance elastography is attractive because magnetic resonance imaging is the diagnostic standard for musculoskeletal imaging; however, it is an expensive and less accessible method compared to other imaging modalities such as sonography. Sonography is a readily available, portable, and inexpensive imaging modality, suitable for use in a physiatrist’s office to complement physical examination, guide therapeutic interventions, and evaluate treatment outcomes. Previously, our group showed the feasibility of sonography for visualizing trigger points and the surrounding soft tissue.12 We were able to visualize trigger points and score them on an ordinal scale using gray scale B-mode images, color variance elastography, and Doppler waveforms of local blood flow from 9 patients. The goal of this study was to expand on these preliminary findings to study a larger set of patients and develop quantitative measures of the soft tissue environment of myofascial trigger points. We hypothesize that the soft tissue environments of active trigger points, latent trigger points, and normal muscle differ as measured by sonoelastography and Doppler imaging. We also wanted to explore the sensitivity of these measures to change after a common treatment, dry-needling therapy, to evaluate the feasibility of this method for monitoring posttreatment clinical outcomes.