Muscle Ultrasound

It is fitting to start with muscle because this is the portion of the peripheral nervous system first studied by ultrasound. Here it is helpful to distinguish neuromuscular ultrasound, which encompasses the primary disorders of nerve and muscle, from musculoskeletal ultrasound, which covers bone, tendon, and unsubtle secondary disorders of muscle such as hematoma, trauma, calcification, lacerations, tears, and neoplasms, disorders in which patients can typically point to where something is wrong because it is tender, painful, or swollen. The challenge of neuromuscular ultrasound imaging is knowing where to look for pathology because neuromuscular disorders often manifest in nerve segments and muscles not readily apparent to patients and referring physicians. Clinical expertise in localization is limited to a small group of neuromuscular/electrodiagnostic specialists, and it is therefore these individuals, who are likely to be accomplished practitioners of neuromuscular ultrasound.

At first glance, it may seem that secondary, musculoskeletal disorders of muscle, because of their frequency in the general population, might significantly outweigh the prevalence of primary disorders of muscle. However, neuromuscular causes of muscle disease are quite common as well. The most common, although benign, neuromuscular finding detectable by ultrasound is likely to be distal (benign) fasciculations of muscle, which may occur in up to 43% of the general population (see Chapters 3 and 10). Neurogenic changes in distal muscles caused by significant diabetic neuropathy are likely to be detectable by ultrasound in 1.5% of the general population. There are also a variety of other neurogenic causes of muscle atrophy or altered echogenicity including radiculopathies and compression mononeuropathies. Less common are primary diseases of muscle, the myopathies. Regardless of one’s specialty orientation, there is ample demand for both musculoskeletal and neuromuscular ultrasonographers.

Ultrasound is likely to change the evaluation of muscle disease in several ways. Based on trends in the literature, ultrasound will become an increasingly quantitative tool in the study of muscle (Fig. 13.2). Its simplest and most obvious use will be measuring muscle size.³ Currently normative data are available for muscle thickness,^4–7 and it seems likely that accurate estimates of muscle volume, either by the development of nomograms, linear measures, or by improved experience using three-dimensional (3D) ultrasound, will further enhance the ability to assess normal muscle size, atrophy, and hypertrophy. As effective therapies for wasting disorders of nerve or muscle are discovered, ultrasound measurements of muscle size may become a useful biomarker of disease progression.⁸

Fig. 13.2 A graph of the percentage of any article on nerve and muscle devoted to ultrasound findings versus electrodiagnostic findings as of mid-2009. Note the increase in the relative number of publications on ultrasound imaging that parallels the evolution of technology in this field.

One interesting paper has suggested that the thickness of distal muscles in patients with diabetic neuropathy correlates robustly with motor response amplitudes.⁹ One implication of this study, and the common experience of finding low compound muscle action potential amplitudes in atrophic distal muscles, is that axon loss, currently measured by loss of compound muscle action potential amplitude, may also be measurable by relative atrophy in distal muscles by ultrasound. If so, it may be possible that ultrasound imaging could reduce the need for uncomfortable nerve conduction testing in situations in which demyelination is unexpected or in patients who cannot tolerate electrical stimulation, particularly if serial nerve conduction studies are needed for follow-up.

A number of lines of research have established that, in addition to atrophy, increased echogenicity is also a sensitive marker of muscle disease.^10–13 Increased echogenicity is helpful in the descriptive classification of some myopathies (see Chapter 10), and further refinement in this approach, perhaps by using quantitative measures, is likely to expand such applications. Echogenicity may also be a particularly robust indicator of disease severity in spinal muscular atrophy (SMA), a disorder primarily found in young adults and children, individuals who may be reluctant to undergo electrodiagnostic testing, particularly serial testing.¹⁴ It can be hoped that user-friendly methods of measuring muscle echogenicity will be available on the next generation of ultrasound instruments to simplify clinical studies investigating muscle disease. Of perhaps greater interest to those not specializing in neuromuscular medicine, muscle echogenicity may also relate to muscle fat content. With aging, muscle storage of fat may be an indicator of susceptibility to vascular disease, and ultrasound of muscle may be a simple noninvasive technique to evaluate patients and improve systemic health through dietary change.

Another area of development in muscle imaging, one that fits in with an evolving field of electrodiagnosis, is the use of ultrasound in the evaluation of pelvic floor muscles.^15–17 Ultrasound can not only assess thickness and echogenicity of pelvic floor muscles but also provide information regarding needle placement, muscle movement, muscle injury, residual urine volumes, bladder neck mobility, urethral integrity, prolapse, and other structural problems. It is likely this technique will prove complementary to electrodiagnosis in the evaluation of patients with bowel and bladder dysfunction.

Considerable interest has focused on measuring the thickness of truncal and paraspinal muscles,^18–22 in large part in an attempt to assess exercise programs that focus on core strengthening and their role in managing chronic back pain. For those interested in measuring the outcomes of certain types of physical therapy, ultrasound offers a noninvasive tool for quantifying exercise effects.

More advanced capabilities of ultrasound allow for the observation of muscle movement. Current instruments are sufficiently sensitive to detect fasciculations.^23–26 Optimal parameters for identifying fibrillations have not yet been worked out^27–28 but it seems likely that instrument-specific enhancements of frame rate, resolution, and display will make it possible to better image fibrillation potentials in muscles, reducing the need for electromyography in select patients.