Neuromuscular ultrasonography: quantifying muscle and nerve measurements

Abstract

Neuromuscular ultrasonography can be used both descriptively and quantitatively in the evaluation of patients with neuromuscular disorders. This article reviews the quantitative use of this technology, particularly measurements of the size and echogenicity of nerve and muscle, as a tool for assessing the severity, progression, and response of these tissues to therapeutic interventions. Neuromuscular ultrasonography has several features, including portability and noninvasiveness, that make it an attractive research tool for advancing the diagnosis and treatment of neuromuscular disorders.

Fig. 1

Two identical cross-sectional images of the extensor digitorum brevis (EDB). On the left, the muscle (EDB), which is just above a small artery (A), is fully relaxed, and the crosses show the thickness at 2.8 mm. On the right, the muscle is fully contracted, displacing the artery, with a thickness of 5.1 mm.

Fig. 2

The percentage increase in diaphragm thickness after a dedicated exercise program in (A) 10 patients with cystic fibrosis, (B) 6 healthy controls, (C) 10 healthy controls, and (D) 4 healthy controls. Although the N is small, the results from different studies show good concordance.

Fig. 3

Bar graph using a (A) standard size scale and (B) a log scale. Note that the log scale adjustment significantly diminishes the apparent magnitude of the difference. Use of B-mode imaging, by displaying amplitude as brightness, tends to reduce perceived echo intensity differences because of the different way the human brain processes size versus illumination.

Fig. 4

(A) Normal cross section through the tibialis anterior of a healthy adult showing the tibia (TIB), a faint outline of the fibula (FIB), and a prominent central aponeurosis (AP) in the muscle. At the top of the image is a very thin layer of subcutaneous tissue (SC) with arrows pointing to the upper edge of the muscle. (B) Cross section of the tibialis anterior from a young adult with hereditary motor and sensory neuropathy. The subcutaneous tissue (SC) layer is thicker with the muscle edge deeper (arrows), the fibula (FIB) and interosseous membrane (IM) that connects the fibula and tibia (TIB) show markedly reduced depth of the muscle. The muscle is distinctly more echogenic, which tends to obscure the bone edge of the tibia, with a sort of moth-eaten appearance more typical of neurogenic than myopathic change.

Fig. 5

Cross-sectional image of the median gastrocnemius muscle (MG) with a thin layer of subcutaneous tissue (SC) above it. The vertical line through the center of the muscle is displayed below, using M-mode (motion mode) ultrasonography. The 1 line of ultrasound data, from a single transducer element from a linear array of multiple elements, is displayed below as it changes over 4 seconds. At about half way during this 4-second epoch, a fasciculation occurs in the muscle, marked by a focal perturbation that lasts 253 milliseconds. Both contraction (thickening) and relaxation is seen during this transient motor unit discharge. The duration exceeds that of electrically recorded fasciculations because ultrasonography captures the mechanical behavior of muscle, whereas electrodiagnostic measurements only capture the brief change in muscle membrane potential.

Fig. 6

(A) Cross-sectional image of the wrist showing a markedly enlarged and somewhat flattened median nerve. The cross-sectional area of the nerve (green tracing) is 31 mm², which is about 4 times the normal size. Note that the nerve is hypoechoic, particularly when compared with the tendons immediately below it that are much brighter. (B) Sagittal view of the median nerve using color flow Doppler. Note the linear blue elements within the nerve, which demonstrate a stable pulsating blood flow within it, a finding not seen in normal healthy nerves. There is random transient noise in nearby structures such as the tendons (T).