Ultrasound
Ultrasound (US) is a well-established and validated diagnostic imaging method in the evaluation of patients with suspected muscle disorders [10]. It is a relatively cheap and easily applicable method, allowing the visualisation of striated muscle with a high temporal resolution (>0.1 mm). The major advantage of US is the lack of any radiation exposure, which makes it the perfect imaging method for the evaluation of children. It even allows dynamic imaging of contracting muscles and can visualise pathological muscle activity such as fasciculation [11–13].
A major drawback of US is that its application is limited to superficial muscle groups. Sound wave reflection and absorption lead to difficulties in displaying the deeper structures. This effect becomes even more pronounced when multiple muscle groups overlap. Another disadvantage is the relatively low inter-observer agreement and intra-observer agreement (depending on the level of experience), which makes a strictly standardised examination (e.g. according to certain anatomical landmarks) crucial [2, 14]. However, ultrasound is a reliable method concerning the measurement of muscle thickness and muscle echo intensity. In addition, by measuring muscle echo intensity it is possible to further characterise age-related or pathological changes of the striated muscles with special regard to dystrophic changes in terms of fatty degeneration and replacement of muscle by connective tissue (Fig. 1). In order to quantify the degree of fat deposition there are several ratings scales available (e.g. the Heckmatt score) as well as computed-assisted quantification methods of muscle echo intensity [15–19].
Fig. 1 Ultrasound images of the quadriceps muscle in normal control (a) and a patient with Duchenne muscular dystrophy (b). In the healthy subject (a), muscle appears largely black with few perimysial septa. Note the increased homogeneous fine granular echogenicity of muscle due to increased replacement of normal muscle by connective and fatty tissue in the patient with muscular dystrophy (b)
Ultrasound applications are widely and routinely used in neuromuscular disorders in terms of assessment of changes in muscle morphology (atrophy, hypertrophy, changes in muscle architecture). In particular it is a useful screening tool during the initial diagnostic phase, especially in children. Depending on the disease entity, the sensitivity of detecting dystrophic changes ranges from 25% in non-dystrophic myopathies up to 100% in dystrophic myopathies (Duchenne muscular dystrophy) [10, 19, 20]. The detection of pathological changes can be helpful in guiding muscle biopsy, and the description of the muscle involvement pattern might help in the differential diagnosis [10, 21].