Muscle ultrasound: Present state and future opportunities

Overview

This invited review by Wijntjes and van Alfen, published in Muscle & Nerve in 2021, describes the current role of muscle ultrasound in neuromuscular disease and what stands in the way of wider use. The authors regard it as a valuable, patient-friendly and non-invasive addition to the clinic and to research, but one that is not yet used to its full potential.

What ultrasound shows

Diseased muscle is replaced by fat and fibrous tissue, which makes the image brighter. Healthy muscle has a typical 'starry night' look on transverse scans. Muscular dystrophies give a diffuse 'ground glass' appearance, inflammatory myopathies show focal or generalised increases in echogenicity and sometimes calcifications or thickened fascia, and long-standing neurogenic disease can produce a 'moth-eaten' pattern. Ultrasound can also detect fasciculations, which makes it useful when motor neuron disease such as ALS is suspected, and it can measure muscle thickness and blood flow.

Ways to assess muscle

Visual assessment has a sensitivity of about 70% and the four-grade Heckmatt scale about 71 to 76%. Quantitative analysis of grayscale or backscatter is the most sensitive method, at up to 92%, but it needs reference values that are specific to each ultrasound device. Ultrasound performs less well in metabolic myopathies and in children under 3 years.

Barriers and future directions

Key obstacles are dependence on observer experience, differences between devices and settings, limited equipment and training, and the lack of regulatory approval for ultrasound as a trial outcome. The authors expect simpler scoring systems, dedicated muscle ultrasound systems, deep-learning image analysis and inclusion in training curricula and guidelines to improve implementation.