Ultrasound Beyond Joints: A Review of Extra-Articular Applications in Rheumatology

Abstract

Purpose of review: This review highlights key ultrasound applications for evaluating extra-articular involvement in rheumatic diseases, including the lungs, vessels, salivary glands, muscles, nerves, skin, and nails. It explores recent advances, emerging areas of assessment, and future research directions. Additionally, the review examines current limitations in the routine use of ultrasound for these purposes and considers the potential of new technologies, such as shear-wave elastography, contrast-enhanced ultrasound, and artificial intelligence, to enhance the early detection and monitoring of extra-articular manifestations in rheumatic diseases.

Recent findings: Extra-articular manifestations in patients with rheumatic diseases are crucial for diagnosis, management (including treatment strategies), and prognosis, making accurate assessment essential. Growing evidence supports the role of ultrasound in assessing these manifestations for diagnosis, monitoring, and gaining insights into disease pathogenesis. Recent studies emphasize the significant utility of ultrasound in evaluating extra-articular involvement across various organ systems, including the lungs, vessels, salivary glands, muscles, nerves, skin, and nails. Technological advances, such as shear-wave elastography, contrast-enhanced ultrasound, and artificial intelligence, are expanding the scope and precision of ultrasound applications. Despite its potential, challenges such as operator dependency, lack of standardized protocols, and the need for specialized training hinder its widespread adoption. Ultrasound is a non-invasive, cost-effective, and radiation-free imaging modality with high diagnostic accuracy, making it a valuable tool for assessing extra-articular manifestations in rheumatic diseases. Emerging technologies may further enhance its clinical utility. However, efforts to standardize techniques and improve accessibility are necessary to optimize its integration into routine practice.

Keywords: Extra-articular; Interstitial lung disease; Large vessel vasculitis; Muscle; Nail; Nerve; Salivary gland; Skin; Ultrasound.

Fig. 1

Ultrasound in lung involvement (interstitial lung disease). A shows a lung ultrasound using a convex probe, demonstrating a normal pleural line. B highlights pleural line irregularities, visible as nodular thickening (white circle), and C reveals a B-line (indicated by the white arrow) in patients with rheumatoid arthritis. Asterisks = pleural line; LU = lung parenchyma

Fig. 2

Ultrasound in the assessment of vessels (large vessel vasculitis). A shows the ultrasound scan of the superficial temporal artery in a healthy subject, demonstrating normal vessel wall thickness in both transverse (A) and longitudinal (A’) views. The vessel wall disappears upon probe compression (A’’). The figure also includes images of the superficial temporal artery in a patient with giant cell arteritis, displaying the halo sign in transverse (B) and longitudinal (B’) views (marked with asterisks). In B’’, the thickened arterial wall remains visible despite external compression, indicating the compression sign. C presents a normal axillary artery in transverse (C) and longitudinal (C’) views in a healthy subject. D and D’ display the halo sign in the axillary artery in transverse and longitudinal views, respectively, in a patient with Takayasu disease

Fig. 3

Ultrasound in the assessment of salivary glands. A and B present ultrasonographic images of the parotid and submandibular glands, respectively, showing the homogeneous structure of gland parenchyma in a healthy subject. A’ and B’ depict grade 3 inhomogeneity in patients with Sjögren disease, characterized by anechoic and hypoechoic areas affecting the entire gland surface (parotid and submandibular gland, respectively). A’’ and B’’ show diffuse atrophy and a loss of clear demarcation between the gland border and surrounding tissue in other patients with Sjögren disease (parotid and submandibular gland, respectively)

Fig. 4

Ultrasound in muscle involvement (inflammatory myositis and sarcopenia). A shows a transverse scan, and B a longitudinal scan, both demonstrating widespread increased echogenicity without loss of the underlying bone signal (“see through” appearence) of the rectus femoris muscle with preserved muscle structure in a patient with polymyositis. Additionally, an example of a recently developed semi-quantitative scale for assessing muscle echogenicity (quadriceps muscle) in patients with sarcopenia is provided (reference no. 112). Images were obtained from patients with rheumatoid arthritis. f = femur; rf = rectus femoris muscle; vi = vastus intermedius muscle

Fig. 5

Ultrasound in the assessment of nerves (compressive neuropathty). A shows a transverse view of a normal ulnar nerve at the elbow in a healthy subject, while A’ displays an enlarged ulnar nerve (increased cross-sectional area) in a patient with cubital tunnel syndrome. B presents a longitudinal view of a normal median nerve at the wrist in a healthy subject, and B’ shows an enlarged median nerve with power Doppler signal (red spots) in a patient with carpal tunnel syndrome. fds = flexor digitorum superficialis; fdp = flexor digitorum profundus; l = lunate; m = median nerve; me = medial epicondyle; o = olecranon; r = radius; u = ulnar nerve

Fig. 6

Ultrasound in the assessment of skin (scleroderma), sub-cutaneous soft tissues (calcinosis cutis) and nails (nail psoriasis). A presents a comparative ultrasound image of skin layers in the atrophic phase of systemic sclerosis, obtained with a high-frequency probe (22 MHz). The epidermis (red) appears as a thin hyperechoic layer, while the dermis (green) is hypoechoic with tightly packed, regular connective tissue fibers. The subcutaneous tissue (blue) is more hypoechoic, with looser connective tissue fibers. In A’, the epidermal, dermal, and subcutaneous layers of a healthy subject are clearly delineated, following the same echogenic patterns as in A, though with notable differences in thickness, especially in the dermis. B shows an ultrasound image of subcutaneous calcinosis, highlighting calcifications of variable shapes and sizes (asterisks) accompanied by acoustic shadowing. In B’, a surrounding power Doppler signal (red spots) is visible in a patient with systemic sclerosis. Finally, C provides an ultrasound comparison of a physiological nail, showing a normal trilaminar structure, while C’ depicts a psoriatic nail with loss of the ventral plate and increased nail bed thickness. de = dermis; dp = distal phalanx; ep = epidermis; hyp = hypodermis; nb = nail bed; p = nail plate. A, A’, B, and B’ kind courtesy of Prof. L. Idolazzi