Ultrasonography in Rheumatology: Beyond the Joints and the Vessels!

The scope of ultrasound (US) in rheumatology is ever-evolving. Musculoskeletal US (MSKUS) has brought a paradigm shift in the diagnosis and assessing disease activity in various rheumatological conditions presenting with arthritis. Its relatively lower cost, ease of use, and lack of apparent contraindications have resulted in this modality becoming an integral part of rheumatology practice, both for diagnostic and therapeutic purposes.^[1] Apart from the assessment of synovial proliferation, vascularity, and erosions, MSKUS is also helpful to diagnose various periarticular conditions such as enthesitis, dactylitis, tenosynovitis, and bursitis. US is now commonly used in the diagnostic algorithm of polymyalgia rheumatica^[2] and gout. Vascular US is another established area where US plays a crucial role in diagnosing giant cell arteritis (GCA) and assessing disease activity in both GCA^[3] and Takayasu arteritis.^[4,5]

In this current issue of the journal, Koshy et al.^[6] published a descriptive study on the role of US in evaluating patients with interstitial lung disease (ILD) associated with autoimmune connective tissue disease (AICTD). Forty-two patients with AICTD patients were recruited for lung US and compared with the gold standard of high-resolution computed tomography (HRCT) of the thorax. They reported the presence of B-lines in US as a marker of ILD. Considering HRCT as the gold standard, there were one false positive and two false negatives. Lung US had good diagnostic accuracy for ILD in AICTD (sensitivity - 93.55%, specificity - 90.91%, positive predictive value - 89.38%, and negative predictive values - 94.51%). Although the results were comparable with HRCT, the technique of lung US has several limitations for the diagnosis of ILD. It is challenging to assess thickened and distorted lesions in the deep interstitium, vascular bronchial bundle, small nodules distributed around the bronchial vessels, deep-seated ground-glass opacities or small nodules, and mediastinal lymph nodes using lung US.^[7] Lung US also might miss ILD, particularly early ILD, compared to HRCT, which in turn might impact the management and long-term outcome of such patients. Despite several advantages such as lower cost, portability, and no radiation hazard, the assessment of ILD using lung US is still in a nascent stage and can miss more ILD, especially early. Before we conclude the diagnostic utility of lung US in ILD, we need both longitudinal data as well as multicentric data in a larger number of patients.

In recent times, US has shown to be helpful in some other relatively rare conditions for diagnosis and assessing disease activity. One such condition is relapsing polychondritis, where the assessment of auricular chondritis can be done using USG. The classic findings are increased thickness with reduced echogenicity of subcutaneous tissue and positive power Doppler signals, which denoted active inflammation of the area.^[8] US has also been shown to be useful in other rheumatological conditions such as myositis and Sjogren syndrome for diagnosis and disease assessment. Typically, the muscle tissue is hypoechoic, thought to be the result of high blood flow in muscle tissue.^[9] On the transverse plane, normally, there is a moderately echogenic speckled pattern due to the reflection of perimyseal connective tissues. In contrast, the longitudinal plane gives a linear pennate or triangular pattern. Epimysium normally appears white or hyperechoic.^[1] Patients with myositis have several changes, such as muscle turning whiter due to fat replacement and fibrosis. Heckmatt et al. in 1980, for the first time, described these observations and subsequently proposed a grading system known as “Heckmatt criteria” having four different grades: grade 1: normal echogenicity, Grade 2: muscle echo increased and bone still distinct; Grade 3: markedly increased muscle echo and bone echogenicity reduced, and Grade 4: very strong muscle echo with and complete loss of bone echogenicity.^[10] However, since US is operator-dependent, several factors can influence the assessment. Patient position, machine settings, and the plane of view can affect the echogenicity of the muscle. Echogenicity also differs in muscle groups, increases with age, and is higher in females, in those with more subcutaneous fat and postexercise state.^[11] USG of muscle is a promising tool for future follow-up assessment of disease activity. Incorporating a deep learning algorithm for detecting myositis and its subsequent assessment may further improve its accuracy in future.

Salivary gland US is emerging as a very sensitive tool for early diagnosis of primary Sjogren syndrome. Typically, parotid glands appear as homogenous structures, and once the glands are affected by the inflammatory process, they become inhomogeneous. Based on the severity of the inhomogeneity, a grading system has been proposed: Grade 0: normal parotid glands, Grade 1: mild unspecific inhomogeneity, Grade 2: evident inhomogeneity, and Grade 3: gross inhomogeneity.^[12] Interestingly, salivary gland USG correlates with minor salivary gland biopsy and unstimulated salivary flow. As a noninvasive modality, it could be an integral part of diagnosing and assessing primary Sjogren syndrome. Apart from this, US has also been used for the assessment of peripheral nerve entrapments such as the median nerve in carpal tunnel syndrome.^[13]

Figure 1 summarizes the various diseases where US has a role in rheumatology. Applying US in various rheumatological conditions can be a handy futuristic tool, more so when combined with artificial intelligence and deep learning for image analysis.

Figure 1: Indications for US in Rheumatology. US: Ultrasound