Key ultrasound features that help in diagnosing large vessel vasculitis

Although ultrasound is being used to diagnose large vessel vasculitis (LVV), namely giant cell arteritis (GCA) and Takayasu’s arteritis (TA), since last two decades, it has become more popular in recent years following release of new classification criteria of GCA by ACR/EULAR, where temporal artery ultrasound has been given the same weightage as temporal artery biopsy. Nowadays ultrasound of bilateral common superficial temporal arteries (STA), its parietal (PTA) and frontal (FTA) branches and axillary arteries (AA) is considered as the first imaging modality in GCA. Involvement of the axillary artery may indicate more severe extra-cranial involvement. In ultrasound hypoechoic wall thickening of affected arteries, known as ‘halo sign’, indicative of vessel wall inflammation. As the involvement is segmental, the whole length of the above arteries needs to be examined in long and short axes. In TA also ultrasound is a useful tool as it can assess the involved parts of the aorta and its main branches except the thoracic aorta, as suggested by EULAR (2023). Its main advantages over those other modalities, namely MR angiogram, CT angiogram or FDG-PET,  are its easy accessibility in the OPD, cost-effectiveness and the fact that it can be repeated frequently if necessary during follow up.

Ultrasound as an aid in differentiating LVV from other mimicking conditions

In both atherosclerotic arterial disease and vessel wall inflammation in LVV intima-media thickness is increased. But the ‘halo’ sign is hypoechoic, concentric and homogeneous whereas atherosclerotic plaques are eccentric and often echogenic. ‘Slope sign’ and ‘macaroni sign’ are seen in TA and involve large vessels in GCA and are very characteristic of these diseases. For fibromuscular dysplasia the most characteristic ultrasound finding is a “string-of-beads” appearance, indicating alternating segments of narrowing and dilation of arteries, commonly present in the carotid and renal arteries.

Role of Doppler ultrasound in assessing disease activity and monitoring therapeutic response in LVV

In GCA two scoring systems are available – ‘halo count’ and ‘halo score’. For that total 8 arteries are needed to be examined ultrasonographically – STA, PTA, FTA and AA bilaterally. ‘Halo’ count (0-8) is nothing but the total number of these arteries where halo sign is present. ‘Halo’ grading is done based on the maximum ‘halo’ thickness in an examined artery. For example, in STA a ‘halo’ thickness of ≤0.3mm, 0.4mm, 0.5mm, 6-7mm and 8mm are graded as grade 0, 1, 2, 3 and 4 respectively. From the halo thickness grading, halo scores are derived. Both halo count and halo score can independently predict ocular ischaemia and halo score correlates with CRP. Thus we can have an indirect assessment of disease activity.

In TA a ultrasound based scoring system to assess disease activity is available – CDUS-K, which based on the presence of stenosis and altered flow patterns of affected arteries.