Gout of hand and wrist: the value of US as compared with DECT

The incidence of gout has tripled over recent decades and now represents the most common inflammatory arthritis in men and women [1‐4].

Long-term hyperuricemia results in deposition of monosodium urate (MSU) crystals in joints and soft tissues, triggering gouty arthritis and, if not properly treated, the formation of gouty tophi [5‐8]. With respect to imaging methods Bongartz et al. [4] showed that DECT has a significant impact on clinical decision-making when gout is suspected. In the studies by Dalbeth [9] and Zhang et al. [10], DECT is described as an advanced imaging method for longitudinal follow-up to monitor treatment response when looking at tophus size regression [11‐13].

Few studies have evaluated the feasibility of ultrasound (US) as compared to dual-energy CT (DECT), which can differentiate calcium-rich material (high attenuation) from MSU crystal-rich material (low attenuation) for the diagnosis of gout [14‐16]. Manger et al. [17] described periarticular gouty deposits in 9/12 hands and wrists, whereas Huppertz et al. [18] presented overall results for multiple joints (knees, feet, elbows and hands) without detailed sonographic results for hands and wrists in their patient-based evaluation.

Therefore the aim of our study was to evaluate US findings compared to DECT in terms of intra- and extra-articular gouty deposits in patients presenting with acute arthritis of the hand and/or wrist with suspected gout.

The DCS was the most frequently detected US sign for the diagnosis of gouty hand and wrist arthritis with a sensitivity of 76% for intra-articular gout, with an overall sensitivity of 65% (Fig. 4). For the extra-articular compartment in terms of gouty deposits in tendons DECT was superior for delineation and characterization [22]. Our study is the first to give a detailed description of MSU depositions in both the intra- and extra-articular compartments of the hands and wrists with comparison of US to DECT.

US has been advanced as an imaging modality of choice for the diagnosis and management of gout [21, 23‐25]. In fact, in addition to a positive DECT result, the DCS has been incorporated in ACR/EULAR guidelines 2015 as being the most specific sign for the diagnosis of gouty arthritis [24, 26].

Among patients with a positive DCS, a wide range of sensitivity for DCS has been reported, even for experienced sonographers [26‐30]. However, a positive DCS and a negative DECT study might be explained also by a low concentration of MSU crystals or early disease with gouty deposits not yet visible on DECT. Thiele and Schlesinger [28] detected the DCS in 92% of 23 patients, but that study included primarily the first MTP joint and knees and only four MCP joints. Ottaviani et al. [31] reported a sensitivity of 57% and specificity of 98% for DCS, but again with MTP joints and knees. Ogdie et al. [32] reported a pooled sensitivity for DCS of 0.83 (0.72–0.91) and specificity of 0.76 (0.68–0.83) in a literature review of 11 studies. A recent study by Naredo et al. [33] showed a sensitivity of 84.6% and specificity of 83.3% for DCS in upper and lower limbs in a 12-structure US assessment. On the basis of these studies, the DCS has been incorporated into the ACR/EULAR guidelines [32].

As detailed in our results, 63 of the 66 patients with a positive DCS had evidence of MSU deposits on DECT. A phantom study by Diekhoff et al. [34] demonstrated that gouty deposits can reliably be detected by DECT at a concentration of 12.5%, and CPPD at a concentration of 6.25%, corresponding to deposits with mean Hounsfield unit values of 59.8 for MSU and 101.1 for CPPD [29]. For in vivo assessment the estimated lower limit of uric acid concentration per voxel has to be 35% or higher to be detectable by DECT [4, 32, 35, 36]. It is possible that the three cases of positive DCS in our study with negative DECT findings may represent low concentration MSU disease or early disease with gouty deposits not yet visible on DECT. Furthermore, the presence of a DCS in our study population was related to an elevated mean uric acid level. Careful follow-up for further signs of gouty arthritis both clinically and by US is planned for these three subjects.

As recorded in our series, only 71/180 patients (39.4%) had a definitive diagnosis of gout with US versus 97/180 patients (53.9%) by DECT. Indeterminate US cases frequently had small deposits, suggesting that cases with less severe findings may be more challenging. Our study shows that osteoarthritis (OA) can be challenging in the US differential diagnosis, as 50.6% of the DECT negative patients were diagnosed to have OA with loose bodies by using coronal reformed CT images, and many of these patients were positive or indeterminate by US. US shows echogenic findings which may be secondary to calcium or MSU deposits, while DECT is capable of discriminating urate from non-urate compounds [22] (Figs. 3 and 4). This limitation of US was confirmed in our study (Table 2) as US showed fewer extra-articular gouty deposits (extensor tendon 22.5%, flexor tendon 15.5%) versus DECT (extensor tendon 30%, flexor tendon 65%). A prospective blinded randomized study by Choi et al. [35] showed a sensitivity of 84% and a specificity of 93% for detection of MSU deposits by DECT, but the most common location of MSU deposits in 20 evaluated patients were in lower extremities, and included only 6 hands/wrists out of 40 joints. The role of DECT has also been shown by Hu et al. [37] reporting a high sensitivity and specificity of 75.2% and 92.7% of DECT for the detection of gouty arthritis in upper and lower extremities. Zhu et al. [38] compared the diagnostic accuracy of DECT and US in various joints in 40 patients, also showing that DECT was superior to US in upper limbs, whereas no difference between the two methods was seen for the lower limbs. Finkenstaedt et al. [11] reported a change in the treatment plan in 23/43 patients with DECT, including 37 hands/wrists in their study.

Our study has several limitations. A major limitation of our study is that synovial fluid (SF) aspiration was not performed, because feasibility is limited in clinical practise [14, 21, 22]. Given the small dimensions of the hand/wrist joint recesses, fluid aspirations with small amounts of joint effusion often fail and are painful for the patients. Aspiration of MSU crystals may fail due to periarticular crystal depositions, or due to sedimentation of MSU with aspiration of crystal-free SF. Bongartz et al. [4] showed that in several patients with a negative SF analysis, DECT demonstrated evidence of MSU deposition in tendon sheaths and enthesial sites, suggesting that a significant number of patients could be tested false negative by SF examination alone [36]. Glazebrook et al. reported a sensitivity of 100% for DECT and a specificity of 79% and 89% (in two readers) for DECT compared to MSU positive joint aspiration results, but only 43 patients were evaluated and only 14 patients showed MSU deposits in wrists [16, 36]. Furthermore SF aspiration may not be possible if the patient refuses or is on anticoagulation. Given the difficulty of using SF analysis as a gold standard, US and DECT assume greater importance.

The evaluation of US and DECT by a single radiologist precludes our ability to evaluate inter-observer variations. Finally, we included patients with short duration of gout or a lesser degree of hyperuricemia, which may lead to MSU crystal deposition below the threshold of detection of DECT. The limit of detection of DECT is generally considered to be 2 mm, so microscopic tophi may be missed [36]. A further limitation of our study is the high number of indeterminate cases by US (47 patients) limiting our specificity to 51%, increasing to 96.4% when indeterminate cases are counted as negative. These cases seem particularly to benefit from DECT evaluation and should be addressed in further studies.

In summary the percentage of gouty deposits detected by US was significantly lower than that by DECT, especially in the extra-articular spaces.

Although the sensitivity of US for diagnosis of gouty hand and wrist arthritis is limited, it can be used as a first-line imaging modality in the presence of the DCS.

DECT was superior on overall detection and characterization, which might be of importance not only for diagnosis but for therapeutic follow-up in terms of delineation of gouty burden.