Performance of Ultrasound in the Clinical Evaluation of Gout and Hyperuricemia

Objective To evaluate monosodium urate (MSU) crystal deposition and related lesions in the joints of patients with gout and hyperuricemia (HUA) using ultrasound. To explore the association between ultrasound findings and clinical features in gout and HUA. Methods A total of 202 patients with gout and 43 asymptomatic patients with HUA were included. The clinical data and ultrasonic assessment results were collected and statistically analyzed. Results Deposition of MSU crystals was found in 25.58% (11/43) of patients with asymptomatic HUA and 76.24% (154/202) of patients with gout. Of the 1,082 joints from patients with gout examined, 33.09% (358/1082) displayed MSU crystal deposition. In the joints with MSU crystal deposition, 77.37% (277/358) had a history of attacks. Among the joints of gouty arthritis, double contour sign (DCS), hyperechoic aggregate (HAG), and tophi were found in 32.65% (159/487), 7.80% (38/487), and 24.64% (120/487) of the joints, respectively. DCS and tophi, but not HAG, increasingly appeared with the extension of gout duration. In patients with more than 15 years of gout history, DCS, Tophi, and HAG were found in 48.18%, 40.00%, and 6.36% of US assessed joints, respectively. In patients with gout, synovial lesion and bone erosion were found in 17.74% (192/1082) and 7.58% (82/1082) of joints, respectively. The synovial lesion was related to HAG, while bone erosion was related to tophi and DCS. Nephrolithiasis was detected in 20.30% (41/202) of patients with gout and 4.65% (2/43) of HUA patients, indicating that nephrolithiasis occurred in more patients with gout than in patients with HUA. Conclusion HAG is an early performance of MSU crystal deposition in joints of gout and HUA. Both DCS and tophi are risk factors for bone erosion. Early urate-lowering therapy (ULT) should be considered in patients with gout, DCS, or tophi.


Introduction
Gout is a common inflammatory disease induced by the deposition of monosodium urate (MSU) crystals in joints and surrounding soft tissues, and hyperuricemia (HUA) is a critical factor for developing symptomatic gout. Most HUA patients do not have gouty arthritis, although MUS crystals are detected in their joints [1,2]. Chronic pain, soreness, or numbness in the joints is reported in some patients without convincing clinical evidence of a gouty attack, and it is difficult to differentiate gout from osteoarthritis or other chronic arthritis. Noninvasive imaging evidence of urate deposition in joints is valuable and helpful for differential diagnosis [3].
Ultrasound (US), a noninvasive, free of ionizing radiation, convenient, and inexpensive approach, has recently been used to identify MSU crystal deposits for diagnosing gout [4,5]. A standardized definition of ultrasound lesions with the elementary morphostructural changes in gout has been established in an international consensus [6]. Double contour sign (DCS), hyperechoic aggregates (HAG), and tophi are considered characteristic US abnormalities of MSU crystal deposits. Furthermore, synovial lesions (i.e., synovial hypertrophy and synovitis) and bone erosion are regularly detected by ultrasound in gout and HUA patients [2].
US findings are known to be crucial evidence for the diagnosis or differential diagnosis for gout [7,8]. The various US phenomena may indicate various joint injuries. Tophi detected by US is reported to be associated with worse foot pain and disability [9]. However, US evidence of MSU crystal deposition can be found in asymptomatic joints in gouty patients, while MSU crystals may not always be detected in the gouty joints with attacks [10]. Furthermore, joint US may show the sign of MSU crystal deposition in asymptomatic HUA and normal people [11]. These findings suggest that a reliable correlation between MSU crystal deposition and gouty attack or erosion remains unclear.
In this study, we retrospectively analyzed the joints' US results from patients who were diagnosed with gout or HUA to validate the US assessment of MSU crystal deposition and lesion in joints and evaluate the clinical value of US application in the diagnosis and prevention of gout and HUA.

Study Cohort and Methods.
Patients aged 18-75 years, diagnosed with gout or asymptomatic HUA in a goutspecialized clinic in Huashan Hospital from Aug 1 st , 2016, to Feb 28 th , 2019, were eligible for this study. A total of 245 patients (202 with gout, 43 with asymptomatic HUA) were included. The required inclusion criteria for patients with gout matched the gout classification criteria of the American College of Rheumatology (ACR)/European League Against Rheumatism (EULAR) (2015). The exclusion criteria for patients with gout were prior diagnosis of other crystalrelated arthropathies, such as calcium pyrophosphate deposition disease with visible tophi. Urate levels in fasting serum in all 43 HUA patients were greater than 420 μmol/L, and this was confirmed at least twice. Besides the US results, all the following data, including demographics (i.e., sex, age, and disease duration), body mass index (BMI), and the clinical features of affected joints, were collected.
In the 202 patients with gout, 1,082 joints, including the first metatarsophalangeal joint (MTP1), ankle, knee, acrotarsium, elbow, wrist, and hand joints, were analyzed using US. In the 43 HUA patients, 256 joints were examined. Most of the joints were the three vulnerable pairs of lower joints (MTP1, ankle, and knee), and the others were the joints with mild clinical manifestation, including numb, slight pain, or discomfort. The rheumatologist decided which and how many joints underwent US examination based on clinical judgment (the joints with symptoms and potentially affected joints).
Ethical approval for the study was obtained from the Institutional Review Board for Human Studies at Huashan Hospital, Fudan University (approval number: 2012137). All the included patients had provided a fully informed written consent form before data collection.
2.2. US Assessment. The US examination was performed by skilled sonographers who had more than 10 years of experi-ence in the musculoskeletal US in Huashan Hospital. Aplio i900 color ultrasonic diagnostic apparatus (probe frequency 5-18 MHz) was used for US examination. According to the international consensus of the standardized definition of US gout lesion published by OMERACT (Outcome Measures in Rheumatology), US Gout Task Force was employed [6,12], and MSU crystal deposition in joints was diagnosed based on DCS, hyperechoic aggregates (HAG), and tophi. Synovial hypertrophy, synovitis, and bone erosion were considered joint damage.
2.3. Statistical Analysis. The analysis was performed using SPSS 19.0 (IBM). All data were presented as mean ± SD or proportions. Comparisons of baseline data between the two groups were tested for statistical significance using a t -test or one-way analysis of variance test with a least significant difference multiple comparison test. p values <0.05 were considered significant. Pearson's correlation coefficient was calculated to examine correlations between variables.  (18.5-23.9), and there was no significant difference between the two groups of patients. Serum uric acid (SUA) in patients with gout (524:24 ± 79:68) was higher than in HUA patients (493:40 ± 66:85), which might be due to the longer course of HUA in the gout group.

Demographics and Clinical
The urate crystal deposits in the kidney directly result in chronic urate nephropathy. In Table 1, the creatinine clearance rate (Ccr) in patients with gout (95:55 ± 2:55 mL/ minute) is lower than in HUA patients (106:42 ± 5:54 mL/ minute). Ccr < 80 ml/minute was detected in 39.60% (80/202) of patients with gout and 20.93% (9/43) of HUA patients. Hyperlipidemia and hyperglycemia were known to coexist with HUA and gout. In our study, both fasting blood glucose (GLT) and triglyceride (TG) were found to be higher in patients with gout than in HUA patients (Table 1). In patients with gout, our results showed more common comorbidities, such as kidney damage and metabolic disorder.

Clinical Characteristics of Gouty Attacks in the
Population with Gout. We further analyzed the relationship between ultrasound abnormalities signs and clinical manifestations in the gout patients (Table 2, Figure 1). There were a total of 531 joints, including 187 (

Course Time and the MSU Crystals in Patients with Gout.
The progression of gout can be defined by four pathophysiological stages: hyperuricaemia without evidence of monosodium urate crystal deposition or gout, crystal deposition without symptomatic gout, crystal deposition with acute gout flares, and advanced gout characterized by tophi, chronic gouty arthritis, and radiographic erosions. When the urate level in serum exceeds its saturation concentration, the precipitated urate crystals deposit in joints and soft tissues. In this study, we found that MSU crystal deposition was correlated with the SUA level (p < 0:01) and the duration (p < 0:01). Furthermore, many patients had no awareness of their SUA level before seeking clinical specialists. In 202 patients with gout, only 17.33% (35/202) of the patients came for treatment at their early stage (course < 1 year). Most had more than one year of gout history (Table 5(a)).
In patients with gout, the proportion of US-positive signs of MSU crystal deposition gradually increased, especially DCS and tophi during gout. In patients with more than 15 years of gout history, DCS and tophi were detected in 48.18% and 40.00% of joints, respectively, whereas in patients with less than one year of gout history, DCS and tophi were found only in 6.29% and 5.03% of joints, respectively. HAG showed no notable rising as gout duration extension. HAG was found in 5.03% (8/159) of joints of the patients who had less than one year of gout course, and it was 6.36% (7/110) in patients with more than 15 years of gout course (Table 5(a)).

Correlation of Joint MSU Crystal Deposition with
Nephrolithiasis. Patients with gout were reported to be prone to have nephrolithiasis, acute renal colic, and hematuria, although it is difficult to determine the type of crystal in the kidney. In this study, we further analyzed the US data of the kidney. Nephrolithiasis was defined as US signs of calculus or crystal deposition in the kidney. We found that nephrolithiasis was detected in 20.30% (41/202) of patients with gout and 4.65% (2/43) of HUA patients, indicating that nephrolithiasis occurred more commonly in patients with gout than in hyperuricemia patients. The findings also   Journal of Immunology Research showed that nephrolithiasis was remarkably relevant to MSU crystal deposition in joints in (p < 0:05) ( Table 6).

Discussion
The demographics of cohorts in this study show that the average ages of gout and HUA patients in this study were 46.90 and 44.47 years old, respectively, which were younger than the previously reported average age of 52.69 years from the HUA data in a Chinese national cross-sectional survey in 2014 [14]. This suggests that gout and HUA are increasing in the younger population due to lifestyle changes [15]. Our results are consistent with previous reports that high BMI, hyperlipidemia, and hyperglycemia are notably complicated with gout and HUA, especially gout [16]. Therefore, middle-aged males with certain metabolic syndromes should be listed in the attention-demanding high-risk population of gout and HUA.
Our data indicate that lower limb joints are more vulnerable in patients with gout. MTP1 (49.01%), followed by the ankle (31.20%) and acrotarsium (10.40%), is the most affected joints. The upper limb joints, including hands, wrist, and elbow, are rarely involved. This result strongly supports the gout diagnostic value of MTP1 and ankle attacks, especially at an early stage.
For patients without classic symptoms, MSU crystal deposition could be identified by image-based examination, such as dual-energy computed tomography (DECT) and US. It has been reported that the MSU burden volume, which is predictive of the risk of flares [17], can be measured using DECT [13]. Due to being noninvasive, free of ionizing radiation, convenient, and inexpensive, US is more widely used in the clinic. Our study shows that MSU deposition can be detected by US, and the most frequently affected joint is MTP1 in the gout patients. This result is consistent with clinical findings reported by other groups [7].
Previous research suggested that MSU deposition in joints is a crucial factor of gouty arthritis attack [4]. In this study, 28 gout patients came to outpatients at their first acute attack and took a joint examination by US. In the 28 patients, only 39.29% (11/28) attacked joints were found with MSU deposition. Furthermore, among the total 11 gouty joints with initial attacks, large joints such as the knee and ankle were the majority of MSU deposition-positive joints. Only one MTP1 was positive of MSU crystal deposition detected by US.
Moreover, of the 17 patients who were most attacked on MTP1 at an early stage (course < 1 year), 47.06% (8/17) were positive for MSU crystal deposition in the MTP1. These results raise a serious question about how sensitive the US is to detect MSU crystal deposition in joints, particularly in smaller joints of the lower extremities at an early stage. Surprisingly, 77.37% of the patients with US-detected MSU  [2] 0 0 Pearson's chi-squared test, X 2 = 5:571, p = 0:018 (p < 0:05 considered as significantly difference). 5 Journal of Immunology Research crystal deposits in joints had gouty attacks in the past. Due to practical reasons, the selection of joints for US examination is based on rheumatologist personal judgment in this retrospective study, and a certain bias cannot be ruled out. This interpretation is consistent with previous research, as ultrasound features of MSU crystal deposition had a high positive predictive value but more limited sensitivity for early gout [18]. Further investigation is warranted to validate the US's sensitivity and specificity in detecting MSU crystal deposition in various joints.
According to "2018 updated European League Against Rheumatism evidence-based recommendations for the diagnosis of gout" [19], MSU crystal deposition is crucial evidence for gout diagnosis. In our study, DCS and tophi were frequently detected by US in gout patients, and they are notably increased with the disease history of gout. HAG was reported to be the most common sign at the early stage, especially in asymptomatic joints [20]. In an animal experiment where MSU was injected into the knees of rabbits, HAG was found frequently displayed in 75% of knees at an early stage (day 7 after MSU injection) [21]. Collectively, our data support that HAG is the US sign for early MSU crystal deposition, while DCS and tophi are a useful signature of MSU crystal deposition for chronic gout.
HAG was found to be associated with synovial lesions in this study. We found synovial lesion was notably detected at an early stage and gradually increased with the gout course extension. It has been reported that urate-lowering therapy (ULT) with febuxostat for 24 months reduced synovitis detected by MRI in patients with acute gout [22]. ULT may diminish HAG and improve synovitis and synovial hypertrophy. Bone erosion was known to be an irreversible injuriousness in gout [23]. In our study, tophi and DCS were both found to be correlated with bone erosion. Timely, ULT may minimize tophi and DCS and block bone injury in the gout patients.
Male, diabetes, obesity, low pH in urine, hyperuricosuria, and low urine volume have been reported to be the main etiologic factors of nephrolithiasis [24,25]. Due to oversaturated urate and deposition of its crystals in the kidney, it is not surprising that a higher prevalence of nephrolithiasis is found in gout patients [26]. In this study, nephrolithiasis is remarkably relative to MSU crystal deposition in gouty arthritis. The result suggests that the patients with MSU crystal deposition in joints may be prone to suffer nephrolithiasis. Although MSU crystal deposition is related to high serum uric acid [27], systemic conditions such as genetic predisposition, geographical location, dietary indiscretion, and various metabolic characteristics should be considered as other risk factors [28,29].

Conclusion
Ultrasound is a clinically convenient approach to detect MSU crystal deposits in joints for supporting the diagnosis of gout. HAG is considered an early sign of MSU crystal deposition in joints, whereas DCS and tophi correlate with bone erosion. Early ULT might be effective in the reduction of HAG and partially prevent synovitis and synovial hypertrophy. ULT should be considered when gout patients are detected with DCS or tophi in joints.

Data Availability
The datasets used and analyzed during the current study are available from the corresponding author on reasonable request.

Disclosure
The funders had no role in the study design, data collection and analysis, decision to publish, and manuscript preparation.