Gout represents a common inflammatory arthritis conditions in which the monosodium urate (MSU) crystals are deposited in joints and other tissues producing attacks of acute arthritis/ chronic arthropathy with important destructive lesions in articular or juxta-articular spaces.
High resolution ultrasonography (US) is able to detect these deposits and their consequences: hyaline cartilage pathology, joint effusions, synovitis, bone erosions, tendonitis, bursitis, and other soft tissues involvement. Moreover, there are characteristic findings of this aggregates that can be detected by US, helping the clinician for positive and differential diagnosis. In this review we will present the main US findings in gout. The utility of the method in diagnosis and treatment response will be discussed. Also, comparison with other imaging techniques in terms of sensibility and specificity of the methods for this pathological situation will be made.
Keywords: gout, ultrasonography, double contour sign, monosodium urate aggregates, tophus
The place of musculoskeletal ultrasonography in gout diagnosis
, Rodina Nestorova2
, Violeta Vlad3
, Mihaela Micu4
12nd Internal Medicine Department, “Iuliu Hatieganu” University of Medicine and Pharmacy, Cluj-Napoca, Romania,
2Centre of Rheumatology “St. Irina”, Sofia, Bulgaria, 3Sf. Maria Clinical Hospital, Bucharest, Romania, 4Rheumatol- ogy Division, Department of Rehabilitation II, Rehabilitation Clinical Hospital, Cluj-Napoca, Romania
Received 22.09.2013 Accepted 18.10.2014 Med Ultrason
2014, Vol. 16, No 4, 336-344
Corresponding author: Daniela Fodor, MD, PhD 2nd Internal Medicine Department
“Iuliu Hatieganu” University of Medicine and Pharmacy, Cluj-Napoca, Romania 2-4 Clinicilor str,
400006 Cluj Napoca, Romania Phone: 004 0264591942/442 Email: [email protected]
Gout is a term representing a heterogeneous group of metabolic disorder found exclusively in humans, cha- racterized by increased serum urate concentration for a long period of time, resulting into tissular deposition of monosodium urate (MUS). Hippocrates described acute gout in the 4th century BC but Thomas Sydenham made the classic description of acute gout in the 17th century.
Gout is a common inflammatory arthritis conditions, causing morbidity, disability, and poorer quality of life.
The disease is frequently associated with other co-mor- bidities (type 2 diabetes mellitus, cardiovascular disease, chronic kidney disease, or obesity) . It is more com- mon in men that women and the prevalence increases with age. The gout is the consequence of deposition of
MSU crystals in joints and other tissues as a result of per- sistent hyperuricaemia . Traditionally, four phases are described in the disease evolution: asymptomatic hyper- uricaemia, acute, intercritical, and chronic gout  but recently a new revised staging system for hyperuricaemia and gout was proposed (table I) .
Diagnosis of suspected gout is based on typical clini- cal and laboratory (ie, hyperuricaemia) findings, while definitive diagnosis requires identification of MSU crys- tals in aspirated synovial fluid or tophi . The crystals of MSU are proved to be in and around the joints (car-
Table I. Proposed staging system for gout (from  adapted).
Stage Symptomatology Definition
A Asymptomatic Hyperuricaemia without MSU crystal deposition
B MSU crystal deposition but with-
out signs or symptoms of gout C Symptomatic MSU crystal deposition with prior
or current episodes of acute gout flares
D Advanced gout (tophaceous gout
and chronic gouty arthropathy) requiring specialized interventions
tilage, epiphyses, synovial membrane, tendons, or liga- ments). These aggregated deposits produce local necrosis and, with the exception of avascular tissues, the inflam- matory response can lead to a fibrous proliferation .
In the early stage of gout radiography is less useful for diagnosis due to the non-specific soft-tissue swelling overlying the inflamed joint; abnormal findings specific for radiology will generally appear after years of evolution .
CT and MRI have been used in few studies [7-10] especial- ly to evaluate the tophi, but these methods cannot be used as routine examinations. Ultrasonography (US) instead, can be used for diagnosis and management of gout from the first attack. Crystalline material found in gouty joints reflects ultrasound waves more strongly than surrounding tissues (such as un-mineralized hyaline cartilage or syno- vial fluid) and can thus be readily distinguished .
The US findings in gout are specific or non-specific  and for some of these findings OMERACT defini-
tions were recently established  (table II) (for some of the gout findings the previous OMERACT definitions should be used ).
The MSU crystals can precipitate theoretically in every anatomical structure but there are specific places where these deposits have to be searched (table III).
Effusions are less specific for gout. Intra-articular fluid can be strictly anechoic (simple joint effusion) at the first gouty attacks (early disease) but in time MSU ag- gregates with variable echogenicity will appear [3,6,14- 17]. The hyperechoic spots have less than 1 mm and their presence within the synovial fluid realize the “starry sky”
sign . During the real time examination, at a delicate pressure with the transducer under the examined joint, the spots will float inside the joint cavity creating a “snow-
Table II. The ultrasonographic findings in gout and OMERACT definitions [12,13].
Specific findings OMERACT definition [12,13]
Double contour sign “Abnormal hyperechoic band over the superficial margin of the articular hyaline cartilage, independent of the angle of insonation and which may be either irregular or regular, continous or intermittent and can be distinguished from the cartilage interface sign” 
Aggregates “Heterogeneous hyperechoic foci that maintain their high degree of reflectivity even when the gain set- ting is minimized or the insonation angle is changed and which occasionally may generate posterior acoustic shadow” 
Tophus “A circumscribed, inhomogeneous, hyperechoic and/or hypoechoic aggregation (which may or may not generate posterior acoustic shadow) which may be surrounded by a small anechoic rim” 
Synovial fluid “Abnormal hypoechoic or anechoic (relative to subdermal fat, but sometimes may be isoechoic or hyper- echoic) intra-articular material that is displaceable and compressible; does not exhibit Doppler signal”
Synovial hypertrophy “Abnormal hypoechoic (relative to subdermal fat, but sometimes may be isoechoic or hyperechoic) intra- articular tissue that is nondisplaceable and poorly compressible; may exhibit Doppler signal” 
Power Doppler signal Bone surface changes
(erosions) “An intra- and/or extra-articular discontinuity of the bone surface (visible in 2 perpendicular planes)” 
Table III. Specific structures for monosodium urate depositions and ultrasonographic findings.
Anatomical structure US findings
Hyaline cartilage •Double contour: focal or diffuse enhancement of the superficial articular cartilage layer •Slightly irregularities of the superficial margin of the hyaline cartilage;
•Findings independent of the insonation angle
Synovial fluid •Hyperechoic spots (<1 mm) floating within the joint cavity •“Snowstorm” appearance
Bone •Intra-articular erosions, profound and destructive
•Extra-articular breaks of the bony cortex (intra-osseous tophaceous deposits) Tendon •Microdeposits: ovoid-shaped hyperechoic densities
•Small hypoechoic halo in inflammatory response
Soft tissue deposits •Soft tophi: nodular, small, hypoechoic, homogenous structures, soft on palpation
•Hard tophi: hyperechoic band with posterior acoustic shadow, harder in consistency on palpation •Mixed tophi
storm” appearance [14,15,19]. Crystal aggregates have to be differentiated from joint debris or proteinaceous ma- terials . Some authors described hyperreflective foci in joint fluids that were related to gas bubbles (probably nitrogen), bubbles formed as a result of joint movement and capsular distension producing fluid agitation .
Lowering the gain during examination could help in cor- rect interpretation.
The gold standard for gout diagnosis is the visuali- zation of MSU crystals (in effusion, synovial biopsies, tophi aspirates, etc) . By US the joints with effusions are easily identified and US-guided aspiration can be per- formed, being of great help for clinicians, especially in undiagnosed cases.
The synovial hypertrophy (fig 1) is a frequent non- specific finding. The increased vascularisation in the proliferated synovium can be interpreted as inflamma- tory reaction and may precede erosions and destruction of the adjacent bone [similar with rheumatoid arthritis (RA)], especially if the bone is in contact with a tophus . No specific aspects of the synovitis or vascularisa- tion are seen in order to differentiate the acute gout from other acute arthritis. Only the presence of hyperechoic spots or hyperechoic cloudy areas inside the synovium has high grade of specificity for gout . The presence of the power Doppler signal (especially grade 2 or 3),
correlates well with the acutely affected joints. Also, the power Doppler signal disappears with treatment .
The pathological vascularisation can be detected also in apparently non-affected joints, demonstrating that power Doppler is significantly more sensitive in identification of the active gouty joints that is the clinical examina- tion . The main utility of US from this point of view seems to be the monitoring of the treatment in gouty pa- tients .
The bone erosions, step-down lesions in the bone contour detectable in two perpendicular plans, were posi- tive correlated with the number of gout attacks, long dis- ease, and the presence of tophi . Common sites for searching the bone erosions in patients with gout are the medial side of the first metatarsophalangeal (MTP) joint and the metacarpophalangeal (MCP) joints [12,18,25].
The erosions in gout detected by US at the MTP-1 are significantly more frequent (67%) comparing with other arthritis (43%) or healthy subjects (6%) .
Comparing US with x-ray findings, the erosions sug- gestive for gout are detected more often by US [25,26]
proven to be a moderately specific (69%) but with a low sensitivity imaging method (24%) . The main limita- tion of the method are the lack of clear criteria for differen- tiation the gout erosions from erosions of other inflamma- tory arthritis (RA, spondyloarthritis, etc). The erosions are
Fig 1. a) Longitudinal scan of the first metatarsophalangeal (MTF) joint- acute gout, gray scale: fluid and proliferated synovium with few hyperechoic spots; b) acute gout- hypervascularised proliferated synovium; c) intercritical gout, second MTF joint – small quantity of transonic fluid and proliferated synovium; note the double contour of the hyaline cartilage; d) longitudinal scan of the tibialis anterior tendon- intratendinous hyperechoic deposits with alteration of the fibrillar structure and tenosynovi- tis; e) and f) transversal scan of the tibialis anterior tendon, gray scale and color Doppler- hyperechoic deposits, tenosynovitis and hypervascularisation.
usually more deeper and destructive comparing with RA [14,27] but only the association of erosions with findings suggestive for MSU deposits confer higher specificity.
Extra-articular breaks of the bony cortex due to tophaceous deposits are easily detectable when the de- posit is less compact and allows the penetration of the US beam (fig 2, a,b,c).
No standardized US scoring system for erosions in gout exists, but the scoring system created by Wakefield et al  for RA (small <2 mm, moderate 2–4 mm, and large erosions >4 mm) can be applied also for gout.
The US aspect of the normal hyaline cartilage is char- acterized by a homogeneously anechoic layer delimited by two sharp hyperechoic margins. The superficial mar- gin is thinner than the deeper one and its visualization requires perpendicular ultrasound beam on the cartilage surface (avoiding anisotropy) . MSU have a particu- lar predilection for crystallize on the superficial margin of the cartilage and so the MSU crystals are deposited mainly in the superficial hyaline cartilage. By this mech- anism a focal or diffuse enhancement of the superficial margin of the articular cartilage (thickness similar to the subchondral bone), with the reflectivity independ- ent from the angulations of the ultrasound beam – the double contour (DC) sign – is created [11,19,26,29] (fig 2 d,e,f,g). Slightly irregular surface of the hyperechoic band may be seen [11,19,29]. This sign is considered to be highly specific for gout [6,30].
The DC sign has 43.6% sensitivity and 99% specific- ity for gout diagnosis . More important, the sign was found only in gouty patients; no other articular disease or
healthy subjects had this type of DC, the sign been a useful finding for the differential diagnosis between different arth- ritides . Moreover, inter-reader reliability in assessing the DC sign was found to be excellent [29,31], making this sign one of the most important US finding in gout.
The DC sign is very well visualized in MTP, knees, MCP and less visualized in joints with thin cartilage (tarsal joints) or with osteoarthritis . It is important to differentiate this sign from the normal cartilage interface (well seen in the presence of the joint effusion due to the posterior enhancement) or from the modifications en- countered in osteoarthritis (thin and irregular cartilage) and calcium pyrophosphate deposits (aggregates within the cartilage) [6,15,19].
The presence of the DC sign is not strictly related of the stage of the gout. The sign was found even in the asymptomatic hyperuricemia phase. In a recent study De Miguel et al  found DC sign in 27% (7 patients) of the 26 enrolled patients with asymptomatic hyperurice- mia (sensitivity 100%, specificity 88%). All the 7 pa- tients had MSU crystals in fluids from joint aspiration.
Taking in account EULAR definition  these patients should be interpreted as gout despite the lack of symp- toms. More studies are necessary to clarify this aspect. In the other phases of gout the DC sign could be consider a marker of disease severity , urate levels being posi- tively correlate with this sign in knee and MTF joints (but not with the presence of tophi) . Ottaviani et al 
found in patients with crystal-proven gout the DC sign with sensitivity of 67% for MTPs, 57% for knees, and 21% for MCPs, and very high specificity (>98%).
As concerning the responsiveness to treatment, the DC sign disappearance was found to be a good indicator for an efficient treatment [35-37].
Fig 2. a), b) and c) Small, moderate, and large erosions of the medial part of the first metatarsal head; d) normal aspect of the hyaline cartilage, femoral condyle; e) focal enhancement of the superficial margin of the articular cartilage: the double contour sign; f) diffuse enhancement of the superficial margin of the articular cartilage; g) irregular surface of the hyperechoic band.
MSU aggregates (hyperechoic spots, hyperechoic cloudy areas, tophi)
At a concentration over 6.8 mg/dL the serum urate ex- ceeds the limit of solubility and supersaturation of urate in the serum and other extracellular spaces takes place . There are still an-answer questions about the trigger mechanisms of the disease but the deposition of MSU aggregates is consider to be the landmark for gout. These aggregates can be found theoretically in every structure of the organism.
The MSU aggregates reflects ultrasound beams stronger than the surrounding tissues and so it can be eas- ily distinguished and their reflectivity is less influenced by the insonation angle [6,15,19,38]. The US aspect of these structures has high variability depending of their size, localizations, distribution, local reactions, or the disease phase  (fig 3).
Hyperechoic spot, less than 1 mm, is the most en- countered US finding. When hyperechoic spots are found in joint effusion, the “snowstorm” appearance could be demonstrated. Their presence in proliferated synovium is considered to be a specific sign for gout.
Hyperechoic cloudy areas (HCA) or “cottony imag- es”  are aggregates less then 1cm, well or poorly defi- nite, generally homogeneous without posterior acoustic shadow  and are considered to be elementary lesions in gout . HCA were found to be significantly influ- enced by treatment .
The tophi, extracellular deposits of MSU surrounded by foreign-body giant cells and mononuclear cells form- ing a granuloma-like structure , have US aspect de- pending on their age and size. Tophi may occur at any site but their frequent locations are the toes, especially
MTP I (dorsal more frequent that volar), wrist, fingers, knee, and ankle [17,18]. Three types of tophi were de- scribed according to the degree of compaction of the de- posits: soft, hard, and mixed [14,15,19,40].
At the beginning the tophi are nodular, small, hypo- echoic, with homogenous structure, and soft on palpation – soft tophi. A hyperechoic rim could be found . As they grow they become more echogenic, with hyperecho- ic edges and well differentiated from the surrounding tis- sues. They can dislocate the normal structures of the area or even compress them (tendons, vessels, nerves, bone).
For example there are cases with carpal tunnel syndrome caused by tophaceous gout . In this stage a clear dif- ferentiation from rheumatoid nodules is difficult . So- noelastography was recently used to differentiate rheuma- toid nodules from tophi and the result are promising .
After years the tophi might develop into pseudotu- moral forms and become dishomogenous, harder in con- sistency on palpation – hard or mixed tophi, even with calcification foci, with posterior acoustic shadow. A hypoechoic halo can appear if there is an inflammatory reaction around the tophus. This rim is formed by mac- rophages, lymphocytes and large foreign body giant cells  and if this kind of tophus is in contact with the bone, the presence of the inflammatory cells can explain why the tophus erodes the bone surface . In patients with gouty flare a surrounding edematous and hypervascular- ised soft tissues is observed . Moreover, a persistent low-grade of inflammation in asymptomatic chronic tophaceous gouty arthritis was found and this may ex- plain the silent development of erosions .
The prevalence of tophi in gout becomes higher dur- ing the disease evolution. Filippucci et al  found nearly half of the gouty patients having tophi in at least
Fig 3. Monosodium urate deposits: a) hyperechoic spots, less than 1 mm in the proliferated synovium of the suprapatelar bursa of the knee; b) hyperechoic spots and hyperechoic cloudy areas in the first metatarsophalangeal joint, dorsal longitudinal scan;
c) hyperechoic cloudy areas in the patellar tendon, longitudinal scan; d) hyperechoic cloudy area in the synovial sheath of the tibialis anterior tendon, longitudinal scan; note the fluid in the ankle joint; e) soft tophi in the middle and distal phalanx, volar scan; f) mixed tophi near the elbow; g) hard tophus in the Achilles tendon.
one of the examined joints. Ottaviani et al  detected articular urate deposits in 60% of gouty patients (the prevalence of tophi for knees and MTPs being 26.7%), more common in patients with higher serum uric acid.
The tophi were found as well in patients with asympto- matic hyperuricaemia not only in synovium but also in tendons and soft tissue [45,46].
As concerning the measurement of the tophi Perez- Ruiz et al  compared the utility of MRI and US in evaluating intraarticular and articular tophi. US detected at least one tophus in all joints but MRI found nodules considered to be tophi. The aspiration of nodules suspect- ed as tophi confirmed MSU crystals in 83 %. The study found good correlation but only fair agreement between US and MRI. More important is the higher intraobserver intraclass correlation for diameters and volume, suggest- ing that US is a reliable method for measure the tophi.
This aspect is important in monitoring the gout evolution and response to treatment. In patients with adequate con- trol of serum uric acid levels a decreased in diameter and volume of the tophi was observed .
Intratendinous tophi can be frequently found inside the patellar ligament, Achilles, triceps, quadricipital, an- terior tibialis tendons, and plantar fascia. They are hypo/
hyperechoic and inhomogeneous, with poorly defined margins, producing focal alteration of the fibrillar struc- ture of the tendon and generating posterior shadow. Hy- perechoic linear bands deposits of MSU within the ten- don substance were also described [6,15,17-19,48,49].
These lesions are areas of minimal resistance and sponta- neous tears can occur .
Among the periarticular bursae, the pre-patellar and olecranon bursae are the most commonly involved sites [46,49]. Fernandes et al  found that hyperechogenic- ity (91.7%), poorly defined contours (88.6%), multiple grouped nodules (85.6%), and heterogeneity (68.6%) are the most characteristics findings of the tophi from olecra- non bursa. Peritendinous bursae (infrapatellar, preachil- lian) were found to be inflamed in cases with concom- mitant tendon pathology .
Subcutaneous tophi should be differentiated from other types of nodules, especially rheumatoid nodules. In 80% of the gouty nodules the masses are heterogeneous and hyperechoic [27,52].
The complex deposits of MSU in digits (tendon, sub- cutaneous tissue, etc) could create the aspect of gouty dactylitis, a rare finding, seen in chronic polyarticular disease [53-55].
Enthesopathy, although uncharacteristic for gout, was found around patella, quadriceps tendon, and patellar lig- ament insertions and around calcaneus at Achilles tendon and plantar fascia insertions .
One of the problems in US investigation of gouty pa- tients is which sites to be investigated (only symptomatic joints or structures or more extensive examination) and what findings to be search (MSU deposits, effusion, syn- ovial hypertrophy, vascularisation) in order to have good sensibility and specificity of the method.
In a recent published paper by Naredo et al  the authors found that US examination of 12 anatomical sites (bilateral radio-carpal joint, first metatarsal head cartilage, talar cartilage, second metacarpal head, knee femoral condyle cartilage, patellar and triceps tendons) for DC sign and hyperechoic aggregates gives the best results for sensitivity and specificity (84.6% and 83.3%, respectively). This approach had high positive and good negative predictive values (92% and 71%, respectively) for diagnosing gout.
Peiteado et al  studied the presence of six types of lesions in gouty patients: hyperechoic spots in the synovial fluid, HCA, bright stippled aggregates, DC sign, erosions, and the Doppler signal in 17 joints and 8 tendons. They found that the knees and MTP joints are the most frequent affected sites (in 93% of patients) and a six-minute US ex- amination of four joints (knees and the 1st MTPs) allowed the detection of HCA or DC sign in 97% of cases.
Clinical diagnosis of gout is sometimes difficult, even if the manifestations are in MTF-1 (classical podagra).
Kienhorst et al  recently demonstrated that in patients with MTP-1 arthritis the gout was the correct diagnosed only in 77% of cases but the general practitioner sup- posed gout in 98% of cases. In these cases US exami- nation of the joint could simplify the diagnosis process.
This fact was demonstrated by Lamers-Karnebeek et al  in patients with monoarthrithis in which 48% of pa- tients had MSU crystals-proven gout. In this cases sen- sitivity of DC sign and any US abnormality (DC sign, tophi, or snowstorm appearance of the joint effusion) was 77 and 96 %, respectively. The authors considered that the DC sign is an important informative finding for clini- cian, with positive predictive value of 74 % and negative predictive value of 78 %.
The new US techniques probably will increase the US capacity to detect the crystals aggregates. MicroPure, a new US image processing function designed to improve the visualization of microcalcifications, was used by Yin et al  in patients with gout. Significantly more mi- crocalcifications were seen with MicroPure compared to gray scale US and the level of agreement between inves- tigators was consistently improved. The method seems to be useful especially for the cases where the presence of gray scale US artifacts or unspecific findings make
difficult the interpretation. The use of different imaging methods (dual energy CT, MRI, positron emission to- mography) may improve the description of lesions .
The main question is if there is a really need in clinical practice for such a complex approach.
The need for good education and training for identifi- cation and interpretation of the US findings encountered in gout was underline by Filippucci et al .The authors highlight the necessity for dedicated training-programme to avoid the false positive and false negative results. Af- ter 7 days of training the rheumatologists with limited US experience gain satisfactory skills to identify MSU aggregates in different tissues.
Due to the lower diagnostic utility of the clinical gouty features (exception tophi and response to colchi- cine) , there is an incremental need for imaging tech- niques to confirm the clinicians’ suppositions. The place of the US in patients suspected of gout must be clearly defined  due to the high numbers of patients present- ed with monoarthritis. The research agenda concerning the use of US in gouty patients is still busy despite of the recent achievements in this pathology.
In conclusion in the assessment of the gouty patients US is an important and valuable tool. For a complete ex- amination and a correct interpretation of the imagines the examiner need to have solid US knowledge about normal and pathological aspects of the musculoskeletal struc- tures. Knowing the specific US aspects of MSU deposi- tions will permit a rapid diagnosis. Moreover, when com- bining the clinical examination with US scan in patients with suspicion of gout a proper and rapid decisions for the patient management can be taken.
Conflict of interest: none
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