A Flare in the ED: Using Ultrasound to Diagnose Gout
CASE
A 53-year-old male with hypertension presents to the emergency room with painful swelling of the right first toe at the MTP joint. The patient was in good health until a few days ago, where he was attending a college reunion. The swelling and pain increased overnight. The patient is having difficulty walking this morning due to the pain. He tried taking two Advil for the pain, which helped bring the pain from an 8/10 to a 6/10.
Patient is afebrile to 99.2, BP is 138/90 and the remaining vitals are stable.
Physical exam shows an uncomfortable but otherwise well appearing male. Exam is unremarkable except for his right first MTP, which is erythematous, edematous, and tender to palpation. Range of motion for his right foot is intact, DP/PT pulses 2+ bilaterally, sensation is intact, and motor strength is 5/5 bilaterally. There is pain with weightbearing causing a limp with ambulation.
Labs, including CBC, ESR, CRP, and a BMP were drawn. In addition, the swollen joint was aspirated for synovial fluid analysis and bedside ultrasound imaging was completed in the ED.
DIAGNOSIS
Gout
DISCUSSION
Introduction
Joint pain is a common complaint in the ED, and the differential diagnosis is wide, including trauma, osteoarthritis, rheumatoid arthritis, septic arthritis, gout, pseudogout, cellulitis, and osteomyelitis. With the patient seen above, there is a monoarticular flare leading to pain, swelling, and redness.
Gout is a common inflammatory arthritis caused by the accumulation of monosodium urate crystals in the joints and soft tissues [1]. The prevalence of gout is more than three percent in the adult American population, affecting men more than women, and has increased worldwide [2]. The extracellular fluid urate saturation is reflected by hyperuricemia in the blood (more than 6.8 mg/dL). Uric acid is the result of purine metabolism from natural substances in the body as well as from diet. High purine foods include red meat, seafood, and alcohol. Uric acid can build up either through overproduction or underexcretion by the kidneys. Uric acid crystallizes within the joints and soft tissues, predominantly on superficial positions of articular cartilage [3]. Almost all joints can be affected, although the great toe is common.
Gold standard diagnosis is traditionally found via synovial fluid analysis. In patients with gout, the analysis shows monosodium urate crystals and a white blood cell count between 10,000-100,000 with neutrophil predominance. Under polarized microscopy, the fluid shows needle-shaped, yellow, negatively birefringent crystals.
Imaging modalities including radiography, MRI and CT have been studied for their sensitivity and specificity in the diagnosis of gout. Bony changes leading to gouty tophi or erosions can show up on imaging, but are not usually found with a first gout flare. Ultrasound may be useful for early detection or for monitoring progression [4].
Ultrasound Findings
Diagnostic descriptions of gout on ultrasound include a hyperechoic irregular or linear enhancement known as the double contour sign (DCS), which shows on the articular surface of the hyaline cartilage. Another is hyperechoic cloudy areas that indicate the presence of tophi within the joint or tendons. A “snowstorm-like” appearance with “bright stippled foci” may also show up in the joint space [1,3,5].
Figure 1. Ultrasound of an ankle joint, longitudinal view, showing the double contour sign[1]
Figure 2. Comparing controls to gout patients. (A) dorsal longitudinal (B) plantar longitudinal (C) medial longitudinal [3]
Figure 3. Snowstorm appearance in a right wrist joint [6]
Systematic review shows that the pooled (95% CI) sensitivity and specificity of US DCS were 0.83 (0.72 to 0.91) and 0.76 (0.68 to 0.83), respectively; of US tophus, were 0.65 (0.34 to 0.87) and 0.80 (0.38 to 0.96), respectively [4]. In the Study for Updated Gout Classification Criteria (SUGAR), the double contour sign was statistically significant in diagnosing gout [7].
The advantages of ultrasound are that it is non-invasive, relatively low cost, lacks ionizing radiation, and can be used in settings where more advanced imaging is not readily available. At the same time, ultrasound relies on a good acoustic window to visualize a joint and is less sensitive than MRI [1].
CASE RESOLUTION
The patient’s synovial fluid aspiration revealed a WBC of 70,000 with 80% neutrophils, and an elevated ESR/CRP. Additionally, a double contour sign was identified on ultrasound. The patient was provided with oral indomethacin 50 mg TID and will be followed by his primary care provider to monitor progress.
TAKE-AWAYS
· Ultrasound has become an increasingly accepted method in the diagnosis of gout and for monitoring treatment due to its sensitivity and specificity
· Sonographers should look for the “double contour” sign when suspecting gout
· Synovial fluid analysis remains the gold standard method for diagnosing gout
Keywords & Categories:
Gout, ultrasound, musculoskeletal
AUTHOR: Shital Shah is a fourth-year medical student at The Warren Alpert Medical School of Brown University.
FACULTY REVIEWER: Kristin Dwyer, MD, MPH is the Emergency Ultrasound Division and Fellowship Director at Brown University.
REFERENCES
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