Urate Crystals; Beyond Joints

Abstract

Gout is the most common inflammatory arthropathy caused by the deposition of monosodium urate (MSU) crystals. The burden of gout is substantial with increasing prevalence of gout globally. The prevalence of Gout in the United States has increased by over 7% in the last two decades. Initially, it was believed that MSU crystal deposits occur only in the joints with the involvement of the periarticular soft tissues, but recent studies have shown the presence of MSU crystal deposition in extra-articular sites as well. Human plasma becomes supersaturated with uric acid at 6.8 mg/dl, a state called hyperuricemia. Beyond this level, uric acid crystals precipitate out of the plasma and deposit in soft tissues, joints, kidneys, etc. If left untreated, hyperuricemia leads to chronic gout characterized by the deposition of tophi in soft tissues such as the joints, tendons, and bursae. With the advent of newer imaging techniques such as DECT, MSU crystals can be visualized in various extra-articular sites. Extra-articular deposition of MSU crystals is believed to be the causative factor for the development of multiple comorbidities in gout patients. Here, we review the literature on extra-articular deposition of urate crystals and the role of dual-energy computed tomography (DECT) in elucidating multi-organ involvement. DECT has emerged as an invaluable alternative for accurate and efficient MSU crystal deposition detection. Future studies using DECT can help determine the clinical consequences of extra-articular deposition of MSU in gout patients.

Keywords: DECT; MSU crystal deposition; extra-articular; gout; hyperuricemia.

Figure 1

Cardiovascular urate deposits. (A) Urate crystal (green) deposits in the left main coronary artery, left anterior descending artery, and its first diagonal branch (arrowheads). Note the artifactual green deposits in the coastal cartilage (arrow), as it has similar attenuation profile as uric acid. (B) Mural urate deposit in the descending thoracic aorta (arrowhead). (C) Urate deposit in the right coronary artery (arrowhead) and in the aortic valve calcification (arrow). Note the artifactual green deposit at the edge of vertebral osteophyte (circle).

Figure 2

Urate renal stones. (A) Coronal reformatted, (B) 3D rendered, and (C) axial images demonstrating bilateral renal urate stones (red).

Figure 3

Spinal urate deposits. (A) Sagittal reformatted, (B) coronary reformatted, and (C) 3D-rendered images demonstrating urate deposits in facet joints (green).