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Gout is a crystal arthropathy due to deposition of monosodium urate crystals in and around the joints.
Typically occurs in those above 40 years. There is a strong male predilection of 20:1, with this predilection more pronounced in younger and middle-aged adults. In the elderly, the gender distribution is more equal 13.
Acute gouty arthritis presents with a monoarticular red, inflamed, swollen joint, typically in the lower limb and classically affecting the first metatarsophalangeal joint (podagra) 12. It often manifests during sleep, and can later involve more than one joint to become an oligoarthropathy or rarely, a polyarthropathy 12.
Once the acute phase is over, usually within 7-10 days, there is an intercritical asymptomatic period (intercritical gout) between acute flares 12. This asymptomatic period is unique to crystal arthropathies and varies in length between patients, but often lasts months 12.
Patients with chronic uncontrolled hyperuricemia, such as those with chronic kidney disease, may develop chronic tophaceous gout. In chronic tophaceous gout, there are solid urate crystal collections (tophi) and chronic inflammatory and destructive changes in surrounding connective tissue 12. These tophi are typically yellow-white in color, non-tender, and are typically located within the articular structures, bursae, or the ears 12.
The pathology is characterized by monosodium urate crystals deposition in periarticular soft tissues. The crystals are needle-shaped and are strongly birefringent in plane-polarized light 10. The synovial fluid is generally a poor solvent for monosodium urate and therefore crystallization occurs at low temperatures. The crystals are chemotactic and activate complement.
There are five recognized stages of gout:
- asymptomatic hyperuricemia
- acute gouty arthritis
- intercritical gout (between acute attacks)
- chronic tophaceous gout
- gouty nephropathy
The primary risk factor is hyperuricemia, although only a small proportion of patients with hyperuricemia develop gout, often taking 20 to 30 years to develop. There are two mechanisms by which hyperuricemia can develop:
- undersecretion of uric acid by kidneys (90%)
- overproduction of uric acid (10%)
The reason gout is uncommon in premenopausal women is thought to be due to estradiol having a urate-lowering effect 13.
Usually, has an asymmetrical polyarticular distribution:
- joints: 1st MTP joint most common (known as podagra when it involves this joint); hands and feet are also common
- less common: bones, tendons, bursae
Most radiographic findings include the skeletal system.
Characteristic radiologic changes occur in the chronic stage, though not all patients progress to this. There is a predilection for the small joints of the hands and feet. Chondrocalcinosis is present in ~5%.
- joint effusion (earliest sign)
- preservation of joint space until late stages of the disease
- an absence of periarticular osteopenia
- eccentric erosions
- the typical appearance is the presence of well-defined “punched-out” erosions with sclerotic margins in a marginal and juxta-articular distribution, with overhanging edges, also known as rat bite erosions
- punched-out lytic bone lesions
- overhanging sclerotic margins
- mineralization is normal
Surrounding soft tissues
- tophi: pathognomonic
- olecranon and prepatellar bursitis
- periarticular soft tissue swelling due to crystal deposition in tophi around the joints is common
- the soft tissue swelling may be hyperdense due to the crystals, and the tophi can calcify (uncommon in the absence of renal disease)
While there can be variation in appearance, tophi tend to be hyperechoic, heterogeneous, and have poorly defined contours. They can form multiple groups with surrounding anechoic haloes 8. Additional findings may include:
- echogenic, irregular bands apposed to articular cartilage 15,16
- synovial thickening
- with increased vascular flow
- joint effusions
- with dependent hyperechoic, punctate debris
- bony cortical discontinuities
- associated with adjacent formed tophi
Other described sonographic features on ultrasound include
- double contour sign 11
- snowstorm appearance 11
Findings generally reflect those on the plain radiograph.
Dual-energy CT can distinguish between urate mineralization and calcification, which may be useful for cases where the clinical and biochemical presentation is atypical 11. Allowing for not only visualization and characterization, but also quantification of monosodium urate crystal deposits, it can be used for treatment monitoring as well 14.
Signal characteristics of gouty tophi are usually:
- T1: isointense
- variable 4
- the majority of lesions are characteristically heterogeneously hypointense
- T1 C+ (Gd): tophus often enhances
Treatment and prognosis
Acutely, gout can be managed with non-steroidal anti-inflammatory drugs (e.g. naproxen), colchicine, prednisolone, or newer cytokine blocking agents (e.g. IL-1 blockers such as anakinra or canakinumab) if refractory disease 12. In the long-term, xanthine oxidase inhibitors (e.g. allopurinol or febuxostat), uricosuric drugs (e.g. probenecid), or uricase agents (e.g. pegloticase) may be used to reduce urate levels and prevent further acute flares 12. Tophaceous gout can also be managed with surgical excision of symptomatic lesions 12.
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