Key Points
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Radiography is the main imaging modality for evaluation of progression of gout, but there is a 5- to 10-year latent period between first clinical symptoms and the appearance of specific radiographic findings.
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The classic radiographic appearance of gout is the presence of well-defined erosions with overhanging edges, normal bone mineralization, and relative sparing of the joint space.
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Tophi appear as soft tissue masses with higher density than surrounding soft tissue and occasionally are calcified.
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Computed tomography is useful in the evaluation of complex areas such as the spine and sacroiliac joints.
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Dual-energy computed tomography (DECT) is specific in detecting urate crystal deposition and in distinguishing tophi from calcific crystal deposits. There is noteworthy utility of DECT in diagnosis of gout, and in following the outcome of therapeutic urate lowering.
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Magnetic resonance imaging is primarily used for evaluation of spinal involvement by gout. Tophus deposits often have low to intermediate signal on all sequences.
Introduction
The gold standard for making the diagnosis of gout is the detection of urate crystals in the symptomatic joint. Imaging is often inconclusive early in the disease, as the only finding is nonspecific soft tissue swelling. In typical cases, there is a latent period of about 5 to 10 years between the first clinical symptom and the appearance of specific plain radiographic signs, such as well-defined erosions and tophi. In more advanced disease, imaging provides documentation of extent and distribution of the disease and allows assessment of response to treatment. High-resolution ultrasound, an increasingly useful modality for detection of early disease in gout, and for differential diagnosis of gout and calcium pyrophosphate dihydrate (CPPD) deposition disease, is reviewed in detail in Chapter 26 .
General Considerations
Soft Tissue
An early finding of gout is soft tissue swelling around the joint during the acute gouty attach ( Fig. 24-1 ), this soft tissue swelling resolves when the symptoms subside. Later in the disease, with chronic gout there is deposition of urate crystals in the soft tissues with the formation of tophi. On radiographs, tophi appear as masses slightly denser than surrounding soft tissues ( Fig. 24-2 ). Occasionally, tophi may calcify ( Fig. 24-3 ) and rarely ossify. Calcification of tophi suggest the presence of coexisting calcium-containing crystals such as hydroxyapatite or an underlying abnormality of calcium and mineral metabolism such as renal failure or hyperparathyroidism.
Joint Space
Although erosion can be present, the joint space is often well maintained until late in the disease ( Fig. 24-4 ). The presence of a relatively normal joint space in patients with extensive erosions helps in distinguishing gout from other type of arthritis. Obliteration of the joint space and bony ankylosis are rare.
Bone Mineralization
Mild osteopenia is occasionally observed in films taken during an acute gouty attack, but more often the bone mineralization is normal; when osteopenia is present, it may be mediated by effects of urate crystals on osteoblast and osteoclast function, as well as disuse.
Erosions
Bone erosions can be marginal ( Fig. 24-5 ), intraarticular ( Fig. 24-6 ), or away from the joint ( Fig. 24-7 ). Erosions are usually well defined and may have a sclerotic border. A characteristic of gouty erosion is the presence of overhanging edges. These spurlike formations are thought to be related to a combination of the bony resorption beneath the gradually enlarging tophus and the concomitant bony thickening at the edge of the involved cortex.
Intraosseous Calcifications
Intraosseous calcifications ( Fig. 24-8 ) can be seen especially in the hand and foot and are caused by intraosseous tophi. Intraosseous tophi are usually seen in patients with longstanding tophaceous gout with hyperuricemia and chronic renal failure.
Differential Diagnosis
When reviewing radiographs of patients with arthritis, it is important to remember the “ABCDs of arthritis” ( A lignment, B one mineralization and proliferation, C artilage, D istribution and S oft tissues). The alignment of the joints is usually normal with gout, while it is frequently abnormal in rheumatoid arthritis, in spondyloarthritis malalignment is a late manifestation. Bone mineralization is usually normal with gout as in spondyloarthritis; with osteoarthritis, bone density is normal or increased; and with rheumatoid arthritis, osteopenia is common. Bone proliferation is rare in gout, it is absent in rheumatoid arthritis, and it is common with spondyloarthritis and juvenile idiopathic arthritis. The joint space is often normal in gout; joint space narrowing is an early finding in rheumatoid arthritis and osteoarthritis and a late manifestation in spondyloarthritis. Erosions in gout are eccentric, intraarticular or extraarticular, with sclerotic borders and overhanging edges. In rheumatoid arthritis, erosions are marginal or intraarticular and do not have sclerotic borders unless treated. In spondyloarthritis, erosion may be marginal or intraarticular and often associated with periosteal bone proliferation. The distribution of joint involvement is usually asymmetric in gout, while it is symmetric in rheumatoid arthritis. Distribution is variable with spondyloarthritis. Soft tissue swelling is often eccentric in gout and is fusiform in rheumatoid arthritis. In spondyloarthritis, it can be fusiform or involve an entire digit (sausage digit). With osteoarthritis, soft tissue swelling is often absent except in erosive osteoarthritis. Soft tissue calcifications are seen occasionally in gout, but are rare in rheumatoid arthritis and spondyloarthritis. Soft tissue calcifications are common in calcium pyrophosphate deposition disease, hydroxyapatite deposition disease, scleroderma, dermatomyositis, and polymyositis ( Table 24-1 ).
| Gout | Rheumatoid Arthritis | Seronegative Arthritis | |
|---|---|---|---|
| Alignment | Normal | Frequent malalignment | Occasional malalignment |
| Bone proliferation | Rare | Absent | Common |
| Bone mineralization | Normal or mild osteopenia | Moderate to severe osteopenia | Normal or mild osteopenia |
| Cartilage: joint space | Often normal | Early narrowing | Late narrowing |
| Cartilage: erosions | Eccentric, sclerotic borders, overhanging edges, intraarticular or extraarticular | Marginal, no sclerotic borders, intraarticular, no overhanging edges | Marginal, intraarticular |
| Distribution | Asymmetric | Symmetric | Asymmetric or symmetric |
| Soft tissue swelling | Eccentric | Fusiform | Fusiform or sausage-digit |
| Soft tissue calcifications | Occasional | Rare | Rare |
Computed Tomography
The differentiation of tissues in computed tomography (CT) is based on their x-ray attenuation as quantified in Hounsfield units and displayed in shades of gray at different window levels in CT scans. Attenuation is caused by absorption and scattering of radiation by the tissues examined. The attenuation of different tissues varies based on the energy of the x-rays. The variation in attenuation with different beam energy is small for low atomic number tissues such as soft tissues and tophi but is larger for high atomic number materials such as calcium. The main advantage of dual-energy CT (DECT) is that the ratio or change in CT values is independent of density or concentration of the tissue. Using DECT it is possible to color coding for dual energy properties of different materials (i.e., uric acid uroliths, urate crystalline tophi, and calcific crystals).
Single-Energy Computed Tomography
Single-energy CT is used in evaluation of the extent of gout in areas that are difficult to visualize on radiographs, such as sacroiliac joints and spine. The density of sodium urate crystals is approximately 170 Hounsfield units (HU) ± 30 HU ( Figs. 24-9 and 24-10 ), which is very different from muscles (less than 50 HU) or calcium (greater than 400 HU), but differentiation of a calcified tophus from other calcified lesion is difficult on CT. CT is more useful in differentiating tophi from other noncalcified soft tissue masses such as xanthomas or rheumatoid nodules.
Dual-Energy Computed Tomography
DECT is useful to evaluate mixed lesion containing both calcium deposits and urate deposits. DECT can help in difficult cases including atypical gout and subclinical tophi. DECT can distinguish confidently gout from other inflammatory arthritis and calcific deposits of CPPD deposition disease. ( Fig. 24-11 ) It also provides accurate and reproducible volumetric assessment of the tophi size. The specificity of DECT in detecting tophaceous disease could eliminate the need of arthrocentesis in some cases, unless septic arthritis needs to be ruled out (as a prime example).
Nuclear Medicine
Nuclear medicine is rarely used in the evaluation of patients with gout. Abnormal uptake in patients with gout is often an incidental finding on a nuclear medicine examination performed for other reasons.
Scintigraphy
Skeletal scintigrams are obtained 3 to 4 hours after intravenous injection of 99m Tc methylene diphosphonate; this 3- to 4-hour delay is used to give time for urinary excretion to decrease the amount of radiotracer in the soft tissues. The uptake on these delayed images is proportional to the bone turnover. Increased uptake is present on skeletal scintigraphy in the bones affected by gout due to increased bone turnover, but it is not specific. Similar increased uptake can be seen with other types of arthritis such as rheumatoid arthritis or osteoarthritis. To distinguish osteomyelitis from cellulitis, three-phase bone scans are used, but they do not help in distinguishing acute gouty arthritis from septic arthritis or osteomyelitis ( Fig. 24-12 ). During an acute gouty attack, increased uptake is present on all three phases of a bone scan. In the first phase (blood flow phase), increased activity is seen due to hyperemia; in the second phase (blood pool phase), increased activity is seen due to the local inflammation; and in the third phase (delayed images), increased uptake is present due to bone remodeling.
Related posts:
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Overview of Gout Therapy Strategy and Targets, and the Management of Refractory Disease
Calcium Pyrophosphate Dihydrate Crystal Deposition: Epidemiology, Clinical Features, Diagnosis, and Treatment
Basic Calcium Phosphate Crystal Arthropathy
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