Identification of gout in unusual sites by dual energy computed tomography.

A 41-year-old man presented with 2-year history of episodic swelling and pain in the knees. He had been initially diagnosed as having gout at a local hospital and treated with allopurinol and diclofenac. The symptoms progressed and he was referred to our department. He denied a history of other diseases. Physical examination found swelling and tenderness in both knees and the right ankle. A joint aspiration from the right knee on admission found sodium urate crystals in the aspirated fluid under a polarized light microscope. Additionally, an abnormal chest radiograph revealed a bone erosion in the left clavicle region. Bone scintigraphy after 99mTc-methylene diphosphonate (99mTc- MDP) injection showed collective radionuclide in both knees, the right ankle, and the first left rib. Bence-Jones protein in the blood and urine were within the normal reference range. To exclude malignancy on the ribs, a dual energy computed tomography (DECT) was performed, which confirmed urate deposits bilaterally in 1 to 4 ribs, extremitas sternalis clavicularis, extremitas acromialis clavicularis, shoulder blades, presternum and cervical vertebra (Figure 1) and both knees (Figure 2). The joints most commonly involved in gout are knees, meta-tarsophalangeal joints (MTPs), and ankles.1 Ribs, clavicle, sternum, scapula and vertebrae are rarely affected. In this case, all of these unusual sites were included. DECT is an imaging advance in recent years showing a high sensitivity and specificity to gout crystals.2 It is useful in differentiating gout from other lesions, such as malignant tumor. Additionally, DECT can identify and color-code monosodium urate crystals in multiple joints simultaneously and noninvasively.3 Considering its high price and the potential risk of radiation, DECT is not a routine tool in gout diagnosis, but it is still highly recommended and is especially useful in scanning and identifying gout in unusual sites, which are hard to differentiate by conventional radiography or nuclear medicine.

Figure 1

DECT demonstrated the urate crystal deposition (shown in green) in the ribs, scapula, sternum and cervical vertebra.

Figure 2

DECT demonstrated the urate crystal deposition (shown in green) in the knees.