Clinical Pearls & Morning Reports
Published May 4, 2022
Crystal deposition disorders, such as calcium pyrophosphate deposition disease or gout, can cause bony erosions, pain, and fever, thereby mimicking a spinal infection. Read the NEJM Clinical Problem-Solving Article here.
Q: What are some of the risk factors for hyperuricemia?
A: Gout is caused by an excess of urate. Hyperuricemia is largely genetically mediated, but factors including high intake of purine-rich foods, renal dysfunction, and diuretic use can impair urate clearance or increase the serum urate level. Psoriatic arthritis is also a risk factor. Excess urate precipitates within the joints and soft tissues, forming monosodium urate crystals. Crystal aggregates called tophi can deposit throughout the body and erode into adjacent joints.
Q: How are urate crystals typically identified?
A: Unpolarized light microscopy has low sensitivity for the detection of urate crystals. Polarized light that is shone through a compensating filter causes urate crystals to glow yellow when they are parallel to the direction of the polarized light and blue when they are perpendicular to the light. This phenomenon, which is caused by negative deflection of the wavelength of the light along the plane of the crystal, is known as negative birefringence and is the diagnostic standard for the detection of urate crystals. This technique is rarely performed unless specifically requested.
A: Cells of the innate immune system can engulf urate crystals and trigger an interleukin-1β–mediated autoinflammatory cascade, resulting in a gout flare. Flares commonly cause acute swelling, redness, and tenderness in affected joints, but they can also cause fevers and chills and can mimic septic arthritis. Monoarticular attacks often involve the first metatarsophalangeal joint. However, gout can affect any joint and can evolve into a chronic, painful, widespread inflammatory arthritis that mimics rheumatic diseases such as rheumatoid arthritis and psoriatic arthritis. Spinal gout is now considered to be rare. Clinical manifestations can include neck or back pain, radicular symptoms, or neurologic compromise. Signs include fever, leukocytosis, and elevated inflammatory markers; thus, distinguishing spinal gout from infectious spondylodiskitis can be difficult.
A: Traditional radiography, CT, and MRI have low sensitivity and specificity for spinal gout, whereas dual-energy CT has a sensitivity of 78 to 100% and a specificity of 89 to 100% for the detection of monosodium urate. Dual-energy CT can also identify advanced chondrocalcinosis. Dual-energy CT uses two sets of images that are acquired simultaneously at different peak kilovoltages to exploit differences in the chemical composition of tissues at different energies; the differences in attenuation at high and low energy, termed the dual-energy index, is then calculated for each compound. Elements with a high molecular weight, such as calcium, have a higher dual-energy index than those with a low molecular weight, such as monosodium urate. Potential limitations of this technique include noise or scatter, which may cause single green pixels or submillimeter foci to be mistaken for crystals. Low concentrations of urate crystals can also be missed at current scanner resolutions.