Pseudogout is a crystal arthropathy, which may cause an acute inflammatory arthritis from deposition of calcium pyrophosphate.
The principle mechanism underlying pseudogout is deposition of calcium pyrophosphate dihyrate (CPP). In fact, pseudogout is an old term that refers to one of the clinical syndromes within a spectrum of conditions associated with CPP deposition (CPPD) in connective tissue.
The term pseudogout is still used commonly in clinical practice. This is because of the close resemblance to gout in both pathophysiology (crystal arthropathy) and clinical features (acute inflammatory arthropathy).
CPPD is an umbrella term that covers all clinical conditions linked to the deposition of CPP in connective tissue.
The term CPPD was originally proposed by the European League Against Rheumatism (EULAR). When associated with arthritis, it is termed CPPD disease.
In total, there are six clinical syndromes associated with CPPD, but the majority are asymptomatic.
The majority of cases of pseudogout are idiopathic.
Several secondary conditions have been associated CPPD disease. These are particularly important to consider in younger patients.
Calcium pyrophosphate crystal formation and deposition is central to the pathogenesis of pseudogout.
Chondrocytes are the principle cell involved in the formation and deposition of CPP. Chondrocytes are the only cell type found in articular cartilage. They are essential for the generation of the structural component of cartilage, which then embeds them.
In pseudogout, excess pyrophosphate production causes local CPP supersaturation, crystal formation and deposition. CPP crystals stimulate a proinflammatory response leading to activation of phagocytes and neutrophils. The inflammatory response is usually self-limited due to the modification of CPP crystals (e.g. addition of lipoproteins), which reduces the inflammation.
Pseudogout presents with acute severe joint inflammation that is characterised by pain, redness, swelling and restricted movement.
Pseudogout characteristically leads to asymmetrical arthropathy of ‘larger’ peripheral joints. Patients presenting with a first episode affecting the upper extremity should raise suspicion of pseudogout over gout.
Diagnosis of pseudogout is based on synovial fluid analysis and plain film radiography.
A definitive diagnosis of pseudogout (i.e. acute CPPD disease) requires evidence of both CPP crystals on synovial fluid analysis and classical radiographic changes.
Due to the pattern of joints affected in pseudogout, it is difficult to confidently make a diagnosis without evidence of CPP crystals.
Patients with an acutely swollen joint should undergo arthrocentesis. Samples should be sent for microbiology (white cell count + microscopy, culture and sensitivity) and biochemistry (crystal analysis).
NOTE: if there is any concern about septic arthritis a joint aspiration should always be performed.
Conventional radiographs should be used to identify cartilage calcification (CC), previously known as chondrocalcinosis. X-rays are useful for assessing other features of CPPD disease (e.g. degenerative changes).
Typical sites of CC include the menisci of the knee or the triangular fibrocartilage complex (TFCC) of the wrist. In hyaline cartilage, CC will appear as a thin opaque line. Some x-ray changes are suggestive of a secondary disorder (e.g. second and third MCPs in hereditary haemochromatosis).
The treatment of pseudogout involves steroid injections, NSAIDs and/or colchicine.
The majority of acute flares of pseudogout will resolve within 7-14 days. However, treatment is highly effective in reducing the severity of the illness.
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