Musculoskeletal

Crystal Arthropathy

An acute hot-joint syndrome caused by crystal-triggered inflammation; aspirate when uncertain to distinguish monosodium urate, calcium pyrophosphate and septic arthritis, then tailor acute and long-term care to the result.

In a nutshell

Crystal arthropathy presents as an acutely hot, painful joint. First exclude septic arthritis when clinically suspected or the diagnosis is uncertain, then identify the crystal: negatively birefringent needles indicate gout and weakly positively birefringent rhomboids indicate CPPD. Acute treatment is anti-inflammatory; only gout has treat-to-target urate-lowering therapy.

Classic presentation

A rapidly painful, red and swollen joint: first MTP favours gout, while knee or wrist in an older person favours CPPD, but aspiration is needed when the diagnosis or infection status is uncertain.

Key points

Septic arthritis can coexist with crystals; a crystal result does not end the infection assessment.
Gout: monosodium urate, needle-shaped and negatively birefringent; CPPD: rhomboid and weakly positively birefringent.
Serum urate supports and monitors gout but is not a test for CPPD and can be low during a gout flare.
Chondrocalcinosis supports CPPD but does not itself prove that the current hot joint is non-infective.
Treat acute crystal inflammation with an NSAID, colchicine or corticosteroid according to comorbidity and medicine safety.
Gout requires consideration of treat-to-target urate lowering; do not start allopurinol for CPPD.
For early-onset, recurrent or atypical CPPD, consider associated metabolic disease with specialist input.
Recurrent, refractory or diagnostically uncertain disease warrants rheumatology advice.

First-line investigation

Synovial-fluid aspiration for crystal analysis, Gram stain and culture when the diagnosis is uncertain or infection is possible.

Management

Treat the hot joint as potentially septic

Assess systemic illness and infection risk; use the local septic-arthritis pathway and aspirate for microscopy, Gram stain and culture when diagnosis is uncertain.^1,2
Remember that infection and crystals can coexist, so do not let a crystal finding stop the microbiology assessment.^2,3

Identify the crystal and suppress inflammation

Use microscopy to distinguish urate needles from CPP rhomboids, and choose an NSAID, colchicine or corticosteroid after checking comorbidity and the BNF.^1,2,5,6
For a single large joint, aspiration with local steroid injection may be sufficient for acute CPP crystal arthritis after infection is excluded and under appropriate clinical expertise.⁴

Tailor the long-term plan

For gout, review serum urate, flare burden, tophi, CKD and diuretic therapy to decide on treat-to-target urate lowering.¹
For CPPD, do not use allopurinol; manage flares, associated osteoarthritis and any identified metabolic contributor.^3,4

Refer atypical or refractory disease

Seek rheumatology advice for recurrent or refractory attacks, uncertain microscopy, atypical early-onset or polyarticular CPPD, or a treatment plan limited by comorbidity.^1,4
Consider targeted metabolic assessment in early-onset, florid or recurrent CPPD rather than ordering an unfocused panel for every older patient.⁴

Confirm recovery and safety-net

Review response and function, explain the diagnosis and medicine risks, and provide a plan for recurrent attacks.^1,3
Tell the patient to seek urgent help for fever, systemic illness or a worsening hot joint because septic arthritis may develop or coexist.^1,2

Exam traps

Do not call every hot swollen joint gout or pseudogout without considering infection.
Positive crystals do not exclude septic arthritis.
Chondrocalcinosis is supportive of CPPD, not proof that an acute presentation is sterile.
Serum urate is irrelevant to diagnosing CPPD and can be normal during a gout flare.
Allopurinol lowers urate for gout; it is not treatment for CPPD.

Illustrations

Calcium pyrophosphate crystals under polarised lightPolarised-light microscopy showing rhomboid calcium pyrophosphate crystals, which are weakly positively birefringent; compare with the needle-shaped, negatively birefringent urate crystals of gout.Mikael Haggstrom, Wikimedia Commons · CC0

Key sources

This page is exam revision material, not medical advice, and must not be used for patient care. Always check drug doses against the BNF and current guidance. Full disclaimer.