Residency · Residency · Rheumatology

Calcium Pyrophosphate Deposition Disease

Introduction

Calcium pyrophosphate deposition disease encompasses a clinical spectrum ranging from asymptomatic chondrocalcinosis discovered incidentally on imaging to acute pseudogout flares and chronic CPP crystal inflammatory arthritis that can closely mimic rheumatoid arthritis. The prevalence of CPPD increases dramatically with age, affecting approximately 10 to 15 percent of individuals aged 65 to 75 years and 30 to 50 percent of those over 85 years. The condition is frequently underdiagnosed because of its protean clinical presentations, which can mimic rheumatoid arthritis, osteoarthritis, gout, and septic arthritis. The 2023 EULAR terminology recommendations have standardized nomenclature, with "CPPD" replacing older and inconsistent terms and "acute CPP crystal arthritis" replacing the traditional designation of "pseudogout."

Pathophysiology

CPP Crystal Formation

Calcium pyrophosphate dihydrate crystals are rhomboid or rod-shaped structures that demonstrate weak positive birefringence under compensated polarized light microscopy. Unlike monosodium urate crystals in gout, which deposit on the surface of cartilage, CPP crystals form within cartilage itself, predominantly in hyaline cartilage and fibrocartilage. The generation of inorganic pyrophosphate by chondrocytes is mediated by the ectonucleotide pyrophosphatase/phosphodiesterase enzyme ENPP1, which produces extracellular pyrophosphate from nucleotide triphosphates. The transmembrane transporter ANKH, known as ANK in mice, channels intracellular pyrophosphate to the extracellular space, further increasing local pyrophosphate concentrations. When excess extracellular pyrophosphate combines with calcium in the cartilage matrix, CPP crystal nucleation occurs. Aging promotes crystal formation through alterations in the cartilage matrix, including changes in proteoglycan composition and structure that create a more permissive environment for crystal nucleation.

Crystal-Induced Inflammation

When CPP crystals are shed from their cartilage matrix into the synovial fluid, they are phagocytosed by macrophages and neutrophils, triggering an inflammatory cascade. The NLRP3 inflammasome is activated through the same pathway as in gout, leading to caspase-1 activation and IL-1 beta release. Complement activation contributes to the inflammatory response. The intensity of inflammation in CPPD is generally less than that seen with MSU crystals in gout, though substantial overlap exists, and individual flares can be quite severe.

Risk Factors for CPPD

Age is the single strongest risk factor for CPPD, and the disease is rare before the age of 50. Several metabolic and endocrine conditions are associated with CPPD and should be screened for when the diagnosis is made before age 55 or when the disease is florid or polyarticular. Hyperparathyroidism promotes crystallization through elevated calcium levels. Hemochromatosis facilitates crystal nucleation through iron deposition in cartilage, and screening with ferritin and transferrin saturation is indicated. Hypomagnesemia is relevant because magnesium normally inhibits CPP crystal formation, making magnesium levels an essential part of the metabolic workup. Hypophosphatasia leads to elevated pyrophosphate levels due to reduced alkaline phosphatase activity. Wilson disease is a rare association. Hypothyroidism has a mild but recognized association with CPPD. Gitelman syndrome causes CPPD through chronic hypomagnesemia. Familial or hereditary CPPD, caused by autosomal dominant mutations in the ANKH gene, presents with early onset before age 50, polyarticular involvement, and severe disease.

Clinical Presentations

2023 EULAR Nomenclature

The 2023 EULAR nomenclature recognizes four principal clinical presentations of CPPD. Asymptomatic CPPD refers to chondrocalcinosis identified on imaging without accompanying symptoms and represents the most common presentation. Acute CPP crystal arthritis, formerly termed pseudogout, denotes self-limited acute inflammatory flares. Chronic CPP crystal inflammatory arthritis describes a pattern of chronic inflammatory polyarthritis. OA with CPPD refers to osteoarthritis occurring in joints atypical for primary OA where coexistent CPPD is present.

Acute CPP Crystal Arthritis

Acute CPP crystal arthritis presents as an acute monoarticular or oligoarticular flare that is typically less explosive in onset than gout. The knee is the most commonly affected joint, involved in more than 50 percent of cases, followed by the wrist, ankle, shoulder, and elbow. Episodes tend to last days to weeks, generally persisting longer than typical gout flares. Common triggers include surgery, acute medical illness, hospitalization, and rapid changes in calcium or phosphate levels such as those occurring after bisphosphonate infusion or parathyroidectomy. The clinical presentation can closely mimic septic arthritis, making arthrocentesis with synovial fluid analysis mandatory to exclude infection. Pseudogout attacks in hospitalized patients are particularly common, often triggered by the physiologic stress of illness or surgery.

Chronic CPP Crystal Inflammatory Arthritis (Pseudo-RA)

Chronic CPP crystal inflammatory arthritis produces a chronic symmetric polyarthritis that can closely mimic rheumatoid arthritis. The metacarpophalangeal joints, wrists, and knees are commonly involved. Patients experience morning stiffness and have elevated ESR and CRP levels. Low-titer rheumatoid factor may be present in 10 to 15 percent of elderly patients, further complicating the distinction from RA. Distinguishing features that favor CPPD over RA include the presence of chondrocalcinosis on radiographs, CPP crystals identified on joint aspiration, absence of anti-CCP antibodies, and older age at onset.

OA with CPPD

When CPPD occurs in conjunction with osteoarthritis, it characteristically involves joints that are atypical for primary OA, including the wrists at the radiocarpal joint, the metacarpophalangeal joints particularly the second and third, shoulders, ankles, and elbows. This pseudo-OA pattern features progressive degenerative changes in these atypical locations with radiographic evidence of CPPD. The arthritis may exhibit a more inflammatory phenotype than typical primary osteoarthritis.

Crowned Dens Syndrome

Crowned dens syndrome results from CPPD deposition around the odontoid process of C2. It presents with acute severe neck pain and stiffness, often accompanied by fever and markedly elevated CRP, creating a clinical picture that can mimic meningitis, polymyalgia rheumatica, or cervical abscess. The diagnosis is established by CT of the cervical spine, which reveals characteristic calcification around the dens on axial or coronal views, forming a crown-like pattern. Treatment consists of colchicine, NSAIDs, or a short course of glucocorticoids, with most episodes resolving within days to weeks.

<image>A multi-panel clinical presentation diagram of CPPD disease. Panel A: AP knee radiograph showing chondrocalcinosis with linear calcification in the meniscal fibrocartilage and hyaline cartilage of the femoral condyles, with a magnified inset highlighting the calcifications. Panel B: AP wrist radiograph showing TFCC (triangular fibrocartilage complex) calcification and radiocarpal joint CPPD deposition. Panel C: Axial CT of the cervical spine showing calcification around the odontoid process (crowned dens syndrome) with a ring of calcium deposits around the dens. Panel D: Compensated polarized light microscopy image showing rhomboid-shaped, weakly positively birefringent CPP crystals (blue when parallel to compensator axis) within and outside neutrophils. Label all findings clearly and include the birefringence characteristics.</image>

Diagnosis

Synovial Fluid Analysis

Synovial fluid analysis with compensated polarized light microscopy remains the gold standard for diagnosing CPPD. CPP crystals are rhomboid or rod-shaped and demonstrate weak positive birefringence, appearing blue when aligned parallel to the slow axis of the compensator. These crystals are often more difficult to identify than MSU crystals and may require careful, systematic examination of the synovial fluid specimen by an experienced observer. The synovial fluid cell count typically ranges from 5,000 to 30,000 white blood cells per microliter, generally less inflammatory than gout on average. Gram stain and culture must always be sent to exclude septic arthritis, as infection must be ruled out in every case of acute monoarthritis. It is important to recognize that CPP and MSU crystals can coexist within the same joint, and the identification of one crystal type does not exclude the presence of the other.

Imaging

Conventional radiographs demonstrate chondrocalcinosis as linear calcification within fibrocartilage and hyaline cartilage, best visualized in the knee menisci, the triangular fibrocartilage complex of the wrist, the pubic symphysis, and the hip labrum. Chondrocalcinosis may be absent in early CPPD or after crystal dissolution, limiting the sensitivity of plain radiographs. Structural changes including joint space narrowing, subchondral cysts, and sclerosis may be present in atypical OA locations. Ultrasound reveals hyperechoic punctate or linear deposits within the mid-zone of hyaline cartilage, a location that distinguishes CPPD from the surface deposition seen with the double contour sign of gout. OMERACT definitions have standardized ultrasound findings for research purposes. CT is superior for detecting crowned dens syndrome, spinal CPPD, and tendon calcification. Dual-energy CT is less reliable for differentiating CPP from MSU crystals and is better suited for gout diagnosis.

Metabolic Workup (When to Screen)

Screening for underlying metabolic conditions is indicated when CPPD is diagnosed before age 55, when disease is florid or polyarticular, when acute attacks are recurrent, or when there is a family history of CPPD. The recommended screening panel includes calcium, parathyroid hormone, ferritin, transferrin saturation, magnesium, alkaline phosphatase, thyroid-stimulating hormone, and ceruloplasmin in younger patients to evaluate for Wilson disease.

Management

Acute CPP Crystal Arthritis

Intra-articular glucocorticoid injection is the first-line treatment for monoarticular or oligoarticular flares after infection has been excluded by synovial fluid analysis. Triamcinolone acetonide at 40 milligrams for the knee or 10 to 20 milligrams for smaller joints provides rapid and effective relief. Colchicine using the same low-dose regimen as gout, consisting of 1.2 milligrams followed by 0.6 milligrams at one hour, extrapolated from the AGREE trial, is an effective alternative. Prophylactic colchicine at 0.6 milligrams daily to twice daily is used for recurrent attacks. NSAIDs including indomethacin 50 milligrams three times daily or naproxen 500 milligrams twice daily for 7 to 10 days are appropriate options. Systemic glucocorticoids with prednisone 25 to 30 milligrams daily for 3 to 5 days are effective for polyarticular flares. IL-1 inhibition with anakinra 100 milligrams subcutaneously daily for 3 to 5 days may be used off-label for refractory cases, though data are limited. ACTH (cosyntropin) at 40 to 80 international units intramuscularly is an alternative in hospitalized patients.

Chronic CPP Crystal Inflammatory Arthritis

Colchicine at 0.6 milligrams daily to twice daily serves as the first-line agent for both prophylaxis and chronic inflammation. Low-dose NSAIDs are employed when colchicine alone is insufficient or contraindicated. Methotrexate at 7.5 to 20 milligrams weekly is used in practice for chronic inflammatory CPPD, though the evidence base is limited. Hydroxychloroquine at 200 to 400 milligrams daily has been supported by case series and is generally well tolerated. IL-1 inhibition with anakinra has been reported effective in refractory chronic CPP arthritis in case reports and small series. A critically important point is that no urate-lowering therapy equivalent exists for CPPD; no currently available drug reliably dissolves CPP crystals, and management therefore remains exclusively anti-inflammatory.

Treat Underlying Metabolic Disease

When an underlying metabolic condition is identified, targeted treatment may improve CPPD outcomes. Parathyroidectomy for hyperparathyroidism may reduce the frequency of CPPD flares. Phlebotomy and iron reduction in hemochromatosis may slow CPPD progression, though existing crystal deposits may not resolve. Magnesium supplementation, either oral or intravenous, is indicated for hypomagnesemia.

Differential Diagnosis

The differential diagnosis of CPPD includes gout, in which MSU crystals are needle-shaped and negatively birefringent, exhibiting distinctly different crystal morphology. Septic arthritis must always be excluded, as it can coexist with CPPD. Rheumatoid arthritis is distinguished by anti-CCP positivity, RF positivity, symmetric polyarthritis, and the absence of chondrocalcinosis. Basic calcium phosphate disease involves apatite crystals that are not visible on standard polarized microscopy and require alizarin red staining for detection, with Milwaukee shoulder syndrome being the prototypical clinical entity. Hydroxyapatite deposition disease presents as calcific tendinitis, most commonly of the supraspinatus tendon, with dense calcification visible on radiographs.

Key Clinical Pearls

• CPPD before age 55 should trigger metabolic screening (hyperparathyroidism, hemochromatosis, hypomagnesemia, hypophosphatasia)
• CPP crystals are weakly positively birefringent (blue parallel to compensator) vs MSU crystals which are strongly negatively birefringent (yellow parallel)
• Crowned dens syndrome can mimic meningitis or PMR; CT of C-spine is diagnostic
• There is NO equivalent of urate-lowering therapy for CPPD; management is anti-inflammatory only
• Chondrocalcinosis in atypical OA locations (wrist, MCP, shoulder) should raise suspicion for CPPD
• Hospitalized patients with acute joint flares should have CPPD high on the differential

<image>A diagnostic and management algorithm for CPPD disease. Start with "Acute monoarthritis/oligoarthritis in patient >50 years." First step: Joint aspiration (mandatory) → synovial fluid analysis with compensated polarized microscopy, cell count, Gram stain, culture. Branch to: CPP crystals identified → CPPD confirmed. No crystals → consider other diagnoses. For confirmed CPPD, show two pathways: (1) Acute management: IA GC injection (first-line if 1-2 joints), colchicine, NSAIDs, systemic GC. (2) Metabolic screening: If age <55, polyarticular, or recurrent → check calcium, PTH, ferritin, transferrin saturation, magnesium, alkaline phosphatase, TSH. Include a chronic management box: colchicine prophylaxis 0.6 mg daily/BID, HCQ, MTX for chronic inflammatory CPPD. Note "No crystal dissolution therapy available" in a highlighted box.</image>

References

1. Rosenthal AK, Ryan LM. Calcium pyrophosphate deposition disease. N Engl J Med. 2016;374(26):2575-2584.
2. Zhang W, et al. EULAR recommendations for calcium pyrophosphate deposition. Part I: Terminology and diagnosis. Ann Rheum Dis. 2011;70(4):563-570.
3. Abhishek A, et al. EULAR recommendations for calcium pyrophosphate deposition disease. Part II: Management. Ann Rheum Dis. 2011;70(4):571-575.
4. Martinon F, et al. Gout-associated uric acid crystals activate the NALP3 inflammasome. Nature. 2006;440(7081):237-241.
5. Viriyavejkul P, et al. Calcium pyrophosphate deposition disease: pathogenesis, diagnosis, and management. Curr Rheumatol Rep. 2023;25(7):135-147.

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