Background/Purpose: Calcium pyrophosphate deposition disease (CPDD) is an often un-recognized form of acute and chronic arthritis preferentially affecting the elderly.    Some controversy exists about a possible association of rheumatoid arthritis (RA) with CPDD. While early studies suggested that RA and CPDD rarely overlapped, more recent data suggest that CPDD may be more common in patients with RA than in the general population.  Possible explanations for this association include diagnostic confusion, higher imaging rates in patients with existing arthritis,  or  increased deposition of CPP crystals in the damaged RA cartilage. The purpose of our study was to describe a cohort of well-characterized patients with both CPPD and RA, and to generate possible hypothesis about the nature of the relationships between these diseases.

Methods:   After approval from the local IRB, a cohort of patients with both RA and CPDD was collected from a university based rheumatology clinic.   Patients were identified by participating providers and were included if they satisfied ACR criteria for RA and had CPDD noted radiographically or by synovial fluid crystal identification.  A total of 21 such patients seen in the year 2015-2016 were selected for further analysis. Their records were reviewed and analyzed for age, sex, age of RA onset with the diagnosis based on ACR criteria, age of CPDD onset (using either crystal identification or chondrocalcinosis), pattern of joint involvement ,  joint surgeries, and laboratory values, including rheumatoid factor (RF), cyclic citrullinated peptide antibody (CCP), iron studies, PTH , and calcium levels.

Results: This cohort with both RA and CPDD had a mean age of 75 years, with a mean age of RA onset of 53 years. Two thirds were women and most were seropositive.  Medications included typical DMARDs and biologics for 18/21 patients. One was on no medications, and two were on low dose prednisone.   The mean age of CPDD onset was 69.9 years, with the mean RA disease duration prior to the onset of CPDD of 13.4 years.  One patient had a simultaneous diagnosis of both RA and CPDD.  The majority of the patients were diagnosed with CPDD based on the presence of chondrocalcinosis (15/21) and the most commonly involved joint was the knee.  Other involved joints were typical of the joint pattern seen with CPDD.  Three patients underwent joint replacement of the joints involved with CPDD.  No patient had significant abnormalities of PTH levels or elevated iron levels. Calcium supplements were used in about 50% of patients.

Conclusion: The results of our study support existing data that CPDD tends to co-occur with RA after years of disease. The typical demographics of CPDD and its pattern of joint involvement are preserved in patients with RA who develop CPDD, and thus association is not likely to be due to diagnostic confusion.   Further studies to understand the connection between these diseases are warranted.   Table 1. Patient characteristics