To the Editor: Sarcoid arthritis is often underdiagnosed because it may mimic reactive or rheumatoid arthritis. We report a case which was initially misdiagnosed.

A 38-year-old white woman was admitted to hospital because of pain and swelling of her hands and feet. Two years earlier, she had been admitted because of joint pain and erythematous, painful round lesions on her shins. A chest x-ray at that time was normal. She was diagnosed with reactive polyarthritis based on positive serological tests for Rickettsia conorii and Coxiella burnettii. Doxycycline and indomethacin were given and her condition improved.

Three days before the current admission her fingers, wrists and ankles had become painful and swollen. There was tenderness and swelling of the metacarpophalangeal, wrist and ankle joints. She was afebrile. Chest x-rays showed an enlarged left hilum. Computed tomography (CT) of the chest (Box) showed lymphadenopathy in the mediastinum and both hila. Her erythrocyte sedimentation rate was 104 mm/h (normal, 3–12 mm/h). Laboratory test results were normal, except for an elevated serum level of angiotensin-converting-enzyme (ACE). A mediastinoscopy was performed, and specimens obtained for biopsy revealed sarcoidosis. Prednisone (30 mg/day) was prescribed and she was discharged 7 days later with no symptoms.

“Sarcoid arthritis” is a sarcoid process whose main or unique manifestation is joint disease. Some of its characteristics are seasonal clustering (typically in spring), higher incidence among non-smoking patients, and the presence of the human leukocyte antigen DQ2 (DQB1*0201) and DR3 (DRB1*0301) haplotypes. It occurs slightly more frequently in women. The median age of affected patients is 40 years. The process affects mainly ankle and knee joints symmetrically. Acute sarcoid arthritis is a self-limiting joint disease with a benign prognosis, but some patients can develop chronic sarcoidosis of the lungs, specially those who suffer recurrent episodes of arthritis.1,2

In our patient, the first episode, with associated erythema nodosum, was misdiagnosed as a reactive arthritis as there were false positive serological test results for Rickettsia and Coxiella secondary to an immune polyclonal response. In the second episode, the patient had mediastinal and hilar lymphadenopathy and an elevated ACE level. Although sarcoid arthritis is very often associated with lymphadenopathy and erythema nodosum (Löfgren syndrome), we should keep in mind other forms of joint involvement in sarcoidosis.3

Doctors should consider sarcoid arthritis in the differential diagnosis of seronegative arthritis. Chest x-ray and ACE assay are useful in identifying sarcoidosis.

Computed tomography scan showing mediastinal and hilar lymphadenopathy