A case of IgG4oligoarthritis mimicking psoriatic arthritis.

IgG4-related disease (IgG4-RD) is a rare but rapidly emerging immune-mediated fibroinflammatory condition that can affect almost any organ. It is typically associated with involvement of organs such as lymph nodes, submandibular glands, orbits, periaortic region and pancreas. However, IgG4-RD presenting primarily as an inflammatory arthritis is much less recognised. We present a rare case of IgG4-RD mimicking psoriatic arthritis. In spite of normal circulating IgG4 plasma levels, a clinical index of suspicion was required to obtain a histopathological diagnosis. The patient's aggressive arthritis disease did not initially respond to typical disease- modifying agents such as methotrexate. Instead, we report a dramatic response to B cell depletion with rituximab. It is important to emphasise this case as a rare presentation of a rare disease. IgG4-RD is a rapidly emerging condition and the diagnosis should be considered when the alternatives do not seem to fit. © BMJ Publishing Group Ltd (unless otherwise stated in the text of the article) 2018. All rights reserved. No commercial use is permitted unless otherwise expressly granted.