Summary

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.