IgG4-Related Disease Retroperitoneal Fibrosis: An Unusual Cause of Low Back Pain.

IgG4-related disease (IgG4-RD) is a cluster of rare fibroinflammatory diseases that more commonly affect organs such as major salivary glands, biliary tree, periorbital tissues, kidneys, lungs, lymph nodes, retroperitoneum, and less frequently, meninges, aorta, prostate, thyroid gland, pericardium, and the skin. The clinical picture mainly depends on the affected organ and the effects on the surrounding organs, however, the histopathologic findings are very similar regardless of the organ affected. Most patients have a subclinical presentation of the disease and the only clinical manifestation is related to the anatomic location of the disease, whereas some patients may have constitutional symptoms such as weight loss and are often misdiagnosed as having other pathologies (i.e., malignancies, other inflammatory conditions, etc.). Up to 40 percent of patients can have symptoms of allergy or asthma. Patients often have diseases confined to one organ but multiorgan involvement is not uncommon. Patients with multiple organs involvement can have an elevation of up to 30-40 upper limit of normal serum IgG4 concentration; patients with fewer organ involvement can have normal serum IgG4 concentration despite histopathologic findings of the disease.  Idiopathic retroperitoneal fibrosis (RPF) is a commonly encountered subtype of IgG4-RD. Idiopathic retroperitoneal fibrosis accounts for approximately 70 percent of cases and can be divided into IgG4-RD and non-IgG4-RD. Most cases of RPF are incidental findings on radiology studies but should be suspected in any patients complaining of back pain and flank pain, with new-onset kidney dysfunction.