| Summary: | The heavy chain diseases (HCDs) are rare B-cell malignancies characterized by the production of a monoclonal immunoglobulin heavy chain without an associated light chain. There are three types of HCD, defined by the class of immunoglobulin heavy chain produced: IgA (α-HCD), IgG (γ-HCD), and IgM (μ-HCD). Alpha-HCD is the most common and usually occurs as intestinal malabsorption in a young adult from a country of the Mediterranean area. Gamma- and μ-HCDs are more rare and associated to a B-cell non-Hodgkin lymphoma that produces an abnormal Ig heavy chain. These patients may occasionally be diagnosed with a monoclonal gammopathy of undetermined significance (MGUS). Fanconi syndrome, on the other hand, can be primary (inherited) or secondary (acquired). The only exception to this rule is the idiopathic form. Adult acquired Fanconi syndrome can be a rare complication of a monoclonal gammopathy. At diagnosis, most patients have a MGUS or smoldering multiple myeloma, with renal failure and evidence of osteomalacia. During follow-up, patients can develop end-stage renal disease. Chemotherapy provides little benefit on renal function.
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