MM is a malignant plasma cell proliferative disorder characterized by diffuse tumor infiltration into the bone marrow. The median age at diagnosis is 66 years. Malignant plasma cells invade multiple organs and create different symptoms such as bone pain, renal failure, hypercalcemia, fractures, anemia, and symptoms of hyperviscosity. Neoplastic plasma cells occasionally adopt a different growth pattern and engender, in 9% of cases, tumor masses called extramedullary plasmacytomas (EMP) (6).
The majority (97%) of MM patients have a monoclonal protein secreted by malignant plasma cells. Malignant plasma cells may produce immunoglobulin heavy and light chains, light chains, or neither. Their frequencies are IgG (52%), IgA (21%), kappa or lambda light chain only (16%), IgD (2%), X (2%), IgM (0.5%), and negative (6.5%) (8).
IgD MM is a rare type of MM much more common in the Asian population (9, 10). It currently develops at a younger age, predominantly males, with a median age of onset of 58 years (9, 10).
IgD-secreting plasma cells are the product of somatic hypermutation of the IgD region of germinal center B cells. Unlike other myelomas, lambda light chain predominance is a characteristic feature of IgD myeloma and is seen in 70% to 90% of cases (7).
The clinical features of IgD MM are similar to those of other myeloma types (1). Asthenia, weakness, pallor, and bone pain are the most early manifestations (7). The risk of renal damage in IgD MM is increasing and the patients with renal impairment tend to be younger (10). According to the litterature, renal failure is present in 20% to 40% of patients at the time of diagnosis (7) while primary kidney involvement and severe renal insufficiency are seen in only 15% to 20% of cases (4,11, 12). The mechanisms of renal injury can be either due to light chain cast nephropathy or through the direct toxicity caused by intracellular crystals (7,11). When present, renal failure predicts worse prognosis in newly diagnosed IgD MM patients (10).
Extramedullary involvement in IgD MM is seen in 19% to 63% of patients. The usual sites are the chest wall, respiratory tract, gastrointestinal tract tract, skin, lymph nodes, paraspinal areas, and rarely isolated testicular involvement (7, 13). EMP may either be present at diagnosis or develop later during the disease course (7). They can present as nerve root compression or extradural tumor and studies have shown that patients with EMP have decreased progression-free survival (1).
Our patient was North-African. At dignosis of IgD lambda MM, he was 55 and he presented with nonspecific complaints including abdominal pain and vomiting. He also presented with primarily kidney involvement and severe renal insufficiency, which are infrequently seen in IgD myeloma. What seems above all unusual and even surprising in this observation is that the patient presented with a 10-year-old sternal arch, which he neglected and which later turned out to be a plasmacytoma. He would therefore have had a dormant sternal plasmacytoma for ten years which would later turn into MM.
According to the literature, patients with solitary plasmacytoma and minimal bone marrow involvement are at higher risk for progression to MM compared to when there is no detectable plasmacytosis in the marrow (5). This was first demonstrated in a retrospective review of 127 patients at Mayo Clinic (14). In their study, including 50 patients, Hill et al. concluded that the progression occurred in 72% of patients with occult marrow disease detectable by flow cytometry compared with 12% in patients who did not have any demonstrable bone marrow involvement by flow cytometry (15). The median time to development of MM was 21 months, with a 5-year probability of 51% in the series of Knobel et al. (16).
Our patient most likely had a solitary plasma cell for a long time without bone marrow involvement. This would explain the very late evolution towards an MM.
Historically, IgD MM has been thought to be associated with a poor prognosis. Compared with other subtypes, the median survival of patients with IgDtype received traditional chemotherapy was generally less than two years. However, some recent studies have suggested comparable survival rates with other subtypes thanks to autologous hematopoietic stem cell transplantationand novel therapies such as proteasome inhibitors and immunomodulatory agents (9, 17, 18). Unfortunately, our patient could not benefit from these therapies.