Plasma cell crystalline inclusions with rod, rectangular, and needle-like shapes have been described in cases of multiple myeloma and they are believed to be due to accumulation of cytoplasmic immunoglobulins secondary to a block in the protein synthetic pathway [2, 5, 8]. Herein we present a case of plasma cell myeloma with rhomboid intracytoplasmic crystalline inclusions and extracellular crystals deposition. It is of note that it has been postulated that the presence of crystalline inclusions may indicate a nonprogressive clinical course of the disease . However, since this morphological pattern of presentation of multiple myeloma is very rare, its prognostic significance is currently largely unknown. Yet, it is important to consider this association as part of the differential diagnosis in a patient with plasma cell myeloma and, for example, arthritis with crystal deposition. Rectangular, rod-like and rhomboid-like crystals in the joints could be seen in cases of calcium pyrophosphate dihydrate (CPPD) crystal deposition disease or pseudogout. In plasma cell myeloma, a rheumatoid-like seronegative polyarticular erosive arthropathy is an unusual but reported presentation characterized by crystal deposition of cryoglobulins in the synovium and several other tissues . The patient presented here did not report articular symptoms for which we could not yet establish a connection between crystalline deposits in the bone marrow and the synovium.
Renal parenchyma damage in patients with plasma cell myeloma is frequent and well characterized and can involve crystal depositions. Renal biopsy can show intracytoplasmic crystalline inclusions most frequently located in the distal tubule, but that can also be seen in the proximal tubule . So far, the patient presented here has not suffered renal insufficiency and a renal biopsy is not available. Therefore, an association between bone marrow and renal parenchyma crystalline deposition could not be made.
The skin and subcutaneous tissue could also be a site of crystalline depositions in patients with plasma cell myeloma [11, 12]. In such cases, crystalloid structures have been identified within histiocytes and the entity interpreted as a cutaneous crystal storing histiocytosis . Interestingly, a case of bilateral ecchymoses and corneal crystal deposition have been reported as initial presentation of plasma cell myeloma in a 55 year-old man . This patient presented with amyloidosis and Bence-Jones proteins but crystalline deposits in the bone marrow were not reported.
The differential diagnosis in cases of plasma cell intracytoplasmic crystalline inclusions should also include reactive processes, since it has been shown to occur in cases of helicobacter associated gastritis . In the case presented here, a reactive process is ruled out due to the monotypic nature of the plasma cell population.
Finally, crystalline inclusions similar to those seen in plasma cell myeloma have been described in cases of granulocytic sarcoma . Electron microscopy demonstrated homogeneously dense, bipyramidal structures, similar to Charcot-Leyden crystals. Should the morphology be equivocal, immunophenotypical and molecular studies might be necessary to yield the correct diagnosis.
Of note, somewhat unusual in our patient is the clinical presentation with cognitive deficits. While this could be merely a coincidental event, a possible association with patient's myeloma cannot be completely excluded based on the current data available. Histopathological evaluation of the central nervous system and/or future case reports of similar clinical and histopathological presentations should help to understand whether such a connection exists.
In summary, this case of plasma cell myeloma with rhomboid crystalline inclusions invites review and consideration of plasma cell myeloma in cases presenting with intracytoplasmic crystalline inclusions and extracellular crystals depositions. The literature suggests that plasma cell myeloma and crystal deposition can occur in different sites such as joints, kidneys, skin and cornea in addition to bone marrow. None of the cases reported so far presented with crystal deposition in the bone marrow and in other sites.