Pleural extramedullary haematopoiesis in myelosclerosis.

DESPITE the extensive literature on myelosclerosis and extramedullary haematopoiesis (myeloid metaplasia), reference to the pleura as being the site of extramedullary haematopoiesis is not made in either the standard medical and pathology textbooks or in the radiological literature. We have recently encountered a case of myelosclerosis in which aspiration of what appeared on both clinical and radiological grounds to be a pleural effusion failed to reveal the presence of fluid. Subsequent pleural biopsy carried out at the same site showed tissue, at first regarded as tumour, but later shown to be a manifestation of extramedullary haematopoiesis. Because such a lesion when it occurs at this particular site is liable to be misinterpreted, we feel that a description of this case is justified. Myelosclerosis may be regarded as a form of myelofibrosis with thickening of the bony trabeculae of the marrow on histological examination. The diagnosis of myelofibrosis is made when the presence of fibrosis on bone marrow biopsy is associated with a leuco-erythroblastic anaemia. A mild increase in bone trabeculation may cause an osteosclerosis visible on X-ray examination. Myelofibrosis may be a complicating feature of many diseases including lymphoma, metastatic carcinoma, leukaemia and aplastic anaemia, under which conditions it is termed secondary myelofibrosis (Pitcock et al., 1962). They emphasize that in secondary myelofibrosis, evidence of the underlying disorder may be found on examination of bone-marrow sections as well as the presence of fibrous tissue. Furthermore, in such cases, the blood picture may be distinctive. Myelofibrosis without an associated haematological, reticuloendothelial or neoplastic disorder is classed as being idiopathic. The above authors, however, consider that myelofibrosis following polycythaemia vera should be included in the idiopathic myelofibrosis group, because it cannot be differentiated pathologically once fibrosis has developed in the bone marrow. The case to be described belongs to the idiopathic myelofibrosis category. Extramedullary haematopoiesis accompanies many types of bone marrow disease including idiopathic myelofibrosis, carcinomatosis, lymphoma, leukaemia, marble bone disease, erythroblastosis, haemolytic anaemia, pernicious anaemia, thalassaemia, Gaucher's disease, osteitis deformans, osteomalacia, osteitis fibrosa cystica, tuberculosis and exposure to toxic agents such as benzene, fluorine and irradiation. In a recent report, sicklecell anaemia was found to be associated with extramedullary haematopoiesis (Seidler & Becker, 1964). Rarely, extramedullary haematopoietic tissue may occur in patients not suffering from diseases of the blood or bone marrow. Dodge & Evans (1956) describe a presacral retroperitoneal fatty tumour with haematopoietic foci, for which they suggest myelolipoma as a suitable descriptive term. It should be pointed out that extramedullary haematopoiesis does not necessarily occur in myelofibrosis: Korst, Clatanoff & Schilling (1956) found it in only eleven of their twenty-three cases.