DIAGNOSIS Multiple intracranial meningeal extramedullary hematopoiesis
EMH occurs as a compensatory mechanism in a variety of disorders characterized by a chronic hematopoietic deficit such as thalassemia major and intermedia, congenital spherocytosis and sickle cell anemia, and bone marrow dysfunction seen in myelofibrosis and carcinomatosis. Lymphoma and leukemia can also result in EMH. Intracranial EMH is rare. The pathway of EMH involvement of the dura is unknown, but it is postulated that the dura has hematopoietic capacity in the fetus and EMH may originate from primitive rests. In addition, the epidural space is a potential space that can harbor metastatic or heterotopic tissue deposits. Various hypotheses exist to explain the development of hemopoietic tissue to supplement marrow production. Lyall  proposed the possibility of direct extension from marrow into epidural space. Knoblich  proposed that multipotential cells in the epidural space can be transformed into marrow under conducive circumstances
EMH is typically asymptomatic but can cause neurological symptoms when the mass compresses adjacent tissue such as nerve roots in the spine or the optic nerve . Intracranial EMH patients can present with headaches, seizures, hemiplegia, altered consciousness and cranial nerve compression symptoms, although asymptomatic patients have been reported . The multiple intracranial extra-axial masses can mimic multiple meningiomas, lymphoma, myeloma, leukemia and neuroblastoma and other metastatic malignant disease. In addition, the differential diagnoses include epidural hematoma, abscess, granulomatous diseases like tuberculosis and sarcoidosis, and pachymeningeal thickening related to rheumatoid disease.
Diagnosis of EMH is based on the clinical circumstances, laboratory data and the use of different diagnostic imaging modalities. MRI is the examination of choice in patients suspected of having meningeal involvement or cord or nerve compression. Multiplanar enhanced views are helpful in defining the extent. These masses are usually lobular, well-circumscribed masses of intermediate signal intensity on T1-weighted images and low signal intensity on T2-weighted images. These masses may show significant enhancement after gadolinium administration. In this case, meningeal tail sign was displayed on T1-weighted image after gadolinium administration. Meningeal tail sign is not only indication of meningeoma, and can be showed in some diseases encroaching on meninges such as lymphoma, metastatic carcinoma, and chronic inflammation. This is the reason why our case was misdiagnosed after MRI scan on admission.
Confirmation of EMH can be made by tissue biopsy. Grossly, EMH usually forms a soft, red mass resembling a hematoma on its cut surface . Histologically, all hematopoietic elements found consist of myeloid cells, erythroid cells at various stages of maturation, and megakaryocytes . In our case, hematoxylin and eosin stain showed collections of large immature cells resembling immature reticuloendothelial cells or hemocytoblasts just like immature granulocytes and mononuclear cells. And the clustered nucleated erythroblasts presented smaller rounded or oval cells with hyperchromatic nuclei, frequently admixed with mature red blood cells. So the lesions consisted of hyperplasia of trilineage hemopoietic stem cells, including myeloid cells (granulocytes and mononuclear cells), mature and immature erythroid cells, and megakaryocytes. On immunohistological staining, intense expression of CD61 was observed in megakaryoblasts and megakaryocytes. The erythroid cells expressed the glycophorin A. The myeloid elements were positive for myeloperoxidase.
Management of EMH masses includes hypertransfusion, cytotoxic drugs, radiotherapy or combinations of these modalities. Hematopoietic tissue is highly sensitive to low-dose radiation . Treatment of EMH is usually unnecessary except when complications occur . Since hematopoietic tissue is highly sensitive to irradiation, radiotherapy is the treatment of choice for patients with nerve compression. Neurological improvement has been achieved in 3-7 days after initiation of treatment . Occasionally, the patient may need surgical intervention . Surgery confers the immediate relief of nerve compression and provides tissue for histological diagnosis. It has the disadvantages of incomplete excision, operating on anemic individuals who are poor surgical candidates, and it has the potential risk of profuse bleeding from the surgical site . Therefore, surgery is only indicated where immediate relief of spinal cord or nerve root compression is required to prevent permanent neurological damage. Blood transfusion and iron chelation with desferrioxamine is the ideal treatment for asymptomatic individuals. It relieves anemia and suppresses EMH, and is most effective when used as an adjunct to surgery or radiotherapy .
Contributed by Wen-Xia Jiang, Shao-Qiang Zheng, Pei-Jun Wang