Normal CSF is typically sparsely cellular with rare, mature-appearing lymphocytes and monocytoid cells. These usually occur as singly dispersed cells.
Bacterial meningitis in CSF contains mostly neutrophils. Similar to lymphocytes and monocytoid cells, neutrophils occur as singly dispersed cells.
In contrast to a CSF with normal findings, a specimen from a patient with aseptic meningitis is more cellular and has a mixture of lymphocytes, plasma cells, and monocytoid cells.
Medulloblastoma, as seen in this figure, is a small round blue cell tumor that occurs primarily in children. Identification of nuclear molding is useful in distinguishing medulloblastoma from lymphoma/leukemia, another small round blue cell tumor. Metastatic small cell carcinoma also demonstrates nuclear molding, but it affects primarily adults.
Small cell carcinoma has cells with scant cytoplasm, nuclear molding, and no nucleoli. The lung is often a source of occult primary tumors presenting as brain metastases. There is morphological overlap between small cell carcinoma and medulloblastoma, but the latter predominantly affects children.
