If it does not clear, blood indicates subarachnoid hemorrhage. Erythrocytes from subarachnoid hemorrhage are cleared in 3 to 7 days. A few neutrophils and mononuclear cells may also be present as a result of meningeal irritation. Xanthochromia may also be seen with hemorrhagic infarcts, brain tumors, and jaundice. Polymorphonuclear pleocytosis indicates acute suppurative meningitis.
Tumor cells indicate dissemination of metastatic or primary brain tumors in the subarachnoid space.
The most common among the latter is medulloblastoma. They can be best detected by cytological examination. A mononuclear inflammatory reaction is often seen in addition to the tumor cells. A more severe increase occurs in the Guillain-Barr syndrome and acoustic and spinal schwannoma. Oligoclonal bands are also seen occasionally in some chronic CNS infections.