The normal leukocyte cell count varies in children from 0 to 30 cells per mL in neonates to the adult level of 0–5 cells per mL in the older child; however, most patients without infection have counts on the lower end of the spectrum, which make these specimens quite hypocellular [1]. The cell counts are highest in neonates and monocytes usually predominate over lymphocytes. As the child gets older, there is a decrease in the number of white blood cells, and lymphocytes start to predominate over monocytes. In addition to the rare lymphocytes and monocytes (mononuclear cells), rare neutrophils can also be seen in the normal CSF (Fig. 10.1b). However, the presence of plasma cells is almost always associated with a disease process [3, 4].

Incidental findings in normal CSF include anucleate squamous cells from the skin (most common), blood and bone marrow cells (Fig. 10.1c), ependymal lining and choroid plexus cells (Figs. 10.1d and 10.2a), capillary vessels (Fig. 10.2b), and osteoclasts and osteoblasts (Fig. 10.2c, d). These findings tend to be seen more in samples obtained from neonates and infants. Evidence of bone marrow contamination is usually heralded by the presence of immature cells from all hematopoietic lineages, particularly nucleated red blood cell precursors (e.g., basophilic or polychromatophilic normoblasts), or cartilage (dense metachromatic material with chondrocytes in lacunae), given that cases with bone marrow contamination are from lumbar punctures rather than shunt specimens. Megakaryocytes (large cells with abundant cytoplasm and multilobulated nuclei) are very rarely seen with bone marrow contamination in the CSF. In intraoperative washing specimens and specimens obtained during shunt placement, it is not uncommon to see glial fibrillary elements or neurons with prominent nucleoli. When interpreting these specimens, it is important not to misinterpret neurons, megakaryocytes, or chondrocytes as malignant cells.

Ventricular lining cells (e.g., ependymal or choroid plexus cells): These are uncommonly seen in CSF specimens and have similar cytomorphology. Usually they are seen as clusters of bland-appearing columnar cells with basally oriented, round nuclei. The overall nuclear-to-cytoplasmic ratio is low and nucleoli are not typically seen. Sometimes the clusters will form round aggregates so that the columnar nature is not evident in the CSF.

Trauma versus traumatic tap: Blood dilution of a CSF sample is more commonly caused by a traumatic tap related to the procedure than a subarachnoid hemorrhage. The possibility of subarachnoid hemorrhage due to accidental trauma, child abuse, or other causes may be in the differential diagnosis in certain pediatric patients. In general, neutrophils are not considered a normal cellular component of CSF; thus, the presence of neutrophils should raise concern for meningitis. However, in a bloody specimen, the significance of a few neutrophils is diminished given the possibility of peripheral blood contamination and the fact that mild pleocytosis can be seen in infants without CNS infection [3, 4]. One study suggested that a white blood cell (WBC) to red blood cell (RBC) ratio of less than or equal to 0.01 is considered not infected [3]. Table 10.1 compares cytologic findings of CSF in CNS hemorrhage versus traumatic tap.

Common infections (viral and bacterial): Viral infections (Fig. 10.4a, b) are more common than bacterial infections (Fig. 10.4c, d) in the CSF of children, especially in the absence of prior surgical intervention and/or intraventricular shunt placement. Viral infections typically manifest with a predominance of lymphocytes or a lymphomonocytosis. Many of these cases will not have a pathogen detected by usual studies (e.g., culture, gram stain) and are considered “aseptic meningitis.” In some viral infections, particularly herpes simplex virus (HSV) meningitis, the monocytes can have lobulated nuclei imparting a “footprint in the sand” appearance, which are called “Mollaret cells” [5]. The cytologic predominance of neutrophils is seen in the majority of bacterial meningitis, and this predominance is not limited to the first 24 h of disease [6]. CSF abnormalities associated with bacterial meningitis are rarely obscured by blood contamination from a traumatic tap [7]. Specific types of bacteria predominate in certain age groups. In a recent study by Nigrovic et al., Group B streptococcus, Streptococcus pneumoniae, and Neisseria meningitidis predominated in children 1–3 months, 3 months to 10 years, and 10–19 years of age, respectively [8]. Other organisms include gram-negative bacilli (enteric pathogens) and Listeria in the first 2 months of life, in addition to group B streptococcus. Haemophilus influenzae type b has dramatically decreased given the effective vaccination against this organism.

Less common infections (fungal): Fungal infections of CSF are extremely rare, except in two patient groups:

Ancillary studies: Gram stain, microbial culture, and PCR or other molecular studies for identification of pathogens.

Pearls: Neutrophils and other inflammatory cells start to degenerate quickly, within 2 h of collection; thus, immediate preparation of the specimen is critical. Some cases of bacterial meningitis may not have a neutrophil predominance and show more of a pleocytosis or lymphocytosis. The mixed inflammatory cells seen in bacterial infection may include clumps of reactive monocytes that may mimic the appearance of neoplastic cells (Fig. 10.5d).

Hematopoietic tumors, especially lymphoblastic leukemia, represent the most commonly encountered neoplasms involving the CSF of children. Primary CNS tumors constitute the second most common group of tumors (e.g., medulloblastoma, ependymoma, etc.) (refer to Sect. 10.2). Other small blue cell tumors of childhood may involve the CSF due to direct extension or metastasis but are least commonly encountered.