Cerebral Spinal fluid (CSF) is the fluid that protects and nourishes the brain and spinal cord. It is a filtrate of arterial blood that is produced by the choroid plexuses of the lateral and fourth ventricles of the brain. CSF flows inside all the ventricles, the central canal of the spinal cord, and throughout the subarachnoid space of the brain. The body produces up to a maximum of 150 mL of CSF for adults and 60 mL for neonates. This volume remains constant as the ventricles secrete and reabsorb CSF at a rate of approximately 840 mL a day. The blood-brain barrier refers to the structure and function of the capillaries inside the choroid plexuses of the ventricles. All pre and post capillary vessels in the brain are covered by an extension of the subarachnoid space called the perivascular space. The capillaries are not covered by the perivascular space. They make contact with the endothelial cells in the choroid plexuses and have a special structure that only allows for the movement of certain substances between the blood and the CSF. The result is that small lipophilic molecules such as oxygen and carbon dioxide can move freely between the two structures, whereas larger molecules such as glucose and amino acids require the help of transporters. The blood-brain barrier protects the brain from toxic substances, drugs, and other foreign materials. In the event that the blood-brain barrier is disrupted, materials that are normally kept out of the CSF can gain entry into the brain. Inflammatory mediators and malignant brain cells are examples of things that can cause disruptions of the structures in the blood-brain barrier.
CSF can be collected for both therapeutic and diagnostic purposes. In the event that intracranial pressure is increased CSF can be collected in order to relieve pressure inside the brain. CSF obtained for laboratory analysis can reveal the presence of inflammation, trauma, infections, or even malignancies. CSF can be collected from a variety of sites, however it is most often collected using a lumbar puncture. In this case a needle is inserted into the space between vertebrae in the lumbar portion of the spine in order to remove CSF. In the event that a lumbar puncture cannot be performed a cisternal puncture or a ventricular puncture can be used. A cisternal puncture is done with fluoroscopy, and involves placing the needle below the occipital bone of the skull in order to obtain a sample. A ventricular puncture is usually done in the operating room and involves inserting a needle directing into one of the ventricles of the brain. Lastly samples can be obtained from shunts that have been placed either in the spinal column or in the ventricles. Up to 20 mL of fluid can be obtained for analysis and is collected 2-4 mL at a time into three separate sterile tubes. Each tube should be labeled in the order that it is collected. The first tube collected is used for the analysis of chemistries, serology, and if needed a beginning cell count. The second tube is used for microbiological analysis. The third tube is used for a final cell count and morphology. Cell counts should be done within one hour of collection since cells degrade rapidly in CSF. Cell counts can also be paired (performed on tubes 1 & 3) in order to rule out the presence of contamination due to a traumatic tap.
In a normal healthy patient CSF is clear, colorless, and contains only a few cells. It can have up to five leukocytes per cmm but should not have any erythrocytes present. The leukocytes most often seen are lymphocytes and monocytes. Because the brain requires proteins and glucose for nourishment it is normal to find these molecules in CSF. A normal sample of CSF has a total protein level around 15-45 mg/mL and a glucose level around 40-70 mg/mL. These values can change in response to a disruption in the blood-brain barrier. There are several instances where CSF protein levels will be increased. Mild increases can occur due to inflammation caused by diseases such as meningitis, encephalitis, presence of tumors, hemorrhage, and stroke. Bacterial meningitis will cause a much larger increase in CSF protein concentration. Severe increases in protein concentration can be found in patients with Guillain-Barre syndrome. For patients with multiple sclerosis the increase in protein concentration is mild however there will be a specific elevation in IgG which can be quantified via separate testing. A decrease in CSF protein concentration is a sign that the body is producing CSF rapidly either due to illness or injury. Increases in CSF glucose are a reflection of high serum glucose levels. Most often CSF glucose concentrations will present as normal to decreased. A decrease in CSF glucose can be a sign of bacterial or fungal infection as these organisms utilize the glucose available to them. Tumors and leukocytes will also utilize glucose and increased concentrations of these cells can lead to a decrease in the concentration of CSF glucose. When performing a cell count and differential it is important to note the numbers and types of cells found in CSF. Increases in the amount of leukocytes present can be indicative of infection, stroke, or tumors. Neutrophils will be more abundant in bacterial infections while lymphocytes and monocytes will be more abundant in viral infections and malignancies. The presence of erythrocytes can be indicative of a bleed either in the brain or the spinal cord. However blood can be introduced into a CSF sample as the result of a traumatic tap. It is also important to note that high levels of erythrocytes can falsely increase the concentration of CSF protein. This can occur even when the sample is full of degraded erythrocytes and appears xanthochromic in color.
Cerebral spinal fluid analysis is an important tool available to doctors when diagnosing patients. The brain and spinal cord are well protected by the body; however in the event of illness or injury they can become compromised. It is imperative to perform analysis of samples quickly in order to gain the most accurate picture of a patient’s condition. This valuable analysis allows clinicians to properly treat patients and save lives.
*Turgeon, M. L., Clinical Hematology Theory and Procedures. Boston/Toronto/London: Little, Brown & Company (1993). p. 406.
*Cerebrospinal Fluid. (2008). Chapter Fourteen: Cerebrospinal Fluid. Neuropathology Web. Retrieved on May 29, 2010 from http://www.neuropathologyweb.org/chapter14/chapter14CSF.html
*Cerebral Spinal Fluid Collection. (2010). Cerebral Spinal Fluid Collection. Medline Plus. Retrieved on May 29, 2010 from http://www.nlm.nih.gov/medlineplus/ency/article/003428.htm