What is cerebrospinal fluid (CSF)?
Cerebrospinal fluid (or CSF) is formed within the brain, primarily at specialized sites called the choroid plexuses.
"A major function of CSF is to provide a 'cushion' for the brain and spinal cord."
CSF is found within the brain and in the space that surrounds both the brain and the spinal cord (this space is called the subarachnoid space). A major function of CSF is to provide a 'cushion' for the brain and spinal cord.
When is collection of CSF indicated?
The collection of CSF is usually indicated when the patient shows clinical signs such as seizures, incoordination, circling behavior, neck or back pain, and no obvious cause (such as recent trauma) is known.
How is CSF collected?
CSF can be collected from two different sites, the cerebellomedullary cistern, located at the back of the head and the lumbar cistern, located at the lower part of the spinal column, near the pelvis. CSF collection requires a general anesthetic. Your veterinarian will choose one of the two collection sites, based upon the clinical signs that your pet is exhibiting. The sample site is shaved and the skin is thoroughly cleaned with alcohol or another skin disinfectant. A small amount of CSF (about ½ teaspoon in total) is withdrawn into a sterile collection tube, using a special spinal needle.
Are there any risks associated with CSF collection?
Yes. Some risks include those associated with any general anesthetic. Therefore it is important to evaluate the general health of your pet prior to anesthesia by a series of screening tests including a complete blood count, a serum biochemistry profile, and a urinalysis (handouts are available on these subjects). Other risks are specific to CSF collection, and include the risk of trauma to the spinal cord from the spinal needle, the possibility of brain herniation if the pet has increased pressure in the central nervous system, and the possibility of introducing bacteria through the collection site. These specific risks are minimized by careful attention to technique and a thorough evaluation of your pet prior to undertaking this procedure.
How is the CSF sample assessed?
A CSF sample must be processed within 30 to 60 minutes of collection or the cells will deteriorate. Therefore, your veterinarian may refer your pet to a facility that is capable of immediately processing the sample. The veterinary pathologist will evaluate the sample for a total nucleated cell count, a red blood cell count, a total protein determination, and a concentration of the cells in the sample.
"A CSF sample must be processed within 30 to 60 minutes of collection or the cells will deteriorate."
The total nucleated cell and red blood cell counts are done in a specialized counting chamber called a hemocytometer. The protein content is determined by methods that can detect very low protein concentrations (microprotein techniques). The cells in the sample are concentrated and placed on microscope slides, which are evaluated by a veterinary pathologist familiar with the normal appearance of CSF cells.
What can the evaluation of CSF tell us?
The evaluation of CSF frequently does not provide a specific diagnosis, but it will often provide evidence to support the most likely diagnosis when additional factors such as the breed, age, and clinical signs of the animal are taken into account.
"...evidence to support the most likely diagnosis..."
Occasionally a specific diagnosis can be reached. For example, the presence of bacteria or fungal organisms may be detected along with increased numbers of inflammatory cells, leading to a diagnosis of bacterial or fungal infection. Occasionally, neoplastic cells may be found, indicating an underlying tumor within the brain or spinal cord.
Can any further evaluation of a CSF sample be undertaken?
Yes. Further testing can include protein electrophoresis if the CSF protein content is markedly increased (see our handout on protein electrophoresis). Electrophoresis will indicate what type of protein is involved in the increase, which will suggest its most likely cause. If bacteria are identified within a CSF sample, or if large numbers of poorly preserved inflammatory cells are found, then a bacterial culture may be suggested.
Occasionally, a sample may be evaluated for the presence of specific antibodies against infectious agents such as Toxoplasma or Ehrlichia.