Cerebrospinal Fluid Tap
Summary of Procedure:
| Indications | To obtain a sample of cerebrospinal fluid for cytology, chemical analysis, and microbial culture in the work-up of central nervous system disorders. Cisternal puncture is also used for myelography and intrathecal chemotherapy. |
| Limitations | Most useful in identifying/characterizing meningitis (especially immune-mediated).
Limited utility in confirming/characterizing neoplasia (except lymphoma). Bacterial culture may be negative in septic meningitis. |
| Complications | Cervical spinal cord injury (respiratory arrest, vestibular dysfunction, paresis/paralysis).
Tentorial/foramen magnum brain herniation. Contamination of sample with blood. CNS infection (rare). |
| Contraindications | Anatomic abnormality, elevated CSF pressure (increased risk for herniation), bleeding disorders, infection of overlying tissues. |
| Supplies/instruments | Spinal needle (usually 22ga, 1.5 in), red top tube, urine dip stick. |
| Position | Lateral recumbency (right for right-handed people, left for left-handed people), with head and cervical vertebrae at edge of table. Elevate the nose so that the head is perfectly parallel to the table and flex the neck such that the head is perpendicular to the long-axis of the body. |
| Restraint | General anesthesia. |
Landmarks:
For right-handed person, identify the wings of the atlas with thumb and index finger of the left hand. Draw an imaginary line caudally from the occipital crest with the right hand. Where this line crosses the cranial border of the wings of the atlas is the site of penetration.
Details of Procedure:
- Surgically prepare a 6 cm2 area over the atlanto-occipital area.
- Kneel for the procedure. For the right-handed person, hold the needle in your right hand. Advance the needle (bevel facing rostrally) into the skin. Stabilize the hub of the needle with your left hand (which should rest against the animal), withdraw the stilette and observe for CSF. If none is seen, replace the stilette and advance the needle another 1-2 mm and recheck for CSF by withdrawing the stilette. Proceed in this manner. Do not rely on a popping sensation, which may be felt as the cisterna magna is entered. If bone is encountered, withdraw the needle and begin again, in a slightly more caudal location (the most common error is to be too far cranial) along the imaginary line. It may help to aim the needle towards the angle of the jaw.
- Allow CSF fluid to drip out into collecting tube (do not aspirate). Remove the needle in one smooth motion; do NOT replace the stilette. Hold needle vertically and allow remaining fluid to drip into tube. (For this lab, remove the needle as soon as CSF is seen so that group members may repeat the procedure.)
- If there is not enough CSF collected to perform cytology and protein count, let drip one drop of CSF on the appropriate targets (blood and proteins) of a urine dip stick in order to evaluate protein content and presence for blood contamination. This is an empirical measurement, but still a valid method to detect an increase in protein content (in the absence of blood) typical of CNS pathology.
- Note: fluid can be difficult to retrieve from the cadaver.