Cerebrospinal fluid
Objectives: Diagnosis of bacterial or fungal meningitis by microscopic examina-tion and culture with identification and susceptibility test of the isolated organism. Diagnosis of viral meningitis (polio, rabies, some arboviruses) by isolation and identification of the virus in cell culture.
The aspiration of the cerebrospinal fluid (CSF) is contraindicated in the following situations: elevated cerebrospinal fluid pressure (except if there is a strong suspicion of meningitis or subarachnoid haemorrhage); suspicion of a space-occupying lesion (e.g. abscess) in the posterior fossa; bleeding diathesis or anticoagulant therapy (relative contraindication); and infected lumbar area (relative contraindication).
Test material: Cerebrospinal fluid (CSF), subarachnoid fluid, subdural fluid, cisternal fluid, ventricular fluid, ventricular shunt fluid.
Collection time: As soon as the suspicion arises, preferably before antibiotic treatment is started.
Equipment: Request form, labels, surgical gloves, sterile cotton wool sponges, antiseptic solution, 1% lidocaine with 1% epinephrine, syringe, needle, spinal puncture needle, manometer, sterile specimen tubes, plaster.
Procedure: 1. The collection should only be performed by a physician.
2. Inform the patient about the procedure and get his or her acceptance. Fill in the request form.
3. Do a funduscopic examination and look for signs of elevated cerebrospinal fluid pressure (bilateral disc elevation, distended retinal veins with absent pulsation). If this is present, use a smallbore (25 gauge) lumbar puncture needle and be prepared to treat herniation (see below).
4. Place the patient on his side with his back at the edge of the bed. Ask the patient to bend his knees, hip, back, and neck as much as possible to decrease the normal lumbar lordosis and spread the spinous processes.
5. Wash your hands and put on surgical gloves. The puncture site is normally between the spinous processes of the 4th and 5th lumbar vertebrae and can be found where a line joining the iliac crest crosses the columna. The space between the 3rd and 4th vertebrae may be used in adults, but not in infants and children.
8. Advance slowly until a definite resistance followed by a "give" is felt as the dura is passed. Remove the obturator stylet. If no "give" is felt when the needle has reached a depth of 4 cm (adult), remove the obturator stylet at each 2 mm advancement of the needle.
9. Ask the patient to straighten the legs and neck and connect the manometer to the needle. Use the needle as a zero point and measure the initial cerebrospinal fluid pressure in millimetres. The normal pressure is 60-200 mm and more than 300 mm is definitely increased. Disconnect the manometer.
11. If the patient develops signs of herniation, stop the collection and start treatment (see below).
12. Remove the lumbar puncture needle gently and if iodine has been used as an antiseptic, wash it away with alcohol. Apply a plaster over the puncture site.
14. Instruct the patient to remain flat in bed for at least 4 hours.
Other investigations: If the CSF is cloudy: blood culture, haemoglobin, blood leukocytes, serum electrolytes, serum creatinin, plasma glucose.
Complications: Transient headache
Aetiology: occurs in 15-30% of all punctures, more in younger males, usually suboccipital, lasts 1-10 days.
Prevention: Use a small-gauge spinal needle; instruct the patient to remain flat in bed for at least 4 hours; do not remove an excessive amount of spinal fluid.
Bloody tap
Aetiology: Laceration of epidural venous plexus with the spinal needle.
Transient backache
Aetiology: Multiple puncture attempts.
Prevention: Infiltrate adequately with local anaesthetic; avoid multiple or traumatic pass.
Dry tap
Aetiology: Excessive lateral location of needle tip.
Prevention: Accurately determine midline and intraspinal position.
Meningitis or epidural or subdural empyema
Aetiology: Contaminated needle, inadequate skin disinfection, local sepsis of the skin.
Prevention: Use meticulous aseptic technique, avoid puncture in presence of local infection in the lumbar area, avoid lumbar puncture in septicemic patients without signs of meningitis.
Subdural haematoma
Aetiology: Removal of too large a volume of spinal fluid in an elderly patient, resulting in tearing of the perforating vein.
Prevention: Check prothrombin (Quick) time or prothrombin-proconvertin test (thrombotest) in patients on anticoagulant therapy.
Herniation (unconsciousness, bradycardia, respiratory depression, bilateral pupil dilatation).
Aetiology: Removal of spinal fluid from below a site of impaction of neural tissue.
Prevention: Do not tap in the presence of papilloedema or focal neurological signs unless a mass lesion is ruled out.
Treatment: • Do not remove the lumbar puncture needle.
• Attach the manometer to the lumbar puncture needle.
• Administer mannitol (20%) intravenously (10 mL/kg over 30-60 minutes in adults), until the cerebrospinal fluid pressure is below 200 mm Hg.
• Remove lumbar puncture needle and immediately give dexamethasone phosphate 10 mg intravenously or intramuscularly.
• Control pressure with dexamethasone phosphate, 4 mg every 6 hours intramuscularly, or mannitol intravenously.
Exacerbation of paraparesis
Aetiology: Removal of spinal fluid from below a complete intraspinal block.
Prevention: Do not remove excessive amounts of CSF in a patient suspected of intraspinal mass lesion.
Transient radicular pain
Aetiology: Nerve root hit by spinal needle.
Prevention: Advance a spinal needle with bevel parallel to the axis of the spine.
Storage: Refrigeration at 2-8 °C.
Transportation: Short transportation time is essential. If the transportation time is more than one hour, the specimen should be sent in a cooling box (28 °C).
Reporting: Results of the microscopy and all positive cultures of CSF are reported immediately to the treating physician. Negative bacterial results are sent out 72 hours after the CSF is received. Viral isolation may take at least 7 days.
Comments: Total and differential white cell count is essential, particularly in the differentiation of bacterial and nonbacterial meningitis. In bacterial meningitis the glucose level is usually low and protein level is high, whereas in viral meningitis the glucose is within normal value and might increase slightly. In partially treated bacterial meningitis, e.g. in patients who have received oral antibiotic therapy before meningitis was considered, the differentiation between bacterial and viral meningitis can be extremely difficult, but a low glucose level should be taken as an indication of the need for the start of intravenous antibiotic therapy.
Early in the infection of bacterial meningitis one may find an abundance of bacteria with only a few leukocytes. This is particularly true in pneumococcal and staphylococcal infections. Initially polymorphonuclear leukocytes predominate, but as recovery takes place these are gradually replaced by lymphocytes. Polymorpho-nuclear leukocytes may also be present, along with lymphocytes, in tuberculous meningitis, cerebral abscess, listeria meningitis, and in the early stages of poliomyelitis. A predominance of lymphocytes is the typical picture in most viral meningitis and encephalitis, active neurosyphilis, and tuberculous meningitis cases. Eosinophilic pleocytosis is seen with certain helminthic infections of the central nerve system (angiostrongyliasis, gnathostomiasis, paragonimiasis and cestode larval disease).
In tuberculous meningitis "classical signs" of meningitis may be missing in a substantial number of cases. Choroid tubercles, miliary tubercles in the lung or a primary complex may give the clue. To identify acid-fast bacilli by microscopy, adequate volumes of CSF should be investigated. On a single specimen there is a less than 40% chance of identifying acid-fast bacilli, but this increases to nearly 90% on four specimens, drawn daily for four days.
If the CSF is collected it may also be used for immunoassays (antigen detection) for certain infections.