Lumbar Puncture and Cerebrospinal Fluid
INDICATIONS
When central nervous system (CNS) infection (meningitis, encephalitis) is suspected, one must examine the cerebrospinal fluid (CSF). However, there is an exception: lumbar puncture (LP) should not be performed if one suspects brain abscess or another significant space-occupying mass lesion.
An LP is performed to diagnose subarachnoid hemorrhage when that diagnosis is strongly suspected, and the computed tomography (CT) scan result is negative.
An LP is done when cerebrospinal fluid (CSF) chemistries may have diagnostic value (e.g., gamma globulin and oligoclonal banding in multiple sclerosis).
LP is needed for the study of CSF pressure:
To check for increased pressure in suspected pseudotumor cerebri.
To check for low pressure in spontaneous intracranial hypotension headache.
In normal pressure hydrocephalus, when removal of CSF may improve gait and mentation.
LP is done for cytology, when carcinomatous or lymphomatous meningitis is suspected. Remember to take large volumes for cytology. First tap results are often negative, and a second or third tap may be required to demonstrate abnormal cells.
Therapeutically, an LP may be done to inject methotrexate or ara-c for central nervous system (CNS) leukemia, or amphotericin B for fungal meningitis, or to remove fluid as treatment for benign increased intracranial pressure.
CONTRAINDICATIONS
Contraindications to LP include the following:
Infection at the site of the LP.
Severe thrombocytopenia or uncorrected bleeding disorder.
When a cerebral mass lesion (brain abscess, brain tumor, etc.) is suspected, particularly in a patient with lateralized neurologic signs or a possible mass in the posterior fossa.
Intracranial hemorrhage. This condition is best diagnosed by CT scan. In these instances, a CT scan, or magnetic resonance imaging (MRI) for definition of the hemorrhage, and a search for a mass lesion should be done before the LP.
Spinal block. This is a relative contraindication to a lumbar puncture (LP); a CT scan or MRI should be done before considering an LP.
Presence of papilledema (a check of the fundi must precede each LP) without radiologic examination. An LP ultimately may be done in a patient with papilledema (e.g., in pseudotumor or if CSF examination is crucial), but only after neurologic or neurosurgical consultation.
In the presence of spinal epidural abscess. Performing an LP in this setting may spread infection to the CSF.
Clopidogrel may be a contraindication to LP, due to increased risk of epidural bleeding. Similarly anticoagulants are a contraindication to LP.
COMPLICATIONS
Post-lumbar puncture (LP) headache occurs in 10% to 30% of patients. It is characteristically exacerbated by sitting or standing, and it is relieved by lying flat. It is seen within the first 1 to 3 days after the LP; it usually lasts 2 to 5 days, although it may persist for weeks. Treatment consists of bed rest and fluids. The mechanism of the headache is believed to be continued CSF leakage through the dural hole at the site of the LP, with subsequent intracranial traction on the meninges. Caffeinated beverages, ergot preparations, and theophylline may be helpful in this headache. Severe leaks can be treated by the placement of a blood patch by an anesthesiologist or neurosurgeon. Post-LP headache may be minimized by using a small-gauge needle (22) (type A recommendation), inserting the needle parallel to the dural fibers so they are spread apart rather than torn, and having the patient turn prone before removing the needle. The diagnostic lumbar puncture data for noncutting needles versus cutting needles are inconclusive. Having the patient lie in bed after LP does not appear to reduce the incidence of post-LP headache, and no longer is recommended. Patients with migraine are particularly prone to post-LP headaches.
When there is an unexpected increased opening pressure (it must stay elevated after the patient has relaxed with legs extended, and a few minutes have elapsed from the onset of the LP), remove minimal fluid needed for studies. One may leave the needle in with the stopcock closed to prevent further leakage.
Neurologic or neurosurgical consultation should be obtained, the use of mannitol or steroids should be considered, and the patient should be watched carefully over the ensuing hours for signs of deterioration. Patients with meningitis may have markedly elevated pressures, but these pressures are not as severe a risk as increased intracranial pressure secondary to a focal lesion. Remember, hypercarbia, water intoxication, and hypertensive encephalopathy are remediable causes of increased intracranial pressure. When the intracranial pressure is increased, and there is neurologic deterioration immediately or during the hours after the LP, treatment with osmotic dehydrating agents and steroids is indicated (see Chapter 31). In cryptococcal meningitis, acutely elevated CSF pressures may lead to blindness from optic nerve pressure, even when CT scan is normal. Treatment with continuous lumbar or ventricular drainage, in combination with antibiotics, is usually effective.
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