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Lumbar puncture

In medicine, a lumbar puncture (colloquially known as a spinal tap) is a diagnostic procedure that is done to collect a sample of cerebrospinal fluid (CSF) for biochemical, microbiological and cytological analysis.



The most common indication for a lumbar puncture is to collect cerebrospinal fluid in a case of suspected meningitis. Subarachnoid hemorrhage, hydrocephalus, benign intracranial hypertension and other diagnoses may be supported or excluded with this test.

Lumbar punctures may also be done to inject medications into the cerebrospinal fluid.


A lumbar puncture requires aseptic technique and performance by qualified and skilled medical practitioners.

In performing a lumbar puncture (in an adult), first the patient is usually placed in a left (or right) lateral position with his/her neck bent in full flexion and knees bent in full flexion up to his/her chest, approximating a fetal position as much as possible. It is also possible to have the patient sit on a stool and bend his/her head and shoulders forward. The area around the lower back is prepared using aseptic technique. Once the appropriate location is palpated, a spinal needle is inserted, usually between the lumbar vertebrae L3/L4 or L4/L5 and pushed in until there is a "give" that indicates the needle is past the dura mater. The stylet from the spinal needle is then withdrawn and drops of cerebrospinal fluid are collected. The opening and closing pressures of the cerebrospinal fluid may be taken during this collection. The procedure is ended by withdrawing the needle while placing pressure on the puncture site. The patient should then lie on his/her back for at least six hours and be monitored for any signs of neurological problems. The technique described is almost identical to that used in spinal anesthesia, except that spinal anesthesia is more often done with the patient in a sitting position.

Patient anxiety during the procedure can lead to increased CSF fluid pressure, especially if the person holds their breath, tenses their muscles or flexes their knees too tightly against their chest. Diagnostic analysis of changes in fluid pressure during lumbar puncture procedures requires attention both to the patient's condition during the procedure and to their medical history.


Adverse side effects from a lumbar puncture range from introduced infection to headache. Headache is the most common complication. Serious complications of a lumbar puncture include damage to the spinal cord or spinal nerve roots resulting in weakness or loss of sensation, or even paraplegia. The latter is very rare.

The procedure is not recommended when epidural infection is present or suspected, when topical infections or dermatological conditions pose a risk of infection at the puncture site or in patients with severe psychosis or neurosis with back pain. Elevated or reduced pressure in the brain also poses risks during lumbar punctures. Withdrawal of fluid when initial pressures are too low could result in spinal cord compression or cerebral hernia.

Removal of cerebrospinal fluid resulting in reduced fluid pressure has been shown to correlate with greater reduction of cerebral blood flow among patients with Alzheimers disease. Its clinical significance is uncertain.


Increases in CSF pressure after withdrawal of fluid can indicate congestive heart failure, cerebral edema, subarachnoid hemorrhage, hypo-osmolality resulting from hemodialysis, meningeal inflammation, purulent meningitis or tuberculous meningitis.

Decreases in CSF pressure can indicate complete subarachnoid blockage, leakage of spinal fluid, severe dehydration, hyperosmolality, or circulatory collapse . Significant changes in pressure during the procedure can indicate tumors or spinal blockage resulting in a large pool of CSF, or hydrocephalus associated with large volumes of CSF.

The presence of white blood cells in cerebrospinal fluid is called pleocytosis . Counts above 500 usually indicate a purulent infection. Usually present as granulocytes, white cells can also indicate reaction to repeated lumbar punctures, reaction to a needle contaminated with detergent, reactions to prior injections of medicines or dyes, central nervous system hemorrhage, leukemia or a metastatic tumor.

Several substances found in cerebrospinal fluid are available for diagnostic measurement.

  • Measurement of chloride levels can detect the presence of tuberculous meningitis .
  • Decreased glucose levels can indicate fungal, tuburculous or pyogenic infections, lymphomas, leukemia spreading to the meninges, meningoencephalatic mumps or hypoglycemia.
  • Increased glucose levels in the fluid can indicate diabetes.
  • Increased levels of glutamine are often involved with hepatic encephalopathies, Reye's syndrome, hepatic coma, cirrhosis and hypercapnia
  • Increased levels of lactate can occur the presence of cancer of the CNS, multiple sclerosis, low blood pressure, low serum phosphorus, respiratory alkalosis, idiopathic seizures, traumatic brain injury, cerebral ischemia, brain abscess, hydrocephalus, hypocapnia or bacterial meningitis.
  • The enzyme lactate dehydrogenase can be measured to assess meningial infections as being of bacterial origin, which are often associated with high levels of the enzyme, or in the absence of unusual levels as likely being of viral origin.
  • Changes in protein content of cerebrospinal fluid can result from pathologically increased permeability of the blood-cerebrospinal fluid barrier , obstructions of CSF circulation, meningitis, syphilis, brain abscesses, subarachnoid hemorrhage, polio, collagen disease or Guillain-Barré syndrome, leakage of CSF, increases in intercranial pressure or hyperthyroidism.

Last updated: 05-07-2005 08:03:04
Last updated: 05-13-2005 07:56:04