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In medicine, a lumbar puncture (colloquially known as a spinal tap) is a diagnostic and at times therapeutic procedure that is performed in order to collect a sample of cerebrospinal fluid (CSF) for biochemical, microbiological, and cytological analysis, or rarely to relieve increased intracranial pressure.
The most common indication for a lumbar puncture is to collect cerebrospinal fluid in a case of suspected meningitis, since there is no other reliable tool with which meningitis can be excluded and it is often a life-threatening but highly treatable condition. Young infants commonly require lumbar puncture as a part of the routine workup for fever without a source, as they have a much higher risk of meningitis than older persons and do not reliably show signs of meningeal irritation (meningismus). For instance, most experts agree that any infant less than 2 months of age with fever of 38 degrees Celsius or greater and no identifiable source requires a lumbar puncture. In any age group, Subarachnoid hemorrhage, hydrocephalus, benign intracranial hypertension and many other diagnoses may be supported or excluded with this test.
A lumbar puncture requires aseptic technique (sterile) and performance by qualified and skilled medical practitioners.
In performing a lumbar puncture, 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, local anaesthetic is infiltrated under the skin and then injected along the intended path of the spinal needle. 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 The stylet from the spinal needle is then withdrawn and drops of cerebrospinal fluid are collected.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 pressure of the cerebrospinal fluid may be taken during this collection by using a simple column manometer. The procedure is ended by withdrawing the needle while placing pressure on the puncture site. In the past, the patient would often be asked to lie on his/her back for at least six hours and be monitored for signs of neurological problems, though there is no scientific evidence that this provides any benefit. 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.
The seated position is advantageous in that there is less distortion of spinal anatomy which allows for easier withdrawal of fluid. It is preferred by some practitioners when a lumbar puncture is performed on an obese patient where having them lie on their side would cause a scoliosis and unreliable anatomical landmarks. On the other hand, opening pressures are notoriously unreliable when measured on a seated patient and therefore the left or right lateral (laying down) position is preferred if an opening pressure needs to be measured.
Patient anxiety during the procedure can lead to increased CSF 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.
The risks and benefits of a lumbar puncture should be discussed in detail with the patient by the practitioner who will be performing the procedure. Almost always, when it is indicated, the benefits significantly outweigh the risks.
Headache with nausea is the most common complication; it often responds to analgesics and infusion of fluids and can often be prevented by strict maintenance of a supine posture for two hours after the successful puncture. Merritt's Neurology (10th edition), in the section on lumbar puncture, notes that intravenous caffeine injection is often quite effective in aborting these so-called "spinal headaches." Contact between the side of the LP needle and a spinal nerve root can result in anomalous sensations (paresthesia) in a leg during the procedure; this is harmless and patients can be warned about it in advance to minimize their anxiety if it should occur. A headache that is persistent despite a long period of bedrest and occurs only when sitting up may be indicative of a CSF leak from the lumbar puncture site. It can be treated by more bedrest, or by a "blood patch", performed by a neurosurgeon injecting the patient's own blood back into the site of leakage under radiologic guidance to cause a clot to form and seal off the leak.
Serious complications of a properly performed lumbar puncture are extremely rare. They include spinal or epidural bleeding, and trauma to the spinal cord or spinal nerve roots resulting in weakness or loss of sensation, or even paraplegia. The latter is exceedingly rare, since the level at which the spinal cord ends (normally the inferior border of L1, although it is slightly lower in infants) is several vertebral spaces above the proper location for a lumbar puncture (L3/L4 or L4/L5). There are case reports of lumbar puncture resulting in perforation of abnormal dural arterio-venous malformations, resulting in catastrophic epidural hemorrhage; this is exceedingly 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. Some authorities believe that withdrawal of fluid when initial pressures are abnormal could result in spinal cord compression or cerebral herniation; others believe that such events are merely coincidental in time, occurring independently as a result of the same pathology that the lumbar puncture was performed to diagnose. In any case, computerized tomography of the brain is often performed prior to lumbar puncture if an intracranial mass is suspected.
Removal of cerebrospinal fluid resulting in reduced fluid pressure has been shown to correlate with greater reduction of cerebral blood flow among patients with Alzheimer's disease. Its clinical significance is uncertain.
Increased CSF pressure can indicate congestive heart failure, cerebral edema, subarachnoid hemorrhage, hypo-osmolality resulting from hemodialysis, meningeal inflammation, purulent meningitis or tuberculous meningitis, hydrocephalus, or pseudotumor cerebri.
Decreased 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. Lumbar puncture for the purpose of reducing pressure is performed in some patients with idiopathic intracranial hypertension (also called pseudotumor cerebri.)
The presence of white blood cells in cerebrospinal fluid is called pleocytosis. A small number of monocytes can be normal; the presence of granulocytes is always an abnormal finding. A large number of granulocytes often heralds bacterial meningitis. White cells can also indicate reaction to repeated lumbar punctures, reactions to prior injections of medicines or dyes, central nervous system hemorrhage, leukemia, recent epileptic seizure, or a metastatic tumor. When peripheral blood contaminates the withdrawn CSF, a common procedural complication, white blood cells will be present along with erythrocytes, and their ratio will be the same as that in the peripheral blood.
Several substances found in cerebrospinal fluid are available for diagnostic measurement.
- Measurement of chloride levels may aid in detecting the presence of tuberculous meningitis.
- Glucose is usually present in the CSF; the level is usually about 60% that in the peripheral circulation. A fingerstick or venipuncture at the time of lumbar puncture may therefore be performed to assess peripheral glucose levels in order to determine a predicted CSF glucose value. Decreased glucose levels can indicate fungal, tuburculous or pyogenic infections; lymphomas; leukemia spreading to the meninges; meningoencephalitic mumps; or hypoglycemia. A glucose level of less than one third of blood glucose levels in association with low CSF lactate levels is typical in hereditary CSF glucose transporter deficiency.
- Increased glucose levels in the fluid can indicate diabetes, although the 60% rule still applies.
- 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, heritable mitochondrial disease, 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 help distinguish meningitides of bacterial origin, which are often associated with high levels of the enzyme, from those of viral origin in which the enzyme is low or absent.
- Changes in total 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 intracranial pressure or hyperthyroidism. Very high levels of protein may indicate tuberculous meningitis or spinal block.
- IgG synthetic rate is calculated from measured IgG and total protein levels; it is elevated in immune disorders such as multiple sclerosis, transverse myelitis, and neuromyelitis optica of Devic.
- Numerous antibody-mediated tests for CSF are available in some countries: these include rapid tests for antigens of common bacterial pathogens, treponemal titers for the diagnosis of neurosyphilis and Lyme disease, Coccidiodes antibody, and others.
- The India ink test is still the standard for detection of meningitis caused by Cryptococcus neoformans.
- CSF can be sent to the microbiology lab for various types of smears and cultures to diagnose infections.
- Polymerase chain reaction (PCR) has been a great advance in the diagnosis of some types of meningitis. It has high sensitivity and specificity for many infections of the CNS, is fast, and can be done with small volumes of CSF. Even though testing is expensive, it saves cost of hospitalization.
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