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© Dirk Biddle
A lumbar puncture is an examination of the patient's cerebrospinal fluid (CSF). It is also known as a spinal tap. The cerebrospinal fluid surrounds the brain and spinal cord, and acts as a buffer, like a cushion, protecting the brain and spine from injury. CSF is normally clear and colourless, and changes in its colour, quantity or composition may be an indication of neurological damage or disease.
The patient lies still, in the foetal position, while a local anaesthetic is administered to his or her lower back. Sometimes the test is done with the person sitting up, but bent over. A needle is carefully inserted into the patient's spinal column, between the third and fourth vertebrae, into the fluid-filled area that surrounds the nerve roots. Once the needle is properly positioned, spinal fluid pressure is measured, and a little fluid is removed for testing. The needle is then removed, the area is cleaned, and a bandage is placed over the needle site. Nervous patients may be given a mild sedative or sleeping pill, and the examination carried out while they are asleep. Some patients complain about headaches, hours or days after the examination. Patients are advised to lie flat for 1-3 hours after the spinal tap. Very rare side effects include bleeding and infection.
Additional conditions under which the test may be performed include Chronic inflammatory polyneuropathy, Dementia due to metabolic causes, Encephalitis, Epilepsy, Febrile seizure (children), Generalized tonic-clonic seizure, Hydrocephalus, Inhalation anthrax, Normal pressure hydrocephalus (NPH), Pituitary tumour and Reye's syndrome (1).
Interpretation of abnormal results:
Pressure:
Increased pressure - increased intracranial pressure (pressure within the skull) from trauma or infection,
Decreased pressure - obstruction to the flow of CSF above the puncture site (spinal cord tumour), shock, fainting, diabetic coma.
Appearance:
Cloudy - infection, white blood cells in the CSF, protein in the CSF, microorganisms, Bloody or reddish coloured - bleeding within the brain or subarachnoid space, spinal cord obstruction, traumatic lumbar puncture (first specimen bloody, rest clear),
Brown, orange, yellow colour - elevated protein in the CSF, old (greater than 3 days) blood in the CSF.
Protein:
Increased protein - blood in the CSF, diabetes, polyneuritis, tumours, trauma, any inflammatory or infectious condition,
Decreased protein - rapid CSF production.
Increased Gamma globulin - demyelinating disease such as multiple sclerosis, neurosyphilis, Guillain-Barre syndrome.
Glucose:
Increased Glucose - systemic hyperglycaemia (elevated blood sugar),
Decreased Glucose - systemic hypoglycaemia (low blood sugar), bacterial or fungal infection (such as meningitis), tuberculosis, carcinomatous meningitis.
Leucocytes:
Increased WBC - active meningitis, acute infection, beginning of a chronic illness, tumour, abscess, brain infarction (stroke), demyelinating disease (such as multiple sclerosis)
Erythrocytes:
RBC - bleeding into the spinal fluid, traumatic lumbar puncture.
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1) http://www.nlm.nih.gov/medlineplus/ency/article/003428.htm
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