Investigations: Cerebrospinal fluid
BMJ 2003; 327 doi: https://doi.org/10.1136/sbmj.0311408 (Published 01 November 2003) Cite this as: BMJ 2003;327:0311408
- Suneeta Kochhar, final year medical student1,
- William Marshall, reader and honorary consultant in clinical biochemistry1
- 1Guy's, King's, and St. Thomas's School of Medicine, London
When to take a cerebrospinal fluid sample
After taking a history and doing a physical examination it is important to order appropriate investigations and interpret the results in order to formulate a diagnosis and make treatment decisions.
Cerebrospinal fluid (CSF) investigations are usually done when meningitis is suspected. But examining CSF may also be useful when diagnosing suspected subarachnoid haemorrhage, multiple sclerosis, and certain polyneuropathies, such as Guillain-Barré syndrome and chronic inflammatory demyelinating polyneuropathy.
CSF is produced by the choroid plexus and circulates in the subarachnoid space--a sample is usually obtained by lumbar puncture. A lumbar puncture is also done for intrathecal administration of drugs to treat pain and severe spasticity, measurement of CSF pressure when there is clinical evidence of rising intracranial pressure, and for the therapeutic removal of CSF in benign intracranial hypertension to prevent visual disturbances and secondary brain injury.
Obtaining a CSF sample
Contraindications
A lumbar puncture should not be done if intracranial pressure is raised, or if an intracranial lesion with mass effect is a possibility. This is because doing a lumbar puncture with raised intracranial pressure carries a risk of herniation of the cerebellar tonsils--also called coning. You may suspect this in a patient with a severe headache, nausea, focal neurology, or papilloedema.
Other relative contraindications to consider include clotting abnormalities, as there may be a risk of prolonged bleeding, and skin infection, which potentially would lead to contamination at the site of needle insertion. When considering whether or not to do a lumbar puncture, always ask someone more senior for help.
SIMON FRASER/RVI NEWCASTLE/SPL
Doctor performs lumbar puncture on patient
Procedure
The patient usually lies …
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