William Blows, PhD, BSc, RMN, RGN, RNT, OStJ
Lecturer in applied biological sciences, St Bartholomew School of Nursing and Midwifery, CityUniversity, London.
What is it?
A lumbar puncture is the insertion of a hollow tube needle under local anaesthetic into the subarachnoid space of the spinal canal to obtain a sample of cerebrospinal fluid (CSF) for clinical investigations or to inject medication (Fig.1).
The three coverings of the brain and spinal cord (meninges), are separated by small spaces. The outer membrane, the dura mater, is in two layers, with a space between containing fluid, blood vessels and venous sinuses. Beneath the dura mater is a small subdural space, followed by the arachnoid mater, which has a spider’s web-like structure. Below this, the subarachnoid space is filled with CSF. The innermost layer, the pia mater, rests on the brain and cord surface.
The procedure is carried out by the doctor, who will insert the needle in the space between the third and fourth lumbar vertebrae (Fig 1).
Insertion any higher up the spine would risk injury to the spinal cord. The level is found by imagining a line drawn from one iliac crest to the other, and noting where this line crosses the vertebrae.
CSF is derived from blood and is produced in the ventricles of the brain. It passes to the subarachnoid space through tiny canals. In the subarachnoid space it flows down the spinal cord and returns to the space surrounding the brain. Here it is absorbed back into the blood. Thus the CSF creates a cycle, and is changing constantly.
CSF has a number of functions. It:
- Acts as a shock absorber, protecting the central nervous system (the brain and cord) against trauma;
- Provides buoyancy - the brain’s apparent weight is nearly 30 times lighter than its dead weight due to ‘floating’ in CSF;
- Helps to compensate for pressure changes within the skull (for a limited time), for example when a space occupying lesion (SOL) develops – this is achieved through changing in the rate of CSF production or absorption;
- Helps to keep the brain’s biochemical environment stable by providing some nutrition and removing waste metabolites for excretion.
Why is it done?
Lumbar puncture is done for one of three reasons:
- To acquire a sample of CSF for analysis;
- To measure the CSF pressure;
- To introduce drugs into the spinal canal (called an intrathecal injection).
Samples of CSF are taken for:
- Taking cell counts (a tiny number of white cells may normally be present);
- Measuring glucose and protein (also present in small quantities);
- Cytology, i.e. looking for abnormal cells;
- Immunoglobulin (antibody) studies;
- Bacterial or viral tests;
- Biochemical analysis.
Lumbar puncture may also be carried out to introduce a contrast radio-opaque medium (one that shows up on X-ray), to provide radiographic images of the spinal canal that do not show on ordinary X-rays. This type of X-ray is called a myelogram and is used for two purposes - the diagnosis of spinal lesions and to help plan surgery by isolating the level of the lesion and selecting the most suitable spinal segment for operation.
- Normal CSF is mostly water, but it also contains:
- Protein (15-45mg per 100ml);
- Glucose (40-80mg per 100ml);
- Lactate (1.1-1.9mmol/l);
- Lymphocytes (0-5 cells per mm3).
- Normal CSF pressure measured with a manometer is 60-160mm of water.
Normal CSF is a clear watery fluid, but the presence of many cells, extra protein or blood can alter its appearance.
- Cells, either lymphocytes in larger than normal numbers, or pus cells, can cause turbidity, and this can indicate the presence of an infection.
- Fresh blood in the CSF may be due to injury to a vertebral vein by the LP needle, and the sample will become clear as it continues to drip from the needle (Sharief, 2000). Continuous blood contamination indicates bleeding into the subarachnoid space, usually due to a very recent (usually within hours) ruptured aneurysm in the head, possibly within the Circle of Willis (Blows, 2001).
- A continuous yellow stain to the fluid (xanthochromia) is usually due to an older subarachnoid bleed that has had time for the red cells to settle out, leaving other blood plasma components such as proteins and other cells in the CSF.
- High protein counts may be due to infection of the meninges (meningitis) or the brain (encephalitis), or due to brain tumours or multiple sclerosis (Sharief, 2000).
- Other changes of CSF contents as a result of central nervous system infections are reduced or absent glucose levels and a disturbance of the lactate levels – these being bigger changes seen in bacterial rather than viral infections.
- Increased CSF pressure is indicative of raised intracranial pressure due to SOLs such as tumours or intracranial bleeds. It is also seen in hydrocephalus (excess CSF covering the brain in some children) (Blows, 2001).
- Lumbar puncture is a strict aseptic technique requiring full sterile procedures.
- Encourage patients to drink well before and after the procedure.
- Positioned the patient carefully, laying on one side in a curled up position with the lumbar spine exposed (knees drawn up to the chest). Moving the patient’s back closer to the edge of the bed will make access to the lumbar spine easier. Support the patient in this position throughout the procedure.
- A small local sterile dressing is applied to the spinal site after removal of the needle.
- Headache is a common complaint following lumbar puncture. The patient should lay flat for 6-12 hours afterwards, as sitting up may make any headache worse.
- In myelograms, the patient’s head should be kept raised for up to 24 hours afterwards to prevent contrast medium in the spinal canal from entering the skull. This may caused seizures if it passes around the brain (Blows, 2002).
Blows, W.T. (2002) Lumbar puncture. Nursing Times; 98: 36, 25-26.
Blows, W.T. (2001) The Biological Basis of Nursing: Clinical Observations. Abingdon: Routledge.
Sharief, M. (2000) Lumbar puncture and CSF examination. Medicine; 28: 6, 36-38.