A lumbar puncture is a procedure that is used to examine the cerebrospinal fluid (CSF).
The cerebrospinal fluid is the fluid which circulates around the brain and membranes around the brain and spinal cord (the meninges). It is mainly used in aiding diagnoses pertaining to infection, inflammatory diseases and traumatic injuries. It can also be used therapeutically and for anaesthesia, however this document is mainly concerned with the investigative use of a lumbar puncture.
To begin with the procedure is usually carefully explained to the patient, including the risks and benefits. In experienced hands, a lumbar puncture is a relatively safe procedure.
Proper positioning of the patient is extremely important. The patient is asked to lie on their side, with their back toward the examiner and then curl into a ball. This involves the patient flexing their neck and lower spine, whilst drawing up their thighs toward their chest. The shoulders and pelvis should be vertically aligned without forward or backward tilt.
In approximately 94% of individuals the spinal cord terminates at the level of the L1 vertebrae. In the further 6% of individuals the spinal cord can extend to the L2-L3 interspace. Therefore a lumbar puncture is generally performed at or below the L3-L4 interspace. As a general anatomical rule, the line drawn between the posterior iliac crests often corresponds closely to the level of L3-L4. The interspace is selected after palpation of the spinous processes at each lumbar level.
Once the area for needle insertion has been ascertained, the examiner puts on a mask and sterile gloves – this decreases the risk of infection. The skin is then cleansed with alcohol and usually an iodine based disinfectant and the area is draped with a sterile cloth. A local anaesthetic, commonly 1% lignocaine, is injected into the subcutaneous area – this should be ideally done at least 5 minutes prior to insertion of the lumbar puncture needle.
The lumbar puncture needle is typically a 20 – 22 gauge needle and it is inserted into the target area and slowly advanced. The bevel of the needle is maintained in a horizontal position (with the flat portion of the bevel pointing up) and it should be parralel to the direction of the dural fibers. In most cases the needle is advanced 4 – 5cm before the subarachnoid space is reached – this is characteristically recognized by a sudden decrease in resistance and sometimes a ‘popping’ sound may be heard.
Once a subarachnoid space has been reached, a manometer can be attached to the needle to record the opening pressure. Fluid is then usually obtained for collection. Fifteen millilitres of CSF is usually sufficient for a sample.
Analysing Cerebrospinal Fluid
The fluid is then taken and may be analysed for a number of parameters (according to the clinical presentation) including:
- cell count with differential
- protein and glucose concentrations
- culture – bacterial, fungal, mycobacterial and viral
- smears – gram stains and acid-fast bacilli smear
- antigen tests and serology (e.g. cryptococcal antigen, latex agglutination, limulus lusate tests)
- PCR (polymerase chain reaction) tests to amplify DNA or RNA of micro-organisms
- antibody tests
- cytology or cell studiesSome normal values for CSF parametes are as follows:
- Glucose 2.22-3.89mmol/L
- Lactate 1-2mmol/L
- Protein 0.15-5g/L
- Red Blood Cells 0
- IgG 0.009-0.057g/L
The minor risks and complications associated with a lumbar puncture include backache, post lumbar puncture headache, radicular pain and numbness. Major complications that rarely occur include infection, haemorrhage, damage to the spinal cord or nerve roots and herniation of cerebral tissue in patients with pre-existing increased intracranial pressure.