Lumbar puncture is the insertion of a specialised needle into the lumbar subarachnoid space to gain access to the cerebrospinal fluid. This may be required:

The procedure may be performed on patients who are inpatients or day patients, according to their clinical status. A lumbar puncture should not be performed if raised intracranial pressure is suspected as the raised pressure might cause the brain stem tissue to herniate through the foramen magnum. This is known as ‘coning’ and could be fatal (Boon et al 2004).

A lumbar puncture is performed by a medical practitioner using an aseptic technique. The patient is helped into the correct position. An area of skin above the 3rd, 4th and 5th lumbar vertebrae is prepared and cleansed with alcohol-based antiseptic solution prior to the administration of local anaesthesia. Once the local anaesthetic has taken effect, a special lumbar puncture needle is inserted between the 3rd and 4th, or 4th and 5th, lumbar vertebrae in order to gain access to the subarachnoid space below the spinal cord in the region of the cauda equina (Fig. 23.1). The needle is hollow with a stilette to ease introduction to the subarachnoid space. Once in position, the stilette of the needle is removed. A disposable manometer is attached to the end of the needle via a two-way tap, and the cerebrospinal fluid (CSF) is allowed to flow into the manometer to record the intraspinal pressure. A normal CSF pressure is 60–150 mmH₂O. The pressure will fluctuate with respiration and heart beat. Coughing will cause the pressure to rise (Clarke 2002).

When the pressure recording has been completed, the manometer is occluded, and 2–3 ml of cerebrospinal fluid is allowed to flow into each of the three separate sterile specimen containers as required while still maintaining asepsis. The specimen containers should be pre-labelled, numbers 1, 2 and 3 because the first specimen may contain blood as a result of the needle being introduced. The containers should collect the specimens sequentially to avoid misinterpretation of results. The medical practitioner will note the colour, consistency and opacity of the cerebrospinal fluid as well as observing the presence or absence of blood. On completion of this stage, the needle is removed, and the puncture site is covered by a small sterile dressing or plastic sealant spray. Following this procedure, according to local policy and the patient’s clinical status, appropriate neurological observations, wound-site assessment and the monitoring of any localised pain or headache should be performed (Lindsay & Bone 2004).