Lumbar puncture - Investigations and procedures

Lumbar puncture

This is the procedure by which cerebrospinal fluid (CSF) is aspirated by an approach between the lumbar vertebrae.

Indications

When any of the following are suspected:

·        Infection (meningitis, encephalitis, fungal infections or neurosyphilis).

·        Multiple sclerosis.

·        Subarachnoid haemorrhage (with a normal CT head). Guillain–Barre´ syndrome.

·        Meningeal carcinomatosis (malignant meningitis) including lymphoma.

CSF pressure measurement is useful – particularly in the diagnosis of idiopathic (benign) intracranial hypertension, where CSF removal may be a therapeutic manoeuvre.

Lumbar puncture (LP) is also required for intrathecal administration of contrast media (in myelography), and drugs such as antimicrobials and chemotherapy.

Contraindications

Infection at the site of LP.

Suspected intracranial mass lesion – focal neurology or depressed GCS should lead to a CT brain scan prior to LP.

Suspected raised intracranial pressure or papilloedema before CT evaluation.

Suspected spinal cord compression.

Bleeding disorders should be corrected first (including a platelet count of <40 × 109/L, anti-coagulant drugs such as heparin or warfarin).

Congenital lumbosacral lesions such as meningomyelocele, because the cord may be tethered or low.

Procedure

After giving consent, the patient is positioned on their left side on a firm surface, with the back at the edge of the couch. The knees are drawn up as far as possible and the neck flexed, to open up the spinous processes of the lumbar vertebrae.

The aim is to insert the needle between L3–L4 or L4–L5 in adults (below the level of the spinal cord). L4 normally lies at the level of the iliac crests. The area is cleaned and infiltrated with lidocaine.

The lumbar puncture needle is inserted in the midline with its stylet in place aiming slightly towards the umbilicus. The needle is advanced slowly ∼4–5 cm, and a slight give is often felt as it penetrates the dura mater. The stylet is withdrawn and if no CSF appears, it is reinserted and the needle advanced slightly – this is repeated until CSF appears. If the needle encounters firm resistance, it should be withdrawn and another approach tried.

Sometimes the patient will feel a pain radiating into the leg or back – this is due to the needle touching a root – if it persists, the needle will have to withdrawn and a slightly different angle attempted.

Once CSF appears, the CSF pressure can be measured by attaching a manometer (normal 6–15 cm H2O). CSF is collected in three sterile tubes (sent for microscopy and culture, protein and cytology) and an additional sample is sent for glucose measurement. A simultaneous blood sample for glucose should be sent. Oligoclonal bands are identified using paired CSF and serum samples.

Complications

The most common complication is headache, which may be treated by lying flat and adequate hydration.

Bleeding, infection, arachnoiditis, exacerbation of spinal cord compression, cerebral herniation and spinal cord or root injury are all recognised risks, but occur rarely.