Lumbar puncture

Lumbar puncture (LP) is the procedure in which one can obtain and analyse cerebrospinal fluid. Cerebrospinal fluid can be assessed in the evaluation of many neurological conditions. The CSF pressure can also be measured during lumbar puncture.

Indications

• Assessment of CSF, in the evaluation of
  • CNS infection or inflammation (meningitis, encephalitis)
  • CNS malignancy or paraneoplastic syndrome
  • Subarachnoid haemorrhage
  • Gullain-Barré syndrome
  • Multiple sclerosis
  • Dementia
• CSF pressure measurement, in the evaluation of:
  • Normal pressure hydrocephalus (tap test)
  • Pseudotumour cerebri
• Intrathecal administration of drugs

Procedure of lumbar puncture

Performing lumbar puncture while there is increased ICP can cause cerebral herniation, and so ruling out increased ICP is important before performing LP. This can be performed by ophthalmoscopy (by looking for papilloedema) or by head CT.

LP can be performed with the patient in sitting position or lying on their side. In either case, the patient should flex their back maximally to enlarge the space between the vertebrae. If lying on their side, the patient should have a pillow between their legs to prevent the back from twisting. Lying position gives less postprocedural headache.

A line is drawn from the top of both iliac crests to the spine. This line crosses the spine at approximately L4 level. One can puncture between L3-L4 or L4-L5, as the spinal cord ends at L1-L2.

Sterile technique must be used. Local anaesthesia may be used, although the most painful part is needle penetration through the skin and subcutaneous tissue, and applying local anaesthesia means that the patient experiences two needle punctures rather than one. The penetration of the LP needle beyond the subcutaneous tissue is usually not painful, but rather uncomfortable.

The needle is introduced in the midline, although paramedian entry is also a possibility, but mostly for those who are very experienced. The needle bevel should point to the patient’s side, to allow the needle to spread the ligaments rather than cut them. The needle should be angled slightly cranially, as if aiming for the umbilicus. When the needle penetrates the ligamentum flavum and dura mater, a sudden loss of resistance is felt, which is a good sign. While progressing the needle, repeatedly remove the stylet to observe for CSF flow, which indicates successful entry into the subarachnoid space. When flow is achieved, CSF can be collected.

Contraindications

• Increased intracranial pressure due to expansive process in the skull (may cause herniation)
• Infection above or near the point of entry of LP, for example epidural abscess
• Thrombocytopaenia < 40 000
• Anticoagulation (should be stopped a few days before)

Interpretation

Compared to plasma, the CSF has less protein and very few cells. The electrolyte composition is also different, but this isn't of clinical relevance.

Parameter	Normal range	Bacterial meningitis	Viral meningitis	Multiple sclerosis
Pressure	8 – 20 cmH2O	Elevated	Normal	Normal
Appearance	Clear, like water	Turbid	Clear	Clear
RBCs	A few	Normal	Normal	Normal
WBCs	< 5 cells/µL	Elevated (mostly granulocytes)	Elevated (mostly lymphocytes)	Elevated (mostly lymphocytes)
Protein	< 0,5 g/L	Elevated	Normal	Normal
Glucose	> 60-70% of blood glucose	Decreased	Normal	Normal
Oligoclonal bands (on electrophoresis)	Absent	Absent	Absent	Present

Microbiological examinations (culture, PCR), immunoglobulins, as well as measurement of certain biomarkers, like those of dementia or prion disease, can also be measured in CSF.

Complications

• Headache (relatively common)
• Spinal haematoma
• Infection

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