What is a Lumbar Puncture?
A lumbar puncture (also called spinal tap) is a medical procedure in which a needle is inserted between the vertebrae in the lower back to collect cerebrospinal fluid (CSF) for diagnostic or therapeutic purposes. 1
The Procedure
The procedure involves the following key steps:
- A needle is inserted into the subarachnoid space (the area surrounding the spinal cord) in the lumbar region of the spine, typically between the L3-L4 or L4-L5 vertebrae 2, 3
- CSF is collected through the needle either by passive gravity flow or gentle aspiration for laboratory analysis 2
- The patient is positioned either lying on their side (lateral recumbent position) with knees pulled to chest and neck flexed, or sitting upright 1, 2
Why It's Performed
Lumbar puncture serves both diagnostic and therapeutic purposes:
Diagnostic Indications
- Suspected central nervous system (CNS) infections including meningitis and encephalitis—this remains the most common indication 4, 5, 3
- Diagnosis of Alzheimer's disease and other dementias by measuring CSF biomarkers for amyloid-β and tau proteins 1, 6
- Suspected subarachnoid hemorrhage when CT scan results are normal 5
- Autoimmune encephalitis to detect neuronal autoantibodies 3
- Inflammatory and neoplastic diseases affecting the CNS 7
Therapeutic Indications
- Administration of intrathecal medications such as antisense oligonucleotides for spinal muscular atrophy 3
- Reduction of CSF pressure in certain conditions 7
What Information It Provides
CSF analysis obtained through lumbar puncture includes:
- Cell count and differential to identify white blood cells, red blood cells, and determine if infection is viral (lymphocytic) or bacterial (neutrophilic) 8
- Protein and glucose levels to help differentiate between infectious and non-infectious causes 8, 4
- Microorganism identification through Gram stain and culture 8
- Molecular testing including PCR for pathogens like herpes simplex virus 4, 3
- Biomarkers for neurodegenerative diseases such as amyloid-β and tau in Alzheimer's disease 1
- Opening pressure measurement when performed in lateral recumbent position 2
Safety Profile
Lumbar puncture is a generally safe procedure when performed correctly, with serious complications occurring in less than 1% of patients. 2, 5, 7
Common Side Effects
- Post-dural puncture headache occurs in 0.9% to 9.0% of patients when atraumatic needles are used, and up to 35% with conventional needles 1, 2
- Back pain and soreness at the puncture site 1, 2
- Transient leg numbness 2
Serious Complications (Rare)
- Cerebral herniation in patients with elevated intracranial pressure and mass lesions 5, 9
- Bleeding in patients with coagulation disorders 2, 4
- Infection at the puncture site 9, 7
Key Safety Measures
- Use atraumatic (pencil-point) needles of 22-25 gauge rather than conventional cutting needles—this reduces post-puncture headache from 11% to 4.2% 1, 2
- Position patient in lateral recumbent position rather than sitting to minimize complications 1, 2
- Allow passive CSF flow rather than active aspiration with syringe 2
- Limit attempts to fewer than 4 punctures—risk doubles with 2-4 attempts and increases five-fold with ≥5 attempts 2
When It Should NOT Be Performed
Absolute contraindications include:
- Elevated intracranial pressure with mass lesion or space-occupying lesions 4, 5
- Severe coagulation abnormalities (INR >1.5 or platelets <40-100 × 10⁹/L) 2, 4
- Skin infection at puncture site 2, 7
- Impaired consciousness (Glasgow Coma Scale <13) 4
- Focal neurological signs or papilledema suggesting increased intracranial pressure 4