Lumbar puncture (LP)

What is Lumbar Puncture (LP)?

Lumbar Puncture (LP), commonly known as a spinal tap, is a medical procedure performed to access the subarachnoid space in the lumbar (lower back) region of the spinal canal. Its primary purposes are diagnostic (collecting cerebrospinal fluid (CSF) for analysis) and therapeutic (injecting medications or anesthetics, or relieving pressure).

For diagnostic purposes, lumbar puncture provides crucial information about the cerebrospinal fluid (CSF) and the environment within the central nervous system:

• Measuring CSF Pressure: The opening pressure of the CSF is measured using a manometer connected to the spinal needle. Normal pressure in a relaxed adult lying on their side is typically between 60 and 200 mmH₂O (millimeters of water), although slightly higher values up to 250 mmH₂O may be considered acceptable by some institutions. Elevated pressure can indicate conditions like hydrocephalus, infections, bleeding, or tumors.
• Assessing Subarachnoid Space Patency: Dynamic tests (like Queckenstedt's test, though less commonly performed now due to risks and the availability of imaging) can help assess if there is a blockage (e.g., from a tumor or spinal injury) hindering the flow of CSF within the spinal canal.
• Analyzing CSF Composition: The collected CSF sample is sent to the laboratory for analysis. This includes assessing its appearance (color, clarity), cell count (white blood cells, red blood cells), protein levels, glucose levels, and performing specific tests like cultures (for bacteria, fungi, viruses), cytology (for malignant cells), and specialized assays (e.g., for specific antibodies or proteins associated with diseases like multiple sclerosis).

Lumbar puncture is particularly valuable and often considered the gold standard for diagnosing certain conditions:

• Subarachnoid Hemorrhage (SAH): Detecting red blood cells or their breakdown products (xanthochromia) in the CSF confirms bleeding into the subarachnoid space, especially when CT scans are negative or equivocal.
• CNS Infections: Identifying bacteria, viruses, fungi, or elevated white blood cell counts in the CSF is crucial for diagnosing meningitis, encephalitis, and myelitis.
• Inflammatory Conditions: Detecting specific markers like oligoclonal bands can support the diagnosis of multiple sclerosis or other inflammatory diseases like Guillain-Barré syndrome (which typically shows elevated protein with normal cell count - albuminocytologic dissociation).
• Malignancy: Identifying malignant cells (carcinomatous meningitis, leptomeningeal metastases) through CSF cytology.

Changes such as increased protein or decreased glucose concentration in the CSF can also point towards various infectious, inflammatory, or neoplastic processes.

Lumbar Puncture Procedure Technique

The patient is typically positioned lying on their side (lateral decubitus position) with their knees drawn up towards the chest and neck slightly flexed, resembling a fetal position. This position helps to open the spaces between the lumbar vertebrae. Alternatively, the procedure can be performed with the patient sitting upright and leaning forward over a stable support.

The standard insertion sites are the interspaces between the spinous processes of L3-L4 or L4-L5 vertebrae. These levels are below the termination of the spinal cord (conus medullaris) in adults, minimizing the risk of cord injury. The L4 spinous process can be located by palpating the highest points of the iliac crests and drawing an imaginary line between them (Tuffier's line).

Strict aseptic technique is paramount. The skin over the puncture site is cleaned thoroughly, typically with an antiseptic solution like chlorhexidine or povidone-iodine. If iodine is used, it should be removed with alcohol before puncture to prevent carrying iodine particles into the subarachnoid space. Sterile drapes are applied.

Local anesthetic (e.g., 1-2% lidocaine) is usually injected into the skin and deeper subcutaneous tissues at the puncture site using a fine needle to minimize discomfort during the spinal needle insertion.

A specialized spinal needle with a tightly fitting stylet is then inserted through the anesthetized track. Standard needles are typically 20-22 gauge and around 9 cm (3.5 inches) long for adults, though different sizes are available. The needle is advanced slowly in the sagittal plane, angled slightly cephalad (towards the head). It passes through the skin, subcutaneous tissue, supraspinous ligament, interspinous ligament, ligamentum flavum, epidural space (containing fat and veins), dura mater, and finally the arachnoid mater to enter the subarachnoid space.

A distinct "pop" or feeling of decreased resistance is often felt as the needle penetrates the dura mater and ligamentum flavum. Once entry into the subarachnoid space is suspected, the stylet is removed to check for CSF flow. If CSF appears, a manometer is attached to measure the opening pressure. CSF is then collected sequentially into sterile tubes (typically 3-4 tubes) for various laboratory analyses. The fluid should be allowed to drip out passively; aspiration is generally avoided. The volume collected depends on the tests required but is usually around 8-15 mL in adults.

After collection, the stylet is reinserted before withdrawing the needle to reduce the risk of post-LP headache. Pressure is applied to the puncture site, and a sterile dressing is applied.

Indications for Lumbar Puncture Procedure

Lumbar puncture may be indicated for various diagnostic and therapeutic reasons, particularly in neurology and neurosurgery:

Diagnostic Indications:

• Suspicion of CNS infection (meningitis, encephalitis, myelitis)
• Suspicion of subarachnoid hemorrhage (especially if CT is negative)
• Evaluation of inflammatory conditions (e.g., Multiple Sclerosis, Guillain-Barré Syndrome, CNS vasculitis, arachnoiditis)
• Diagnosis of CNS malignancies (leptomeningeal metastases, primary CNS lymphoma)
• Evaluation of unexplained neurological symptoms where CSF analysis might provide clues
• Measurement of CSF pressure (e.g., idiopathic intracranial hypertension, normal pressure hydrocephalus)
• Checking patency of the spinal subarachnoid space (less common, usually adjunct to imaging)
• Evaluation of suspected CSF leak (liquorrhea) after trauma or surgery (specific tests on fluid)

Therapeutic Indications:

• Administration of intrathecal medications (e.g., chemotherapy, antibiotics for certain CNS infections, anesthetics for spinal anesthesia)
• Reduction of CSF pressure (e.g., temporary relief in idiopathic intracranial hypertension, CSF drainage after surgery)
• Introduction of contrast agents for myelography (though largely replaced by MRI) or air/oxygen/ozone for specific procedures (less common).

Therapeutic LP (removing larger volumes, e.g., 10-20 mL or more) can sometimes be used to aid CSF circulation dynamics or remove blood/inflammatory products, although this requires careful consideration of risks and benefits.

Contraindications to Lumbar Puncture Procedure

Performing an LP when contraindicated can lead to serious complications. Key contraindications include:

• Signs of Significantly Increased Intracranial Pressure with Mass Effect: Especially suspected intracranial hematoma, large tumor, or brain abscess (particularly temporal lobe or posterior fossa location). Performing LP in these situations carries a high risk of **cerebral herniation** (downward displacement of brain structures), which can be fatal. Brain imaging (CT or MRI) is often required before LP if raised ICP with mass effect is suspected (e.g., papilledema, focal neurological signs, altered mental status).
• Signs of Impending Brainstem Herniation: Clinical signs like altered consciousness, pupillary changes, abnormal posturing, or Cushing's triad strongly contraindicate LP.
• Infection at the Puncture Site: Skin or soft tissue infection overlying the intended needle insertion site in the lumbosacral area increases the risk of introducing infection into the CNS.
• Severe Bleeding Diathesis or Anticoagulation: Uncorrected severe thrombocytopenia (low platelets) or coagulopathy (impaired clotting) significantly increases the risk of spinal epidural hematoma. Anticoagulant medications often need to be temporarily stopped or reversed according to specific guidelines before LP.
• Severe Spinal Deformity or Previous Surgery: May make the procedure technically difficult or impossible without imaging guidance.

Relative Contraindications / Situations Requiring Caution:

• Traumatic Shock or Massive Blood Loss: Stabilize the patient first.
• Comatose Patient: LP may be considered carefully if the cause of coma is unclear and CSF analysis is critical for diagnosis (e.g., distinguishing traumatic, vascular, metabolic, infectious causes), but only after ruling out significant mass effect with imaging if possible.

Preventing Lumbar Puncture-Related Complications

While generally safe when performed correctly, LP carries potential risks. Prevention strategies are crucial:

1. Careful Patient Selection: Strictly adhere to indications and contraindications. Obtain brain imaging prior to LP whenever increased intracranial pressure with mass effect is suspected.
2. Aseptic Technique: Meticulous skin preparation and use of sterile equipment are essential to prevent infection.
3. Proper Needle Technique:
   • Use an appropriate-gauge spinal needle (smaller gauge, e.g., 22-25G, may reduce headache risk).
   • Use atraumatic (pencil-point) needles (e.g., Sprotte, Whitacre) instead of cutting (Quincke) needles, as they are associated with a significantly lower incidence of post-LP headache.
   • Ensure the needle bevel is oriented parallel to the longitudinal dural fibers (bevel up/down if patient is lateral, bevel sideways if patient is sitting) during insertion and removal.
   • Reinsert the stylet before withdrawing the needle to minimize CSF leakage.
4. Limit CSF Removal: In diagnostically unclear or high-risk situations (e.g., borderline elevated pressure), remove only the minimum amount of CSF necessary for essential tests (often 1-2 mL per tube). Avoid excessive or rapid removal of CSF, especially if opening pressure is high.
5. Post-Procedure Care: While prolonged bed rest is controversial and its effectiveness debated, advising the patient to lie flat for a period (e.g., 1-2 hours) and ensuring adequate hydration may help some individuals minimize post-LP headache risk. Clear instructions should be given regarding symptoms to watch for (severe headache, fever, neurological changes).
6. Manage Coagulopathy: Ensure platelet counts and coagulation parameters are within acceptable limits, or take corrective measures before the procedure. Follow guidelines for managing anticoagulant/antiplatelet therapy.

The most common complication is post-lumbar puncture headache (PLPH), thought to be caused by CSF leakage from the dural puncture site. Other potential complications include back pain, infection (meningitis, epidural abscess), bleeding (spinal hematoma), nerve irritation or injury, and, rarely, cerebral herniation if performed when contraindicated.

References

1. Doherty CM, Forbes RB. Diagnostic Lumbar Puncture. Ulster Med J. 2014 May;83(2):93-102. PMID: 25075138; PMCID: PMC4113153. (Review of technique, indications, contraindications)
2. See husen DA, Reeves MM, Fomin DA. Cerebrospinal fluid analysis. Am Fam Physician. 2003 Sep 15;68(6):1103-8. PMID: 14524396. (Overview of CSF analysis)
3. Roos KL. Lumbar Puncture. Semin Neurol. 2003 Mar;23(1):105-14. DOI: 10.1055/s-2003-40758. PMID: 12870105. (Neurological perspective on LP)
4. Engleborghs S, Niemantsverdriet E, Struyfs H, Blennow K, Brouns R, Comabella M, et al. Consensus guidelines for lumbar puncture in patients with neurological diseases. Alzheimers Dement (Amst). 2017 Jun 21;8:111-126. DOI: 10.1016/j.dadm.2017.04.007. PMID: 28656185; PMCID: PMC5484180. (Consensus guidelines)
5. Turnbull DK, Shepherd DB. Post-dural puncture headache: pathogenesis, prevention and treatment. Br J Anaesth. 2003 Nov;91(5):718-29. DOI: 10.1093/bja/aeg231. PMID: 14570796. (Detailed review of post-LP headache)